Understanding the interior processes shaping Venus surface is one of the fundamental tasks for the next decade of Venus science. Even in the absence of plate tectonics, Venus’ surface is covered with many tectonic and/or volcanic structures, including the so-called “coronae” (Latin for crown). Coronae range in diameter from 60 km to up to 2600 km and are defined by rings of closely spaced concentric fractures.  These enigmatic structures are widely distributed across the surface and are considered crucial to understanding the planet's geodynamic regime and resurfacing history. Coronae showcase diverse topographic morphologies, volcanic features, and fracture patterns. Geodynamic modelling studies have provided various scenarios for corona formation, often involving some type of plume-lithosphere interaction.

In this work, four end-member geodynamic cases for large coronae formation from  Gülcher et al. (2020) are used to compute the expected gravity signal for different scenarios through time. Results demonstrate that the addition of gravity field constraints helps to distinguish different formation scenarios that may lead to similar surface expressions. A detailed classification of coronae based on gravity requires a resolution that is greater than currently available. By carefully rescaling our models to Magellan resolution we assess a set of macro-characteristics allowing us to apply this methodology to current datasets, notably pertaining the end-member formation class and the evolution stage of the corona. By applying these concepts to the Magellan topography and gravity maps of Venus we classify several coronae as active/inactive and assess whether their formation could imply crustal recycling. Importantly, we find that 47 coronae bear signatures of present-day plume-induced activity with various styles of tectonic processes at play.

The next decade will be marked by multiple missions flying to Venus aiming to gather updated, and fundamentally more accurate gravity. Anticipating the future VERITAS mission’s gravity data, we provide an overview of the resolving power of the higher resolution dataset and what additional information on coronae formation may be forthcoming.

Gülcher, A. J. P., Gerya, T. V., Montési, L. G. J., and Munch, J. (2020). Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus. Nature Geoscience, 13(8):547–554. https://doi.org/10.1038/s41561-020-0606-1

How to cite: Cascioli, G., Gülcher, A., Mazarico, E., and Smrekar, S.: Revealing tectonic processes at large coronae on Venus by joint analysis of modelled topography and gravity, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-223, https://doi.org/10.5194/epsc2024-223, 2024.

ABSTRACT
Recent analysis of NASA’s Magellan radar data revealed that magmatic activity is ongoing on Venus. The geodynamic regime is, however, poorly constrained. Many scenarios have been proposed to explain Venus’ evolution being one of them the “plutonic-squishy” lid regime, which involves the presence of magmatic intrusions. We investigate the effects of magmatic intrusions on mantle’s thermal evolution and the present-day thermal state of the interior. We vary the percentage of extrusive magmatism and compare our results to available geophysical constraints to select successful models. Our findings support recent observations of a magmatically active planet, with Venus being primarily governed by highly intrusive magmatic processes.

INTRODUCTION
A recent study examined synthetic aperture radar data from NASA’s Magellan mission and identified a volcanic vent that changed shape in the 8-month interval between two radar images, which was interpreted as a sign of ongoing volcanic activity on Venus [1]. Moreover, the large number of volcanoes recently compiled in a global catalog [2] indicate that the planet’s evolution and present-day state has been dominated by volcanic processes.

Venus’s geodynamic regime and surface tectonics are, however, poorly constrained and several scenarios including catastrophic resurfacing, episodic plate tectonics, and the  “plutonic-squishy lid” regime were proposed [3]. This last one, which has been suggested to be representative for the tectonic regime on early Earth [4], considers that part of the melt that formed in the interior rises to the surface but a significant part remains trapped in the crust and lithosphere as magmatic intrusions.

Here, we investigate effects of intrusive magmatism on the evolution and present-day state of Venus. We evaluate our models by comparing our present-day constraints based on studies of the elastic lithosphere thickness [5,6,7] and interior viscosity of Venus [8].

METHODS
We use the geodynamic code GAIA in a 2D spherical annulus geometry [9,10]. The viscosity is temperature- and depth-dependent, following an Arrhenius law [11]. The thermal conductivity and thermal expansivity in our models are also pressure- and temperature-dependent [12]. We assume a homogeneous distribution of the heat sources and account for the decay in time of radioactive elements (i.e. ²³⁸U, ²³⁵U, ²³²Th, and ⁴⁰K) and for core cooling.

Melting occurs when the temperature of the mantle rocks is higher than the solidus, for which we assume similar values as proposed for the Earth’s interior [13]. We model that the melt can reach the surface (extrusive magmatism), but also that part of this melt can remain trapped in the crust and lithosphere (intrusive magmatism). Fig.1 shows the melt treatment in our models for fully extrusive and fully intrusive cases. We vary the extrusive melt between 0% (fully intrusive) and 100% (fully extrusive) in steps of 20%, and we vary it between 10km and 90km in steps of 20km.

RESULTS

Our results show that magmatic intrusions affect the temperature, viscosity, and velocity across the mantle. The intrusive cases have more vigorous convection than the fully extrusive one and larger temperature variations in the convecting mantle with clearly distinguishable mantle plumes (Fig.1). Moreover, the intrusive melt depth seems to control the growth of the stagnant lid: the more intrusive magmatism, the thinner the lid is (Fig.3d).

On average, Venus models with highly intrusive magmatism are characterized by a cooler interior, produce more melt at shallower depths and colder temperatures, and have higher thermal gradients, and consequently higher surface heat fluxes during their entire evolution (Fig.2).

At present day, our models show that depending on the percentage of extrusive melt and the depth of magmatic intrusions, the maximum thermal gradient varies from a few K/km up to almost 40K/km, with higher values obtained for higher percentages of intrusive melt and shallower magmatic intrusions (Fig.3a). Models in which the extrusive magmatism is higher than 60% and the depth of magmatic intrusions lies deeper than 50km cannot explain high local thermal gradients as suggested by studies of elastic lithosphere thickness [5,6,7].

In a recent study [8], the presence of a low viscosity layer (LVL) in the shallow Venusian mantle has been suggested to be related to the presence of partial melt. The LVL starts beneath the lithosphere at depths shallower than 200km. This places constraints on the depth of melting that we can use to select successful models. Models that are compatible with partial melting starting at a depth of 200km or less beneath the surface require <40% extrusive magmatism and magmatic intrusions located strictly deeper than 10km (Fig.3b).

Regarding the melt properties, the distinction between intrusive and extrusive cases starts very early in the evolution. Cases that have fully extrusive magmatism present hotter melt and melt deeper than the cases with some percentage of intrusions (Fig.3c). The fully extrusive cases produce partial melt as deep as ~11GPa, the pressure at which melt is more compressible than the solid silicate matrix, and becomes denser which prevents it from rising to the surface [14]. We do not consider this deep melt in our simulation since do not contribute to the magmatic processes investigated here.

SUMMARY

Our analyses indicate that magmatic processes play a major role in the interior evolution of Venus. Our models that are compatible with available geophysical constraints support the scenario of a magmatically active Venus, primarily governed by highly intrusive magmatism and is compatible with the plutonic-squishy lid regime.

The melt depth and melt temperature estimations will be used to inform high-pressure-high-temperature laboratory experiments that will be carried out at the University of Münster to investigate the mantle compositions of Venus that explain the Venera and Vega data (Jennings et al., this meeting).

REFERENCES

[1] Herrick & Hensley, Science, 2023; [2] Rolf et al., SSR, 2023; [3] Hahn & Byrne, JGR, 2023; [4] Lourenco et al., G³, 2020; [5] Borelli et al., JGR, 2021; [6] Maia et al., JGR, 2022; [7] Smrekar et al., Nat.Geoscience, 2023; [8] Maia et al., GRL, 2023; [9] Hüttig et al., PEPI, 2013; [10] Fleury et al., G³ 2024.; [11] Karato et al., Philosophical Magazine, 2003; [12] Tosi et al., PEPI, 2013.; [13] Stixrude et al., EPSL, 2009; [14] Ohtani et al., Chem.Geol., 1995.

How to cite: Herrera, C., Plesa, A.-C., Maia, J., Jennings, L., and Klemme, S.: Effects of intrusive magmatism on the thermal evolution and present-day state of Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-666, https://doi.org/10.5194/epsc2024-666, 2024.

Crustal plateaus are large, high elevation physiographic features on Venus associated with the strongly deformed tessera terrains. They present positive, low magnitude gravity anomalies, and they are stratigraphically the oldest surfaces on the planet [1]. Previous investigations of the gravity and topography signatures of the plateaus have shown that these regions are consistent with shallow support via crustal thickening [2, 3]. In addition, surface emissivity data obtained by Venus Express have shown that the plateaus are associated with low emissivity anomalies, which could be indicative of a felsic composition [4]. Given these observational data sets that are summarized in Figure 1 for Alpha Regio, crustal plateaus could be analogues to the continents on Earth, which would have major implications for our understanding of the tectonic and geodynamic processes that operated throughout Venus’ evolution. Therefore, a careful investigation of the origin and evolution of these features is one of the main objectives of the future missions to Venus. 

The goal of this study is to quantify how much the future gravity datasets, in particular obtained by the VERITAS mission, will help us constrain the interior structure of plateaus. We focus on crustal density estimations, since the density would give essential information about the potential felsic composition of the crustal plateaus. 

Here, we build on the research and methods of Maia & Wieczorek [3], but we adopt a different inversion approach and focus only on surface loads. To this end, we use  a two-layer (mantle, crust+load) lithospheric flexural model. In our inversion, the admittance, a measure of the response of the gravity field to variations in topography, is the physical quantity that is compared between model and observations. The admittance is then spatially and spectrally localized using the windowing functions developed by Wieczorek & Simons [5], to focus the analysis on the plateau regions of interest (here, Alpha Regio and Ovda Regio). A Bayesian inversion to retrieve the parameters of crustal plateaus (i.e., crustal thickness Tc, elastic thickness Te, crustal density ρ_c) is carried out using the Dynesty package [6] that implements a Nested Sampling algorithm. 

Results using current low resolution gravity data show some improvements on the uncertainties of crustal and elastic thickness in comparison to [3] (Figure 2). In particular, our inversions were able to retrieve a lower bound of about 5 km for the elastic thickness of Ovda Regio. However, as expected, the plateau’s crustal density remains unconstrained within the explored range (2400-3000 kg/m3). The slight variability of the results for different window sizes is caused by uncorrelated gravity anomalies not related to the crustal plateau.

These resolution limitations will be soon overcome, thanks to the improvements expected with the VERITAS and EnVision missions, to be launched in the early 2030s to study Venus. In particular, thanks to its quasi-circular orbit, VERITAS will obtain a gravity model with degree strength globally larger than degree 160 and over degree 200 in some regions [7]. Thus, the second focus of this study is to provide a first impression of the extent to which this new dataset will improve our knowledge about the interior structure of the crustal plateaus.

For the simulated gravity dataset, we use flexural models up to spherical harmonic degree 180, for which we  consider 6 different variations of the parameters of interest (Tc=20 km, Te=15/30 km, ρ_c=2650/2800/2950 kg/m3). In a second step, simulated noise is added, and the synthetic data is then mixed with the currently measured gravity field with a sigmoid (logistic function). By changing parameters of this function, such as the steepness of the curve (k) and the degree at which the growth occurs (x0), the mixing can be controlled. This new simulated gravity dataset is used together with our inversion process to study the impact of the higher degree gravity field on our results and test our ability to retrieve the parameters of the crustal plateaus (Tc, Te, ρ_c).

We observe that, by including more synthetic data compared to the measured gravity field, our inversion is able to retrieve the parameters of the crustal plateaus within 10% of the true value known from the synthetic models (see results in Figure 3 for Alpha Regio). We show that the uncertainties of the crustal plateau parameters substantially decrease when the maximum degree of the gravity field that is used in our inversions increases. Our results demonstrate that such an approach would be able to distinguish between a high density, basaltic crust and a low density more felsic crust.

Future developments will investigate different sources of noise that can be added to the admittance, such as geologic signals not related to flexural support. A natural next step is to apply this analysis to different crustal plateaus to determine the potential of retrieving crustal density estimates at various locations on Venus. Finally, we will perform additional sensitivity tests for our inversion approach to assess its robustness to various additional input parameters.

References:

[1] Ivanov M.A. & Head J.W. (2011) Planet. Space Sci., 59; [2] Grimm R.E. (1994) Icarus, 112 ; [3] Maia J.S. & Wieczorek M.A. (2022) JGR Planets, 127; [4] Gilmore M.S. et al. (2015) Icarus, 254; [5] Wieczorek M.A. & Simons F.J. (2007) JFAA, 13; [6] Speagle J.S. (2020) MNRAS, 493; [7] Cascioli G. et al. (2024) 55th LPSC.

How to cite: Baccarin, J., Maia, J., Cascioli, G., Plesa, A.-C., Mueller, N., Mazarico, E., Breuer, D., and Smrekar, S.: Constraining the interior structure of crustal plateaus on Venus in the context of future space missions., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-817, https://doi.org/10.5194/epsc2024-817, 2024.

Often termed the twin sister of the Earth, Venus represents an alternative outcome of the evolutionary path taken by large terrestrial planets. Given its extreme surface conditions, lack of surface water, and the absence of plate tectonics, the present-day thermal state of its mantle is likely very different from the Earth. Venus also remains the most enigmatic of terrestrial worlds in terms of interior structure. Both its tidal Love number k₂ and moment of inertia factor, the main sources of information on the core size and interior structure, are known with a large uncertainty of about 10% [1, 2], and the magnitude of tidal dissipation, sensitive to the planet’s thermal state, has only been determined indirectly [e.g., 3]. Yet, the set of observables acquired by the Magellan and Pioneer missions can still be used to put constraints on the interior structure.

