Pictures taken by the Phoenix Mars Lander and released yesterday by NASA, like the one seen here, seem to be evidence of buried ice on the planet. A few days ago, white stuff was spotted in a trench dug in the Martian soil by the lander's robotic arm, and scientists thought it was either bits of ice or salt. When they couldn't tell for certain with the first chemical analysis, they decided to test the material against the thin atmosphere. If the white stuff disappeared when exposed to the air, it was ice. In a process called "sublimation," frozen water can turn directly into vapor.
When scientists compared the photos of the trench taken on Sunday with ones from yesterday, some white spots had clearly vanished. "It must be ice," Peter Smith, a research scientist at the University of Arizona and the mission's principal investigator, said in a NASA release. "These little clumps completely disappearing over the course of a few days, that is perfect evidence that it's ice. There had been some question whether the bright material was salt. Salt can't do that."
While scientists already knew there is ice on Mars, the discovery is a sign that the lander is digging in the right region. The Phoenix landed on the planet's northern plains late last month to search for evidence that liquid ice existed there and to look for traces of organic compounds in the soil, trying to determine whether it could have supported primitive life. You can follow the lander's progress with its updates on Twitter. —Heather Wax