High-resolution subsurface water-ice distributions on Mars

Theoretical models predict that the depth at which water ice is stable in the subsurface of Mars will vary due to local surface features such as rocks and slopes, and the thermal inertia of the ground above the water. So far, measurements have been limited to overviews of the several-hundred-kilometre footprint covered by the γ-ray spectrometer on-board the Mars Odyssey spacecraft. But Joshua Bandfield has now used the seasonal temperature response of the martian surface (from Odyssey's THEMIS package) to map water ice distribution at the sub-kilometre scale. The observations reveal significant regional and local water-ice depth variability at potential landing sites for the Mars Phoenix mission due for launch later this year. The Phoenix lander's robotic arm will be able to dig up to half a metre or so into the soil to search for water ice, and these results suggest that it may encounter considerable variation in the depth to the ice layer. The seasonal temperature response of the Martian surface is used to map local surface heterogeneities (such as rocks, slopes and thermal inertia of ground cover) at subkilometre scale. These observations show significant regional and local water-ice depth variability, and indicate that the depth to the water-ice table is highly variable in the potential Phoenix spacecraft landing ellipses. Theoretical models indicate that water ice is stable in the shallow subsurface (depths of <1–2 m) of Mars at high latitudes1,2,3,4,5,6,7. These models have been mainly supported by the observed presence of large concentrations of hydrogen detected by the Gamma Ray Spectrometer suite of instruments on the Mars Odyssey spacecraft8,9,10. The models and measurements are consistent with a water-ice table that steadily increases in depth with decreasing latitude. More detailed modelling has predicted that the depth at which water ice is stable can be highly variable, owing to local surface heterogeneities such as rocks and slopes, and the thermal inertia of the ground cover11,12,13. Measurements have, however, been limited to the footprint (several hundred kilometres) of the Gamma Ray Spectrometer suite, preventing the observations from documenting more detailed water-ice distributions. Here I show that by observing the seasonal temperature response of the martian surface with the Thermal Emission Imaging System on the Mars Odyssey spacecraft14, it is possible to observe such heterogeneities at subkilometre scale. These observations show significant regional and local water-ice depth variability, and, in some cases, support distributions in the subsurface predicted by atmospheric exchange and vapour diffusion models. The presence of water ice where it follows the depth of stability under current climatic conditions implies an active martian water cycle that responds to orbit-driven climate cycles15,16,17. Several regions also have apparent deviations from the theoretical stability level, indicating that additional factors influence the ice-table depth. The high-resolution measurements show that the depth to the water-ice table is highly variable within the potential Phoenix spacecraft landing ellipses, and is likely to be variable at scales that may be sampled by the spacecraft.