Transverse Aeolian Ridges (TARs) on Mars

Abstract

Aeolian processes are probably the dominant ongoing surface process on Mars; Large Dark Dunes (LDDs), particularly common aeolian landforms, were first recognized in the early 1970s. Recent, higher resolution images have revealed another, morphologically distinct, large population of smaller, ripple-like aeolian bedforms that have been termed "Transverse Aeolian Ridges" (TARs) as it is unknown whether they formed as large ripples or small dunes. We have begun a new study of TARs that examines their distribution, orientation, and morphology using N 10,000 high-resolution Mars Orbiter Camera (1.5 to 8 m/pixel resolution) images in a 45°longitude wide, pole-to-pole survey. The aim of this study is to assess whether TARs are active, to identify possible sediment sources and pathways, and to determine the volumes of sediment that they comprise. We present results from the first half of this study, in which we examine the northern hemisphere, and describe a new three-part classification scheme used to aid the survey. Our results show that TARs are abundant but not ubiquitous: preferentially forming proximal to friable, layered terrains such as those found in Terra Meridiani -the location of the ongoing Mars Exploration Rover "Opportunity" mission. TAR distribution in the northern hemisphere shows a strong latitudinal dependence with very few TARs being found north of ∼ 30°N. We also find that in most cases TARs are less mobile than LDDs, a conclusion possibly explained by Mars Exploration Rover Opportunity observations that show TARlike ripples to have a core of fine material armored by a monolayer of granule-sized particles. This could disallow significant bedform movement under the current wind regime. That TARs are essentially inactive is confirmed by superposition relations with slope streaks and LDDs and by observations of superposed impact craters. We suggest that observations made by the Opportunity Rover in Terra Meridiani indicate that the small aeolian bedforms common here are ripples and not small dunes. Farther south, these bedforms transition into larger features indistinguishable from TARs, suggesting that TARs (in the Meridiani area at least) are ripples and not dunes.

Large wind ripples on Mars: A record of atmospheric evolution

“…2B) is the signature of TARs (48,51,52). TARs may form as a result of coarse-grain armoring, giant saltation trajectories, or deposition of dust transported in suspension (52)(53)(54)(55), and are distinct from the large ripples in activity and morphology: (i) activity of TARs has not been detected (46,56), (ii) their wavelengths are generally larger and more widely distributed (e.g., Table S2), (iii) they have symmetric topographic profiles (54), and (iv) they tend to have a much higher albedo than the dark, active, mafic sands. Thus, the large martian ripples are distinct from TARs.…”

Bedform migration on Mars: Current results and future plans

“…Global studies show that bright TARs are generally older than dark dunes and lack any clear evidence of recent activity (Balme et al, 2008;Bridges et al, 2012a). This seems confirmed by evidence of cratered and eroded TARs reported from several areas of Mars (Reiss et al, 2004;Golombek et al, 2010;Kerber and Head, 2011).…”

Recent developments in planetary Aeolian studies and their terrestrial analogs

“…8), landforms that develop from significantly different emplacement processes; this ambiguity led to the use of the nongenetic term 'transverse Aeolian ridge' (TAR) to describe these features in order to preserve an open mind as to their formation mechanism (Bourke et al, 2003;Wilson and Zimbelman, 2004). Assessments of TARs have been conducted at both regional and global scales, making use of MOC and THEMIS images (Balme et al, 2008) and, more recently, HiRISE images (Zimbelman, 2010;Berman et al, 2011). The evidence indicates that both small dunes and large ripples (megaripples) are found at the 10-m scale, so that continued use of a non-genetic term still seems warranted (Zimbelman, 2010;Berman et al, 2011).…”