Seasonal ice cycle at the Mars Phoenix landing site: 2. Postlanding CRISM and ground observations

The combination of ground observations from the Mars Phoenix Lander and orbital data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provided a detailed view of the formation of late summer surface water ice at the landing site and surrounding regions. CRISM observations of the landing site during and immediately after Phoenix operations were analyzed to track the seasonal and diurnal ice cycles during the late spring to late summer, and a nonlinear mixing model was used to estimate grain sizes and relative abundances of water ice and dust. The surface around the Phoenix landing site was ice‐free from late spring through midsummer, although transient patches of mobile ices were observed in an 85 m diameter crater to the northeast of the landing site. At the ∼10 km diameter Heimdal Crater, located ∼10 km east of the landing site, permanent patches of water ice were observed to brighten during the late spring and darken during the summer, possibly as fine‐grained water ice that was cold trapped onto the ice during late spring sintered into larger grains or finally sublimated, exposing larger‐grained ice. CRISM spectra first show evidence of widespread ice during the night at solar longitude (L s ) ∼ 109°, ∼9 sols before Phoenix's Surface Stereo Imager detected it. CRISM spectra first show evidence of afternoon surface ice and water ice clouds after L s ∼ 155°, after Phoenix operations ended.