The water cycle is critically important to understanding Mars system science, especially interactions between water and surface minerals or possible biological systems. In this thesis, the water cycle is examined at the Mars Phoenix landing site (68.22°N, 125.70°W), using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), High-Resolution Imaging Science Experiment (HiRISE), the Phoenix Lander Surface Stereo Imager (SSI), and employing non-linear spectral mixing models. The landing site is covered for part of the year by the seasonal ice cap, a layer of CO2 and H2O ice that is deposited in mid-fall and sublimates in mid-spring. During the mid-summer, H2O ice is deposited on the surface at the Phoenix landing site. CO2 ice forms at the site during fall. The onset date of seasonal ices varies annually, perhaps due to variable levels of atmospheric dust. During fall and winter, the CO2 ice layer thickens and sinters into a slab of ice, ∼30 cm thick. After the spring equinox, the CO2 slab breaks into smaller grains as it sublimates. Long before all of the CO2 ice is gone, H2O ice dominates the near-infrared spectra of the surface. Additional H2O ice is cold-trapped onto the surface of the CO2 ice deposit during this time. Sublimation during the spring is not uniform, and depends on the thermal inertia properties of the surface, including depth of ground ice. All of the seasonal ices have sublimated by mid-spring; however, a few permanent ice deposits remain throughout the summer. These are small water ice deposits on the north-facing slopes of Heimdal Crater and adjacent plateaus, and a small patch of mobile water ices that chases shadows in a small crater near the landing site. During the late spring and early summer, the site is free of surface ice. During this time, the water cycle is dominated by vapor exchange between the subsurface water ice deposits and the atmosphere. Two types of subsurface ice were found at the Phoenix landing site: a pore water ice that appears to be in diffusive equilibrium with the atmosphere, and an almost pure water ice deposit that is apparently not in equilibrium. In addition to vapor and solid phases of the water cycle, there is strong evidence of a liquid phase. Patches of concentrated perchlorate salt are observed in trenches dug by the lander. Perchlorate is believed to form at the landing site through atmospheric interactions, which deposit the salts on the surface. The salts are then dissolved and translocated to the subsurface by thin films of liquid water. These thin films may arise due to perchlorate interactions with the atmospheric water vapor or seasonal ices. It is possible that the winter CO2 ice slab may act as a greenhouse cap, trapping enough heat for the underlying fall-deposited water ice to react with the perchlorate to form thin films of brines. Alternatively, the brines may form when summertime water vapor interacts with perchlorate on the surface, when temperatures rise above the perchlorate brine eutectic.
- Pub Date:
- January 2010
- Geology;Planetology;Remote Sensing