Observations of Water Frost on Mars with THEMIS: Application to the Presence of Brines and the Stability of (Sub)Surface Water Ice

• Main Authors: Lange, L, Piqueux, S, Edwards, C, S, Forget, F, Naar, Joseph, Vos, E, Szantai, A
• Other Authors: Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL), Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), Department of Astronomy and Planetary Science Flagstaff, Northern Arizona University Flagstaff, PLANETO - LATMOS, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)
• Format: Report
• Language: English
• Published: HAL CCSD 2024
• Subjects: [SDU]Sciences of the Universe [physics]; Sion; Ice cap
• Online Access: https://insu.hal.science/insu-04595139; https://insu.hal.science/insu-04595139/document; https://insu.hal.science/insu-04595139/file/2405.08713v1.pdf

Characterizing the exchange of water between the Martian atmosphere and the (sub)surfaceis a major challenge for understanding the mechanisms that regulate the water cycle. Herewe present a new dataset of water ice detected on the Martian surface with the Ther-mal Emission Imaging System (THEMIS). The detection is based on the correlation be-tween bright blue-white patterns in visible images and a temperature measured in theinfrared that is too warm to beassociated with CO2 ice and interpreted instead as wa-ter ice. Using this method, we detect ice down to 21.4°S, 48.4°N, on the pole-facing slopesat mid-latitudes, and on any surface orientation poleward of 45° latitude. Water ice ob-served with THEMIS is most likely seasonal rather than diurnal. Our dataset is consis-tent with near-infrared spectroscopic data predictions by the Mars Planetary ClimateModel. The water frost average temperature is 170 K, and the maximum temperaturemeasured is 243 K, lower than the water ice melting point. We show that the meltingof pure water ice on the surface is unlikely due to cooling by latent heat during its sub-limation. However, 243 THEMIS images show frosts that are hot enough to form brinesif salts are present on the surface. The water vapor pressure at the surface, calculatedfrom the ice temperature, indicates a dry atmosphere in early spring, during the reces-sion of the CO2 ice cap. When it sublimes, the frost acts as a vapor source that is wet-ter than the near-surface atmosphere, which stabilizes the subsurface ice.