OBSERVATIONS OF THE MARTIAN ATMOSPHERE: THEMIS-VIS CALIBRATION, MESOSPHERIC CLOUDS, AND THE POLAR VORTEX

Abstract

We present observations of the Martian atmosphere derived from two instruments: the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor space- craft, and the visible light subsystem of the Thermal Emission Imaging System (THEMIS-VIS) on the Mars Odyssey spacecraft. For TES, we start with vertically resolved temperatures derived as described by Conrath et al. (2000, JGR, 105), and from them we derive horizontal winds and Ertel potential vorticity on a time series of regular three-dimensional grids. The Ertel potential vorticity is used as a dynamical tracer and diagnostic tool to study the behavior of the martian polar vortices. We find that, in contrast to the terrestrial polar vortices, the martian polar vortices? Ertel potential vorticity typically has an annular maximum well away from the pole. We also find that the martian northern winter vortex is better organized than the southern winter vortex, and thus is likely to be a more effective barrier to mixing. For THEMIS-VIS we develop a complete radiometric calibration pipeline. This pipeline is used for standard data processing to convert Engineering Data Records (EDRs) to the Reduced Data Records (RDRs) released by NASAs Planetary Data System. We use THEMIS-VIS nadir-pointed images to detect clouds in the 40 km to 80 km altitude range, measuring altitude from parallax and velocity from cross-track motion during the imaging sequence. We have observed 5 cases of aphelion season equatorial high-altitude clouds during late afternoon, all located in the eastern Tharsis / Valles Marineris region, and 30 cases of high-altitude cloud features in the northern winter (perihelion season) mid-latitudes, all but one in the Acidalia region. A simple radiative transfer model yields optical depths greater than 0.2 for the equatorial clouds, as well as constraints on their composition. The mid-latitude high-altitude features are visible only in twilight, a geometry for which our simple plane parallel radiative transfer model is not valid. Comparing the zonal velocity of the clouds with a radiative transfer model, we find good agreement in the northern winter mid-latitudes, but poorer agreement for equatorial clouds.