In this study, we combine the calculation of global tidal deformation with a 1-d parameterised model of mantle convection in the stagnant-lid regime [4,5] and we perform a Bayesian inversion of several observational and theoretical constraints (such as the tidal Love number, maximum elastic thickness, or absence of intrinsic magnetic field) to gain insight into the present-day interior structure and thermal state of Venus. The convection model is based on the thermal boundary layer theory and incorporates partial melting, crustal growth, and inner core crystallization. The elastic structure of the mantle for three selected mineralogical models is obtained from the software Perple_X, based on the minimisation of Gibbs free energy [6]. Lastly, to find the tidal parameters, we calculate the tidal deformation of a layered compressible viscoelastic sphere, using the formalism of the normal mode theory [7]. The mantle is described as governed by the Andrade rheological model, which has proven essential for distinguishing between a fully solid and a fully or partially liquid Venusian core [8].

We vary a large set of rheological, structural, and thermodynamic parameters and predict a range of mantle temperatures consistent with previous stagnant-lid models, average mantle viscosities between 10²⁰-10²² Pa.s, and tidal quality factor Q=48⁺⁸⁴_-22. Specifically, the mean value of the tidal quality factor, a parameter inversely proportional to the rate of tidal dissipation, is close to the values derived from studies of equilibrium between solid-body and atmospheric tides [3]. We also investigate the effect of assumed mantle composition on the predicted core size and density and address the question of how likely is the existence of a fully solid or fully liquid core (Figure 1). Finally, we discuss the improvements in data expected from future missions VERITAS and EnVision.

References

[1] Konopliv & Yoder (1995), GRL, 23(14):1857-1860, doi:10.1029/96GL01589.

[2] Margot et al. (2021), Nat. Astron., 5:676-683, doi:10.1038/s41550-021-01339-7.

[3] Correia & Laskar (2003), Icarus, 163(1):24—45, doi:10.1016/S0019-1035(03)00043-5.

[4] Morschhauser, Grott, & Breuer (2011), Icarus, 212(2):541-558, doi:10.1016/j.icarus.2010.12.028.

[5] Baumeister et al. (2023), Astron. Astrophys., 675(A122):23, doi:10.1051/0004-6361/202245791.

[6] Connolly (2009), G3, 10:Q10014, doi:10.1029/2009GC002540.

[7] Takeuchi & Saito (1972), Methods in Computational Physics, 11:217-295, doi:10.1016/B978-0-12-460811-5.50010-6.

[8] Dumoulin et al. (2017), JGR: Planets, 112(6): 1338-1352, doi:10.1002/2016JE005249.

How to cite: Walterova, M., Plesa, A.-C., Baumeister, P., Rückriemen-Bez, T., Wagner, F. W., and Breuer, D.: Constraining the interior structure and thermal state of Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-214, https://doi.org/10.5194/epsc2024-214, 2024.

Abstract

Venus remains a relatively unknown planet. The aim of this study is to determine the Venus' rotational state using the Precise Orbit Determination (POD) method with radio tracking data from Venus Express (VEX) spacecraft. We found a spin period of 243.0206 +/- 1.0028 x 10^-4 days. This value is consistent with those found in the literature using different methods.

Introduction

Although Venus is often considered as the Earth's twin sister, our knowledge of its internal structure, including the size and state of its core are limited [1]. Its rotational state, although measured by several techniques (from the orbit or from ground-based radar), is not very well constrained and shows temporal variations that remain unexplained [2]. The Venus Express (VEX) spacecraft was launched by the European Space Agency (ESA) in November 2005 and orbited the planet for nearly 8 years, from mid-2006 to 2014. This mission was dedicated to the study of the atmosphere of the planet, however, the radio-tracking data performed for navigation purposes can also be used to determine the rotational state of the planet.

Method

This study investigates the Doppler tracking data acquired by ESTRACK Network and collected by VEX Radio Science Experiment (VeRa). The data were analyzed by using the Precise Orbit Determination (POD) method. It consists of performing an iterative least-squares adjustment between Doppler data generated form a numerical integration of a force model driving spacecraft's motion and observed tracking data on successive data arcs of 7 days. For this purpose, we use the GINS (Géodésie par Intégrations Numériques Simultanées) software, developed by French space agency (CNES).

As outputs, two kinds of parameters are fitted: the local and global parameters. Local ones correspond to parameters calculated for each arc whereas the global parameters are derived from all data-arcs. The rotational state of the planet is part of global parameters such as coefficients of its gravity field and the Love number k₂.

Results

We found a spin period of 243.0206 +/- 1.0028 x 10^-4 days. This result is consistent with previous solutions obtained using different tracking data (PVO, Magellan) or methods, such as radar or thermal images form the ground or spacecraft. Figure 1. represents some estimates of Venus' rotation period with respect to their measurement time baseline since 1980. References and methods used for these studies are listed in Table 1.

Figure 1 . Estimates of Venus' rotation period, along with their measurement time baseline and corresponding error bars (1 sigma). The outcome of our study is highlighted in red.

Table 1. References for estimates of Venus' rotation period rotation presented in Figure 1.

Spin period varies- from 243.018 days to 243.023 days, corresponding to a difference of approximately 7 minutes. Furthermore, we observe that the determined values differ depending on the method employed. Our study is in good agreement with the value from Konopliv et al. (1999), who employed the same method as us, as well as the result reported by Campbell et al. (2019) which used Earth-Based radar images. However, our spin period differ from that reported by Margot et al. (2021) end those using other methods such as spaceborne radar [5] or thermal imaging [6].

Conclusion

In this study, an estimate of Venus' rotation period is provided. This result is consistent with the values published in the literature. It shows that navigation tracking data of planetary orbiters, such as VEX, can also be used for science investigation. Further work is ongoing to constrain other geophysical parameters that may help us to better understand Venus.

Acknowledgements

The authors want to thank Bernd Häusler and the VEX Radio Science Experiment (VeRa) team, the Centre de Calcul Intensif des Pays de la Loire (CCIPL) and the ESOC Flight Dynamics.

References

[1] Dumoulin, C. et al.: Tidal constraints on the interior of Venus, J. Geophys. Res. Planets 122, 1338–1352, 2017

[2] Cottereau, L. et al.: The various contributions in Venus rotation rate and LOD, Astron. Astrophys. 531, A45, 2011.

[3] Konopliv A. S. et al.: Venus Gravity: 180th Degree and Order Model. Icarus 139, 3–18, 1999

[4] Margot, J. L. et al.: Spin state and moment of inertia of Venus, Nat Astron 5:676–683, 2021

[6] Mueller, N. T. et al.:Rotation period of Venus estimated from Venus Express VIRTIS images and Magellan altimetry, Icarus 217, 474-483, 2012.

[7] Campbell, B. A. et al.: The mean rotation rate of Venus from 29 years of Earth-based radar observations, Icarus 332, 19-23, 2019.

How to cite: Lévesque, M., Rosenblatt, P., Dumoulin, C., Marty, J.-C., and Pätzold, M.: Determination of Venus’ rotation state using radio-tracking data from the Venus Express spacecraft, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-489, https://doi.org/10.5194/epsc2024-489, 2024.

With one Venusian day being 243 Earth days, the rotational bulge of Venus has the thickness of a few tens of centimetres only, making the Earth’s hotter twin the least rotationally stable planet in the Solar System. There could be a unique link between internal and rotational dynamics on such slowly rotating bodies. This is because the redistribution of mass driven by mantle convection produces perturbations of the body’s inertia tensor that are comparable in amplitude with those associated with the rotational flattening. In effect, Venus may respond to mantle convection by large-amplitude wobbling (Spada et al., 1996), that is, the orientation of Venus with respect to its rotation axis may cyclically change. Wobbling is detectable when both the rotational and the figure axes are measured accurately. The present-day estimate for the angle between the two axes, i.e. the wobble amplitude, is 0.5°, but it is based on gravity models with a limited resolution (Konopliv et al., 1999). Future missions to Venus, namely VERITAS and EnVision, are likely provide a more robust measurement.

The geodynamic regime of Venus’ mantle remains enigmatic. Observational data does not support the existence of continuous plate tectonics on its surface, but some recent evidence of ongoing tectonic and volcanic activity (e.g. Herrick and Hensley, 2023) and crater statistics analyses (e.g. O'Rourke et al., 2014) indicate that the planet is unlikely to be in a stagnant lid regime (see also Rolf et al., 2022).

Here we perform 3D spherical mantle convection simulations of the different possible tectonic scenarios and compute the resulting reorientation (or true polar wander, TPW) of Venus. The TPW path is accompanied with a wobble whose average amplitude we evaluate and compare to the present day estimate of 0.5° (Konopliv et al., 1999). We show that it is unlikely that the present-day wobble of Venus is triggered by mantle convection. For most simulated scenarios, the convection-induced wobble has at least one order of magnitude smaller amplitude when compared to the observed value.
The wobble amplitude is proportional to the rate at which the main inertia direction of mantle convection (MID-MC) changes – the largest wobble is thus obtained in cases with rapid surface mobilization. In simulations with a catastrophic resurfacing, the MID-MC rate reaches its maximum during the lithospheric overturn, and the convection-induced wobble gets closer to the observed value. In a few millions of years after the resurfacing event, however, the wobble amplitude drops again.

References
Herrick, R., Hensley, S., 2023. Surface changes observed on a venusian volcano during the magellan mission. Science
doi:10.1126/science.abm7735.

Konopliv, A., Banerdt, W., Sjogren, W., 1999. Venus gravity: 180th degree and order model. Icarus 139, 3–18. doi:10.1006/icar.1999.6086.

O'Rourke, J.G., Wolf, A.S., Ehlmann, B.L., 2014. Venus: Interpreting the spatial distribution of volcanically modified craters. Geophys. Res. Lett. 41, 8252–8260. doi:10.1002/2014gl062121.

Rolf, T., Weller, M., G ̈ulcher, A., Byrne, P., O’Rourke, J.G., Herrick, R., Bjonnes, E., Davaille, A., Ghail, R., Gillmann, C., Plesa, A.C., Smrekar, S., 2022. Dynamics and evolution of venus’ mantle through time. Space Science Reviews 218, 70. doi:10.1007/s11214-022-00937-9.

Spada, G., Sabadini, R., Boschi, E., 1996. Long-term rotation and mantle dynamics of the Earth, Mars, and Venus. J. Geophys. Res. Planets 101, 2253–2266. doi:10.1029/95JE03222.

How to cite: Patočka, V., Maia, J., and Plesa, A.-C.: Linking internal and rotational dynamics: the amplitude of mantle convection driven wobble of Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-768, https://doi.org/10.5194/epsc2024-768, 2024.

Last year, the first active lava flow on Venus was discovered by Herrick & Hensley (2023), adding to the growing body of evidence that Venus is currently volcanically active with frequently erupting volcanoes (Byrne & Krishnamoorthy, 2022; Van Zelst, 2022). This discovery immediately begged the question as to whether Venus is seismically active at present as well. Indeed, more and more theoretical studies show that Venus could be seismically active today (Van Zelst et al., 2024). In the next decade, we will unravel some of Venus' secrets through a multitude of Venus-bound missions, but determining the seismic activity of the planet is not one of their main goals. Missions to Mars and the Moon have shown the wealth of information that can be gleaned from seismic studies of a planet. We argue here that the seismological exploration of Venus — ‘the quest for quakes on Venus' — should be the next priority of space agencies. 

We first estimate upper and lower bounds on the expected annual seismicity of Venus by scaling the seismicity of the Earth. We consider different scaling factors for different tectonic settings and account for the lower seismogenic zone thickness of Venus. We find that 95 — 296 venusquakes equal to or larger than moment magnitude (M_w) 4 per year are expected for an inactive Venus, where the global seismicity rate is assumed to be similar to that of continental intraplate seismicity on Earth. For the active Venus scenarios, we assume that the coronae, fold belts, and rifts of Venus are currently seismically active. This results in 1,161 — 3,609 venusquakes equal to or larger than M_w 4 annually as a realistic lower bound and 5,715 — 17,773 venusquakes equal to or larger than M_w 4 per year as a maximum upper bound for an active Venus.  

To assess whether any quakes could occur at all at Venus’ high temperatures, we estimate the seismogenic thickness of the planet in three independent ways: through estimates from flexure, through geodynamic models, and through estimates from mantle density anomalies. For all these estimates, we look at the depths of the 600°C and 800°C isotherms as the maximum limit of brittle failure and hence the maximum depth of the seismogenic zone. The seismogenic thickness estimates we find show a large range depending on the assumptions we make for each different method, but in general show that the seismogenic thickness on Venus is on average approximately 10 to 35 km globally. 

To learn more about venusquakes and deduce the interior structure of Venus from them, the detection of seismic waves is crucial. Various concepts to measure seismic waves on Venus have already been explored in the past decades. These detection methods include typical geophysical ground sensors already deployed on Earth, the Moon, and Mars; pressure sensors on balloons; and airglow imagers on orbiters to detect ground motion, the infrasound signals generated by seismic waves, and the corresponding airglow variations in the upper atmosphere. Here, we provide a first comparison between the detection capabilities of these different measurement techniques and our estimates of Venus' seismic activity. In addition, we discuss the performance requirements and measurement durations required to detect seismic waves with the various detection methods. 

We also briefly suggest target regions with a high likelihood of significant surface deformation and/or seismicity for current and future missions. These targets are particularly useful for the upcoming VERITAS (Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy) and EnVision missions. They would specifically benefit from the repeat pass interferometry of VERITAS, which detects surface deformation and can therefore in principle constrain the maximum displacement of surface faulting at locations that are visited twice during the mission.

Our extensive study into the potential seismicity of Venus could be used to drive the design of future mission concepts aiming to study the seismicity of Venus.

References 

Byrne, P. K., & Krishnamoorthy, S. (2022). Estimates on the frequency of volcanic eruptions on Venus. Journal of Geophysical Research: Planets, 127(1), e2021JE007040. 

