TP2

The evolution of surface conditions on Venus has recently made a return to the forefront of planetary sciences questions. Due to both the striking similarities and the obvious differences between Earth and Venus, understanding Venus might hold some of the keys to how terrestrial planets become habitable, either in our solar system or beyond. The question of the origin and persistence of water at the surface/in the atmosphere of Venus determines, in a large part what the planet's evolutionary path has been. The critical difference between Earth and Venus might even be settled during their primordial evolution. Since no sample of Venus can be studied yet, as would be the case for Earth, we turn on alternative methods of investigation. We track the evolution of volatiles at the surface of the planet during its history, since the end of the magma ocean phase. We compare these scenarios with present-day observation and derive limits on maximum amounts of volatiles in the atmosphere of Venus through time, on volatile exchanges, and on water delivery.  

We have developed coupled numerical simulations of the evolution of Venus, modeling mechanisms that govern its surface conditions and atmosphere composition. Currently, the simulations include modeling of mantle dynamics, core evolution, volcanism, surface alteration, atmospheric escape (both hydrodynamic and non-thermal), greenhouse effect, and the feedback mechanisms between the interior and the atmosphere of the planet. Focusing on Late Accretion, we have modeled the effects of large meteoritic impacts on long term evolution through three aspects: atmosphere erosion, volatile delivery and mantle dynamics perturbation due to energy transfer.  The models are constrained by present-day observation and atmosphere composition, with the requirement that scenarios fit reasonnably the current state of the planet.

We produce scenarios that fit present-day conditions and feature both early mobile lid regime (akin to plate tectonics) as well as late episodic lid regime with resurfacing events. However, water outgassing during late evolution could be dampened by high surface pressures. Therefore, it is during the early history of Venus, in particular, that we observe the largest volatile exchanges. That era seems to have large repercussions on long term evolution and present-day state, as it determines volatile inventories and repartition.

The effects of impacts dominate the volatile and mantle evolution during Late Accretion. Large impacts are shown to have essential consequences for volatile repartition. The atmosphere erosion they cause is marginal and doesn’t deplete the atmosphere as much as swarms of smaller bodies, they instead act as a significant source of volatiles. Indeed, if Late Accretion is mainly composed of volatile-rich bodies; it is very difficult to reach the observed present-day state of Venus; instead the atmosphere may become too wet. Likewise, the likely mass received by Venus during volatile-rich Late Accretion, if completely outgassed into the planet's atmosphere, could lead to masses of CO₂ and N₂ 3-6 times higher than observed at present-day.

Simulations show that the maximum contribution of wet material impactors (carbon-chondrites-like) is about 5-10% (mass.) of the total accreted mass during Late Accretion (the larger portion of the Late accretion being composed of enstatite-chondrite-like bodies). In less volatile rich scenarios, water brought by collisions is then lost, either quickly or over billions of years. A small amount of water is then slowly reinserted in the atmosphere by volcanic outgassing.

In wet scenarios, water is efficiently brought to the surface of Venus and loss mechanisms are not able to remove it later, through solid surface oxidation and atmospheric escape. This then leads to water-rich atmosphere, unlike what we observe today.

Those results are consistent over a large range of simulations with variations of late accretion timing, impactors mass-size distribution, composition, efficiency, mantle parameters and so on. Water should have been delivered early to the terrestrial planets in the solar system, during main accretion, before the last giant impact, as is suggested for Earth from isotopic measurements. 

Acknowledgments: CG acknowledges support from the FNRS ET-HOME project and Brussels Free University. CG further acknowledges support from the CLEVER Planets group, supported by NASA.

How to cite: Gillmann, C., Golabek, G., Raymond, S., Tackley, P., Schoenbaechler, M., Dehant, V., and Debaille, V.: The Consequences of Late Accretion Volatile Delivery and Loss Mechanisms on Venus’ Evolution, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-62, https://doi.org/10.5194/epsc2022-62, 2022.

