MOBILITY IN MIDDLE AND HIGH SOUTHERN LATITUDE DUNE FIELDS

Repeat images of martian aeolian features acquired by the High Resolution Imaging Science Experiment (HiRISE) have led to the detection of changes and migration of bedforms (dunes and ripples) in some locations, while other locations do not yet show evidence of change or movement. While all image pairs of bedforms in high northern latitudes (>60° N) show evidence of aeolian driven movement (likely due at least in part to strong katabatic winds from the high elevation polar cap), the same is not true for bedforms observed in high southern latitudes.

Here we analyze all currently available HiRISE image pairs covering dune fields at latitudes south of 40° for evidence of movement. We also apply a dune stability index (SI) based on morphological characteristics (0-6, with higher numbers indicating more morphological indications of stability and erosion) based on previous work by Fenton and Hayward (2010). Of the 71 locations investigated, ~65% show evidence of wind driven movement or bedform migration. Results indicate a general trend of decreasing sand mobility with increasing latitude and SI. Over 75% of the sites between 40-60° S latitude show changes while ~50% of sites south of 60° S, and over 60% of sites south of 70° S show no detectable movement. This is consistent with previous morphological studies that have suggested that bedforms in high southern latitudes are stabilized by agents, such as ground ice, which likely limit sediment movement (i.e. sand availability). Bedforms also show a general decrease in migration rate in higher latitudes, although the highest rates occur in Noachis Terra and Icaria Planum and may be related more to regional trends in wind strength rather than the presence or absence of seasonal (polar) processes. Bedforms with lower (more active-appearing) SIs are more active while those with higher SIs (5-6, less active-appearing) are dominantly immobile. Migration rates also generally decrease with increasing SI. Interestingly, most ripples with an SI of 3 exhibit migration. This combination of actively migrating bedforms along with dune field morphologies consistent with both stability and activity suggests possible competing influences of aeolian and polar processes, or perhaps a shift from earlier conditions dominated by polar processes to a more recent increase in aeolian activity.