Martian Slopes in the Laboratory: Pressure Dependence of the Insolation Driven Knudsen Compressor

Slopes on Mars can be categorized into a number of classes from Recurring Slope Lineae over dune faces to slope streaks. They all have specific average inclinations  within a certain range. But which processes are responsible to set the angle in a given setting on Earth or Mars? There might be more than friction which sets what is known as angle of repose.

Among lifting processes that act on grains on Mars but not on Earth is thermal creep gas flow within the soil. This results in a sub-soil Knudsen compressor if the soil is illuminated, e.g. by sunlight. This support for particles on the surface can induce particle motion and reduce equilibrium slope angles, eventually. 

However, the microphysics is very complex. As one approach, we therefore carry out laboratory experiments to gather empirical data. In the study reported here, we focus on the pressure dependence of the Knudsen compressor. To study this, we did form piles of granular samples of JSC Mars 1 with an average grain size of 75 micrometer. We used air at various ambient pressure. The slope angles are initially set by the angle of repose at about 30 degrees. We then illuminated the samples and observed avalanches which result in a lower slope angle. This process takes somewhat longer than 100 s.  If the light is turned off, the angles are reduced further for a few seconds. For the sample studied, the reduction in slope angle follows a log-normal distribution in pressure and is strongest at 4.5 mbar during illumination and peaks at 6.7 mbar after the light is turned off (Bila and Wurm, submitted). This places the effect well into the pressure range found on Mars.