How sunlight facilitates particle motion on Martian slopes

In an experiment at Martian gravity (on parabolic flights), Martian pressure, and Martian insolation we simulated the motion of particles on Martian slopes with an inclined particle bed, representing Martian soil. 

The insolation generates temperature gradients within the soil which lead to thermal creep gas flow. This generates a sub-soil overpressure millimetres below the surface. The increased pressure supports particles against gravity. Here, we show for the first time that this support allows particles to move downward at much shallower slopes than on non-illuminated parts of the slopes. We also show for the first time, that on inclined slopes, the low Martian light flux is already sufficient to induce this motion (Bila et al. 2024). 

While low ambient pressure is usually not in favour of particle motion as it requires high wind speeds to pick up grains, this is not the case in the presence of thermal creep gas flow. In this case, the low ambient pressure on Mars is rather a booster for particle motion. The pressure at the surface of Mars is exactly in the pressure range, where thermal creep works best (Bila et al. 2023).

At a given pressure, thermal creep depends on grain size. For soils of predominantly large sand-sized particles, no support will be present on Mars but for soils with significant fractions of smaller grains and dust, the flow features on slopes might be influenced, i.e. shaping Recurring Slope Lineae. Also, an easier downward motion of grains might increase the amount of dust that becomes entrained into the atmosphere.

• T. Bila, G. Wurm, K. Stuers, K. Joeris, and J. Teiser, Dry Downhill Particle Motion on Mars, Planetary Science Journal, (accepted), 2024.
• T. Bila, J. Kollmer, J. Teiser, and G. Wurm, Thermal Creep on Mars: Visualizing a Soil Layer under Tension, Planetary Science Journal, 4:16 1-8, 2023.