for National Geographic News
Every night during Mars's winter, water-ice crystals fall from high, thin clouds over the north pole, new data from NASA's Mars Phoenix Lander have revealed.
The clouds resemble cirrus clouds on Earth, noted lead study author James Whiteway, an atmospheric physicist at York University in Toronto.
And the precipitation, he said, is similar to ice crystals that fall through the air in the Arctic in the middle of winter, called diamond dust.
All told, though, there's very little water locked up in the drifting ice crystals, said co-author Peter Smith, principal investigator for the Phoenix mission and a planetary scientist at the University of Arizona, Tucson.
"If you melted it all in a pan, [you] would be barely wetting the surface," Smith said.
"Mars is awfully dry. That's why it's surprising that you see snowfall."
The Phoenix lander arrived near Mars's north pole in May 2008 and collected data for five months before shutting down due to the extreme conditions of Martian winter.
Phoenix first spotted nightly clouds in early September, as winter began to set in, via an onboard weather instrument called LIDAR.
The probe sends laser beams through the atmosphere and records the reflected light from dust and clouds.
"We made more and more late-night observations of these clouds, and noticed streaks coming out the bottom of them," Smith said.
"As the season progressed, these streaks came closer to the surface until they were finally reaching the surface. Basically, we're seeing snowfall."
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