Local Time Variation of Water Ice Clouds on Mars as Observed by THEMIS

Abstract:

The move of the Odyssey spacecraft to an orbit with local time near 6:00 AM and PM enables systematic retrieval of water ice clouds at a time of day not accessible from Mars Global Surveyor, Mars Reconnaissance Orbiter, or previous Odyssey observations. Although surface temperature is reduced near sunrise and sunset compared to afternoon, THEMIS observations show that there is still sufficient thermal contrast between the surface and atmosphere over a range of latitudes near the sub-solar point to retrieve accurate aerosol optical depth. Because water ice clouds form by condensation, relatively small changes in atmospheric temperature can cause clouds to form or sublimate quickly, and there can be large changes in water ice cloud optical depth over the course of a day. Here we present recent retrievals of water ice aerosol optical depth from THEMIS observations. These retrievals show significant differences in cloud locations and opitcal depth compared against THEMIS retrievals from previous Mars Years that were taken at an earlier local time.