Constraints on dust aerosols from the Mars Exploration Rovers using MGS overflights and Mini-TES

Overflights of the Mars Exploration Rovers (MER) by the Mars Global Surveyor (MGS) provide a unique opportunity to examine some of the basic properties of dust aerosols, starting with one of the most fundamental, the indices of refraction (m = n + ik) in the infrared. The upward-viewing geometry of the Miniature Thermal Emission Spectrometer (Mini-TES) and the combined contemporaneous observations from both MER and MGS are powerful tools. Their use allows atmospheric retrievals to directly determine n and k while offering constraints for the menagerie of other radiative transfer input parameters. We exploit these coordinated observing campaigns, along additional data sources, to carry out series of radiative transfer analyses that ultimately return the set of refractive indices. We apply the resulting m to a larger sample of Mini-TES data to both further validate our approach and retrieve several other aerosol properties, including dust optical depth, dust size, and a measure of the vertical mixing profile. We find good agreement with the empirical approach of Smith et al. (2006), in terms of both the optical depths themselves and the frequency dependence of their extinction cross section and single scattering albedo. The retrieved dust sizes vary from near 1.3 μm to 1.8 μm within the selected sample, with a precision estimated to be ≃-0.1-0.2 μm. The vertical mixing profile evolves from well-mixed to appreciably confined by L_s ∼ 30°. For Spirit (MER-A), there is an abrupt transition back to a more well-mixed vertical profile with the onset of regional dust activity at L_s ∼ 140°. We discuss the lack of a definitive detection of water ice clouds in Mini-TES observations and the potential effects of vertical gradients in particle size distribution. Finally, as part of coordinated overflight analyses, an atmospherically corrected TES Lambert albedo map is derived and presented in Appendix A.