Venus appears to be an “alien” planet drastically and surprisingly different from the Earth. The early space missions revealed the world with remarkably hot, dense, cloudy, and very dynamic atmosphere filled with toxic species likely of volcanic origin. During more than 8 years of operations ESA’s Venus Express spacecraft performed a global survey of the atmosphere and plasma environment of our near neighbour. The mission delivered comprehensive data on the temperature structure, the atmospheric composition, the cloud morphology, the atmospheric dynamics, the solar wind interaction and the escape processes. Vertical profiles of the atmospheric temperature showed strong latitudinal trend in the mesosphere and upper troposphere correlated with the changes in the cloud top structure and suggesting convective instability in the main cloud deck at 50-60 km. Observations revealed significant latitudinal variations and temporal changes in the global cloud top morphology, which modulate the solar energy deposited in the atmosphere. The cloud top altitude varies from ~72 km in the low and middle latitudes to ~64 km in the polar region, correlated with decrease of the aerosol scale height from 4 ± 1.6 km to 1.7 ± 2.4 km, marking vast polar depression. UV imaging showed for the first time the middle latitudes and polar regions in unprecedented detail. In particular, the eye of the Southern polar vortex was found to be a strongly variable feature with complex dynamics.

Solar occultation observations and deep atmosphere spectroscopy in spectral transparency “windows” mapped distribution of the major trace gases H₂O, SO₂, CO, COS and their variations above and below the clouds, revealing key features of the dynamical and chemical processes at work. A strong, an order of magnitude, increase in SO₂ cloud top abundance with subsequent return to the previous concentration was monitored by Venus Express specrometres. This phenomenon can be explained either by a mighty volcanic eruption or atmospheric dynamics.

Tracking of cloud features provided the most complete characterization of the mean atmospheric circulation as well as its variability. Low and middle latitudes show an almost constant with latitude zonal wind speed at the cloud tops and vertical wind shear of 2-3 m/s/km. Surprisingly the zonal wind speed was found to correlate with topography decreasing from 110±16 m/s above lowlands to 84±20 m/s at Aphrodite Terra suggesting decelerating effect of topographic highs. Towards the pole, the wind speed drops quickly and the apparent vertical shear vanishes. The meridional cloud top wind has poleward direction with the wind speed ranging from about 0 m/s at equator to about 15 m/s in the middle latitudes. A reverse equatorward flow was found about 20 km deeper in the middle cloud suggesting existence of a Hadley cell or action of thermal tides at the cloud level. Comparison of the thermal wind field derived from temperature sounding to the cloud-tracked winds confirms the validity of cyclostrophic balance, at least in the latitude range from 30S to 70S. The observations are supported by the General Circulation Models.

Venus Express detected and mapped non-LTE infrared emissions in the lines of O₂, NO, CO₂, OH originating near the mesopause at 95-105 km. The data show that the peak intensity occurs in average close to the anti-solar point for O₂ emission, which is consistent with current models of the thermospheric circulation. For almost complete solar cycle the Venus Express instruments continuously monitored the induced magnetic field and plasma environment and established the global escape rates of 3·10²⁴s⁻¹, 7·10²⁴s⁻¹, 8·10²²s⁻¹ for O⁺, H⁺, and He⁺ ions and identified the main acceleration process. For the first time it was shown that the reconnection process takes place in the tail of a non-magnetized body. It was confirmed that the lightning tentatively detected by Pioneer-Venus Orbiter indeed occurs on Venus.

Thermal mapping of the surface in the near-IR spectral “windows” on the night side indicated the presence of recent volcanism on the planet, as does the high and strongly variable SO₂ abundance. Variations in the thermal emissivity of the surface observed by the VIRTIS imaging spectrometer indicated compositional differences in lava flows at three hotspots. These anomalies were interpreted as a lack of surface weathering suggesting the flows to be younger than 2.5 million years indicating that Venus is actively resurfacing. The VMC camera provided evidence of transient bright spots on the surface that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associated volcanism on Earth, provide strong evidence of their volcanic origin and suggests that Venus is currently geodynamically active.

Alongside observations of Earth, Mars and Titan, observation of Venus allows the opportunity to study geophysical processes in a wide range of parameter space. Furthermore, Venus can be considered as an archetype of terrestrial exoplanets that emphasizes an important link to the quickly growing field of exoplanets research.

The talk will give an overview of the Venus Express findings including recent results of data analysis, outline outstanding unsolved problems and provide a bridge, via the Akatsuki mission, to the missions to come in 2030s: EnVision, VERITAS and DAVINCI.

