The effects of dust content on surface sediment transport by carbon dioxide ice sublimation on Mars

Abstract

During the martian year the surface temperatures in winter dip below the condensation temperature of carbon dioxide and it freezes onto the surface. In the spring, it sublimates directly back into the atmosphere and observations reveal that this cycle of condensation-sublimation results in identifiable sediment transport on the martian surface. We use data from the Colour and Stereo Surface Imaging System (CaSSIS) on ESA's Exomars Trace Gas Orbiter to illustrate the range of landforms thought to be created by these sublimation processes. Previous experiments have revealed that condensation of CO2 ice into the regolith pore space and its subsequent sublimation can result in downslope sediment transport. they also showed that aeolian sand was less prone to sediment motion triggered by sublimation than martian regolith simulant and it was suggested the presence of dust could be responsible for this difference. As dust is an important component of the martian atmosphere and surface, in these experiments we explore the influence of dust content on the sediment transport processes and capacity for sediment transport.Our experimental setup consists of a liquid nitrogen cooled copper sample holder ~30cm long by 20 cm wide within which the sediment is formed into a slope at 30° (max. depth 10 cm). This container is placed inside the Open University’s Mars Chamber which has has a length of 2 m and a diameter of 1 m. One experiment typically takes 2hrs, and the preparation takes 12-14hrs. First the chamber is evacuated and backfilled with CO2 gas twice to purge terrestrial gases including H2O. Once this is complete the sample holder is cooled with liquid nitrogen until all the sediment temperatures reach the condensation temperature of CO2. The experiment then starts and a heat lamp is used to force the CO2 sublimation.  The experiments are monitored by an array of cameras for photogrammetry, a high definition video camera to record the processes, pressure gauges to maintain/monitor the pressure and thermocouples to monitor the sediment and surface temperature.In this series of experiments we vary the dust content in an aeolian sand matrix from 0 to 20% by weight by adding the clay fraction of the MSC simulant. We find no significant difference in the results between 0 and 5% dust content, then at higher values the transported volume and activity increases suddenly and the transported volume and activity remains stable at a higher level from 10% dust upwards. Our results reveal that a sediment transport threshold seems to exist between 5% and 10% dust content and therefore this factor must be considered when studying seasonally active surface processes on Mars.