It's snowing on Mars

(09-29) 17:22 PDT SAN FRANCISCO -- It's snowing on Mars and winter is icumen in - to misquote the Middle English paean to springtime.

Scientists studying the coded signals from the lander Phoenix on the planet's arctic surface detected the snow falling lightly from clouds drifting across the sky some 2 1/2 miles above the spacecraft, said James Whiteway, an atmospheric scientist from York University in Canada.

"Nothing like this has ever been seen on Mars before," he said.

Whiteway, whose team built the weather station aboard Phoenix, said the ice crystals appeared to vaporize before they reached the red Martian ground.

Not that snow was unexpected. Whiteway said his instruments have watched the clouds drifting across the horizon every morning after the sun rises. His Lidar instrument - an acronym for laser detection and ranging - on the spacecraft shoots a green laser beam up into the clouds 100 times a second, and the falling snow reflects brightly in each pulse.

Meanwhile, NASA scientists continue to decipher the signals sent back to Earth from Phoenix, now busy analyzing the soil and ice on the planet's surface.

In a telephone briefing for reporters Monday, the project's chief scientist, Peter H. Smith of the University of Arizona, said that five months of rooting around in the soft and dry soil of the landing site with the spacecraft's mobile robotic digging arm has confirmed that there's a "skating rink" of water ice a little more than 2 inches below the red soil.

That surface soil itself seems so dry remains a mystery.

Measurements of the ice indicate it has an alkaline level - known as pH - of 8.3, and is very similar to seawater, said Michael Hecht, a physicist at NASA's Jet Propulsion Laboratory in Pasadena. There also appears to be plenty of calcium carbonate in the tiny particles of the icy soil, he said.

Additionally, there are signs of several clay minerals whose sheeted structures hold molecules of water vapor, said William Boynton of the University of Arizona, whose miniature ovens aboard Phoenix heat the soil and ice to drive off identifying vapors.

"We can now begin rewriting the book of Martian chemistry," said Hecht.

The scientists must work fast now, for winter is indeed approaching. During the spacecraft's first three months on Mars, the sun never sank below the horizon, leaving plenty of solar energy for Phoenix to power all its instruments. Now, however, the sun sinks beneath the horizon for more than four hours every Martian night, according to Barry Goldstein, the Phoenix project manager and chief engineer at the Jet Propulsion Laboratory. It's also getting colder, and that drains power from the instruments because they must be kept warm in order to operate, he said.

Before the end of October, Goldstein said, there won't be enough power left to keep the lander's robotic arm operating, so digging into the soil and scraping ice samples from beneath the soil will have to stop. By November, Phoenix will be standing rigidly in the pitch dark, and carbon dioxide in the atmosphere will encase it in ice like some otherworldly frozen mummy - at more than 150 degrees below zero Fahrenheit.

That should make a weird picture, and Goldstein said the Phoenix team will ask NASA to have the Mars Reconnaissance Orbiter high above send images to Earth of what it looks like.

There isn't much chance that Phoenix will recover once sunlight returns next year, Goldstein said. His engineers have created a "Lazarus mode" software program to wake the spacecraft up.

"But I don't really believe it's possible," he said.

E-mail David Perlman at dperlman@sfchronicle.com.