Dust Aggregate Bedforms in Low Pressure, Low Gravity Environments: Evidence from High Resolution Imaging of Mars

The HiRISE camera provides unprecedented detail of aeolian and other geologic features on Mars. Images of low thermal inertia regions on and near the Tharsis and Elysium vocanic provences reveal a surface covered in reticulate ridges with a spacing of 1-2 m. There are two dominant textures: 1) long ridges bridged by shorter ones, with the long ridges generally oriented circumferentially to the volcano summits, 2) ridges organized into a honeycomb-like pattern. The former are typically found in smooth areas whereas the latter are common within pits and craters, although these relationships are not exclusive. Areas near the caldera edges contain fewer ridges than on the volcano flanks or within the calderas. Many of the ridges are located near yardangs, such as on the western flank of Olympus Mons. The morphology, orientation, and distribution of the ridges is consistent with them being aeolian bedforms. The low thermal inertia and high albedo indicate the materials are composed of dust, probably accumulated over time from duststorm fallout. From wind tunnel studies conducted at Earth and Mars atmospheric pressures, cohesive and electrostatic effects are known to inhibit the saltation of dust-sized particles. Therefore, the bedforms are probably composed of dust aggregates blown by nighttime katabatic winds with freestream velocities of 30-100 m/s. Saltation must occur under constrained conditions of windspeed and particle size such that material is removed from the surface, yet is not suspended. The morphology of the reticulate bedforms is similar to the texture seen on some Martian yardangs. This suggests that some reticulate bedforms become cemented into a fairly weak “duststone” rock that is abraded by subsequent aeolian activity. However, it is also possible that the resemblance is coincidental, with the yardang texture formed purely from abrasion.