Herrick, R. R., & Hensley, S. (2023). Surface changes observed on a Venusian volcano during the Magellan mission. Science, 379(6638), 1205-1208. 

Van Zelst, I. (2022). Comment on “Estimates on the Frequency of Volcanic Eruptions on Venus” by Byrne and Krishnamoorthy (2022). Journal of Geophysical Research: Planets, 127(12), e2022JE007448. 

Van Zelst, I., Maia, J., Plesa, A.-C., Ghail, R. C., Spühler, M. (2024). Estimates on the possible annual seismicity of Venus. EarthArxiv, 10.31223/X5DQ0C

How to cite: van Zelst, I., Crameri, F., De Toffoli, B., Garcia, R. F., Ghail, R., Gülcher, A. J. P., Horleston, A., Kawamura, T., Klaasen, S., Lefevre, M., Lognonné, P., Maia, J., Näsholm, S. P., Panning, M., Plesa, A.-C., Sabbeth, L., Smolinski, K., Solberg, C., and Stähler, S.: The quest for quakes on Venus , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1010, https://doi.org/10.5194/epsc2024-1010, 2024.

Detecting ongoing volcanic activity on Venus presents significant challenges due to the planet's dense atmosphere, which hinders direct observation of the surface via satellite optical imagery. The most comprehensive data on Venus’s surface morphology to date have been provided by radar images acquired by the Magellan spacecraft, which orbited the planet from 1990 to 1994 (Ford, 1993).

Evidence of geologically recent volcanic activity, on the scale of millions of years, has been supported by indirect markers such as variations in atmospheric SO₂ levels (Esposito, 1984), changes in surface thermal emissivity (Smrekar et al., 2010), and detailed morphological analyses of volcanic-related features (Gülcher et al., 2020). Furthermore, the direct observation of a volcanic vent deforming between successive Magellan observations has been interpreted as indicative of active volcanic processes (Herrick & Hensley, 2023). However, until now, clear evidence of active lava flows on the Venusian surface has not yet been found.

To address this, we processed and coregistered Magellan images and applied a change detection technique similar to those used for radar data acquired on Earth (Lee et al., 2023). Specifically, we utilized mosaics generated from Full-Resolution Basic Image Data Records (F-BIDRs), having a spatial resolution of 75 m/px (Saunders et al., 1992). The regions of Venus observed by Magellan were specifically chosen where images from at least during Cycle 1 (mid-September 1990 to mid-May 1991) and Cycle 3 (mid-January to mid-September 1992) were available. This ensured consistent comparisons as both cycles employed a leftward-looking radar-viewing geometry, minimizing discrepancies caused by different viewing direction. Our processing involved more precise image realignment using tie points identified with ArcGIS pro georeferencing tools, speckle noise removal using dedicated despeckling techniques, and  change detection algorithms based on the threshold evaluation method described by Kittler & Illingworth (1986).

The analysis revealed evident changes in radar backscatter in two specific regions: the western flank of Sif Mons and the western part of Niobe Planitia. The western flank of Sif Mons, a broad shield volcano located in Eistla Regio, showed a notable increase in radar backscattering in correspondence of lava flows, covering approximately 30 km². This intensification in Cycle 3 images, superimposed on the darker flows from Cycle 1, strongly indicates ongoing volcanic activity. Similarly, in the western Niobe Planitia, a volcanic plain characterized by the presence of numerous small shield volcanoes, new, bright, sinuous lineations were observed extending northeastward, covering about 45 km². These features, visible only in Cycle 3 data, also suggest the deposition of new lava during the Magellan mission.

With the upcoming VERITAS and EnVision missions from NASA and ESA respectively, which will acquire new radar images of Venus’s surface with significantly improved spatial resolutions compared to the Magellan probe (Ghail et al., 2018; Smrekar et al., 2022), further change detection analyses will become feasible. These advancements will not only enhance image quality but also enable comparisons between historical Magellan data and new mission images, providing a more comprehensive understanding of the planet's geological changes over time (Whitten et al., 2021).

Acknowledgements

G.M., D.S. and M.M. acknowledge support from the Italian Space Agency (Grant No. 2022-15-HH.0).

References

Ford, J. P. Guide to Magellan Image Interpretation. National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology (1993).

Esposito, L. W. Sulfur dioxide: episodic injection shows evidence for active Venus volcanism. Science 223, 1072–1074 (1984).

Smrekar, S. E. et al. Recent hotspot volcanism on Venus from VIRTIS emissivity data. Science 328, 605–608 (2010).

Gülcher, A. J. et al. Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus. Nat. Geosci. 13, 547–554 (2020).

Herrick, R. R. & Hensley, S. Surface changes observed on a Venusian volcano during the Magellan mission. Science 379, 1205–1208 (2023).

Lee, J. Y., Jung, S. W., & Hong, S. H. Mapping lava flow from the Kilauea eruption of 2018 in the east rift zone using space-based synthetic aperture radar. GISci. Remote Sens. 60(1), 2176275 (2023).

Saunders, R. et al. Magellan mission summary. J. Geophys. Res.: Planets 97, 13067–13090 (1992).

Kittler, J. & Illingworth, J. Minimum error thresholding. Pattern Recognit. 19, 41–47 (1986).

Ghail, R. C. et al. VenSAR on EnVision: taking Earth observation radar to Venus. Int. J. Appl. Earth Obs. Geoinf. 64, 365–376 (2018).

Smrekar, S. et al. VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy): a discovery mission. In 2022 IEEE Aerospace Conference (AERO) 1–20 (IEEE, 2022).

Whitten, J., Smrekar, S., Hensley, S., Dyar, M., & Nunes, D. (2021, December). VERITAS: Change Detection and Community Involvement in Target Selection. In AGU Fall Meeting Abstracts (Vol. 2021, pp. P34B-05).

How to cite: Sulcanese, D., Mitri, G., and Mastrogiuseppe , M.: Ongoing Volcanic Activity on Venus: Insights from Magellan Radar Data, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-891, https://doi.org/10.5194/epsc2024-891, 2024.

The observation of low viscosity lava flows and shield volcanoes on radar maps, combined with in-situ X-ray fluorescence analyses performed by three Soviet landers, strongly suggests that Venus' crust is primarily basaltic. Still, some of the most intriguing features of Venus are its crustal plateaus, characterized by heavily deformed terrains, which have long been suggested to bear a superficial resemblance to Earth’s continental crust and mountain ranges. Infra-red emissivity spectra from the Galileo and Venus Express missions tend to support the presence of a larger fraction of felsic minerals in the plateaus compared to the surrounding basaltic plains [1-2].

On Earth, flux melting of the mantle wedge at subduction zones, followed by fractional crystallization or partial melting of hydrous basalts, are believed to be the two primary mechanisms generating the large volumes of intermediate to felsic rocks that make up the continental crust. By contrast, igneous differentiation of water-poor basaltic melts typically yields negligible amounts of felsic melts. The possibility that highland plateaus are dominated by intermediate to felsic rocks will be evaluated by the EnVision and Veritas missions, in the hope of providing evidence for the presence of water oceans and, therefore, habitable conditions in Venus' distant past.

In this work, we show, using thermodynamic calculations (Perple_X), that the melting of dry eclogite is another viable mechanism that can produce large volume of intermediate to felsic melts, in the absence of water. An average basaltic crust, of composition identical to the ones analyzed at the Venera 14 and Vega 2 landing sites, would transform into a quartz eclogite at a depth of 50 to 60 km. Partial melting of this material can produce 15-25 % of dacitic melts. The crust of Venus could have reached this depth under crustal plateaus, according to gravity and topography investigations [3]. It has also been suggested that the current young surface of Venus could indicate that abundant basaltic material was recycled to the mantle [4]. Remelting of this material could fuel occasional but large-scale magmatic events and account for the formation of felsic crustal plateaus in the absence of water. Confirmation that crustal plateaus are dominantly felsic by future missions might therefore not necessarily indicate that wetter and more hospitable conditions prevailed on early Venus.

[1] G. L. Hashimoto et al. (2008) Felsic highland crust on Venus suggested by galileo near-infrared mapping spectrometer data. J. Geophys. Res.: Planet 113, E00B24.

[2] M. S. Gilmore, N. Mueller, J. Helbert (2015) VIRTIS emissivity of Alpha Regio, Venus, with implications for tessera composition. Icarus 254, 350–361.

[3] J. S. Maia, M. A. Wieczorek (2022) Lithospheric structure of Venusian crustal plateaus. J. Geophys. Res.: Planet. 127, e07004.

[4] S. E. Smrekar, A. Davaille, C. Sotin (2018) Venus Interior Structure and Dynamics. Space Science Reviews 214(5), 88.

How to cite: Collinet, M., Maia, J., Plesa, A.-C., Klemme, S., and Wieczorek, M.: Formation of intermediate to felsic crustal plateaus on Venus: Is water required?, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1110, https://doi.org/10.5194/epsc2024-1110, 2024.

Simulations of spectra during a volcanic event

The composition and the variability of the lower atmosphere of Venus are critical to understanding the surface-atmosphere interactions, the atmospheric evolution and the volatile exchanges between the interior and the atmosphere [1, 2, 3]. The lower atmosphere can be investigated on the near-infrared spectral windows on the nightside, centered at 1.18 µm (0-15 km), 1.74 µm (20-30 km) and 2.3 µm (30-45 km) [4, 5], which provides a chance to detect volcanic species such as H₂O, SO₂. CO, HCl and HF, which are common species on Earth volcanoes [3].

The Planetary Spectrum Generator (PSG) [6, 7, 8] was used to simulate the nightside 2.3 µm spectrum of Venus, in nadir-geometry, using a VIRA atmospheric template for temperature and molecular vertical abundance profiles and four aerosol modes, of effective radii of 0.49 µm, 1.18 µm, 1.56 µm and 4.25 µm [9, 10]. The spectral resolution was defined as R = 7500.

An H₂O volcanic gas plume was modeled using a Johnson unbounded distribution, with a maximum value of volume mixing ratio (vmr) at a chosen altitude and a slow decrease towards lower altitudes. The profile was defined as a perturbation of the VIRA nominal H₂O profile (~ 30 ppm). with a peak vmr in the range 10-70 ppm and a peak altitude in the range 10-58 km. For each plume, the total integrated column density was conserved, defined as a scaler of the VIRA value, N(total)(H₂O) ~ 13.23 kg m^-2.

An H₂O plume at Z = 30 km with a 10 ppm peak increase results in a radiance difference of ~ 10% at 4150 cm^-1. Ground-based observations and future space missions, such as iSHELL/IRTF and EnVision’s VenSpec-H spectrograph [11], respectively, should have the high-spectral resolution and sensitivity required to detect such H₂O variations due to a possible volcanic eruption event.

Spectral Retrievals of iSHELL/IRTF spectra

On this framework, we are currently investigating the sensitivity of iSHELL/IRTF to retrieve the abundance of H₂O and its temporal and latitudinal variability using the near-infrared spectral window at 1.74 µm (20-30 km) and 2.3 µm (30-45 km) [4]. The retrieval is ongoing with the ASIMUT tool (Vandaele+2006) [12].

We performed 6 hours of observations, in February 2^nd-3^rd 2024, of Venus dayside and nightside using the K3 (2.26-2.55 µm) and H3 filters (1.64-1.82 µm) on the iSHELL/IRTF instrument. The slit width used was 0.375’’ x 5’’ (R ~ 75 000) and it was positioned parallel to the axis of rotation. iSHELL is a cross-dispersed high-spectral resolution 1.1-5.3 µm echelle spectrograph (R ~ 80 000), located in Maunakea, Hawaii [13]. Wavelength calibration, flat fielding, dark correction and sky subtraction were performed using the Spextool data reduction software [14, 15].

The dayside straylight contribution on the nightside, in order 306, was removed by scaling of the dayside spectra with a scaler computed as the nightside/dayside ratio, in order 295 (1.764-1.771 µm), where we modeled expected counts ~ 0.

We identified several CO₂ absorption lines in order 306 (1.702–1.709 µm) and 305 (1.708-1.714 µm) and some hints of H₂O and HCl absorption lines in order 300 (1.735-1.742 µm). The absolute flux calibration and retrieval of the H₂O and HCl column densities are currently ongoing, using ASIMUT [12].

References. (1) Gilmore et al. 2023, Space Sci Rev. (2) Gillmann et al. 2022, Space Sci Rev (3) Wilson et al. 2024, Space Sci Rev (4) Bézard et al. 2009, JGR Planets (5) Tsang et al. 2009, Journal of Quantitative Spectroscopy and Radiative Transfer (6) Villanueva et al. 2018, Journal of Quantitative Spectroscopy and Radiative Transfer; (7) Smith et al. 2013, JGR Planets (8) Dias et al. 2022, Atmosphere (9) Zasova et al. 2006, Cosmic Research; (10) Haus et al. 2010, Icarus (11) Robert et al. 2021, EPSC. (12) Vandaele et al. 2006, Proc of the First 'Atmospheric Science Conference' (13) Rayner et al. 2022, PASP 134 (14) Cushing et al. 2004, PASP 116 (15) Vacca et al. 2003, PASP 115.

Funding. This work was supported by FCT through the research grants UIDB/04434/2020 and UIDP/04434/2020 and a fellowship grant 2022.09859.BD. Funded by ESA in the framework of MWWM - Mars Wind and Wave Mapping project. SR acknowledges funding by BELSPO with financial and contractual support coordinated by the ESA Prodex Office (PEA 4000137943, 4000128137).

Acknowledgements. We kindly thank Maxence Lefévre, from the University of Oxford, for the discussions on volcanic gas plumes, and Shohei Aoki, from the University of Tokyo, for the collaboration concerning the data reduction.

