Mars aerosol studies with the MGS TES emission phase function observations: Optical depths, particle sizes, and ice cloud types versus latitude and solar longitude

Emission phase function (EPF) observations taken in 1999-2001 by Mars Global Surveyor Thermal Emission Spectrometer (MGS TES) support the broadest study of Martian aerosol properties to date. TES solar band and infrared (IR) spectral EPF sequences are analyzed to obtain first-time seasonal/latitudinal distributions of visible optical depths, particle sizes, and single scattering phase functions. This combined angular and wavelength coverage enables identification of two distinct ice cloud types over 45°S-45°N. Type 1 ice clouds exhibit small particle sizes (r_eff = 1 - 2 μm) and a distinctive backscattering increase. They are most prevalent in the southern hemisphere during aphelion, but also appear more widely distributed in season and latitude as topographic and high-altitude (≥20 km) ice hazes. Type 2 ice clouds exhibit larger particle sizes (r_eff = 3-4 μm), a distinct side-scattering minimum at 90-100° phase angles (characteristic of a change in particle shape relative to the type 1), and appear most prominently in the northern subtropical aphelion cloud belt. The majority of retrieved dust visible-to-IR optical depth ratios are indicative of r_eff = 1.5 ± 0.1 μm, consistent with Pathfinder and Viking/Mariner 9 reanalyses. However, increased ratios (2.7 versus 1.7) appear frequently in the northern hemisphere over L_s = 50-200°, indicating substantially smaller dust particles sizes (r_eff = 1.0 ± 0.2 μm) at this time. In addition, larger (r_eff = 1.8-2.5 μm) dust particles were observed locally in the southern hemisphere during the peak of the 2001 global dust storm. Detailed spectral modeling of the TES visible band pass indicates agreement of EPF-derived dust single scattering albedos (0.92-0.94) with the spectrally resolved results from Pathfinder observations.