An Evolving Understanding of Enigmatic Large Ripples on Mars

• Published in: Journal of geophysical research. Planets 2021-02, Vol.126 (2), p.n/a
• Main Authors: Lapôtre, Mathieu G. A., Ewing, Ryan C., Lamb, Michael P.
• Format: Article
• Language: English
• Subjects: Aerodynamics; Atmosphere; Atmospheric pressure; Dunes; eolian processes; Mars; Mars atmosphere; Mars climate; Mathematical models; Numerical models; Paleoclimate; Ripples; Sand; Synthesis; winds
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2020JE006729

Two scales of ripples form in fine sand on Mars. The larger ripples were proposed to have an equilibrium size set by an aerodynamic process, making them larger under thinner atmospheres and distinct from smaller impact ripples. Sullivan et al. (2020, https://doi.org/10.1029/2020JE006485) show that large ripples can develop in a numerical model due to Mars’ low atmospheric pressure. Although their proposed growth‐limiting mechanism is consistent with an aerodynamic process, they argue that the ripples in their model are simply large versions of impact ripples, not a separate class of ripples. Here, we explore this debate by synthesizing recent advances in large‐ripple formation (including initiation and subsequent evolution to equilibrium). Although significant knowledge gaps remain, it is clear that large Martian ripples in well sorted sand are larger under thinner atmospheres, and thus remain a powerful paleoclimate indicator. Plain Language Summary Earth's sandy deserts host small ripples and large dunes, but Mars’ dune fields also host a third type of sedimentary pattern—large ripples with meter‐scale spacings. The large ripples were previously proposed to be distinct from small Earth‐like ripples, and their size was suggested to result from the low atmospheric pressure on Mars. In contrast, Sullivan et al. (2020, https://doi.org/10.1029/2020JE006485) show that, in a numerical model, small ripples can grow large on Mars. Here, we explore this debate by synthesizing recent advances in large‐ripple formation. While significant knowledge gaps remain, it is clear that large Martian ripples in well sorted sand are larger under thinner atmospheres, and thus remain a powerful tool to probe Mars’ ancient climate. Key Points A new model predicts small windblown ripples should grow to be meter‐scale on Mars That model cannot explain some observations that appear to be more consistent with an aerodynamic origin for large Martian ripples Answering remaining questions on large‐ripple formation will require new observations, both in situ and from wind‐tunnel experiments