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

Abstract

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.

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Figure 1: Evolution of RSL at Garni Crater, Valles Marineris, Mars.
Figure 2: Knudsen pump in the Martian soil.
Figure 3: Angle of repose, modified by the Knudsen pump as a function of slope aspect (expressed as the clockwise angle from the north) and season (expressed in Ls).

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Acknowledgements

We acknowledge support from the ‘Institut National des Sciences de l’Univers’ (INSU), the ‘Centre National de la Recherche Scientifique’ (CNRS) and ‘Centre National d’Etude Spatiale’ (CNES) through the ‘Programme National de Planétologie’, HRSC/MEX, OMEGA/MEX and PFS/MEX programmes. Computational work was supported by the Slovak Research and Development Agency under contract No. APVV-14-0017. This work is supported by the Center for Data Science, funded by the IDEX Paris-Saclay, ANR-11-IDEX-0003-02. We thank S. Piqueux for fruitful remarks.

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F.S. led the project. F.A. was in charge of the angle of repose theory. F.C. interpreted the RSL geomorphology. M.K., A.G.M. and F.S. adapted the thermal profile calculation. All co-authors contributed to the analysis of the results and the redaction of the article.

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Correspondence to Frédéric Schmidt.

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The authors declare no competing financial interests.

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Schmidt, F., Andrieu, F., Costard, F. et al. Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows. Nature Geosci 10, 270–273 (2017). https://doi.org/10.1038/ngeo2917

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