The Compositions, Particle Sizes, and Distributions of Ice Aerosols in the Mars Mesosphere from 2009-2016 CRISM Visible-NearIR Limb Spectra

Since 2009, the CRISM visible-nearIR imaging spectrometer onboard the Mars Reconnaissance Orbiter (MRO) has returned over 70 orbits of Mars limb image scans over the 0-130 km altitude range. Pole-to-pole latitudinal coverage is obtained from the near-polar, sun-synchronous (LT 3pm) MRO orbit for a limited set of surface longitudes centered on Tharsis, Valles Mariners, Meridioni, and Hellas regions. Seasonal coverage extends over the full seasonal range (Ls=0-360°), as accumulated over 2009-2016 (MY 29-33), supporting a range of aerosol and airglow studies (Smith et al., 2013; Clancy et al., 2012, 2013). The 0.4-4.0 μm wavelength range of these CRISM limb observations proves particularly suitable to characterizing aerosol composition and particle sizes, particularly for the Mars mesosphere (z=50-100 km), which has only recently been observed with any dedication by MCS (Sefton-Nash et al, 2013) and CRISM limb measurements. Dust and H2O, CO2 ice aerosols are clearly distinguished by their distinct scattering and absorption behaviors over the key 2-4 μm wavelength region, and their particle sizes are well determined by the 0.4-3 μm wavelength region. Several key attributes are determined for Mars mesospheric aerosols. Dust aerosols are largely undetected, and are apparently injected to such heights only during global dust storms (Clancy et al, 2010). Ice clouds are generally common at 55-75 km altitudes, although in separate halves of the Mars year. CO2 and H2O ice clouds are most prominent during the aphelion and perihelion portions of the Mars orbit, respectively. CO2 ice clouds, which occur at low latitudes over specific surface longitudes, present distinct particle size populations ranging from 0.5 to 1.5 μm (Reff). Mesospheric H2O ice clouds exhibit somewhat smaller particle sizes (Reff=0.3-1 μm) and extend over low to mid latitudes. This orbital dependence for mesospheric ice aerosol composition indicates extreme annual (orbital) variation in mesospheric water vapor.