Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021

TP12

Atmospheres and Exospheres of Terrestrial Bodies

Space missions have provided a wealth of data on the atmospheres and aeronomy of rocky planets and moons, from the lower layers up to the external envelopes in direct contact with the solar wind. A recent emerging finding is evidence that the atmosphere behaves as a single coherent system with complex coupling between layers.

This session solicits contributions that investigate processes at work (chemistry, energetics, dynamics, electricity, escape etc...) on the terrestrial bodies of the Solar System and includes studies of the coupling between the lower/middle and upper atmospheres. Contributions based on analysis of recent spacecraft and ground-based
observations, comparative planetology studies, numerical modelling and relevant laboratory investigations are particularly welcome. The session will consist of invited and contributed oral talks as well as posters.

Convener: Anni Määttänen | Co-conveners: Francisco González-Galindo, Dmitrij Titov

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: Francisco González-Galindo, Anni Määttänen, Dmitrij Titov
EPSC2021-541
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ECP
Jake Eager, Nathan Mayne, and Tim Lenton

The interaction between reduced gases and the pre-oxygenic photosynthesising microbial population is key to determining the potential methane greenhouse that warmed the Archean. The potential biotic methane flux could have sustained methane concentrations between 100 to 35,000ppm [1]. Looking at the effect of these methane fluxes and concentrations in a 3D atmosphere are crucial to understanding these processes in more detail.

Here, I will present results from a state-of-the-art 3D climate model [2] along with a low dimensional biogeochemical model [3], exploring the potential biotic methane fluxes and subsequent concentrations. From this, we can examine the potential climate conditions during the Archean. We have extended a 1D exploration of methane’s diminished greenhouse potential during the Archean [4] by looking at how methane concentrations affect cloud distribution, atmospheric dynamics and the surface temperature.

We find that global surface temperature peaks at pCH4 ~100 Pa, with this peak shifting at different CO2 concentrations. Equator-to-pole temperature differences also has a peaked response, with the impact strongly dependant on the CO2 concentration. These changes come about from the balance between the effect of methane and carbon dioxide on atmospheric dynamics. This is due to changes in the heating structure of the atmosphere, which also affects the cloud distribution.

[1] Kharecha, Kasting & Siefert (2005) Geobiology 3, 53-76.

[2] Mayne et al. (2014) Geosci. Model Dev. 7, 3059–3087.

[3] Lenton & Daines (2017) Ann. Rev. Mar. Sci. 9:1, 31-58.

[4] Byrne & Goldblatt (2015) Clim. Past 11, 559–570.

How to cite: Eager, J., Mayne, N., and Lenton, T.: Methane’s non-linear effect on climate in a three-dimensional Archean atmosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-541, https://doi.org/10.5194/epsc2021-541, 2021.

EPSC2021-653
Keishiro Muto and Takeshi Imamura

Cloud tracking has been used to measure motions of planetary atmospheres remotely without direct observations. Cloud tracking is a method to track the movements of cloud parcels using temporally-continuous cloud images to obtain cloud motion vectors. Since it is considered in most of the cases that clouds move at the same speed and the same direction as the surrounding atmosphere, the wind direction and wind velocity can be obtained by tracking the movement of clouds. This method has been applied to the atmospheres of the planets, such as Venus and Jupiter, where direct observation is difficult as well as that of the Earth's atmosphere.

In the cloud tracking methods developed so far, only the parallel movement of the characteristic pattern is assumed, and the rotation of the pattern is not directly measured. Here we developed a new algorithm to track the parallel movement and the rotation of cloud patterns using the rotation invariant phase-only correlation method. In this method, the tracking region is Fourier-transformed before applying the phase correlation method for measuring parallel movement, and logarithmic polar coordinate conversion is performed to the amplitude spectra so that the rotation and enlargement/reduction motions can be obtained as parallel movements. With this method, not only the parallel movement but also the rotational movement of the characteristic pattern can be detected at the same time.

We first applied the newly-developed method to simulated image pairs. The rotation rate of the cloud pattern and the vorticity derived from the velocity field were compared in three velocity patterns: solid body rotation, irrotational vortex, and sinusoidal velocity field in the latitude and longitude directions. As a result, in the case of a solid body rotation, the wind speed field and the rotation angle were determined correctly. Large-scale rotations can be measured more accurately by the new method than by the calculation of vorticity from the cloud-tracked velocity. When the scale of the velocity structure is decreased, the number of missing cloud tracking vectors increases, and thus the spatial pattern of the vorticity becomes difficult to obtain. Even in such cases, the spatial pattern of the rotation rate can be relatively well retrieved although its amplitude is underestimated.

