Instruments on NASA's Mars Reconnaissance Orbiter estimated that the newfound ice is 99 percent pure.
Previous spacecraft have spied ice lurking below the Martian surface. Before the Phoenix lander froze to death last year, it dug trenches and touched ice specks at its arctic landing site.
Last year, radar observations from the Reconnaissance Orbiter pointed to the presence of buried glaciers in the Martian mid-latitudes. The latest discovery came as a surprise because scientists did not expect to find shallow ice this far south.
The findings will be published in Friday's issue of the journal Science.
This means "the recent climate of Mars was wetter than what we see today," said lead author Shane Byrne, an associate professor of planetary science at the University of Arizona.
The Reconnaissance Orbiter first spotted ice last year inside five impact craters ranging from the size of a living room to a small house. The ice looked bright against the dark craters. Over several months, the craft noticed the ice, which is unstable when exposed, turned into a gas.
Ice was confirmed by the spacecraft's spectrometer instrument. Though it was confirmed through this method only in one crater, Byrne said the material in the other craters is likely also ice.
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