Seasonal H2O and CO2 ice cycles at the Mars Phoenix landing site: 1. Prelanding CRISM and HiRISE observations

The condensation, evolution, and sublimation of seasonal water and carbon dioxide ices were characterized at the Mars Phoenix landing site from Martian northern midsummer to midspring (L_s ∼ 142° – L_s ∼ 60°) for the year prior to the Phoenix landing on 25 May 2008. Ice relative abundances and grain sizes were estimated using data from the Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment aboard Mars Reconnaissance Orbiter and a nonlinear mixing model. Water ice first appeared at the Phoenix landing site during the afternoon in late summer (L_s ∼ 167°) as an optically thin layer on top of soil. CO₂ ice appeared after the fall equinox. By late winter (L_s∼ 344°), the site was covered by relatively pure CO₂ ice (∼30 cm thick), with a small amount of ∼100 μm diameter water ice and soil. As spring progressed, CO₂ ice grain sizes gradually decreased, a change interpreted to result from granulation during sublimation losses. The combined effect of CO₂ sublimation and decreasing H₂O ice grain sizes allowed H₂O ice to dominate spectra during the spring and significantly brightened the surface. CO₂ ice disappeared by early spring (L_s ∼ 34°) and H₂O ice by midspring (L_s ∼ 59°). Spring defrosting was not uniform and occurred more rapidly over the centers of polygons and geomorphic units with relatively higher thermal inertia values.