Characteristics of large Martian dust devils using Mars Odyssey Thermal Emission Imaging System visual and infrared images

Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Abstract 18 A search for martian dust devils has been carried out, using Mars Odyssey Thermal 19 Emission Imaging System (THEMIS) visible-wavelength images. Simultaneous THEMIS 20 thermal infrared wavelength images were then processed and analyzed, to investigate the 21 thermal properties of the dust devils observed. 3079 images were checked, concentrating 22 on northern spring, summer and autumn (L S from 0 ° to 270 ° , 20 ° S to 50 ° N). Mars Express 23 High Resolution Stereo Camera, Mars Global Surveyor Mars Orbiter Camera and other 24 THEMIS visible images were used for comparison to potentially rule out any ambiguous 25 geological features. Eight clear examples of dust devils have been found in five separate 26 images, with a comparable number of unconfirmed possible devils. The rarity of dust 27 devils observed is believed to result from a combination of the difficulty in identifying 28 dust devils in medium resolution THEMIS data, and that the Mars Odyssey orbit flyover 29 local time is later in the afternoon than would be optimum for dust devil searching. The 30 temporal distribution of dust devil activity appears to be weighted more towards later 31 afternoon, compared to Earth, but this may be a sampling effect due to size variation with 32 time of sol, greater coverage later in the sol, or the small-number statistics. The thermal 33 infrared images indicate that the lofted dust in the column is cooler than the surrounding 34 surface, and must be equilibrating with the atmosphere in the dust devil. This energy 35 transfer is estimated to be about 10% of the heat flux energy that is available to drive the 36 systems. The ground shadowed by the dust column also appears colder than the 37 surroundings, due to reduced solar illumination. From the visible-wavelength images, the 38 shadows of the dust columns were used to estimate the column opacity, which in turn 39 gave estimates of the dust loadings, which ranged from 1.9x10-5 to 1.5x10-4 kg m-3 , 40 3 similar to lander-based observations. No thermal or visible trails are associated with the 41 dust devils, indicating that the surface equilibrates quickly after the devil has …