Mars: Dust Devil Tracks in Hellas Basin and Argyre Planitia

Introduction: Dust devils on Mars could be an important mechanism for delivery of dust into the Martian atmosphere. Suspended dust in the Martian atmosphere is observed to be ~1-2μm in diameter [1,2,3] and calculations of fall-out rate [4,5] imply an ongoing supply of this fine dust to the atmosphere. Laboratory wind tunnel experiments [6,7], however, suggest that entrainment of fine dust into the atmosphere requires surface wind speeds exceeding those measured on Mars. Recent experiments using a laboratory scale vortex generator at low-pressure [8,9] suggest that vortex action (i.e. a dust devil) is more efficient at lifting fine dust than boundary layer winds. Coupled with the many observations of dust devils on Mars [10], these experiments suggest that dust devils are an important contributor to the global dust budget and hence to the global climate. Characterizing the temporal and spatial distribution of dust devil activity on Mars is therefore essential for generating accurate climate models. However, the transient nature of dust devils, the limited spatial coverage of the high resolution images required to actually resolve them, and restricted viewing times of the Mars Orbiter Camera (MOC) makes their detection serendipitous. Several studies have attempted to measure the frequency of active dust devils using both Wide Angle [11] and Narrow Angle (NA) [12] MOC images but, for the reasons given above, they can only sample a small fraction of dust devil activity. However, the passage of dust devils over the surface can leave a surface track that remains visible on the surface for orders of magnitude longer than the dust devil that produced them (fig. 1). Here we investigate the distribution of these dust devil tracks in two study areas: Argyre Planitia (20 to 50W; –50 to -30S) and Hellas Basin (250 to 320W; –50 to -15S). Approach: The study areas were chosen because they a) display a variety of geomorphic units, b) cover a range of elevations, c) are at similar latitudes for seasonal comparisons, and d) contain a reasonable percentage of images with visible tracks. We searched all MOC NA images with resolution 5m/pixel or better from the CAL to E12 phases (March 1999 to February 2002) of the Mars Global Surveyor mission for dust devil tracks. The number of tracks in each image was estimated and then normalized by the surface area of the NA strip to estimate the density of tracks per square km. In total, we searched 637 images in Argyre and 2037 in Hellas. Figure 1. Martian dust devil tracks. (NASA image PIA 02376, Malin Space Science Systems).