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Planetary scientists looking for water ice on Mars have employed a number of tactics to great success in their search. The Phoenix lander dug it up; orbiting radar measurements have seen it under insulating blankets of debris. (Frozen water sublimates to vapor in Mars's climate and so is not stable when exposed at the surface.)
Now a team of researchers has let meteorite impacts do the digging for them—a paper in this week's Science presents observations of fresh impacts and what they turn up from below the surface.
Using instruments on NASA's Mars Reconnaissance Orbiter (MRO), a group led by Shane Byrne, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory, found five recent impact craters in the Martian mid-latitudes, near the boundary where subsurface ice is thought to be no longer tenable. All were relatively small, ranging in size from about four to 12 meters across.
In three cases, the craters were less than a year old, because imagery of the same regions just months prior had shown no such impact scar. But all five showed visual indications of ice, and at the crater with the largest presumed ice deposit, the icy composition was confirmed by the orbiter's spectrometer. (The suspected ice blankets around the other four craters, each occupying only a fraction of a spectrometer pixel, were too small to register in the instrument's readings.)
By checking on the craters over time, MRO saw the bright excavated material fading, consistent with sublimating ice. Modeling the sublimation process for one of the craters, the team concluded that the upturned ice was nearly pure, with a debris content of less than 1 percent. The others appear to be comparably clean also.
Byrne and his co-authors note that the monitoring of such relatively common craters and their ejected ice can give scientists a view of the Martian ice table's boundaries and depth, as well as helping to constrain how much water Mars had in its atmosphere in the past.
Photo of an eight-meter crater surrounded by what appears to be clean ice: NASA/JPL-Caltech/University of Arizona