How to cite: Dias, J., Machado, P., Robert, S., Erwin, J., C. Duarte, J., and Quirino, D.: Volcanic gas plumes’ effects on the spectrum of Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-519, https://doi.org/10.5194/epsc2024-519, 2024.

The temperature and dynamics of the planetary boundary layer (PBL), i.e. the atmosphere closest to the surface, are important to many aspects of Venus science. The characteristics of the PBL are critical for the exchange of angular momentum between atmosphere and solid planet possibly affecting the planets spin rate (Mueller et al. 2012, Navarro et al. 2018, Margot et al. 2020). Stability of surface minerals is temperature dependent and a related temperature albedo feedback has been proposed to stabilize the Venus climate (Hashimoto and Abe 2005). Some gravity science investigations are enabled by thermal tides, which include the PBL (Cascioli et al. 2021). The dielectric behavior of minerals is temperature dependent and apparent changes of radar emissivity with surface elevation have been interpreted in terms of mineralogy (Brossier et al. 2021). Even more relevant for the remote sensing of surface mineralogy is that for determination of surface emissivity in the near infrared the surface temperature has to be known very well (Kappel et al. 2015).

The PBL is not well resolved by in-situ data. The temperature gathered during the descent of the many Venera missions does not have a very high sampling frequency and has high uncertainty so that the PBL is not discernable (Seiff et al. 1985). The temperature sensors of the four Pioneer Venus descent probes all failed above the PBL at about 12 km about mean planetary radius, so that no details of the PBL are included in the Venus International Reference Atmosphere (Seiff et al. 1985). The last descent probe VeGa 2 observed at a higher frequency and better uncertainty but the results between 1 and 6 km were considered implausible, because the observed temperature lapse rate exceeded the calculated adiabatic lapse, i.e. the stratification should have been unstable.

The PBL is not easily accessible to remotes sensing. There are however indirect constraints on the PBL temperature from observations of surface thermal emission through the spectral windows near 1 µm. The Venus Express mission provided with the VIRTIS instrument an extensive data set of thermal emission, that is however mostly limited to the southern hemisphere which does not have highlands reaching far into the layer where VeGa2 found an apparently superadiabatic lapse rate. Mueller et al. 2020 processed the data to a mosaic (Fig. 1) and derived emissivity, again assuming surface temperature corresponding to the VIRA profile. The resulting emissivity is very well correlated with topography in the range from -2 to +2 km relative to the mean planetary radius (MPR) of 6052 km, which is geologically not plausible. The alternative interpretation is again a deviation from the temperature profile assumed in the model instead of a variable emissivity. The model of Mueller et al. 2020 was not used to explore the effect of deviations from the temperature profile but it is possible to estimate the effect. In absence of atmospheric emission, which is approximately true for the 1020 nm window [Meadows and Crisp 1996], the top-of-atmosphere radiance is proportional to the blackbody function at surface temperature. The relative difference between the observed and model TOA radiance can therefore be expressed as the corresponding temperature difference to the model (Fig. 2).

Figure 3 shows the result as function of planetary radius for two regions, Lavinia Planitia and Themis Regio, that were often observed by VIRTIS and were selected because they show large temperature differences at the same elevation and lie on the same latitude band. The differences to the VIRA profile are up to -5K and increase to the lowlands, indicating a lower lapse rate than VIRA. At above 6053 km there is a hint that the lapse rate could reverse and follow the apparently super-adiabatic lapse rate observed by VeGa2, but this is ambiguous. This high location is a single corona and relatively low emissivity would be a geologically plausible alternative explanation [Stofan et al. 2016]. To study the VeGa2 profile, observations at higher elevations are necessary, e.g. those made by Akatsuki IR 1 [Kulkarni et al. 2021] or Parker Solar Probe WISPR [Lustig Yaeger et al. 2023].

The difference in temperature between the two regions is surprising because studies to derive emissivity assumed that the surface temperature was only a function of elevation since heat redistribution by convection is very effective e.g. (Hashimoto et al. 2008). Comparison to the Venus Climate Database, which models the PBL and its interaction with topography (Lebonnois et al. 2018) shows clear similarities (Fig. 4). The midnight surface temperature below 1 km above MPR has a lower lapse rate than VIRA and the Lavinia Planitia basin is warmer than the flanks of the Themis Regio volcanic rise at the same surface elevation. This temperature difference persists over the Venus day in the model. Our working hypothesis is that the relatively constant slope winds of Venus in combination with the different cooling rates of atmosphere and surface at night redistribute heat and thus create these surface temperature differences.

Overall, the differences to VIRA observed by VIRTIS are about two times larger than those in the model. One possibility could be that the approximation for surface temperature exaggerates temperature contrast. This seems unlikely but we will check this using a radiative transfer model. Another possibility is that the difference can be explained by the low resolution of the GCM (~400 km) and the correspondingly more muted topography. In any case, near infrared imaging provides data that can be compared to modeled surface temperatures of GCMs and thus provide indirect evidence on the planetary boundary layer.  Upcoming missions will image these wavelengths with a much-improved signal to noise ratio which may additionally provide surface temperature change rates at night.

Mueller,+(2012).doi:10.1016/j.icarus.2011.09.026; Navarro,+(2018).doi:10.1038/s41561-018-0157-x; Margot,+(2021).doi:10.1038/s41550-021-01339-7; Hashimoto,+(2005).doi:10.1016/j.pss.2005.01.005; Cascioli,+(2021)doi:10.3847/PSJ/ac26c0; Brossier,+(2021).doi:10.1029/2020JE006722; Kappel,+(2015).doi:10.1016/j.pss.2015.01.014; Seiff,+(1985).doi:10.1016/0273-1177(85)90197-8; Mueller,+(2020).doi:10.1016/j.icarus.2019.113400; Meadows,+(1996).doi:10.1029/95JE03567; Stofan,+(2016).doi:10.1016/j.icarus.2016.01.034; Kulkarni,+(2021).doi:10.5194/epsc2021-730; Lustig-Yaeger,+(2023).doi:10.3847/PSJ/ad0042; Hashimoto,+(2008).doi:10.1029/2008JE003134; Lebonnois,+(2018).doi:10.1016/j.icarus.2018.06.006

How to cite: Mueller, N., Kulkarni, S., Lebonnois, S., Das, A., and Rauer, H.: Venus surface temperature derived from VIRTIS on Venus Express in comparison to the Venus Climate Database, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1062, https://doi.org/10.5194/epsc2024-1062, 2024.

1. Introduction
Below 10 km the dynamics of the Venus atmosphere is virtually unknown, only the VeGa-2 has successfully measured the temperature in that region. At the surface, only Venera-9 and 10 directly measured the wind for respectively 49 min and 90 s, and several other probes like Venera-13 and 14 measured indirectly the wind speed. The amplitude of the measured wind speed is inferior to 1 m/s [1] for the first 100 m.
The planetary boundary layer has been studied with a global circulation model (GCM) [2], exhibiting a strong effect of the solar heating on the amplitude of the slope wind in the tropics.
Two dune fields have been observed in radar measurements with Magellan [3], although the lack of knowledge about the spatial and temporal distribution of winds complicates the interpretation of sediment transport. The knowledge of the near-surface dynamics, winds and temperature cycles, is essential as the interaction between the surface and atmosphere is one of the objectives, with DAVINCI measuring temperature and winds at the surface [4], VERITAS and EnVision observing aeolian features [5]. In addition, several projects of long-duration lander at the surface of Venus, like the Seismic and Atmospheric Exploration of Venus (SAEVe) concept [6]. Characterization of the near-surface winds is also crucial to constrain lander trajectories/landing [7].

2. Model
To study the near-surface dynamics, the LMD Venus mesoscale model [8] is used, composed of the Weather-Research Forecast (WRF) non-hydrostatic dynamical core [9] coupled with the IPSL Venus GCM physics package [10]. Two domains were considered for this study, one centered on seven locations of VENERA landing sites, where winds were measured, and one centered on the North Pole where the largest mountain is located. The horizontal resolution is respectively 20 and 15 km. The mesoscale boundary conditions are forced with the IPSL Venus PCM.

3. Results
Fig 1 shows Snapshots map of horizontal winds 10m m above local surface (m/s) at noon (left) and midnight (right), exhibiting the diurnal cycle of surface wind. At night, there is the presence of katabatic winds, going down the slope, whereas at noon the winds are anabatic, going up the slope. This difference
cycle is due to the insolation of the surface. During the night, the potential temperature is significantly colder at the surface on the mountain slopes than at the same pressure level in the atmosphere away from the surface. This slope-induced baroclinicity drives the nighttime katabatic winds. During daytime, the reverse is true: the surface on the slopes is slightly warmer than the atmosphere at the same pressure level, inducing anabatic winds. There is also a strong variability of the amplitude of the wind. At noon, the wind over the first 100 m is below 1 m/s, consistent with the observations. At night however, the amplitude of the wind is larger. The amplitude of the wind is sufficient to engender saltation, especially at night. This diurnal cycle of the winds will have an impact on the surface temperature. In the flat low lands, the variation of the temperature will be around 4 K, whereas in the slope the temperature will vary only by less than 1 K. At high latitudes, the solar flux is much lower, and there is no diurnal cycle of the slope winds.

Figure 1: Snapshots map of horizontal winds 10 m above local surface (m/s) at noon (left) and midnight (right) in the center of the domain.

References
[1] Lorenz, R. D. Surface winds on Venus: Probability distribution from in-situ measurements. Icarus 264, 311–315 (2016).
[2] Lebonnois, S., Schubert, G., Forget, F. & Spiga, A. Planetary boundary layer and slope winds on Venus. Icarus 314, 149–158 (2018).
[3] Greeley, R. et al. Aeolian features on Venus - Preliminary Magellan results. Journal of Geophysical Research 97, 13 (1992).
[4] Garvin, J. B. et al. Revealing the Mysteries of Venus: The DAVINCI Mission. The Planetary Science Journal 3, 117 (2022).
[5] Ghail, R. C. et al. VenSAR on EnVision: Taking earth observation radar to Venus. International Journal of Applied Earth Observation and Geoinformation 64, 365–376 (2018).
[6] Kremic, T. et al. Long-duration Venus lander for seismic and atmospheric science. Planetary and Space Science 190, 104961 (2020).
[7] Knicely, J. J. C., Gilmore, M. S., Lynch, R. J. & Herrick, R. R. Strategies for safely landing on Venusian tesserae. Planetary and Space Science 228, 105652 (2023).
[8] Lefèvre, M., Spiga, A. & Lebonnois, S. Mesoscale modeling of Venus’ bow-shape waves. Icarus 335, 113376 (2020).
[9] Skamarock, W. C. & Klemp, J. B. A time-split nonhydrostatic atmospheric model for weather research and forecasting applications. Journal of Computational Physics 227, 3465–3485 (2008).
[10] Garate-Lopez, I. & Lebonnois, S. Latitudinal variation of clouds’ structure responsible for Venus’ cold collar. Icarus 314, 1–11 (2018).

How to cite: Lefevre, M., Lebonnois, S., spiga, A., and Forget, F.: Venus near-surface dynamics: slope winds and dust transport, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-384, https://doi.org/10.5194/epsc2024-384, 2024.

Venus and Earth share similar mass and radii and likely have a similar bulk composition. The present-day Venus atmosphere composition, surface pressure and temperature point to a distinct climate evolution from Earth's, which occurred at some point in the planet's climate history. The point of climate divergence with Earth and the leading mechanism(s) behind it have been pivotal topics for the climate modelling community. Some studies suggest that Venus was never habitable [e.g., 1]. For instance, warming from nightside stratospheric clouds has been proposed as a crucial climate process, preventing water condensation from the steam atmosphere in the first place [1]. Nonetheless, other studies suggest that different processes might support the Habitability of the paleo-Venus [2, 3, 4]. For instance, dayside cloud-albedo feedback might have supported early and prolonged surface Habitability in a slow-rotator world like Venus [4]. In fact, the high atmospheric D/H ratio in Venus (see 5 for a review) might suggest a large initial water reservoir. In this work, we assume that water condensation occurred and initial conditions in the paleo-Venus were favourable to the existence of a global ocean. The main objective is to study the ocean circulation and main properties of this Ocean (temperature, salinity, mixed layer depth) and how they respond to forcing parameters like insolation, rotation rate and the type of atmosphere (CO₂ rich versus N₂ rich).

We present simulations of a possible ocean on the paleo-Venus using the 3D GCM ROCKE-3D (Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics), developed at NASA Goddard Institute for Space Studies [6]. The model derives from the Earth GCM ModelE2-R [7]. The atmosphere is represented by 20 layers, with the uppermost layer at 0.1 hPa. The model uses a 13-layer, fully dynamic Ocean coupled to the atmosphere [8]. The simulations use a spatial resolution of 4º x 5º (latitude x longitude), and a modern Venus topography was selected, following the NASA/Magellan archive. We use a 310-m deep ocean, corresponding to an ocean volume of 1.4 x 10¹⁷ m³- one order of magnitude smaller than Earth's modern Ocean. This 310-m liquid water equivalent layer occupies the lowest topographic regions, corresponding to a ~ 60% global surface cover of the paleo-Venus. The land and ocean fraction latitudinal distribution can be seen in Figure 1. The simulations use the modern planetary parameters of surface gravity, radius, obliquity and eccentricity. Our baseline simulation simulates the paleo-Venus as a retrograde slow-rotator, with a rotation period of -243 sidereal Earth days. The insolation is 1.396 times the solar constant (1913.6 Wm^-2), corresponding to 4.2 Gyr. The simulation uses a 1 bar CO2 -dominated atmosphere (97% CO₂ and 3% N₂). The first results show a pattern of sea surface temperature marked by the subsolar point's location and the land's latitudinal distribution.

