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Active dark flows known as recurring slope lineae have been observed on thewarmest slopes of equatorial Mars. The morphology, composition and seasonalityof the lineae suggest a role of liquid water in their formation. However,internal and atmospheric sources of water appear to be insufficient to sustainthe observed slope activity. Experimental evidence suggests that under the lowatmospheric pressure at the surface of Mars, gas can flow upwards throughporous Martian soil due to thermal creep under surface regions heated by theSun, and disturb small particles. Here we present numerical simulations todemonstrate that such a dry process involving the pumping of rarefied gas inthe Martian soil due to temperature contrasts can explain the formation of therecurring slope lineae. In our simulations, solar irradiation followed byshadow significantly reduces the angle of repose due to the resulting temporarytemperature gradients over shaded terrain, and leads to flow at intermediateslope angles. The simulated flow locations are consistent with observedrecurring slope lineae that initiate in rough and bouldered terrains with localshadows over the soil. We suggest that this dry avalanche process can explainthe formation of the recurring slope lineae on Mars without requiring liquidwater or CO2 frost activity.

Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows