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Ever looked at something, done a double-take, looked again and yes, that is what you saw but your brain told you it could not be? That is exactly what just happened to me when I read the NASA press release which informed me, in no uncertain terms, that it was snowing on Mars. As if that wasn't confusing enough, NASA even has .

So what's going on, exactly? Good question, and the answer according to NASA boffins is that the Phoenix Mars Lander has "detected snow falling from Martian clouds" a statement which, I have to say, simply blows my mind.

The Phoenix Mars Lander used a laser instrument designed to gather knowledge of how the atmosphere and surface interact on Mars, to detect the snow from clouds some 2.5 miles above the landing site. The gathered data shows this snow vaporising before it hits the surface.

Unsurprisingly, the lead scientist for the Meteorological Station on Phoenix, Jim Whiteway says "Nothing like this view has ever been seen on Mars, we'll be looking for signs that the snow may even reach the ground."

Meanwhile, Peter Smith the Principal Investigator for the Phoenix Project says of the calcium carbonate and particle of a clay-like substance also found on the planet that "we are making good progress on the big questions we set out for ourselves." Big questions such as is this stuff not that same as the carbonates and clays that form on Earth only when there is liquid water around?

NASA is certainly claiming that the fact it has found carbonate points "toward episodes of interaction with water in the past" which is a start.

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— that double-take was justified. What Phoenix actually saw was not a white blanket on the ground but laser-detected fall streaks of tiny water-ice crystals in high, cirrus-like clouds; the crystals were observed to disappear in the thin air before reaching the surface. (jpl.nasa.gov)

The detection came from the Canadian-built Meteorological Station (MET) lidar on Phoenix. The lidar sent short laser pulses upward and recorded backscatter profiles; on Sol 99 (September 3, 2008) the profile showed vertical “fall streaks” and curved streaks at the cloud base (the curvature reflects faster winds near ~3 km), consistent with small water-ice particles being advected and then sublimating. Because temperatures at that altitude were too warm for CO2 ice, the particles were interpreted as H2O ice rather than dry-ice snow. (asc-csa.gc.ca)

Why the headlines mattered: Phoenix’s surface analyses also turned up alteration minerals (calcium carbonate and clay-like material) and soluble salts including perchlorate. The carbonates point toward episodes of interaction with liquid water in the past, while perchlorates change how soils behave (they’re hygroscopic and can lower freezing points), so Phoenix’s combined atmospheric and soil data opened new, testable lines about transient liquid brines and recent aqueous activity. (experts.arizona.edu)

In short, it was “snow” in the sense of falling H2O ice crystals, but Mars’ low pressure makes that snow behave very differently from Earth’s — it usually sublimates before landing. Phoenix converted a surprising press-release line into hard measurements that reshaped thinking about the present-day Martian water cycle and set up the questions later missions have kept investigating. (cjss.ac.cn)

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