INTEGRATED SURFACE AND ORBITAL PERSPECTIVES ON DUNE ACTIVITY ON MARS

On Mars, much longer surface exposure ages, and the relative lack of competing hydrological chemical and physical weathering processes compared to Earth, makes wind arguably the most active agent of landscape modification in the current environment. However, a vexing problem in Martian geological and climate studies has been reconciling evidence for dune and ripple movement as seen in orbital images, and inferred in the geologic record, with wind tunnel and theoretical studies indicating that the frequency of winds above the threshold needed to move sand is much lower than on Earth. A significant missing piece in addressing this issue has been the lack of surface change detection measurements tied to in situ wind measurements. Recently, the Mars Laboratory Curiosity rover has investigated the informally named Bagnold dunes in Gale Crater, the first in situ study of an active dune field on another planetary surface. These results provide an unprecedented opportunity to link changes seen on the surface to in situ wind measurements, and to compare these results to broader-scale data from orbital images and predictions from wind models. HiRISE shows that the bulk dune movement and that of superposed ripples varies seasonally, consistent with predictions from global circulation models. The first phase of the Bagnold Dune Campaign occurred during the lowest wind activity period. Nevertheless, changes were documented, including common occurrences of grain scrambling and minor grain flow. Some activity is correlated to the highest winds measured by the Rover Environmental Monitoring Station Wind Sensor, although other events show no correlation. This suggests that additional processes likely are triggering mechanisms for grain movement, for which even low speed winds can then act as catalysts. These include temperature changes, frost condensation, and upwind mobilization of exposed grains. Although all of these processes are subtle, they may be paramount in a low aeolian activity environment in which short wind gusts will exploit whatever exposed sand is available. Such environments are applicable to many parts of Mars and to long periods in the geologic past under lower obliquity.