NASA's Mars Phoenix Lander, which spent the summer in Mars' northern polar regions performing a variety of science experiments, caused quite a stir when rumors circulated that it had discovered signs of life on the Red Planet. NASA eventually held a press conference to dispel the rumors, promising that more details would eventually be revealed when scientists got around to publishing papers that would describe the experiments in detail. That day has finally arrived; today's issue of Science contains four papers that describe various findings from the mission. There's no sign of alien life, but the studies do reveal an active water cycle on Mars—including night-time snowfall.

The papers rely on evidence from a variety of the instruments on the lander, and the description of the data provides an impressive catalog of the various ways that Phoenix could prod and query the Martian pole. In the months before Martian winter shut the lander down, it managed to dig a dozen trenches, taking soil samples from each. These samples went into wet and dry chemistry labs, had their conductivity tested, and were even examined using an atomic force microscope. Meanwhile, cameras and a LIDAR system (a laser-based range detector) scanned the surroundings.

The overall conclusion is that the northern pole has an active water cycle. This had been suggested by a variety of evidence from orbital sensors, as well early images returned from Phoenix. It's also not a huge shock, given the seasonal growth and retreat of the polar ice cap. Still, Phoenix provided some significant details on the cycling of water in the area where it landed.

For starters, it's clear that there's significant subsurface ice in the area where Phoenix operated. One trench, dubbed "Snow White" by mission controllers, had a clear layer of ice within it, and others had slightly more diffuse signs. The white chunks that were observed sublimating from a trench were confirmed to be ice by an onboard experiment that noticed an endothermic transition occurring at about 2�C, after which its instruments picked up indications of water vapor.

Beyond confirming the presence of water, various instruments gave some indication of what that water was doing. Electrical conductivity in soil samples increased at night, suggesting that the water sublimated off in the sunlight and was returning to the Martian soil at night. The cameras detected clouds and low-lying fog, indicating that the water was taking part in normal atmospheric processes; these were forming at approximately -65�C, much too warm for them to be comprised of frozen carbon dioxide.

But the really amazing data came from the LIDAR instrument, which was able to track the formation of the clouds at the atmosphere's boundary layer. Cloud formation became more pronounced as the summer gradually faded and the atmosphere cooled at night, and the scientists were eventually able to detect cirrus-like clouds as they dropped "tilted vertical sheets" of ice particles back to the surface of the planet. In short, they watched it snow.

The authors of one paper sum up the cycle as follows: "This diurnal cycle deposited ice onto the surface at night, reducing the vapor pressure to low values, sublimated it in the morning, and redistributed it throughout the planetary boundary layer in the turbulent afternoon. Near midnight, ice clouds formed and precipitated a portion of the atmospheric H 2 O back to the surface in the early morning."

That water cycle has clearly had an impact on the minerals detected by Phoenix, which saw signs of calcium carbonate. This mineral is likely to form through a combination of carbon dioxide from the atmosphere, water from the snow, and calcium leeched from the existing minerals. Scientists also confirmed the preliminary reports of perchlorates, which would tend to absorb any liquid water under the current conditions. Nevertheless, this combination suggests that "this region could have previously met the criteria for habitability during favorable Milankovich cycles," when it would receive more sunlight.

All in all, not bad for a mission put together from parts left over from one that failed years earlier.

Science, 2009. DOI: 10.1126/science.1172339

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Listing image by Science/AAAS