THEMIS-VIS Measurements of the Altitude and Velocity of Clouds in the Martian Mesosphere

Details THEMIS-VIS Measurements of the Altitude and Velocity of Clouds in the Martian Mesosphere THEMIS-VIS Measurements of the Altitude and Velocity of Clouds in the Martian Mesosphere

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p21e..03m&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P21E-03

Other

5405 Atmospheres (0343, 1060), 5464 Remote Sensing, 5494 Instruments And Techniques

Scientific paper

Although Mars Odyssey's Thermal Emission Imaging System visible subsystem (THEMIS-VIS) was not designed or intended for stereo imaging or cloud tracking, its multiple exposure color-imaging sequence serendipitously causes a parallax effect that allows the height of high-altitude clouds to be determined, and has sufficient time delay to detect the movement of these clouds. As a result, THEMIS-VIS has acquired exceptionally high resolution (36 or 72 m pixel scale) nadir-pointed images of martian clouds with altitudes in the 60-80 km altitude range, and is providing the first direct measurements of wind speed at these altitudes. We discover high altitude cloud candidates by noticing a severe misalignment of cloud features between any two bands of an image which has been map-projected at the altitude of the local surface. In order to measure altitude and velocity, we reproject the subframes that make up a THEMIS-VIS image at a series altitudes above the local surface, shifting the subframes relative to each other to account for a range of candidate velocities. To select the best fitting altitude and velocity, we manually inspect the reprojected images to find an approximate solution, and then maximize the correlation between the 425 nm band and 540 nm band within a manually selected high-constrast cloud-dominated region of the image in order to refine the solution. The precision of this technique is of course inherently limited by the sharpness of the cloud features. To date we have obtained two high-altitude velocity measurements, and have identified 50 more images with high altitude clouds that are likely to yield velocity measurements. In THEMIS sequence number V06930045, 217 degrees L_s and 47 degrees north latitude, we measure eastward cloud motion of 60 +/- 15 m/s at an altitude of 70 +/- 5 km. In THEMIS sequence number V10526009, 26 degrees L_s and 0.5 degrees north latitude, we measure westward cloud motion of 90 +/- 20 m/s at an altitude of 80 +/- 5 km.

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