An Evolving Understanding of Enigmatic Large Ripples on Mars

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.