The new method was applied to Jupiter and Venus images based on the results above. For Jupiter, many small eddies were found to be distributed in the equatorial region. The spatial scales and the strengths of the eddies resemble those seen in numerical simulations. The observed vortex chains can contribute to the formation of Jupiter's equatorial jet. For Venus, though small-scale eddies are less prominent, a planetary-scale distribution of the rotation rate with a north-south reflection symmetry was seen, such that anti-clockwise rotation occurs in the northern hemisphere and clockwise rotation in the southern hemisphere. Since the large-scale rotation pattern is consistent with the latitudinal shear of the mean zonal wind, the result means that the rotation of small-scale clouds is caused by the large-scale wind. This result suggests that the small-scale streaky features at mid-latitudes, whose origin is poorly understood, are created by the deformation of clouds by large-scale winds.

The newly-developed method can extract smaller scale eddies than those observed in the previous studies; the method has enabled investigation of the interaction between different scales in a wider wavelength range. The method would also enable studies of mesoscale weather systems such as deep convection and also studies of upward energy cascade from small-scale convective storms to planetary scale motions in planetary atmospheres.

How to cite: Muto, K. and Imamura, T.: Analysis of planetary cloud images considering local rotation, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-653, https://doi.org/10.5194/epsc2021-653, 2021.

EPSC2021-295
Pedro Machado, Hermano Valido, Alejandro Cardesin-Moinelo, Gabriella Gilli, Francisco Brasil, and José E. Silva

In 2018 a regional dust storm on Mars has evolved to a global scope, becoming one of the largest dust storms ever observed. These rare and unpredictable events are poorly known. A key factor for its evolution is the role played by the Martian winds. Measuring winds on Mars is a real challenge for remote observations but a global dust storm offers to us a unique opportunity thanks to an innovative technique to measure the Doppler effect of solar Fraunhofer lines back-scattered on the Mars dust cloud. A high spectral resolution is required to resolve the solar lines and to allow us to measure with precision the line shifts due to aerosol motion and in this manner retrieve the related wind map.

We used dedicated ground-based observations made with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory's Very Large Telescope (VLT) facility in Chile. This instrument's high resolution (R ~100000) allows for the dust cloud velocity to be measured, by computing the Doppler shift induced in the Fraunhofer lines (λ of 420-1100 nm) in the solar radiation that is back-scattered in the dust suspended in the Martian atmosphere, by the motion of that same dust particles, with an average error of approximately 5 ms-1.

The processes that allow for the development of global dust storms are poorly understood. Furthermore, the cut-off mechanisms that spur the end of these storms are also without consensus and may even vary from storm to storm. During such events dust can be lifted to heights above 50 km across all latitudes and longitudes, increasing the optical depth along the dust layer in atmospheric suspension and increasing the heat absorbed at each altitude covered by dust [1,2]. Global dust storms are complex stochastic events that can drastically alter the atmospheric dynamics [3,4]. These storms usually develop in the southern hemisphere during southern Summer and Spring ( Ls ≈ 180º - 360º), however, the 2018 storm started developing in the northern hemisphere on Ls ≈ 185º.

Our understanding of both the initiation and decline of global dust storms is only marginal, nevertheless we do know that such events probably originate from the superimposition of three circulation components: the Hadley cell, thermal tides and topographically controlled circulations. This mechanism was suggested by Leovy (1973) [5] and relies on the seasonally increased insolation and dust loading coupled with the above-mentioned components to allow certain storms to become global at the planetary scale. The decay of dust storms is even more obscure as the cause for the halting of the dust lifting hasn't been unambiguously identified. Either the depletion of surface dust available for lifting shuts off the lifting events (which requires replenishment of the surface dust sources) or the decrease in intensity of the various components allows for the surface wind stress to drop below the required threshold for dust lifting [6,7].

Mars' atmosphere is highly transparent in the visible and ultraviolet ranges and the back-scattered radiation in those wavelength ranges in the atmosphere is negligible which precludes the application of the Doppler velocimetry method that we developed and fine-tuned for the case of Venus [8,9,10,11,12]. However, during global dust storms, the opacity of the atmosphere increases and allows for the scattering of enough light in the suspended dust in the middle atmosphere for the application of our Doppler method in an effective way.

The adaptation of our Doppler velocimetry method took in account the geometry of our observations. Spherical geometry was used to locate the observations within the planet, as seen from Earth at the time of each observation, and to compute the de-projection of the radial Doppler velocities from the observer's line-of-sight, for each point of the slit and for each exposure. The rotation velocity's contribution to the overall Doppler shift was removed by computing and subtracting the rotation velocity at each point on Mars sensed by the spectroscopic slit. Furthermore, the contributions made to the total shift by the Young effect were evaluated and deemed negligible under the specific geometry of our observations.

