In the permanently shadowed craters, where radar observations have pointed to the presence of ice, the altimeter recorded dark patches of diminished laser reflectance. “We never see in these regions the large increase of return energy that you would see if it were so cold that ice were exposed on the surface,” Neumann said. One possibility is that the radar-bright deposits, widely believed to be ice, could be overlaid by a dark material, such as a hydrocarbon, that can tolerate somewhat higher temperatures.
That hypothesis was supported by David Paige of the University of California, Los Angeles. He and his colleagues calculated surface and subsurface temperatures for the locations where radar-bright features tend to form and inferred a probable composition of ice pockets blanketed by regolith darkened by organic compounds. Peak temperatures in the shadowed craters, which can often be too warm for exposed water ice, mesh well with conditions at which dark organic molecules would be stable. But just below the surface, temperatures in radar-bright craters tend to be colder, hovering near –170 degrees C. That is exactly the temperature at which water ice would be expected to remain stable, Paige said. MESSENGER's new look at the features long ago spotted by Earth-based radars, he added, demonstrate “fairly conclusively now that they are predominantly composed of thermally stable water ice.”