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

Abstract 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 sizesof Mars ice and dust aerosols over 50–100 km altitudes in the Mars mesosphere. Within the coverage limitations of the CRISMlimb data set, a distinct compositional dichotomy is found in Mars mesosphericice aerosols. CO 2 ice cloudsappear 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, mesosphericH 2 O ice cloudsare 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 mesosphericCO 2 clouds are presented in CRISMspectra 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. CRISMspectra of mesosphericCO 2 clouds also show evidence of iridescence, indicating very narrow particle sizedistributions (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. Mesosphericwater ice cloudsgenerally 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 sizesdecrease 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 mesosphericaerosol measurements indicate near saturation (H 2 O and CO 2 ) conditions for ice cloudsand distinct mesospherictemperature increases associated with mesosphericdust loading. Dayside (3 pm) mesosphericCO 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 mesosphericnighttime CO 2 clouds with considerably larger particle sizes(R eff ∼7 μm). MesosphericCO 2 ice cloudspresent 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. Mesosphericdust aerosols appear infrequently over the non-global (planet encircling) dust stormera of the CRISMlimb 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°) mesosphericdust 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.