  Active dark flows known as recurring slope lineae have been observed on the
warmest slopes of equatorial Mars. The morphology, composition and seasonality
of the lineae suggest a role of liquid water in their formation. However,
internal and atmospheric sources of water appear to be insufficient to sustain
the observed slope activity. Experimental evidence suggests that under the low
atmospheric pressure at the surface of Mars, gas can flow upwards through
porous Martian soil due to thermal creep under surface regions heated by the
Sun, and disturb small particles. Here we present numerical simulations to
demonstrate that such a dry process involving the pumping of rarefied gas in
the Martian soil due to temperature contrasts can explain the formation of the
recurring slope lineae. In our simulations, solar irradiation followed by
shadow significantly reduces the angle of repose due to the resulting temporary
temperature gradients over shaded terrain, and leads to flow at intermediate
slope angles. The simulated flow locations are consistent with observed
recurring slope lineae that initiate in rough and bouldered terrains with local
shadows over the soil. We suggest that this dry avalanche process can explain
the formation of the recurring slope lineae on Mars without requiring liquid
water or CO2 frost activity.
