Tropical Frost on Martian Volcanoes

Abstract

The sublimation and condensation of frost is an important driver of several active geomorphological processes on Mars [1]. It is estimated [2] that there are hundreds of thousands of slope streaks in the equatorial regions - latitudes that are devoid of H 2O frost from 13°S to 32°N [3]. CO 2 frost on the other hand, based on the analysis of surface nighttime temperatures and thermal modeling, was suggested to condense at night in the dusty equatorial regions [4] and on the tallest Tharsis volcanoes [5]. It was proposed that the diurnal sublimation and condensation cycle of CO2 frost may destabilize dusty slopes and trigger slope streaks [4]. Recent studies [6] attempted to detect the putative equatorial frost using early morning observations of the Thermal Emission Imaging System (THEMIS) [7] but were not successful. Using novel observations of the Colour and Stereo Surface Imaging System (CaSSIS) [8] onboard the ESA’s Trace Gas Orbiter (TGO) we discover vast tropical frost deposits on the Arsia (AM), Ascraeus (ASM) and Olympus Montes (OM) calderas. We confirm the detections in OM using a supporting observation from the High Resolution Stereo Camera (HRSC) [9] onboard the ESA’s Mars Express orbiter. Although our observations indicate that frost is present atop the tallest Tharsis volcanoes, slope streaks are absent in these regions. This suggests that the sublimation and condensation of frost may not be the primary cause of slope streak formation. To determine the composition of the observed frost deposits we have recently made joint observations with the Nadir and Occultation for MArs Discovery (NOMAD/TGO) team [10], which will be presented at the conference.