Figure 1. Sea surface temperature (SST) in ºC for the 310-m deep ocean on the paleo Venus at 4.2 Gyr with 1 bar CO₂ – rich atmosphere. The subsolar point is located at latitude 0ºN and longitude 0ºW. The land area is identified in grey.

References

[1] Turbet M., et al., 2021. Nature. 276.

[2] Grinspoon D.H. & Bullock M.A., 2007. In Exploring Venus as a terrestrial planet. AGU.

[3] Way M.J., et al., 2016. GRL. 43.

[4] Way M.J. & Del Genio A.D. (2020). JGR:Planets. 125.

[5] Marcq E., et al., 2018. Space Sci Rev. 214:10.

[6] Way M.J., et al., 2017. ApJS. 213:12.

[7] Schmidt G.A., et al., 2014. J Adv Model Earth Syst. 6.

[8] Russell G.L., et al., 1995. Atmos-Ocean. 33:683.

Acknowledgements: Diogo Quirino acknowledges support from the Fundação para a Ciência e a Tecnologia (FCT) through the PhD Fellowship 2023.05220.BD.

How to cite: Quirino, D., Way, M., Green, M., Duarte, J., and Machado, P.: Ocean circulation on the paleo Venus with ROCKE-3D, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1149, https://doi.org/10.5194/epsc2024-1149, 2024.

The ESA EnVision mission will determine the nature and current state of Venus' geological evolution and its relationship with the atmosphere, to understand how and why Venus and Earth evolved so differently. Perched at the inner edge of the Sun habitable zone, Venus may once have been habitable, with liquid water oceans, before developing the enormous greenhouse warming which renders it uninhabitable today, thus providing a natural laboratory for studying the evolution of habitability. Venus is Earth’s closest sibling geologically: similar in size to the Earth, it has remained active into the present era, unlike the much smaller Mars and Mercury. Venus is essential for understanding the links between planetary geophysical evolution and habitability of terrestrial planets from our own Earth to terrestrial planets and exoplanets everywhere, including those which will be the subject of study by PLATO and ARIEL missions in ESA’s Space Science program.

The VenSpec instrument suite is following the holistic approach of the EnVision mission by studying the coupled system of surface and atmosphere on Venus with three complementary instruments. In combination, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition. A joined VenSpec science team across the whole suite ensures that the synergies between the instruments are fully used.

A good example for the holistic investigation nature of the VenSpec suite is the characterization of volcanic plumes. VenSpec will follow them from close to the surface by the water vapor measurements of VenSpec-M and VenSpec-H, tracing them through the middle atmosphere with VenSpec-H and finally through the clouds to the upper atmosphere with VenSpec-U and VenSpec-H.  This will be complementary with the direct detection of the thermal signature of lava flows on the surface by VenSpec-M as well as change detection by the VenSAR synthetic radar and the sub-surface radar (SRS) on EnVision.

The region above the clouds (65-75 km) will be primarily studied by VenSpec-U monitoring sulphured minor species (mainly SO and SO₂) and the as yet unknown UV absorber in Venusian upper clouds, as well as the day-side observations of VenSpec-H monitoring the SO₂ abundance. The region below the clouds (30-40 km) will be targeted by VenSpec-H with high resolution atmospheric measurements on the night side. The main objective is to detect and quantify variations in SO2, H2O and HDO abundances. The lower atmosphere (0-15km) will be target by combined nightside observation by VenSpec-H and VenSpec-M.

The overlapping altitude coverage of the VenSpec-H and -U channels allows to investigate how the upper atmosphere interacts with the lower atmosphere. Combined with the VenSpec-M information on the surface and in the lower atmosphere (0-15km) measurements of VenSpec-H and VenSpec-M this allows also to study the interaction between the surface and the atmosphere.

The surface will primarily be target by VenSpec-M which will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. VenSpec-M take advantage of the improved altimetry provided by the NASA VERITAS VISAR and Envision VenSAR-derived DEMs. 

How to cite: Helbert, J., Vandaele, A.-C., Marcq, E., Tackley, P., Machado, P., Min, M., Ferus, M., Vinatier, S., Lasue, J., Lara, L. M., Widemann, T., Robert, S., Alemanno, G., Erwin, J., Neefs, E., Bertran, S., Lustrement, B., Hagelschuer, T., Peter, G., and Wolff, F. and the VenSpec Team: The VenSpec suite on the ESA Envision mission – a holistic investigation of the coupled surface atmosphere system of Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-423, https://doi.org/10.5194/epsc2024-423, 2024.

Understanding the composition and distribution of the unknown UV absorber (Figure 1) in the Venusian atmosphere has been an open question in planetary science for close to 100 years. Many candidates for the absorber have been proposed over the years. We focus on two possibilities: ferric chloride (FeCl₃)[1] and the cis- and trans- forms of the SO dimer (OSSO)[2].  

Figure 1: A false colour image of Venus. Regions of UV absorption appear orange. The cause of this absorption is unknown. Image credit: NASA/JPL-Caltech. 

FeCl₃ has been proposed to exist within the sulphuric acid cloud droplets, but the absorption spectrum of FeCl₃ generally used in the literature was measured in ethyl acetate, which is not present on Venus and produces an absorption spectrum with little similarity to the Venusian absorber[3]. We have measured the absorption spectrum of FeCl₃ in sulphuric acid with small quantities of HCl added, and we found that it is much more similar in shape to the observed spectrum of the unknown absorber than prior FeCl₃ spectra available in the literature. 

The spectra measured in sulphuric acid contain contributions from both ferric chloride and ferric sulphate ions. We estimated the molar partitioning of the species in the mixtures by performing least squared fitting to reproduce each measured spectrum from spectra of pure ferric sulphate (measured for Fe₂(SO₄)₃ in 75-87 wt% aqueous H₂SO₄) and pure ferric chloride (measured for FeCl₃ in 5-37 wt% aqueous HCl). We estimated the rate of the reaction converting FeCl₃ toward Fe₂(SO₄)₃ using this method at several temperatures and extrapolate to Venusian temperatures to predict the lifetime of FeCl₃ within the sulphuric acid cloud droplets. 

To test if either FeCl₃ or OSSO are viable candidates we employ three models: the global Planetary Climate Model for Venus (PCM-Venus) to model the photochemistry and 3D transport of the candidates in the atmosphere[4], a 1D sectional aerosol model to predict agglomeration and sedimentation as a transport mechanism of FeCl₃ particles[5], and the 1D multiple scattering radiative transfer model SOCRATES[6]. 

We have added iron chemistry and updated sulphur chemistry into PCM-Venus in order to predict the abundance of gas-phase FeCl₃ produced by the reaction of gas-phase HCl with iron produced by the ablation of cosmic dust particles around 115 km, and of OSSO produced from the recombination of two SO molecules. Mean gas and dynamical profiles from the PCM are used to initialise the agglomeration and sedimentation model, which is then run for many Venus years to reach steady state. 

The potential contributions of each species to the observed absorption were assessed using SOCRATES  to predict the observed absorption from instantaneous dayside PCM-modelled concentrations of OSSO, and to estimate the required concentrations of FeCl₃ in the different cloud modes and the required increase of OSSO from the PCM results to fully explain the absorption measured by MESSENGER/MASCS during its June 2007 Venus flyby (Pérez-Hoyos et al., 2018). 

Agglomeration and sedimentation modelling of FeCl₃ suggests that the PCM-modelled FeCl₃ column abundance above 60 km can account for more than 40% of the observed absorption. The full absorption can be explained by approximately 1 wt% FeCl₃ in the mode 1 cloud droplets (Figure 2). Large ferric sulphate ion concentrations in the laboratory spectra used to model the absorption lead to higher than observed absorption near 300 nm. Work is ongoing to correct the iron partitioning between chloride and sulphate to expected Venusian ratios. 

Results of SOCRATES modelling suggest that an OSSO concentration profile 10³ x larger than the PCM-modelled profile would be required to reproduce the depth of the observed absorption (Figure 2). The SO₂ and SO profiles at 60 - 80km in the PCM are currently ~100 x smaller than observed concentrations, and so increase of 10³ from current concentrations is not unreasonable (as OSSO rate of formation will increase with the square of SO concentration). However, the agreement of the spectral shape with the observations is significantly worse than has been previously reported by Frandsen et al. (2016)[2]. The spectral shape is better approximated when the concentration of trans-OSSO is increased relative to the cis-OSSO, though we have found no chemical justification to make this adjustment. 

Figure 2: Preliminary data comparing the MESSENGER/MASCS spectrum of the unknown absorber (*[3], grey crosses) to SOCRATES-modelled concentrations that best reproduce the depth of the absorption for cis- and trans-OSSO (purple dashed line) and FeCl₃ in mode 1 (modal radius = 0.2 µm) sulphuric acid cloud droplets (orange and red dash-dotted lines). The modelled spectrum with neither OSSO or FeCl₃ included (blue solid line) is included for reference.

We conclude that both candidates show some merit as the unknown absorber and a mixture of both species is likely present on Venus and contributing to the absorption. Work is ongoing to decrease the ferric sulphate contribution in the laboratory-measured FeCl₃ spectrum in line with expected Venusian concentrations (and thereby decrease the high absorption near 300 nm seen in current FeCl₃ spectra shown in Figure 2), and to reproduce observed SO₂ and SO concentrations with PCM-Venus, thereby likely increasing OSSO modelled concentrations. 

References 

[1] Zasova et al., 1981, https://doi.org/10.1016/0273-1177(81)90213-1 

[2] Frandsen et al. 2016, https://doi.org/10.1002/2016GL070916 

[3] Pérez-Hoyos et al. 2018, https://doi.org/10.1002/2017JE005406 

[4] Martinez et al. (2024), doi.org/10.1016/j.icarus.2024.116035 

[5] Frankland et al., 2017, https://doi.org/10.1016/j.icarus.2017.06.005 

[6] Manners et al. (2022), SOCRATES Technical Guide, available at: https://code.metoffice.gov.uk/trac/socrates 

How to cite: Egan, J., Feng, W., James, A., Manners, J., Marsh, D., and Plane, J.: Investigation of potential candidates for the Venusian unknown UV absorber: FeCl3 and OSSO  , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-971, https://doi.org/10.5194/epsc2024-971, 2024.

It is well known that planetary atmospheres have a wide range of time-scale variations that have a significant impact on the surface environment of the planet. Taking the Earth as an example, long-term scale atmospheric variations such as ENSO and QBO are affecting the Earth's weather and climate. On the other hand, whether such long-term scale atmospheric variations exist in planetary atmospheres other than Earth has not been much studied in the past. A change in this situation has been brought by the recent realization of long-term observations of the Venusian atmosphere by Venus Express and Akatsuki: the presence of significant long-term variations in UV albedo and the zonal mean wind speed at cloud levels have been reported (Peralta et al., 2018; Lee et al., 2019; Khatuntsev et al., 2023; Horinouchi et al., 2024). In addition, the measurements using ground-based telescopes also have indicated the long-term variations in the mixing ratios of H₂O and SO₂ gases at the cloud top level (Encrenaz et al., 2023). In this study, we used the vertical profiles of temperature and H₂SO₄ gas mixing ratio obtained by the Venus Express and Akatsuki radio occultation measurements (e.g., Häusler et al., 2006; Imamura et al., 2017; Ando et al., 2020) and investigated their long-term variations in the low latitude region. As a preliminary result, we found that both temperature and H₂SO₄ gas mixing ratio around the cloud bottom level (46–50 km altitudes) in 2006–2014 obtained from the radio occultation measurements by Venus Express are generally higher than those in 2016–2022 from the Akatsuki data. In addition, the trend of the annual variation of H₂SO₄ gas mixing ratio almost follows that of its saturated mixing ratio which is mainly determined by the temperature. In the presentation, we will also discuss the long-term variations of temperature in a wider vertical range from the mesosphere down to the sub-cloud region (~40–85 km altitudes).

How to cite: Ando, H., Noguchi, K., Imamura, T., Sagawa, H., Oschlisniok, J., Tellmann, S., and Pätzold, M.: Long-term variations of temperature and H2SO4 gas mixing ratio in the Venusian atmosphere studied with Venus Express and Akatsuki radio occultation measurements, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-5, https://doi.org/10.5194/epsc2024-5, 2024.

Introduction

The horizontal distribution of sulfuric acid clouds in the Venusian atmosphere is an essential factor that influences the solar energy absorbed by the planet. The cloud layer reflects about 75% of the solar energy [1] and controls the amount of energy input to the planet. Understanding how sulfur dioxide, SO₂, the precursor of sulfuric acid, is transported from the lower atmosphere to the cloud top, where the cloud particles are formed from SO₂ photochemically, is one of the essential subjects for understanding the Venusian climate system. Japanese Venus orbiter Akatsuki [2] has been conducting new observations to understand the supply and consumption process of SO₂. The ultraviolet imager onboard Akatsuki is equipped with two bandpass filters centered at 283 nm for observation of SO₂ and 365 nm for the unidentified UV absorber, taking disk images every two hours at two wavelengths [3]. In recent analysises, the local time mean field of the temperature field obtained by mid-infrared observations of Akatsuki has confirmed that the wavenumber 2 component of thermal tides has a large amplitude at low latitudes, which can be one of the processes that explains the variation of SO₂ at the cloud top altitude [4, 5]. In this study, we have developed a method to retrieve only the effect of absorption by SO₂ from the 283 nm images by using additional images simultaneously taken through a 365 nm filter. We obtained the local time and latitudinal mean distribution of SO₂ to analyze on the transport by thermal tides with large amplitude in the mean field.

