Ices, frosts and clouds on Mars observed by CaSSIS during the first months of TGO’s primary science mission A. Pommerol (1), N. Thomas (1), Z. Yoldi (1), V. Roloff (1), M. Almeida (1), P. Becerra (1), S. Tulyakov (2), L. Tornabene (3), F. Seelos (4), J. Bapst (5), C. J. Hansen (6), G. Portyankina (7), A. Lucchetti (8), M. Pajola (8), S. Douté (9), M. Patel (10) and G. Cremonese (8). (1) Physikalisches Inst., University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland (antoine.pommerol@space.unibe.ch), (2) Ecole Polytechnique Federale de Lausanne, Switzerland, (3) Centre for Planetary Science & Exploration (CPSX), Western University, London, ON, Canada, (4) Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA, (5) LPL, University of Arizona, Tucson AZ, USA, (6) Planetary Science Institute, Tucson AZ, USA, (7) Laboratory for Atmospheric and Space Physics, University of Colorado, USA, (8) INAF-OAPD, Astronomical Observatory of Padova, Italy, (9) Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes/CNRS, France, (10) School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK. Abstract The CaSSIS (Colour and Stereo Surface Imaging System) [1] of the ExoMars Trace Gas Orbiter (TGO) has already acquired numerous images of Mars that show seasonal and diurnal ices and frosts at the surface as well as clouds and fog in the atmosphere. Simulations of the CaSSIS signal in all four colour filters from laboratory measurements with analogues and data from other missions will be helpful to interpret these new observations. 1. Introduction The original 74°-inclination non-Sun-synchronous orbit of the TGO spacecraft coupled with the abilities of CaSSIS to image the surface: - in up to four colour band - with sufficient signal-to-noise to provide good quality images in low-light conditions - with the possibility of quasi-simultaneous stereo acquisitions provide new opportunities to study the seasonal and diurnal cycles of volatiles (H 2 O and CO 2 ) at the surface of Mars. In particular, the regions around 70° latitude in both hemispheres are strongly affected by seasonal changes and can be studied in great details, with short revisit times and possibilities of observations at variable local time during all seasons. In order to interpret the colour images in terms of ice properties relevant for our understanding of volatiles cycles, we have followed two types of approaches in preparation for scientific exploitation of CaSSIS data: - Simulation of the CaSSIS spectral signal from laboratory experiments conducted with well- characterized analogues of Martian icy surfaces. This approach is detailed in a companion abstract [2]. - Simulation of CaSSIS spectral-images from data acquired by the HiRISE, CRISM and CTX imagers of MRO. The simulated data provide a thorough assessment of how the colour capabilities of CaSSIS address the relevant science and will be key for cross-calibration, comparison and change-detection with the actual CaSSIS images [3]. 2. Observations Since the beginning of the primary science phase in April 2018, CaSSIS has already acquired a large number of images, regularly increasing, showing the presence of H 2 O and CO 2 ice and frosts at the surface and occasionally clouds and fogs in the atmosphere. The first observations performed under low-light conditions have confirmed the ability of CaSSIS to provide high-quality images even under challenging illumination conditions. As expected, data from the BLU filter generally show a lower signal-to-noise than the other filters because of the low reflectance of the Martian surface at short visible wavelengths but excel at revealing the presence of even small amounts of ice at the surface or in the atmosphere because of their high reflectance. We are currently focusing our efforts on updating the laboratory calibration of data from the BLU and NIR filters [4] using in-flight data. Colour composite images assembled from data acquired in the BLU filter combined with PAN and either RED or NIR filter can provide a wealth of information on the occurrence and properties of ices and the processes involved in their deposition and evolution. For instance, the RED-PAN-BLU image of the Northern rim of crater Korolev (73° North) shown by [5] and publically released on the 26 th of April 2018 (http://exploration.esa.int/mars/60235- EPSC Abstracts Vol. 12, EPSC2018-272, 2018 European Planetary Science Congress 2018 c  Author(s) 2018 E P S C European Planetary Science Congress