Enhanced Knudsen pumps as a dry formation mechanism of Recurring Slope Lineae

Recurring Slope Lineae or RSL are dark flow-like features seasonally appearing on steep slopes of the Martian surface. They form and lengthen with time during warm and fade during cooler seasons. There are several hypotheses that could explain the formation of RSL based on wet or dry processes. We focus on a dry formation mechanism involving Knudsen pumps. When the sun illuminates the porous soil in the low-pressure environment it induces the upwards directed thermal creep gas flow (Knudsen pump) inside the soil. Consequently, overpressure develops in the near-surface layers of the soil, which in turn reduces the angle of repose and so triggers the formation of RSL. RSL seem to initiate on bouldered sites, where local shadows over the soil are present. In a laboratory study under Martian atmospheric pressure and Earth gravity, we found that the presence of the partial shading enhances the effect of the Knudsen pump and further lowers the angle of repose due to the additional temperature gradients over the shaded area as proposed by Schmidt et al. (2017).

For a more accurate study reduced Martian gravity must be considered. To do so, we develop a new experimental setup. The setup is comprised of a centrifuge carrying two identically build test cells. Inside of each cell, there is a dust bed sample simulating the Martian soil. To recreate the conditions for different altitudes and/or seasons the gas pressure within the cells can be adjusted between 1 mbar and 10 mbar. Furthermore, each cell is equipped with a light source, a camera to observe motion within the sample, a thermal camera and temperature sensors to later calculate the temperature distribution inside the dust bed. The chambers can be tilted to achieve different angles of repose. The experiment will be conducted during microgravity on parabolic flights while the centrifuge will provide Martian gravity.

References:

Schmidt, F., Andrieu, F., Costard, F., Kocifaj, M., & Meresescu, A. G. 2017, Nature Geoscience, 10, 270