Dataset

In this study, we used over 15,000 pairs of 283 and 365 nm UV images taken by the Japanese Venus orbiter Akatsuki. The main absorbers around 283 nm are both SO₂ and the unidentified UV absorber, while the absorption of SO₂ is negligible and that of the unidentified UV absorber is dominant at 365 nm. The UV images are Level 3b data, which are 2880 by 1440 maps of calibrated radiance in the longitude-latitude coordinates expanded from 1024 by 1024 Venus disk images taken by the Akatsuki UV imager. We binned down to 120 by 60 to reduce computational cost.

Retrieval of Sulfur Dioxide

We used radiative transfer calculations to calculate reflectance under various amounts of SO₂ and the unidentified UV absorber to create a look-up table. The calculations were performed under the same geometrical conditions as those of the observation of each pixel on the UV images by using our originally developed calculation code based on the adding-doubling method suitable for the calculation of multiple scattering under a divided icosahedral grid for spherical numerical integration to increase suitability for comparison with observations from orbital positions. The effect of the unidentified UV absorber was considered as the imaginary part, n_i, of the complex refractive index of the sulfuric acid solution cloud droplets. First, n_i is obtained from the 365 nm image under certain initial conditions. The n_i has wavelength dependence, so it is converted to a value at 283 nm using the function of wavelength of n_i estimated from the Venus Express UV spectroscopy observations. Next, the 283 nm image and the n_i value are used to determine the amount of SO₂. The obtained amount of SO₂ is used again to obtain the value of n_i from the 365 nm image. By performing this loop calculation until convergence, the SO₂ and n_i  values were simultaneously retrieved. We performed a sensitivity analysis to cloud altitude. 

Figure 1: An Example of retrieval from two UV images. Top two figures are observed reflectance at 283 and 365 nm. Left bottom is retrived amount of SO₂ and right bottom is n_i.

Local Time Distribution of Sulfur Dioxide

Using all of the horizontal distributions of SO₂ retrieved from 15,000 pairs of UV images, we obtained the mean distribution of SO₂ in local time-latitude coordinates shown in Fig. 2.

Figure 2: Mean distribution of SO₂ on local time and latitude coordinates. The left side of the figure is the afternoon side, and the right is the morning. There is an single maximum around 14-15 in the afternoon.

The mean value of SO₂ is 80 to 200 ppb, which is consistent with the previous study [6]. The distribution has a clear single peak, and the mixing ratio of SO₂ monotonically increases from the morning to around 14:30 and decreases toward the terminator of the evening. In the latitudinal direction, it decreased from low to middle latitudes. The distributions can be explained by the vertical movement of air excited by thermal tides of wavenumber 2. Thermal tides have a structure that is warmer in the afternoon and cooler in the morning. When the background wind passes through this structure, the air is lifted most on the afternoon side. This causes SO₂, which is abundant in the lower layers, to be lifted to the cloud-top region, resulting in a single maximum on the afternoon side. The distribution obtained by this study is qualitatively consistent with the result of a Venus GCM with photochemistry [7] and the wave structures reproduced by the Venus GCM by [8].

References

How to cite: Iwanaka, T., Imamura, T., Aoki, S., Marcq, E., and Sagawa, H.: Distribution and Variation of the Venusian Cloud-top Sulfur Dioxide Derived from Akatsuki UV Images, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-680, https://doi.org/10.5194/epsc2024-680, 2024.

EnVision has been selected in the M5 call of ESA’s Cosmic Vision program as the next European led mission to Venus. It is dedicated to unravel some of the numerous open questions about Venus' past, current state and future and will help to understand why Venus and Earth evolved so differently.

The Radio Science Experiment (RSE) consists of two different experiments: it will perform extensive studies of the gravitational field but also Radio Occultations to sense the Venus atmosphere and ionosphere at a high vertical resolution of only a few hundred metres. These radio occultations provide electron density profiles in the ionosphere and atmospheric density, temperature and pressure profiles in the upper troposphere and mesosphere (~40 – 90 km). Additionally, they allow to study the H₂SO₄ absorption in the Venus cloud layer.

The first radio occultation experiment at Venus was conducted during the Mariner 5 flyby in 1967, followed by Mariner 10, several Venera missions, Magellan, and the Pioneer Venus Orbiter, and Akatsuki. The most extensive radio occultation study of the Venus atmosphere so far was carried out by the VeRa experiment on Venus Express.

EnVision will use two coherent frequencies (X- and Ka-band) to separate dispersive and nondispersive effects. This allows to distinguish between ionospheric wave structures and other noise induced effects in the ionosphere.

The use of Ka-band, which has never been used to sense the Venus atmosphere before, allows to study the H₂SO₄ absorption in the Venus cloud layer due to its high sensitivity to sulfuric acid absorption. Ka-band is also sensitive to liquid H₂SO₄ which provides the opportunity (in combination with X-band) to distinguish between gaseous and liquid H₂SO₄ absorption features on Venus for the very first time.

The short orbital period of EnVision in combination with its very small orbital inclination allows to cover all latitudes, longitudes, local times and solar zenith angles on Venus. Especially short-term variations caused by atmospheric waves can be identified to study traveling or stationary small scale atmospheric structures.

How to cite: Tellmann, S., Oschlisniok, J., Pätzold, M., Dumoulin, C., and Rosenblatt, P.: Radio Sounding of the Venusian Atmosphere with the RSE Experiment on EnVision, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-796, https://doi.org/10.5194/epsc2024-796, 2024.

Context

UV investigations of Venus have revealed interesting features regarding the chemical composition and the complex dynamical processes taking place in the atmosphere. The abundance and variability of sulphured gases like SO₂ and SO, as well as the unidentified “UV absorber” have been monitored, and patterns linked to the dynamical activity such as convection cells and atmospheric waves have been observed at both planetary and smaller spatial scales.

These topics are the main science interests of the UV spectrometer VenSpec-U ^[1], onboard the future ESA mission EnVision. This instrument will focus especially on the upper atmosphere and will perform measurements of the backscattered sunlight on the dayside of Venus, and provide radiance factor spectra derived from the measured radiance of Venus and the knowledge of the solar spectral irradiance. Each of the two channel composing the instrument will be dedicated to particular science goals: the “Low Resolution” (LR) channel will operate in the 190-380 nm range to monitor the abundances and variability of SO₂, UV absorber and dynamical processes at small spatial scale ; while the “High Resolution” (HR) channel will be dedicated to the separate identification of SO and SO₂, by observing in the 205-235 nm range with a 0.3 spectral resolution around the common absorption band of these two species. Observations will be performed with a pushboom strategy, and spatial samplings ranging from 3 to 24 km.

Objective and method

The imaging capabilities of the instrument at small spatial scale are then evaluated in order to estimate the observability of the spatial variability of the key parameters aiming to be measured: the SO₂ mixing ratio at 70 km of altitude, the imaginary part of the refractive index used to consider the UV absorber’s influence, and the cloud-top altitude. To do so, the following steps are implemented:

The first step consists in producing maps of parameters to simulate scenes that could be observed by the instrument. These maps are derived from a mesoscale model of Venus’ atmosphere ^[2], allowing to compute abundance’s vertical profile and spatial variability for the main atmospheric chemical compounds and different latitudes and local times. An observation at the Equator and 12h local time is here considered. For each point of the maps, a radiance factor spectrum is generated with a radiative transfer model ^[3], with consideration of the observation geometry. The second step simulates the instrumental effects and the induced degradations of maps and spectra, including optical processes as well as other perturbations occurring at the detector level and during on-board data treatment. The maps are also spatially binned to match the smallest targeted spatial sampling for VenSpec-U. Finally, the degraded synthetic radiance factor spectra are processed with the inverse RTM to retrieve the associated atmospheric parameters, from which the maps can be recomposed and compared to the unperturbed simulations (Figure 1).

Figure 1: maps of the SO₂ mixing ratio (RTM parameter): - simulated with the mesoscale model (left) - original map averaged to reach a 3 km spatial sampling, with rescaled values to be consistent with observed ranges (centre) - retrieved by the inverse RTM (right)

Further developments

This study is coupled to the development of an instrumental model intending to simulate the various processes involved in the production of the images provided by VenSpec-U. This model aims to be implemented and adjusted over time in order to be representative of the instrument, so that it can be used to perform or refine performance assessments.

References

[1] Emmanuel Marcq, Franck Montmessin, Jérémie Lasue, Bruno Bézard, Kandis L. Jessup, et al. Instrumental requirements for the study of Venus’ cloud top using the UV imaging spectrometer VeSUV. Advances in Space Research, 2021, 68 (1), pp.275-291. 10.1016/j.asr.2021.03.012. insu-03179739

[2] Maxence Lefèvre, Aymeric Spiga, Sébastien Lebonnois. Mesoscale modeling of Venus’ bow-shape waves. Icarus, 2020, 335, pp.113376. 10.1016/j.icarus.2019.07.010. hal-02323587

[3] Emmanuel Marcq, Kandis Lea Jessup, Lucio Baggio, Therese Encrenaz, Yeon Joo Lee, et al. Climatology of SO2 and UV absorber at Venus’ cloud top from SPICAV-UV nadir dataset. Icarus, 2020, 335, pp.113368. 10.1016/j.icarus.2019.07.002. insu-02196097

How to cite: Conan, L., Marcq, E., Lustrement, B., Lefèvre, M., Rouanet, N., Bertran, S., Vandaele, A. C., Helbert, J., and Alemanno, G.: Observability of atmospheric spatial variability by the VeSUV/VenSpec-U instrument onboard ESA’s EnVision mission, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-465, https://doi.org/10.5194/epsc2024-465, 2024.

Venus has long been considered the twin sister of Earth due to their physical similarities, such as mass, radius, and distance from the Sun [1]. This resemblance makes the comparative study between Venus and Earth important to understand an evolution turning point that caused these twin planets to be very different at present [2]. Interestingly, not only Earth experiencing global warming, but also Venus is experiencing ongoing temporal changes. Long-term remote sensing observations of Venus show considerable temporal variations of Venus. Such observations were conducted using spectral intensity, polarization, and imaging measurements. They revealed variations of the SO₂ gas abundance, zonal wind speeds, ultraviolet (UV) brightness, cloud top altitude, and the upper haze vertical structures above the cloud top level (~70 km altitude) [3,4,5,6,7]. The main drivers of the reported variations are unclear but may be associated with surface volcanic activities, the solar cycle, or large-scale oscillations in atmospheric dynamics. To understand possible mechanisms, a long monitoring period is necessary, and reliable data calibration is mandatory. We propose a continuous monitoring project, CLOVE (Chasing the Long-term Variability of Our Nearest Neighbor Planet Venus), utilizing a combination of ground- and space-based facilities to overcome the limitations of using a single dataset. In this project, firstly, we plan a low-Earth orbit CubeSat that will monitor Venus to investigate the cloud top vertical structure, the unknown absorber(s), and the SO₂ gaseous abundance, using bandpass and polarization filters at four selected wavelengths and a total 8 channels including polarization filters. We aim for our first CLOVE CubeSat to be launched in 2026. With its successful operation, we aim to proceed with the subsequent CubeSats that will continue Venus monitoring, replacing the old Sat with a new one to cover at least 15 years of time to complete one Solar Cycle. Secondly, we plan to collaborate with ground-based observation teams to perform coordinated Venus dayside observations with space-based CLOVE observations. The data will be used for cross-check validation and supplementary data to interpret our analysis. In this talk, I will explain what we have seen in the past and current data sets of Venus and the future plan with the CLOVE mission with an emphasis on polarization measurements.

References

[1] Svedhem. H, et al., 2007: Venus as a more Earth-like planet. Nature 450, 7170, 629-632
[2] O’Rourke, J.G., Wilson, C.F., Borrelli, M.E. et al., 2023: Venus, the Planet: Introduction to the Evolution of Earth’s Sister Planet. Space Sci Rev 219, 10
[3] Marcq, E. et al., 2020: Climatology of SO2 and UV absorber at Venus' cloud top from SPICAV-UV nadir dataset. Icarus 335, 113368
[4] Khatuntsev, I.V. et al., 2022: Twelve-Year Cycle in the Cloud Top Winds Derived from VMC/Venus Express and UVI/Akatsuki Imaging. Atmosphere 2022, 13
[5] Lee, Y. J. et al., 2019: Long-term Variations of Venus’s 365nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope. Astronomical Journal 158:126
[6] Coffeen, D. L. and Hansen, J. E. 1974: Polarization Studies of Planetary Atmospheres, in Planets, Stars, and Nebulae: Studied with Photopolarimetry (Edited by T. Gehrels). University of Arizona Press, p.518
[7] Kawabata, K., et al., 1980: Cloud and Haze Properties from Pioneer Venus Polarimetry. Journal of Geophysical Research 85, 8129

How to cite: Lee, Y. J. and the CLOVE team: Long-term Monitoring Plan of Venus using Earth-orbiting CubeSats, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-158, https://doi.org/10.5194/epsc2024-158, 2024.

Recently, the Japanese meteorological satellites Himawari-8 and -9 began to be utilized as a space telescope for planetary science and astronomy. Since its operation started in 2015, Himawari-8 has been taking full-disk images of the Earth at the geostationary orbit every 10 minutes [e.g., 1]. Although Himawari-8 aims to observe the weather in the Asia-Pacific region frequently, its scanning pattern extends to space adjacent to the Earth, where solar-system bodies and stars are captured occasionally. For example, stars at absolute declination below 8.8 degrees can be captured, enabling an observation of the great dimming of the red supergiant star Betelgeuse [2]. In addition, Himawari-8 has taken the Moon more than a thousand times. Because the Himawari-8 can spatially resolve the lunar surface and has 9 bands at the infrared wavelengths, lunar surface geology such as roughness and rock abundance can be inferred from the Himawari-8 infrared images [3]. In principle, these spaceborne observations can extend to other solar system bodies, namely Mercury, Venus, Mars, and Jupiter. In particular, the Venus observation with the Himawari-8/9 infrared cameras could provide valuable data to trace the temporal variation of the cloud-top temperature. At the infrared wavelengths covered by the Himawari-8 and -9, the spaceborne observation of Venus is limited only to the Venera-15 and BepiColombo missions [e.g., 4, 5]. Nevertheless, the Himawari-8 and -9 data have not been utilized to observe these solar-system bodies until the present.

