Megaripple Migration on Mars

Abstract

Aeolian megaripples, with 5‐ to 50‐m spacing, are abundant on the surface of Mars. These features were repeatedly targeted by high‐resolution orbital images, but they have never been observed to move. Thus, aeolian megaripples (especially the bright‐toned ones often referred as Transverse Aeolian Ridges—TARs) have been interpreted as relict features of a past climate. In this report, we show evidence for the migration of bright‐toned megaripples spaced 1 to 35 m (5 m on average) in two equatorial areas on Mars indicating that megaripples and small TARs can be active today. The moving megaripples display sand fluxes that are 2 orders of magnitudes lower than the surrounding dunes on average and, unlike similar bedforms on Earth, can migrate obliquely and longitudinally. In addition, the active megaripples in the two study areas of Syrtis Major and Mawrth Vallis show very similar flux distributions, echoing the similarities between dune crest fluxes in the two study areas and suggesting the existence of a relationship between dune and megaripple fluxes that can be explored elsewhere. Active megaripples, together with high‐sand flux dunes, represent a key indicator of strong winds at the surface of Mars. A past climate with a denser atmosphere is not necessary to explain their accumulation and migration.

Circadian Rhythm of Dune‐Field Activity

“…Based on recent data (Forget et al., 1999; Millour et al., 2012) we believe that Nili Patera, a well‐studied and representative active dune field on Mars (Ayoub et al., 2014), has a daily near‐surface temperature range ∼25% greater than δT 2, cr during the dusty season (Banfield et al., 2020; Text in supporting information). We hypothesize that enhancement of surface winds by nonequilibrium ABL dynamics may help to resolve a current riddle on Mars, where surface winds simulated with MOST are too weak to exceed u ∗, cr predictions and explain actively migrating bed forms (Bridges et al., 2017; Newman et al., 2017; Silvestro et al., 2020).…”

Martian Ripples Making a Splash

“…For example, a systematic variation of wind speed with altitude for the Tharsis data might nudge the interpretation away from an exactly reciprocal density relationship. A recent paper by Silvestro et al (2020) has measured active migration of Martian megaripples from orbit, so we at least know that the process is still active (it is striking that we now have the ability to detect geomorphogical processes on Mars that have velocities about the same as the rate of growth of our hair!) However, with only a single wind measurement station currently operating on the surface of Mars (InSight) we generally have to guess at the winds that are actually causing the motion.…”

Turbulent Shear Flow Over Large Martian Ripples

“…On Earth, aeolian impact ripple wavelengths in well-sorted sands typically are <30 cm and heights are <1 cm (e.g., Sharp, 1963). But on Mars, ripple-like bedforms with crests lacking very coarse grains can be much larger, both in wavelength (commonly >1 m) and height (>10 cm) ( Figure 2) (Bridges et al, 2007(Bridges et al, , 2012Ewing et al, 2017;Lapotre et al, 2016Lapotre et al, , 2018Silvestro et al, 2010Silvestro et al, , 2016Silvestro et al, , 2020Sullivan et al, 2008Sullivan et al, , 2020Vaz et al, 2017). Because crests of these large, meter-scale bedforms are not covered with very coarse grains, these bedforms are not directly analogous to terrestrial megaripples.…”