The distribution of Transverse Aeolian Ridges on Mars

Introduction: Mars is host to a wide range of aeolian forms such as dunes, ripples, dust devils, dust storms, yardangs, and ventifacts. Large dune fields characterized by low albedos and large duneform sizes have been observed and occur mainly around the north polar cap and in the southern mid-latitudes. However, another morphologically and dimensionally distinct population of aeolian bedforms have also been noted. These are generally brighter than the surrounding terrain, are about an order of magnitude smaller than the large, dark dunes (LDDs) and have simple forms. These bedforms have been designated ‘Transverse Aeolian Ridges’, or ‘TARs’ [1]. We have conducted a survey of all high-resolution (~1-11 m/pixel) Mars Orbiter Camera (MOC) images (~10,000 images) in a pole-to-pole swath between 0 and 45° E longitude to identify and classify TARs. This work extends the preliminary survey of [2], and was conducted on the opposite site of the planet. The aims are to determine TAR distributions, orientations, morphologies and morphometries, possible sediment sources, and superposition relationships with LDDs. Approximate percentage of areal coverage of TARs in each MOC image was recorded, as well as classification according to [3] and associations with other features such as LDDs and slope streaks. Distributions and orientations: The geographic distribution of TARs is significantly non-random: in the northern hemisphere, TARs are most commonly found between 0 and 35° N, particularly in the Terra Meridiani region. 668 MOC images in the northern hemisphere contained at least 5% areal coverage of TARs (Fig. 1). In the southern hemisphere, TARs are found between 0 and 55° S. 1591 MOC images in the southern hemisphere had 5% or more areal coverage containing TARs (Fig. 2). TARs tend to be found on crater floors and in regions containing mesas and layered terrains; in short, anywhere where significant mass-wasting can occur. The geographical distribution of different classes of TARs is also non-random: there are a much higher proportion of TARs classified as ‘barchan-like’ in the Meridiani region than anywhere else in the study region. Orientations of TARs (when not influenced by local topography) are consistent over large areas (cf. Fig. 3), suggesting that the wind regimes which control the formation of TARs are also consistent over wide areas. Figure 1. Percent areal coverage of TARs in MOC images for northern hemisphere. White dots represent MOC images with <5% TARs. Green dots show proportional percentages greater than 5%.