First Evidence of Dune Movement on Mars

Many sand dunes on Mars have pristine morphology, fresh grain flows on avalanche faces and sand streamers extending from barchan horns. This suggests that the saltation threshold for sand is exceeded and dune migration is possible under the current Martian climate. However, sand dune movement has not been observed and there is evidence that many of the dunes may be stabilized or indurated. We report the first evidence for the movement of bodies of windblown sand under current climate conditions on Mars. Repeat images of three sand dunes using the Mars Orbiter Camera were acquired between March 1999 and December 2004. We detected the complete removal of sediment from two small dome dunes in a barchan dune field in the North Polar Sand Sea during this time. The third and largest dome dune (77 m wide) in the sample suite had a slight reduction in size, but dune form remained intact. On Earth, dome dunes are circular to oval low mounds of loose, well-sorted, very fine to medium sand. Slip faces are absent or ephemeral and stand only a meter or so high. That these dunes did not migrate, but were eroded, suggests that they were not in equilibrium. Dome dune morphology is not always as effective as e.g., barchan morphology, for trapping sediment, particularly in locations of high velocity winds. In these situations, the removal of sand downwind can lead to the depletion of the dune. Our data confirms that first; the threshold wind speed for saltation is exceeded under present Martian climate conditions. Second, not all dunes on Mars are stabilized or indurated. Third, dune migration is possible under current Martian conditions; however it is likely to be limited to the smallest barchan and dome dunes (i.e. < 20 m wide).