MEGARIPPLES AND TRANSVERSE AEOLIAN RIDGES ACROSS MARS: ON A CONTINUUM FROM ACTIVITY TO LITHIFICATION

Megaripples (MRs) & Transverse Aeolian Ridges (TARs) are unique aeolian indicators that vary significantly in morphology and dynamics. Whereas some are now known to be active in the present Mars climate, many ]show clear evidence of longterm stabilization.

Here, we present results related to the dynamics of MRs & TARs, which we segregate based on their superposition and albedo, from a survey of 85 sites using long baseline (up to 8 Mars years) HiRISE images (Table 1). Their propensity for activity, metrics, and context is a research area of pressing relevance due to the mechanism by which bedforms evolve and migrate. Active bedforms had average wavelengths ranging 5-13 m, and derived heights of 0.5-1.26 m. Migration rates ranged between 0.06­­­-0.27 m/yr with sand fluxes between 0.02­­­-0.14 m³m^-1yr^-1; for context, dunes often range 2 orders of magnitude higher.

Notable areas with active MRs include the north polar erg, Syrtis Major, Hellespontus, & Valles Marineris, while McLaughlin, Ganges Chasma, & two other sites hosted migrating TARs. This MR and TAR activity is most evident among high sand flux dunes, either on the upwind edges of dune fields or within intererg areas near topographic constrictions. Smaller ripples and other mobile sand surfaces were always colocated with active MRs/TARs. Obviously, an abundance of saltation events is required for MRs & TAR mobility, but what’s not clear is whether HiRISE coverage captures effective sediment transport not associated with dark sand patches. For example, upwind (static) TARs in Ganges are adjacent to downwind mobile TARs in high sand supply/flux areas, suggesting there isn’t enough sand transport there (dark or otherwise). Alternatively, some interdune TARs are stabilized and coherent within high flux/volume areas. Examples include 2-4 m tall, sharp crested TARs within the Nili Patera dune field that appear unchanged when buried and bypassed by decameter tall dunes. More extreme evidence can be found with flat topped or fractured TARs pointing to armoring by coarse grains or induration. In this way, some TARs can be mobile, but most may have lost their competition against stabilizing factors and are on a path to lithification.

Table 1.

Activity status	# of sites
Significant migration/changes	27
Minor but unambiguous migration/changes	23
No changes	29
No MRs/TARs present	6