How to cite: Titov, D., Straume-Lindner, A. G., and Wilson, C.: Venus Express as precursor of the Venus Decade, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1120, https://doi.org/10.5194/epsc2022-1120, 2022.

VERITAS is a NASA Discovery mission that was selected on June 2, 2021 and is a partnership between scientists and engineers at NASA/JPL and with the German (DLR), Italian (ASI) and French Space Agencies (CNES). VERITAS would carry two instruments, VISAR, an X-band interferometric radar provided by NASA/JPL with contributions from ASI and DLR, and VEM, an infrared spectrometer provided by DLR. Data from these two instruments would be combined with gravity science data obtained from telecom tracking data to investigate tectonic style and ongoing volcanism. Science data is collected in two phases after arriving at Venus after a 7-month cruise from Earth. During the second phase of science data collections VERITAS plans to collect radar data for image generation, topographic mapping and targeted repeat pass radar interferometry observations to measure surface deformation .

VERITAS has two phases of science operation. The first science phase (SP1) for VEM operations occurs during a 4-month interruption in the middle of 16 month aerobraking phase, which is needed to achieve the desired circular orbit needed for Science Phase 2 observations of both the VISAR and VEM instruments. The final nearly circular polar orbit used for SP2 varies between 180 and 255 km in altitude. SP2 lasts for 4 Venus sidereal days or ‘cycles’, each of which is 243 days in duration owing to the very slow rotation rate of Venus. The orbit is designed as a frozen eccentricity orbit so that its trajectory nearly repeats from cycle to cycle to facilitate repeat pass radar observations.

VISAR nominally collects data for 11 orbits on both ascending and descending passes followed by 5 orbits of data downlink to Earth. Data is processed onboard to multi-looked imagery and interferograms to reduce the data downlink volume up to 1000 fold. The exception to this is when repeat pass radar interferometry (RPI) data is collected for targeted sites on the surface. Since RPI sites are 200 km (along-track) × 200 km (cross-track) in extent, raw data for 20 consecutive orbits (Venus rotates 10 km at the equator during a VERITAS orbit) are needed to map a site. Raw data are downlinked for processing on Earth. Downlinked data volume as well as ∆V and other operational considerations limit the number of RPI sites to approximately12-18.

Fig. 1: VISAR flight configuration and observing geometry are optimized for InSAR DEM acquisition with baseline separation B = 3.1 m.

Venus is expected to be active today. VERITAS requires detection of 2-cm deformations for spatial scales of 1 km (e.g., fault creep) and 2 cm at 40 m horizontal postings for small-scale (e.g., volcanic) features. To access which deformation signals would be discernible given the expected amount of atmospheric noise we simulated volcanic deformation using various localized, spherical sources (a.k.a. Mogi sources) for a range of source depth and delta volumes. We assumed Earth-like magma chambers at depths of 2-28 km and delta volumes of 1-48 km³, in reasonable agreement with the few Venusian volcanoes for which these parameters can be estimated. Figure 2 shows that inflation above magma chambers at a range of depths, even including atmospheric variability, produces a readily discernible deformation exceeding 2 cm, assuming 1.5 cm of atmospheric distortion at 200 km length scale.

Fig. 2: Deformations above 2 cm are detectable with VISAR RPI techniques, after accounting for observed S02 variability. Left: Fringe patterns (2 cm of deformation per color cycle) for 30 Mogi point sources with atmospheric distortion superimposed. Right: De formation above or near the detectability limit are shown as green dots; those obscured by atmospheric distortion are red.

Radar image, topographic and RPI data along with data from the VEM instrument and gravity measurements to answer three essential science questions: 1) What processes shape rocky planet evolution? 2) What geologic processes are currently active? and 3) Is there evidence of past and present interior water? VERITAS’ VISAR instruments is integral to endind many 30-year-old debates for Venus, such as whether volcanism has been steady or catastrophic, why it lacks terrestrial-style plate tectonics, how it loses its heat, and if it has continents.

A portion of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

How to cite: Hensley, S., Smrekar, S., Campbell, B., Mastrogiuseppe, M., Perkovic-Martin, D., Younis, M., and Zebker, H.: VERITAS Radar Observations at Venus, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-751, https://doi.org/10.5194/epsc2022-751, 2022.

