Once the precipitation started, he said, the snow fell every night from clouds about 2.5 miles (4 kilometers) above the spacecraft's landing site.
In the morning the ice crystals sublimated, or turned directly from solid to gas, the researchers found. The water vapor then got mixed back upward by atmospheric turbulence and again became clouds.
Smith suspects the newly discovered weather pattern is confined to the poles, although there's a chance that precipitation could occur at high altitudes, such as the tops of volcanoes.
While Martian snow was a surprise, this isn't the first time clouds have been seen on the red planet, noted Michael Smith, a planetary scientist at NASA's Goddard Space Flight Center who was not involved in the new study.
The orbiting Mars Global Surveyor had revealed cloud patterns that varied with location or the seasons, including so-called polar hood clouds at high latitudes, Smith said.
"Certainly we've [also] had observations before that seem to imply that there are fogs and frost on the ground," he added.
"But these new data show clouds evolving with time, [and] the bottom of the cloud moving down. It's unprecedented to see that level of detail."
Saturn's moon Titan boasts a methane cycle, and its other moon Enceladus appears to spew icy geysers.
But the new results put Mars and Earth in exclusive company: They're now the only solar system bodies with confirmed water cycles.
Clues to Liquid Water?
The snow findings, part of a suite of papers based on Phoenix data appearing in this week's issue of the journal Science, add to the tantalizing hints that Mars may yet support life.
One of the papers shows that a layer of ice water lies 2 to 7 inches (5 to 18 centimeters) under the soil at the north pole, and that liquid water likely helped shape the soil above it.
Two other studies separately confirm the detection on Mars of calcium carbonate—which can control the acidity of water—and perchlorate salts, which lower water's freezing point.
Phoenix lead Peter Smith said all the findings add heft to the possibility of recent, liquid water, a necessity for life as we know it.
"There's plenty of room for disagreement," he said, "but it seems to me that Mars periodically provides a habitable environment."
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