Observations of Water Frost on Mars with THEMIS: Application to the Presence of Brines and the Stability of (Sub)Surface Water Ice
Résumé
Characterizing the exchange of water between the Martian atmosphere and the (sub)surface
is a major challenge for understanding the mechanisms that regulate the water cycle. Here
we 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 the
infrared 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 slopes
at 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 Climate
Model. The water frost average temperature is 170 K, and the maximum temperature
measured is 243 K, lower than the water ice melting point. We show that the melting
of 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 brines
if salts are present on the surface. The water vapor pressure at the surface, calculated
from 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.
Domaines
Planète et Univers [physics]Origine | Fichiers produits par l'(les) auteur(s) |
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