Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

A total of 205 dust devils were detected in 23 High Resolution Stereo Camera (HRSC) images taken between January 2004 and July 2006 with the ESA Mars Express orbiter, in which average dust devil heights were ∼660 m and average diameters were ∼230 m. For the first time, dust devil velocities were directly measured from orbit, and range from 1 to 59 m/s. The observed dust devil directions of motion are consistent with data derived from a General Circulation Model (GCM). In some respects HRSC dust devil properties agree favorably with data from the NASA Mars Exploration Rover Spirit dust devil analyses. The spatial distribution of the active dust devils detected by HRSC supports the conjecture that the ascending branch of the Hadley circulation is responsible for the increase in dust devil activity, especially observed during southern summer between 50° and 60° S latitude. Combining the dust-lifting rate of 19 kg/km²/sol derived from the Spirit observations with the fewer in number but larger in size dust devils from various other locations observed by HRSC, we suggest that dust devils make a significant contribution to the dust entrainment into the atmosphere and to the martian dust cycle.