EnVision was selected as ESA's 5th Medium-class mission in the Agency's Cosmic Vision plan, targeting a launch in the early 2030s. EnVision's overarching science questions are to explore the full range of geoscientific processes operating on Venus. It will investigate Venus from its inner core to its atmosphere at an unprecedented scale of resolution, characterising in particular core and mantle structure, signs of past geologic processes, and looking for evidence of past liquid water. Far more than a simple radar mission, this suite of investigations works together to comprehensively assess surface and subsurface geological processes, interior geophysics and geodynamics, and atmospheric pathways of key volcanogenic gases, which together illuminate how and why Venus turned out so differently to Earth. Recent modeling studies strongly suggest that the evolution of the atmosphere and interior of Venus are coupled, emphasizing the need to study the atmosphere, surface, and interior of Venus as a system. 

EnVision is an ESA Venus orbiter mission formulated in collaboration with NASA; As a key partner in the mission, NASA provides the Synthetic Aperture Radar, VenSAR. The EnVision payload consists of five instruments provided by European and US institutions. The five instruments comprise a comprehensive measurement suite spanning infrared, ultraviolet- visible, microwave and high frequency wavelengths, complemented by the Radio Science investigation exploiting the spacecraft TT&C system. All instruments in the payload have substantial heritage and robust margins relative to the requirements with designs suitable for operation in the Venus environment. This suite of instruments was chosen to meet the broad spectrum of measurement requirements needed to support EnVision science investigations.

How to cite: Widemann, T., Straume-Lindner, A. G., Ocampo, A. C., Voirin, T., Vandaele, A. C., Moreira, A., Campbell, B., Dumoulin, C., Marcq, E., Gilli, G., Helbert, J., Kiefer, W., Carter, L., Bruzzone, L., Mason, P., Hensley, S., and Bocanegra-Bahamon, T.: EnVision: understanding why our closest neighbour is so different, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1097, https://doi.org/10.5194/epsc2022-1097, 2022.

EnVision is a Venus orbiter mission that will determine the nature and current state of geological processes on Venus in the present era, measure how those processes generate and sustain the inhospitable atmosphere and climate of Venus, and piece together the sequence of events – the geological history – that led to that current state. Envision was selected in June 2021 to become the next M-Class mission of ESA's Cosmic Vision Programme. To full fill the science objectives, EnVision employs a suite of instruments optimised for observations from Venus orbit, including an imaging radar for high-resolution surface mapping (provided by NASA), a sounding radar for discerning the geometry of the near subsurface, a multispectral infrared camera capturing the composition of surficial rocks and atmospheric composition, and an infrared and ultraviolet spectrometer, complemented by radio science experiments, to identify the pathways of important volcanogenic gases (water vapour, sulphur dioxide, and others) from the lower atmosphere up and into the clouds and in the upper atmosphere. The radoscience experiment also exploits the precise orbit determination to measure the gravity field and probe the deep interior structure of Venus. Figure 1 shows the instrument payload integrated onto the spacecraft and the country and organization responsible for each payload element. The Synthetic Aperture Radar, VenSAR, will image pre-selected regions of interest at a resolution of 30 m/pixel, and subregions at 10 m/pixel. EnVision will be the first Venus mission hosting a Subsurface Radar Sounder, SRS, characterizing the vertical structure and stratigraphy of geological units including volcanic flows. The spectrometer suite, VenSpec, will obtain global maps of surface emissivity in six wavelength bands using one ultraviolet on the dayside, and five near-infrared spectral transparency windows in the nightside atmosphere, to constrain surface mineralogy and inform evolutionary scenarios; and measure variations of SO2, SO and linked gases in the mesosphere. These variations will be further linked to tropospheric variations and volcanism. The Radio Science Experiment uses the spacecraft-Earth radio link for gravity mapping and atmospheric profiling. EnVision is planned to be launched on an Ariane 62 in 2031 with back-up launch dates in 2032 and 2033. An interplanetary cruise is followed by orbit insertion and then circularisation by aerobraking to achieve the nominal science orbit, a low quasi-polar Venus orbit. NASA is contributing the VenSAR instrument and supplies DSN support. The other payload instruments are contributed by ESA member states, with ASI, DLR, BelSPO, and CNES leading the procurement of SRS, VenSpec-M, VenSpec-H, the USO and VenSpec-U instruments respectively.

Figure 1 EnVision' s payload instruments integrated onto the spacecraft