In this study, we archived all the Venus images captured by Himawari-8 and -9. To extract all the Venus images, we used the Himawari Standard Data (HSD) published by the Japanese Meteorological Agency from July 2015 until April 2024. Because pixels near the Earth’s rim are affected by the terrestrial atmosphere, the Venus images that are at least 8.85 degrees away from the Earth’s center are analyzed. We employed the aperture photometry technique to calculate the disk-normalized radiance after subtracting the background radiance. The analysis methods are described in detail by [2, 3].

Figure 1 displays an example of the Venus data obtained from HSD. Depending on the distance to the Earth, Venus has been captured with an apparent radius ranging from 5” to 33’’. As previously observed by the Venera-15 and BepiColombo missions, spectral depressions at 8 and 13 microns exist possibly due to the absorption of CO₂. In total, 155 spectra have been obtained with similar spectral shapes. The observation can be conducted whenever the Earth hides the Sun, covering a phase angle from 5 to 168 degrees. In Figure 2, the obtained brightness temperatures are shown as a function of time. The brightness temperature at band 12 decreases over time until 2019 and then remains at a similar level. The temporal variation of brightness temperature shows a similar trend among different bands. The correlation coefficients between different bands are always larger than 0.5. Because the imagers onboard Himawari-8 and -9 are calibrated every 10 minutes, this trend cannot be attributed to the instrument degradation but is potentially affected by atmospheric dynamics such as thermal tides. In this presentation, we will discuss the components of the temporal variation in comparison with other Venus datasets.

[1] Okuyama, A. et al., 2018, Validation of himawari-8/ahi radiometric calibration based on two years of in-orbit data. Journal of the Meteorological Society of Japan 96B, 91.

[2] Taniguchi, D. et al., 2022, The Great Dimming of Betelgeuse seen by the Himawari-8 meteorological satellite. Nat Astron 6, 930–935.

[3] Nishiyama, G. et al., 2022, Utilization of a meteorological satellite as a space telescope: the lunar mid-infrared spectrum as seen by Himawari-8. Earth Planets Space 74, 105.

[4] Oertel, D. et al., 1985, Infrared spectrometry of Venus from “Venera-15” and “Venera-16”. Adv. Space Res. 5, 25–36.

[5] Helbert, J. et al., 2023, The second Venus flyby of BepiColombo mission reveals stable atmosphere over decades. Nat Commun 14, 8225.

Figure 1. An example of the Venus data. (a) An image of Venus captured by Himawari-9 on 20 October 2023. The black and white area outside the Earth is space and the outer area of scanning lines. The x- and y-axes correspond to the angular distances from the Earth’s center along the east-west and north-south directions, respectively. (b) The mid-infrared spectrum of Venus obtained from Figure 1-(a). The spectral depression at 8 and 13 microns corresponds to the CO₂ absorption.

Figure 2. The temporal variation of the brightness temperatures of Venus at bands 8, 12, and 16.

How to cite: Nishiyama, G., Suzuki, Y., Uno, S., Aoki, S., Iwanaka, T., Imamura, T., Fujii, Y., and Müller, T.: Temporal variation of Venus brightness temperature seen by the Japanese meteorological satellite Himawari-8/9, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-585, https://doi.org/10.5194/epsc2024-585, 2024.

Optically thick cloud decks shroud the deep atmosphere of Venus from the view of most observing instruments. Limited nightside spectral windows, however, have allowed detection of trace gas species like carbon monoxide (CO) and sulphur dioxide (SO2). The distribution of these species offers clues to the general circulation below the clouds. CO is formed above the clouds by photolysis of CO2 and reaches the deep atmosphere through downwelling at high latitudes associated with a cloud deck Hadley-like cell. SO2, on the other hand, is believed to originate from the surface, potentially from volcanic processes, and varies in abundance above the cloud deck on both short-term and long-term timescales due to a combination of photochemical and dynamical processes.  

Using the Venus Planetary Climate Model (VPCM), a 3-D global climate model of Venus, we performed two age of air simulations to study how the surface, deep atmosphere, cloud decks, and upper atmosphere are connected by dynamical transport processes. In the experiments, a source region was initialised with a passive tracer whose abundance increased linearly with simulation time. The VPCM’s tracer routines transported the tracer from the source region around the model atmosphere. We then used the tracer abundance in each model gridbox to calculate the time since the air in that gridbox was last in the source region, a quantity known as the “mean age of air.” In one experiment (Surface), the tracer source region was the planetary surface, and in the second (Cloud Deck), the source region was the model level just beneath the lowest cloud deck.

Figure 1: Zonal mean age of air for the Surface and Cloud Deck tracer experiments.

In the Surface experiment, air took around 25 Earth years to reach the region below the cloud decks, while in the Cloud Deck experiment, it took 1.5 Earth years to reach the upper atmosphere above the clouds. To put these timescales in context, we define the troposphere-to-stratosphere turnover ratio, a ratio of the transport time through the troposphere to the transport time through the stratosphere, for Venus in comparison to Earth and Mars:

While on Earth, air moves through the troposphere much more quickly than through the stratosphere, this is reversed for Venus, where the upper atmosphere sees much faster turnover. However, we also found a latitudinal gradient in the age of air which varies with altitude, with significantly younger air in polar regions at altitudes from 25-60 km, and older air at the poles both below and above this altitude range.

Figure 2: Six snapshots of the age of air in the Surface simulation at the south pole. The outermost circle is 60° and each radial gridline represents 5°.

To explain this latitudinal gradient, we investigated the circulation between 25-60 km and found that this region of the deep atmosphere generates planetary-scale Rossby waves with a zonal wave number of one. These waves are characterised by a pair of high-latitude cyclonic/anticyclonic structures or gyres in each hemisphere which collect the age of air tracers, leading to a younger age of air near the poles. The waves propagate around the planet with a period of 36 Earth days, as can be seen from the change in longitudinal location of the age of air minimum in Figure 2.  

Figure 3 shows the general circulation at two altitudes within the wave region. Fig. 3a) contains large gyres in the top and bottom left of the panel with an area of upwelling at their western edges. The gyres are less evident in Fig. 3c) because they have become confined to very high latitudes as the superrotating equatorial jet comes to dominate most of the planet. However, time series analysis of the vertical wind at the gyre latitudes found that the waves continue to exist at these altitudes and remain in phase throughout the 25-60 km altitude range.

Traveling high-latitude Rossby waves could be a source of the observed variability in the trace gas distributions below the cloud deck and contribute to the complex, as yet poorly understood dynamics of the Venusian polar regions. The predicted existence of these waves could be tested by long-term or appropriately timed observations of the trace gas species near one or both poles to determine if there is a regular periodic variation in abundance.

Figure 3: Horizonal and vertical wind in the deep atmosphere at 25 km and 47 km altitude, 12 days apart.

References:

[1] Krol, M., de Bruine, M., Killaars, L., Ouwersloot, H., Pozzer, A., Yin, Y., Chevallier, F., Bousquet, P., Patra, P., Belikov, D., Maksyutov, S., Dhomse, S., Feng, W., and Chipperfield, M. P.: Age of air as a diagnostic for transport timescales in global models, Geosci. Model Dev., 11, 3109–3130, https://doi.org/10.5194/gmd-11-3109-2018, 2018.

[2] Waugh, D.W., Toigi, A.D., Guzewich, S.D.: Age of martian air: timescales for martian atmospheric transport, Icarus, 317, 148-157, https://doi.org/10.1016/j.icarus.2018.08.002

How to cite: Cohen, M., Holmes, J., Patel, M., and Lewis, S.: Planetary-scale waves in the Venus deep atmosphere cause variability in tracer distribution, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-165, https://doi.org/10.5194/epsc2024-165, 2024.

Until the ESA Venus Express mission, the Venus mesosphere was a poorly understood region of the Venus’s atmosphere. This mission, which operated between 2006 and 2014, revealed the composition and thermal structure of the mesosphere and lower thermosphere (Titov et al., 2006, Vandaele et al., 2017, Limaye et al., 2018, Marcq et al., 2018) with the help of its suite of instruments: VIRTIS (Drossart et al., 2007), SPICAV-IR and SPICAV-UV (Bertaux et al., 2007), SOIR (Nevejans et al., 2006). The spacecraft counted a suite of eight instruments, amongst which the SOIR (Solar Occultation in the InfraRed) instrument, which was an echelle grating infrared spectrometer operating exclusively in Solar Occultation mode.

This study focuses on the H₂O, HDO, and temperature vertical profiles. The HDO/H₂O ratio in its bulk atmosphere is 120 times Earth’s (Donahue et al., 1982, de Bergh et al., 1991, Gurwell et al., 2007, Krasnopolsky et al., 2013, Encrenaz et al., 2015, Tsang et al., 2017). We report a significant increase in the H₂O and HDO volume mixing ratios with altitude, with the D/H ratio rising significantly from 0.025 at ~70 km to 0.24 at ~108 km. This indicates an increase from 162 to 1519 the Earth’s ratio within 40 km.

Our work explores two hypotheses to explain these observations: isotopic fractionation from photolysis of H₂O over HDO (Liang et al., 2009) or from phase change processes. We show that the first one cannot explain the observations. The latter, involving condensation and evaporation of sulfuric acid aerosols, as suggested by previous authors (Zhang et al., 2010, Karyu et al., 2024), aligns more closely with the rapid changes observed. Vertical transport computations for H₂O, HDO, and aerosols show water vapor downwelling and aerosols upwelling.

We propose a mechanism where aerosols form in the lower mesosphere due to temperatures below the water condensation threshold, leading to deuterium-enriched aerosols. These aerosols ascend, evaporate at higher temperatures, and release more HDO than H₂O, which are then transported downwards. Moreover, this cycle would imply an SO₂ increase in the upper mesosphere, previously observed above 80 km by several authors (Belyaev et al., 2012, Mahieux et al., 2015).

The study highlights two crucial implications. First, altitude variation is critical to determining the Venus deuterium and hydrogen reservoirs. Second, the altitude-dependent increase of the D/H ratio affects H and D escape rates. The photolysis of H₂O and HDO at higher altitudes releases more D, influencing long-term D/H evolution.

These findings suggest that evolutionary models should incorporate altitude-dependent processes for accurate D/H fractionation predictions.

References:

Belyaev, D., et al. (2012), Icarus, 217.

Bertaux, J. L., et al. (2007), Planet. Space Sci., 55.

de Bergh, C., et al. (1991), Science, 251.

Donahue, T. M., et al. (1982), Science., 216.

Drossart, P., et al. (2007), Planet. Space Sci., 55.

Encrenaz, T., et al. (2015), Planet. Space Sci., 113-114.

Gurwell, M. A., et al. (2007), Icarus, 188.

Karyu, H., et al. (2024), The Planetary Science Journal, 5.

Krasnopolsky, V. A., et al. (2013), Icarus, 224.

Liang, M. C., et al. (2009), J. Geophys. Res., 114.

Limaye, S. S., et al. (2018), Space Science Reviews, 214.

Mahieux, A., et al. (2015), Planet. Space Sci., 113-114.

Marcq, E., et al. (2018), Space Science Reviews, 214.

Nevejans, D., et al. (2006), Applied Optics, 45.

Titov, D. V., et al. (2006), Planet. Space Sci., 54.

Tsang, C., et al., 2017, Vol. 49, pp. 502.04.

Vandaele, A. C., et al. (2017), Icarus, 295.

Zhang, X., et al. (2010), Nature Geoscience, 3.

How to cite: Mahieux, A., Viscardy, S., Yelle, R., Karyu, H., Chamberlain, S., Robert, S., Piccialli, A., Trompet, L., Ubukata, S., Nakagawa, H., Koyama, S., Cessateur, G., and Vandaele, A. C.: Unexpected increase in water abundance and the deuterium-to-hydrogen ratio observed in Venus’s mesosphere, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-182, https://doi.org/10.5194/epsc2024-182, 2024.

How to cite: Streel, N., Lefèvre, F., Martinez, A., Stolzenbach, A., and Määttänen, A.: Strong variability of the NO nightglow modelled on Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-387, https://doi.org/10.5194/epsc2024-387, 2024.

Venus is surrounded by sulfuric acid clouds essential to the planet's climate system. The upper part of the cloud is thought to be of photochemical origin, while the lower part is highly variable and will be more affected by atmospheric dynamics causing condensation and evaporation of sulfuric acid. The specific dynamical processes responsible for the variability are poorly understood.

Observations at near-infrared window wavelengths have revealed large opacity variations, which mostly occur in the lower part of the cloud layer. A significant feature is the planetary-scale dark cloud propagating with a period of 4.9-5.5 days, discovered through ground-based observations (Crisp et al. 1991). The IR2 camera aboard the Venus orbiter Akatsuki observed this phenomenon in more detail and found that the planetary-scale cloud discontinuity (disruption) that spans in the north-south direction characterizes the propagating structure (Satoh et al. 2017; Peralta et al. 2020). The relatively large amplitude near the equator and the zonal propagation faster than the background atmosphere indicate that the cloud opacity variation is mainly induced by a Kelvin wave. McGouldrick et al. (2021) analyzed this feature using spectroscopic data taken by Venus Express VIRTIS. They suggested that the cloud particle microphysical properties and trace gas densities change across the discontinuity, and the changes occur in the lower cloud region. Peralta et al. (2020) and McGouldrick et al. (2021) argued that a front associated with a nonlinear Kelvin wave could be responsible for the discontinuity.

