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.

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 (λ = 0.4–4.0 μm), 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. CO 2 ice clouds appear during the aphelion period of Mars orbit (Solar Longitudes, L s ∼ 0–160°) at low latitudes (∼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 H 2 O ice clouds are primarily restricted to the perihelion orbital range (L s ∼160 – 350°) at northern and southern mid-to-low latitudes with less apparent longitudinal dependences. Mars mesospheric CO 2 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 μm). The smaller particle sizes (R eff ≤1 μm) appear concentrated near the spatial (latitude and altitude) boundaries of their global occurrences. CRISM spectra of mesospheric CO 2 clouds also show evidence of iridescence, indicating very narrow particle size distributions (effective variance, V eff ∼ 0.03) and so very abrupt CO 2 cloud nucleation. Furthermore, these clouds are sometimes accompanied by altitude coincident peaks in 1.27 μm 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 μm), although larger particle sizes (R eff = 0.4–0.7 μm) appear infrequently. On average, water ice cloud particle sizes decrease with altitude over 50–80 km in the perihelion mesosphere. Water ice mass appears similar in clouds over a large range of observed cloud particle sizes, with particle number densities increasing to ∼10 cm−3 for R eff = 0.2 μm. Near coincident Mars Climate Sounder (MCS) temperature and aerosol profile measurements for a subset of CRISM mesospheric aerosol measurements indicate near saturation (H 2 O and CO 2) conditions for ice clouds and distinct mesospheric temperature increases associated with mesospheric dust loading. Dayside (3 pm) mesospheric CO 2 clouds with larger particle sizes (R eff ≥0.5 μm) scatter surface infrared emission in MCS limb infrared radiances, as well as solar irradiance in the MCS solar band channel. Scattering of surface infrared emission is most strikingly presented in nighttime (3 am) MCS observations at 55–60 km altitudes, indicating extensive mesospheric nighttime CO 2 clouds with considerably larger particle sizes (R eff ∼7 μm). Mesospheric CO 2 ice clouds present cirrus-like waveforms over extensive latitude and longitude regions (10°×10°), as revealed in coincident Mars Color Imager (MARCI) nadir imaging. Solar tides, gravity waves, and the large orbital variation of the extended thermal structure of the Mars atmosphere influence all of these behaviors. Mesospheric dust aerosols appear infrequently over the non-global (planet encircling) dust storm era of the CRISM limb data set (2009–2016), and exhibit smaller particle sizes (R eff = 0.2–0.7 μm) relative to dust in the lower atmosphere. One isolated case of an aphelion (L s = 96°) mesospheric dust layer with large dust particle sizes (R eff ∼2 μm) over Syria Planum may reflect high altitude, non-local transport of dust over elevated regions. • Characterization of dust and Ice aerosol distributions in the Mars mesosphere • Characterization of Mar mesospheric aerosol compositions and particle sizes • An orbital dichotomy in Mars mesospheric ice cloud composition • Large CO 2 ice cloud particles in the nightside Mars mesosphere • Definition of CO 2 ice cloud iridescence in the Mars mesosphere [ABSTRACT FROM AUTHOR]