Perspective

Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water

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Abstract

Recent liquid water flow on Mars has been proposed based on geomorphological features, such as gullies. Recurring slope lineae — seasonal flows that are darker than their surroundings — are candidate locations for seeping liquid water on Mars today, but their formation mechanism remains unclear. Topographical analysis shows that the terminal slopes of recurring slope lineae match the stopping angle for granular flows of cohesionless sand in active Martian aeolian dunes. In Eos Chasma, linea lengths vary widely and are longer where there are more extensive angle-of-repose slopes, inconsistent with models for water sources. These observations suggest that recurring slope lineae are granular flows. The preference for warm seasons and the detection of hydrated salts are consistent with some role for water in their initiation. However, liquid water volumes may be small or zero, alleviating planetary protection concerns about habitable environments.

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Acknowledgements

Observation planning was funded by the MRO project, and analysis by NASA grants NNX13AK01G and NNX14AO21G. We thank NASA/JPL/University of Arizona and the MRO/HiRISE project for collecting and processing data, the University of Arizona for producing DTMs, and NASA for supporting extended mission investigations. D. Stillman provided helpful comments.

Author information

Affiliations

  1. US Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA

    • Colin M. Dundas
    •  & Moses P. Milazzo
  2. The University of Arizona, Lunar and Planetary Laboratory, Tucson, AZ, USA

    • Alfred S. McEwen
    • , Matthew Chojnacki
    • , Shane Byrne
    •  & Anna Urso
  3. Department of Earth Sciences, Durham University, Durham, UK

    • Jim N. McElwaine
  4. Planetary Science Institute, Tucson, AZ, USA

    • Jim N. McElwaine

Authors

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Contributions

A.S.M., C.M.D. and M.C. planned many of the HiRISE observations to locate and study RSL. C.M.D. designed the study and gathered the slope data. A.O. and M.C. made observations of uphill ripple movement. M.C. assisted with DTM production. All authors contributed to discussion, interpretation and writing.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Colin M. Dundas.

Electronic supplementary material

  1. GranularRSL_SOM

    Supplementary Table 1; Supplementary Figures S1–S4; and caption information for the supplementary Movies 1–2 animations provided as separate files. (Animations S1–S2)

  2. Animation1_Dune_avalanche_lineae_27N.gif

    Animated GIF showing avalanche features on a sand dune slipface, to be compared with RSL

  3. Animation2_Coprates_ripples.gif

    Animated GIF showing upslope movement of ripples at one RSL location