Some Venus GCMs reproduced 5-6 day periodicities in the vertical wind or the thickness of the lower cloud driven by Kelvin waves with a zonal wavenumber of unity (Peralta et al. 2020; Ando et al. 2021). However, the observed sharp discontinuity was not reproduced in the models. The present study proposes mechanisms for the cloud discontinuity from the viewpoint of front generation through hydraulic jump (bore) and associated cloud formation. A simplified dynamical model, in which a Kelvin wave is artificially forced, and a microphysical model are used to reproduce the phenomenon. This study aims to understand the role of the Kelvin wave in the formation of the lower cloud and the conditions necessary for the appearance of the observed sharp discontinuity.

How to cite: Imamura, T., Maejima, Y., Sugiyama, K., Peralta, J., McGouldrick, K., Horinouchi, T., and Satoh, T.: Wave-driven cloud formation on Venus, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-708, https://doi.org/10.5194/epsc2024-708, 2024.

Venus’ atmosphere remains a dynamic and enigmatic engine which not only completely covers the surface of the planet but is believed to have had a dramatic impact on how the planet evolved. The current mechanisms that drive its behaviour and its interactions with both external forces and the surface of the planet have continuously been pieced together for decades, from remote observations to climate modelling, in an attempt to coalesce into a uniform picture of atmospheric circulation.

Atmospheric gravity waves on Venus have garnered increasing attention in the past decade, sparked by the first observation of a planetary scale bow-shape by the Japanese space mission Akatsuki in December 2015. The still ongoing mission has thus far provided a continuous cover of the planet’s cloud layer, including further detection and characterization of stationary waves on the top of the clouds with the Ultraviolet Imager (UVI) which is sensitive to ultraviolet reflected solar radiation, and at slightly lower levels thanks to the capabilities of the Longwave Infrared Camera (LIR) which uses thermal imaging in the mid-infrared. 

For the past decade, most efforts to study gravity waves on Venus have been directed towards understanding both their forcing mechanisms and their influence on atmospheric circulation. Because these waves are driven by such a fundamental force as gravity and also since they can transport energy and momentum across several atmospheric layers, finding these structures on the atmospheres of other planets becomes another tool in understanding their behaviour and contribution to their planets’ energy budget in the atmosphere. Gravity waves can be generated through multiple means, the most common being flow over mountains and convection, which can produce different characteristics on the generated waves. Gravity waves with orographic origins have been widely studied on Earth and a similar mechanism has been proposed for Venus, supported by the presence of the aforementioned stationary waves, which are fixed relative to topography and are commonly associated with prominent mountains on Venus. However, these waves are detected near 60-70 km above the surface and require a positive statically stable environment to be able to propagate, a condition which seems to not always be verified between the surface and the cloud layer. Modelling efforts have been able to reproduce the largest of these structures and how they may propagate from the surface to the top of the clouds where we observe them, but such efforts for smaller-scale features are still ongoing. Since gravity waves feature a wide range of scales and characteristics, other generating sources like convection or shear instability have been proposed. Knowledge of the origin of these waves would also aid in the understanding of their role in the larger picture of Venus’ atmosphere dynamics and quantify their influence at various scales.

Ongoing work for the detection of these waves has already provided some headway with a broadening coverage of past and emerging data sets at 0.283, 3.8, 5 and 10 μm wavelengths [Fukuhara et al. 2017; Fukuya et al. 2022; Kitahara et al. 2019; Kouyama et al. 2017; Peralta et al. 2017], focusing on waves propagating at seemingly different altitudes. The higher spatial resolution of images gathered from several spacecraft allows the identification and characterization of progressively smaller and more varied wave shapes. Such data sets include instruments from Venus Express, namely the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) and from Akatsuki including UVI and the 2 μm Camera (IR2), targeting the upper clouds on the dayside with ~ 20 km/pixel resolution, covering the low latitudes with Akatsuki and high southern latitudes with VEx.

By using these data sets, we hope to expand the current base sample of stationary features detected on Venus and also combine observations at several wavelengths to infer on possible three-dimensional properties of these features and their influence on the atmospheric flow. Additionally, with a larger coverage from several instruments, we will broaden the possible distribution of stationary features to a wider latitudinal coverage and time period, which enables a comparison of the behaviour of these mesoscale structures at different latitudes and possible influence of both the underlying topography and local wind flow regime on the morphology and dynamics of these waves.

References

Fukuhara et al. 2017, Nat. Geoscience; DOI: 10.1038/NGEO2873

Fukuya et al. 2022, Icarus; DOI: 10.1016/j.icarus.2022.114936

Kitahara et al. 2019, JGR Planets; DOI: 10.1029/2018JE005842

Kouyama et al. 2017, Geophys. Res. Lett.; DOI: 10.1002/2017GL075792

Peralta et al. 2019, Nat. Astronomy; DOI: 10.1038/s41550-017-0187

How to cite: Silva, J., Peralta, J., Imamura, T., Hueso, R., Cardesin-Moinelo, A., Lefevre, M., Lee, Y. J., and Espadinha, D.: Mesoscale stationary waves on Venus’ dayside clouds, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-306, https://doi.org/10.5194/epsc2024-306, 2024.

Introduction

We present zonal thermal winds derived by applying the cyclostrophic balance from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) temperature retrievals. VIRTIS was one of the experiments on board the European mission Venus Express [1]. It consisted of two channels: VIRTIS-M and VIRTIS-H. For this study, we analyze the complete VIRTIS-M dataset acquired between December 2006 and January 2010 [2,3], which will allow us to investigate both the day-to-day and local time variability.

Mesosphere dynamics

Venus is a natural laboratory to study the atmospheric circulation on a slowly rotating planet. The dynamics of its upper atmosphere (60-120 km) is a combination of retrograde zonal wind found in the lower mesosphere and solar-to-antisolar winds that characterize the thermosphere, and it is subject to a strong turbulence and a dramatic variability both on day-to-day as well as longer timescales. Moreover, several wavelike motions with different length scales have been detected at these altitudes within and above the clouds and they are supposed to play an important role in the maintenance of the atmospheric circulation. The basic processes maintaining the super-rotation (an atmospheric circulation featuring zonal winds at the clouds level being  60 times faster than the rotation of the planet itself) and other dynamical features of Venus circulation are still poorly understood [4].

Temperature maps

We used temperature retrievals derived from the VIRTIS experiment on board the European mission Venus Express [1]. VIRTIS consisted of two channels: VIRTIS-M and VIRTIS-H. In this study, we analyzed the complete VIRTIS-M dataset acquired between December 2006 and January 2010, corresponding to orbits #23 to #843 [2,3]. Figure 1 shows the latitude-altitude temperature field averaging the whole VIRTIS-M dataset.

Figure 1: Temperature cross-section obtained averaging the whole VIRTIS-M dataset. The data was acquired between 14 May 2006 and 15 August 2008[1]  (orbits 23-843)

Winds maps

Different techniques have been used to obtain direct observations of wind at various altitudes: tracking of clouds in ultraviolet (UV) and near infrared (NIR) images give information on wind speed at cloud top (~70 km altitude) [5] and within the clouds (~61 km, ~66 km) [6], while ground-based measurements of doppler-shift in CO₂ band at 10 μm [4] and in several CO (sub-)millimeter lines [7,8] sound thermospheric and upper mesospheric winds, showing a strong variability.

In the mesosphere, at altitudes where direct observations of wind are not possible, zonal wind fields can be derived from the vertical temperature structure using the thermal wind equation. Previous studies [9,10,11,12] showed that on slowly rotating planets, like Venus and Titan, the strong zonal winds at cloud top can be successfully described by an approximation of the Navier–Stokes equation, the cyclostrophic balance in which equatorward component of centrifugal force is balanced by meridional pressure gradient. Figure 2 shows an example of zonal wind derived from VIRTIS temperature retrievals combining all cubes in orbit #151.

Figure 2: Example of a zonal wind cross-section for orbit #151.

Figure 3 compares 23 zonal wind profiles (thick colored lines) to cloud-tracking measurements (background image, from Goncalves et al., 2020 [5]) at an altitude of 70 km. The wind profiles present a high day-to-day variability, in good comparison with previous observations. As a next step, we plan to analyze more in detail both the day-to-day and the local time variability and compare our results to the Venus Planetary Climate Model [13].

Figure 3: All zonal wind profiles (23 orbits) compared to cloud-tracking observations (from Goncalves R. et al., 2020)  at an altitude of 70 km.

References

[1] Drossart, P. et al. (2007) PSS, 55:1653–1672

[2] Grassi D. et al. (2008) JGR., 113, 2, E00B09.

[3] Migliorini, A. et al. (2012) Icarus 217, 640–647.

[4] Sanchez-Lavega, A. et al. (2017) Space Science Reviews, Volume 212, Issue 3-4, pp. 1541-1616.

[5] Goncalves R. et al. Atmosphere, 12:2., 2021. doi: 10.3390/atmos12010002.

[6] Hueso, R. et al. (2012) Icarus, Volume 217, Issue 2, p. 585-598.

[7] Sornig, M. et al. (2013) Icarus 225, 828–839.

[8] Rengel, M. et al. (2008) PSS, 56, 10, 1368-1384.

[9] Piccialli, A. et al. A&A, 606, A53 (2017) DOI: 10.1051/0004-6361/201730923

[10] Newman, M. et al. (1984) J. Atmos. Sci., 41, 1901-1913.

[11] Piccialli A. et al. (2008) JGR, 113,2, E00B11.

[12] Piccialli A. et al. (2012) Icarus, 217, 669–681

[13] Martinez et al. (2023) Icarus, 389, 115272, doi:10.1016/j.icarus.2022.115272

How to cite: Piccialli, A., Grassi, D., Migliorini, A., Lebonnois, S., Politi, R., Piccioni, G., and Drossart, P.: Analyzing the variability of zonal winds in Venus mesosphere using VIRTIS/VEx temperature maps, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-94, https://doi.org/10.5194/epsc2024-94, 2024.

• Introduction

The Venus PCM (Planetary Climate Model) ^[1,2] has been using a pre-computed net exchange rate (NER) matrix formalism to calculate the infrared radiative transfer in the wavelength range, from 1.7 to 250 μm. This implementation allows the Venus PCM to compute temperature self-consistently. However, this formalism has a drawback, as the cloud model has to be fixed in the computation of the NER matrix, decoupled from the Venus PCM. In this work, we present recent improvements done to the infrared radiative transfer, in order to improve the Venus PCM performances.

• Update of the opacity properties

When calculating the NER coefficients, the high-resolution spectrum data, cloud opacities, and Collision induced absorption (CIA) are integrated ^[3]. As the dominant gas compound, CO₂ dominates most of the energy exchange. In this work, the high-resolution spectrum data of different molecules are updated to the latest version as Figure 1 shows. A new combination of sub-lorentzian far-wing profiles of CO₂ is also considered ^[4,5,6]. The influence of this update on the energy exchange, as discussed in [7], will be presented.

Figure 1: The comparison of the total spectrum absorption coefficient calculated under 1bar 350K for the 2500-3500 cm^-1 wavenumber range. The blue line presents the former database used in the Venus PCM, and the red line is for the results using the updated HITRAN database

• A new IR scheme

A new scheme is now implemented to compute online the radiative transfer both for the solar and for the infrared radiation. This scheme, based on the radiative transfer scheme used in the Generic PCM, allows us to include a better coupling between opacity distributions and dynamics in the model ^[8]. In particular, it will allow a full coupling between cloud distributions (soon to be consistently modeled thanks to a full microphysical module), temperature structure and atmospheric dynamics. This provides more flexibility to study radiative sensitivity to opacity sources.

The different temperature structures simulated in the deep atmosphere using each of the two schemes are compared, as well as the energy exchange rates in the important windows such as 3-7 μm ^[7]. This update will help us to better understand the mechanisms that control temperature and dynamical structure in the deep atmosphere.

References

[1] Lebonnois, S., Sugimoto, N., & Gilli, G. (2016). Icarus, 278, 38-51, doi : 10.1016/j.icarus.2016.06.004

[2] Garate-Lopez, I., & Lebonnois, S. (2018). Icarus, 314, 1-11, doi : 10.1016/j.icarus.2018.05.011

[3] Eymet, V., Fournier, R., Dufresne, J. L., Lebonnois, S., Hourdin, F., & Bullock, M. A. (2009). Journal of Geophysical Research: Planets, 114(E11), doi : 10.1029/2008JE003276

[4] Tran, H., Boulet, C., Stefani, S., Snels, M., & Piccioni, G. (2011). Journal of Quantitative Spectroscopy and Radiative Transfer, 112(6), 925-936, doi : 10.1016/j.jqsrt.2010.11.021

[5] Tonkov, M. V., Filippov, N. N., Bertsev, V. V., Bouanich, J. P., Van-Thanh, N., Brodbeck, C., ... & Le Doucen, R. (1996). Applied optics, 35(24), 4863-4870.

[6] Pollack, J. B., Dalton, J. B., Grinspoon, D., Wattson, R. B., Freedman, R., Crisp, D., ... & Tipping, R. (1993). Icarus, 103(1), 1-42, doi : 10.1006/icar.1993.1055

[7] Lebonnois, S., Eymet, V., Lee, C., & Vatant d'Ollone, J. (2015). Journal of Geophysical Research: Planets, 120(6), 1186-1200, doi :  10.1002/2015JE004794

[8] Turbet, M., Leconte, J., Selsis, F., Bolmont, E., Forget, F., Ribas,I., Raymond, S., Anglada-Escudé, G. (2016). Astronomy & Astrophysics, 596, A112, doi:10.1051/0004-6361/201629577

How to cite: Han, P., Lebonnois, S., and Turbet, M.: Update of the thermal radiative transfer in the Venus PCM, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-344, https://doi.org/10.5194/epsc2024-344, 2024.