The scope of this work is to study the dynamical behaviour of Mars’ middle atmosphere during a global dust storm using ground-based observations made with the high-resolution spectrograph UVES at ESO’s Very Large Telescope and Doppler velocimetry methods, for the first time, to complement observations of orbiter instruments.

The success and validation of the application of this method to the atmosphere of Mars may provide a new tool to investigate the Martian atmosphere during dust storms. We intent is to contribute for a better understanding of the atmosphere's dynamics during planet encircling dust events. We measured the wind velocity and its spatial variability, through high resolution spectroscopy and Doppler velocimetry.

The main goal of this research line is therefore, to provide wind measurements using visible Fraunhofer lines scattered at Mars’ dust hazes, which allows spatial wind variability studies and will make possible to obtain a latitudinal profile of the wind along the cited global dust storm and a wind map of the dust storm as a function of the latitude and local time over the planet as seen from Earth.

Acknowledgements: We acknowledge support from the Portuguese Fundação Para a Ciência e a Tecnologia ref. PTDC/FIS-AST/29942/2017

References: [1] Vandaele, et al., Nature, 568, 2019. [2] Stone, S., et al. Science, Vol. 370, 2020. [3] Zurek, R., and Martin, L., JGR, 98, 1993. [4] Haberle, R.M., Science, 234, 1986. [5] Leovy, C. Journal of Atmospheric Sciences, 30, 1973. [6] Kahre, M., et al., Cambridge University Press, 2017. [7] Pollack, J., JGR, 1979. [8] Machado, P., et al., Icarus, 2012. [9] Machado, et al. Icarus, 285, 2014. [10] Machado, et al. Icarus, 2017. [11] Machado P., et al., Atmosphere, 2021. [12] Gonçalves R., et al., Icarus, vol 335, 2020.

How to cite: Machado, P., Valido, H., Cardesin-Moinelo, A., Gilli, G., Brasil, F., and Silva, J. E.: Final Results of Doppler Velocimetry Winds on Mars' Atmosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-295, https://doi.org/10.5194/epsc2021-295, 2021.

EPSC2021-188
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ECP
Francisco Brasil, Pedro Machado, Gabriella Gilli, Alejandro Cardesín-Moinelo, José E. Silva, Daniela Espadinha, and Brigitte Gondet

We report preliminary result of the systematic detection and characterisation of atmospheric gravity waves on Mars' atmosphere, using observations carried out by the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) [1] imaging spectrometer on board of the Mars Express (MEx) [2].

Gravity waves are mesoscale atmospheric oscillations in which buoyance acts as the restoring force [3]. The presence of gravity waves is observed in many of the Solar System planets atmospheres, from Venus [4], Earth [5], Jupiter [6], and being a crucial factor in the circulation of planetary atmospheres since they transport momentum and energy, which can dissipate at different altitudes and force the dynamics of several layers of the atmosphere.

The source of these waves can be associated with the topographic features (orographic gravity waves) of surface, or with jet streams and atmospheric convections (non-orographic gravity waves). Recent modelling studies showed the strong role of gravity waves on diurnal tides on Mars atmosphere [7], however their characteristic are still not well constrained by observations.

 This work aims to go through the complete OMEGA data set to fully detect and characterise gravity waves observed during the Mars Express space probe. Every image was navigated and processed in order to optimise the detection of the wave packets and accurate characterisation of the wave properties such as the horizontal wavelength, packet width, packet length and orientation.

Due to the longevity of the MEx space mission, acquiring over more than 17 years of observations of the surface and atmosphere of Mars in nadir and limb modes, the OMEGA instrument offers an opportunity to explore the atmosphere dynamics over the years, especially the evolution of gravity waves along the time, due to the time sampling and global coverage of MEx.

The OMEGA images are composed of a hyperspectral cube with a spectral range of 0.38 to 5.1 m, taken with the visible and near-infrared (VNIR) and infrared (SWIR) spectrometers. We retrieved the OMEGA data and its IDL routines through the PSA archive from ESA, to produce OMEGA images which were later navigated and processed individually using ENVI software for optimal detection of wave features and accurate characterisation of wave properties, such as the horizontal wavelengths, packet width, packet length, location and orientation. Since the orbit has a certain resonance with respect to the Martian surface rotation (variable duration along the mission), there will be an overlapping of images taken, allowing the study of the evolution of gravity waves along a period of time and also the study of their activity during dust storms [8].