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Description
Transverse Aeolian Ridges (TARs) are decametre scale bedforms, ubiquitous on the surface of Mars. TARs are are oriented perpendicular to the direction of the prevailing peak winds. Since TARs do not appear to be moving in the present day, their orientation likely records the wind patterns during historic epochs when the environmental conditions on Mars were more conducive to mass movement than they are today.
Using deep learning, we digitised ~20 million TARs in ~10,000 HiRISE Images, a dataset of unprecedented size. Global orientation trends were examined, as were those in a study area covering Isidis, Elysium, and the southern half of Utopia Planitia, as well as adjoining highlands regions in Syrtis Major and along the dichotomy boundary.
We found that in flat plains, TAR orientation trends are consistent over many tens of nearby HiRISE images. This provides a strong signal for wind direction during TAR forming epochs. Rugged areas show less consistent patterns.
We used Earth Movers Distance to compare the distribution of TAR orientations to modelled wind vectors produced by Global Circulation Models (GCMs) of the martian climate. We identified many regions where the GCM predicted wind vectors do not change between climate model scenarios. In most of these regions, the modelled results are always a good match for the TAR orientations, suggesting that conditions here have consistently favoured TAR formation throughout the history of Mars. There is though a preference for a 45° scenario, with that providing the best match for most locations.
In regions where variance between climate scenarios is high, the observed TAR pattern is most consistent with formation under high, 45°, obliquity conditions. This last occurred 5.5 million years ago. This appears to be the latest era during which wind speeds strong enough to mobilise TAR forming materials occurred on a large scale.