The Mars Atmosphere Water Ice Aerosol Climatology by MRO/CRISM: 5 Mars Years of Observations

We use near-infrared spectra returned from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) to provide retrievals of the column-integrated optical depth of water ice aerosols for more than 5 Martian years between Mars Year (MY) 28 at L-s = 112 degrees (27 September 2006) and MY 33 at L-s = 213 degrees (30 August 2016). We have developed a radiative transfer model for this purpose that retrieves aerosol opacity from the water ice aerosol feature near 3.3 mu m using CRISM hyperspectral observations for the first time. The resulting retrievals well depict the main features in the water ice aerosol climatology with repeatable patterns every Martian year. The aphelion cloud belt (ACB) is observed between 10 degrees S and 30 degrees N, with peak optical depth around L-s = 90 degrees. Apart from the global dust storm during MY 28, modest interannual variability in the water ice aerosol optical depth is observed in the ACB. The north polar hood is observed at latitudes poleward of 45 degrees N throughout northern spring and summer seasons and reappears in northern winter. The south polar hood is observed in early to mid-southern autumn, reaching latitudes around 60 degrees S, and re-emerging during southern winter between 45 degrees S and 60 degrees S. A lack of water ice aerosols is repeatedly seen in the southern hemisphere throughout southern spring and summer. High optical depth in the ACB is observed over the volcanoes of Olympus Mons, Elysium, and the Tharsis bulge, as well as over Hellas Basin. Plain Language Summary The atmosphere of Mars can hold water ice crystals when the right temperature and pressure conditions are met. We use observations from an instrument onboard the Mars Reconnaissance Orbiter to track the presence and evolution of water ice in the atmosphere based on how much the ice crystals absorb and scatter sunlight. The observations extend for 5 Martian years (similar to 10 Earth years) and cover all the Martian seasons. We show that there is a concentration of clouds around the equator of the planet, forming during spring and summer seasons, particularly over the volcanoes on Mars. Water ice clouds are also observed at high latitudes near the poles in the form of polar hoods, showing up in the north during northern spring, summer and winter, and in the south during southern autumn and winter. The main features in the retrieved cloud climatology in this study are repeated every Martian year. The amount of absorption caused by the water ice clouds is similar between the equatorial cloud belt, the north and south polar hoods.