MORPHOLOGY OF ANCIENT BEDFORMS ON MARS FROM THE HIGH RESOLUTION IMAGING SCIENCE EXPERIMENT

Sedimentary rocks on Mars attest to a variety of aqueous and atmospheric processes throughout its history. For example, rover investigations at Meridiani Planum and Gale crater have firmly established that some of the ~4 Ga rock record is composed of sediment that was transported by flowing wind and water including partially preserved bedforms. Across the surface other isolated occurrences that meet the morphologic criteria for dunes and ripples are evident in orbital data yet appear degraded and eroded. Here we explore these landforms, termed “paleo-bedforms” – distinct groups of linear to crescentic, positive-relief features of a characteristic wavelength that show evidence for lithification.

Probable candidates were selected using criteria including: 1) identification as a former component of a coherent bedform, and 2) having been rendered dormant in the distant past and show signs of lithification. Our analysis utilized HiRISE images (25 cm/pix) and derived topography (1 m/post).

An initial survey has revealed a wide distribution of areas hosting candidate paleo-bedforms in various geographic contexts (e.g. craters, canyons, basins), most of which are as yet undocumented. The most promising regions include Apollinaris Sulci and Valles Marineris. These sites show variable-sized clusters of broadly crescentic, positive-relief features that resemble barchan/barchanoid dunes with superposed craters, boulders and fractures. These hypothesized paleodunes are 3-25 m-tall and have wavelengths 200-600 m, similar to contemporary (active) barchan dunes, but with slightly less relief, presumably due to erosion. Certain areas display the presence of spatially variable, but eroded mantle deposits, suggesting some level of burial and exhumation. These ancient eolian dunes are largely preserved in an intermediate stage as part of the rock record (~60-90% based on comparisons with contemporary martian dunes), an uncommon circumstance on Earth. Other sites show exposures of decameter-spaced candidate paleobedforms that may be more analogous to ripples or transverse aeolian ridges. To better assess these paleodunes’ geologic history, including relation to past climate systems, future efforts will evaluate their thermophysical and compositional properties, along with paleo-wind regime reconstruction.