Thermal Creep on Mars: Visualizing a Soil Layer under Tension

TL;DR

This study investigates thermal creep effects in Martian soil, revealing pressure variations that could facilitate particle lift and influence surface processes, using laboratory experiments with diffusing wave spectroscopy.

Contribution

It provides experimental evidence of thermal creep-induced pressure maxima in simulated Martian soil, highlighting its potential role in particle lift and surface dynamics.

Findings

Pressure maximum occurs ~2 mm below surface

Motion peaks at ambient pressure of about 3 mbar

Thermal creep supports particle lift on Mars

Abstract

At low ambient pressure, temperature gradients in porous soil lead to a gas flow, called thermal creep. With this regard, Mars is a unique as the conditions for thermal creep to occur in natural soil only exist on this planet in the solar system. Known as Knudsen compressor, thermal creep induces pressure variations. In the case of Mars, there might be a pressure maximum below the very top dust particle layers of the soil, which would support particle lift and might decrease threshold wind velocities necessary to trigger saltation or reduce angles of repose on certain slopes. In laboratory experiments, we applied diffusing wave spectroscopy (DWS) to trace minute motions of grains on the nm-scale in an illuminated simulated soil. This way, DWS visualizes pressure variations. We observe a minimum of motion which we attribute to the pressure maximum ~ 2 mm below the surface. The motion above but especially below that depth characteristically depends on the ambient pressure with a peak at an ambient pressure of about 3 mbar for our sample. This is consistent with earlier work on ejection of particle layers and is in agreement to a thermal creep origin. It underlines the supporting nature of thermal creep for particle lift which might be especially important on Mars.