A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures

Abstract

Aeolian ripples are common in sandy environments on Earth and Mars. On Earth, ripples in sorted dune sands typically are <1 cm high and are erased in high winds. On Mars in similar sands, ripple wavelengths commonly exceed 2 m, with much smaller ripples superimposed. Large Martian ripple sizes and juxtaposition of multiple wavelengths have raised questions about origins and the applicability of terrestrial aeolian physics to different planetary environments. Here, two hypotheses are evaluated for large Martian ripples: (1) fluid/wind drag, analogous to ripples formed under water on Earth, as proposed previously for Martian large ripples; and (2) saltation impact splash, the mechanism creating aeolian ripples of much smaller size on Earth. This study evaluates these hypotheses with numerical experiments and Mars rover observations, and concludes that large Martian ripples develop through the saltation impact splash mechanism. The low-density Martian atmosphere enables aeolian impact ripples to grow much higher into the boundary layer before reaching maximum heights constrained by wind dynamic pressure effects at crests. In this concept, boundary layer conditions influence mature ripple heights more directly than wavelengths. On Mars, low wind dynamic pressures, combined with the impact splash mechanism, also help to explain other distinctively Martian aeolian bedforms, including large longitudinal ripples observed by rovers and orbiters, and transverse aeolian ridges (TARs) distributed widely across the Martian surface. Compared with Earth, low wind dynamic pressures on Mars permit a wider range of ripple sizes, relative ages, morphologies, and orientations in close proximity, as displayed in rover observations. Plain Language Summary On Earth, winds drive sand grains downwind in bouncing motions (called "saltation"). Each high-energy bounce also splashes other surface grains shorter distances, and this impact splash process creates small,~10 cm wavelength ripples with heights <1 cm in typical dune sands. On Mars, sands with similar grain sizes form much larger ripples with wavelengths exceeding 2 m, and their origins have been debated. In this study, numerical simulations and Mars rover observations indicate that aeolian impact ripples can grow much larger on Mars than on Earth because the thin Martian atmosphere does not interfere with the upward growth of ripple crests until ripples are much higher (which in turn constrains minimum, but not maximum, ripple wavelengths). In this concept, wind conditions influence mature aeolian ripple heights more directly than ripple wavelengths. This concept also helps explain other, distinctively Martian aeolian features including large longitudinal ripples, and large transverse aeolian ridges (TARs) observed widely across the Martian surface.

Martian Ripples Making a Splash

“…Ironically (and there is precedent in the eolian business for using the same data, plotted different ways, to support mutually incompatible theories such as those for "booming dunes"; see Lorenz & Zimbelman, 2014), this trend seems incompatible with that predicted by the fluid drag theory (Figure 2). Similarly, Sullivan et al (2020) show that the wind speeds predicted by the fluid drag model to generate observed bedform wavelength would be too low to cause sand to actually move on Mars, so there are some quantitative difficulties with the application of the model here. Lapotre et al's (2016) suggestion of a fluid drag theory was motivated by the two scales of ripples superposed on the Bagnold dunes, "in contrast to the single scale of superimposed terrestrial ripples".…”

“…The large grains give the system a stronger memory, their being only nudged by their more easily blown sand fellows resulting in a very slow "creep" along the surface. This memory brings additional degrees of freedom and time-dependent phenomena such as pattern coarsening to these dynamical systems (captured nicely in sophisticated models such as that of Sullivan et al, 2020;Yizhaq et al, 2014, andLämmel et al, 2018). The work by Lämmel et al (2018) shows that the intermittent removal of grains is an instrumental part of the megaripple evolution process.…”

“…Lapotre et al's (2016) suggestion of a fluid drag theory was motivated by the two scales of ripples superposed on the Bagnold dunes, “in contrast to the single scale of superimposed terrestrial ripples”. Yet a wide range of superposed ripple scales are actually found on Earth, too (e.g., Figure 1), and Sullivan et al (2020) also provide some examples. Furthermore, time‐lapse cameras (Lorenz & Valdez, 2011) let us actually see superposed patterns of straight and sinuous ripples simultaneously forming, evolving, and migrating (see supporting information Movie S1; full resolution movie files and further details are available at http://lib.jhuapl.edu) so the terrestrial situation is perhaps not as simple as was stated, even without introducing additional complication of grain size diversity.…”

“…By the crisis tsunami standards of 2020, it is but a ripple. In 2011 a new text on planetary surface processes (Melosh, 2011; Jay Melosh, from whom this author learned to love geological field study, passed away the week before the present paper was published and will be much missed) noted “The origin of (sand) ripples has currently reached a crisis precipitated by the observations of surface landers and rovers on Mars.” The eolian community continues to be vexed by the challenge of these data, not a bad crisis to have, perhaps, and a new paper by Sullivan et al (2020) lays out a wide suite of observations and simulations that provide a robust challenge to a model proposed by Lapotre et al (2016) as the crisis is, slowly, sorted out.…”

“…So, where do we stand? The work of Sullivan et al (2020) does not show that a fluid drag mechanism can never apply on Mars, only that it is not needed to explain the range of observed bedform wavelengths—the existing impact mechanism seems to provide ample diversity in this respect. Some elements of the Lapotre et al (2016) subaqueous ripple analogy may yet be relevant, however, in a planetary context: Vinent et al (2019) made recent progress in mapping out the phase space of subaqueous and subaerial bedforms in simulations and noted a scale gap due to the hydrodynamics of an internal boundary layer.…”