The distribution, composition, and particle properties of Mars mesospheric aerosols: An analysis of CRISM visible/near-IR limb spectra with context from near-coincident MCS and MARCI observations

摘要

The Compact Reconnaissance Imaging Spectral Mapper (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) obtains pole-to-pole observations (i.e., full MRO orbits) of vertical profiles for visible/near-IR spectra (lambda = 0.4-4.0 mu m), which are ideally suited to identifying the composition and particle sizes of Mars ice and dust aerosols over 50-100 km altitudes in the Mars mesosphere. Within the coverage limitations of the CRISM limb data set, a distinct compositional dichotomy is found in Mars mesospheric ice aerosols. CO2 ice clouds appear during the aphelion period of Mars orbit (Solar Longitudes, L-s similar to 0-160 degrees) at low latitudes (similar to 20S-10N) over specific longitude regions (Meridiani, Valles Marineris) and at typical altitudes of 55-75 km. Apart from faint water ice hazes below 55 km, mesospheric H2O ice clouds are primarily restricted to the perihelion orbital range (L-s similar to 160 - 350 degrees) at northern and southern mid-to-low latitudes with less apparent longitudinal dependences. Mars mesospheric CO2 clouds are presented in CRISM spectra with a surprisingly large range of particle sizes (cross section weighted radii, R-eff = 0.3 to 2.2 mu m). The smaller particle sizes (R-eff <= 1 mu m) appear concentrated near the spatial (latitude and altitude) boundaries of their global occurrences. CRISM spectra of mesospheric CO2 clouds also show evidence of iridescence, indicating very narrow particle size distributions (effective variance, V-eff similar to 0.03) and so very abrupt CO2 cloud nucleation. Furthermore, these clouds are sometimes accompanied by altitude coincident peaks in 1.27 mu m O-2 dayglow, which indicates very dry, cold regions of formation. Mesospheric water ice clouds generally exhibit small particle sizes (R-eff = 0.1-0.3 mu m), although larger particle sizes (R-eff = 0.4-0.7 mu m) appear infrequently. On average, water ice cloud particle sizes decrease with altitude over 50-80 km in the perihelion mesosphere. Water