• Dick writes about Earth and planetary science for Science magazine.

Phoenix Finds Signs of Once-Liquid Water on Mars

30 September 2008 (All day)

NASA/JPL-Caltech/U. Arizona/Imperial College London

Deeper discovery. Beneath the red of Mars, the Phoenix lander has found soil minerals that could only have formed in the presence of now-frozen water.

Scientists announced today the first evidence yet for liquid water in geologically recent times on Mars. The discovery, made by the Phoenix lander, comes in the form of two long-sought soil minerals, which researchers say could have formed only in the presence of liquid water. But team members can't say for certain where the water came from--and time is running out to solve the mystery.

The mineral discoveries come from two Phoenix instruments. The Thermal and Evolved Gas Analyzer (TEGA) recorded the release of water when it heated a soil sample to high temperature. That was "most likely due to clay" water being driven off, TEGA lead scientist William Boynton of the University of Arizona (UA), Tucson, told a bicoastal press conference.

TEGA also detected carbon dioxide being driven off at high temperature. That is unambiguously due to several weight percent of calcium carbonate breaking down, Boynton said. The Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) confirmed the presence of calcium carbonate, said MECA lead scientist Michael Hecht of the Jet Propulsion Laboratory in Pasadena, California. It measured a stable pH of 8.3 in a soil slurry despite the addition of acid, suggesting calcium carbonate was buffering the pH, and it detected calcium ions in solution as well. "These [minerals] are indicators of liquid water in the past," said Phoenix principal investigator Peter Smith of UA.

The working hypothesis behind the mission is that not long ago--tens of thousands to a few millions of years--periodic climate swings might have melted the subsurface ice confirmed by Phoenix to make the soil a few centimeters down a habitable, if temporary, environment for microbial life.

That scenario could still hold, says geochemist Nicholas Tosca of Harvard University. "Liquid water is required to form these minerals," he says. "How much liquid water could be debated. A thin film could do it." That would be enough for microbes too.

The problem is that Phoenix scientists cannot yet show that the minerals formed where they found them. Large deposits of clays that formed more than 4 billion years ago have been detected from orbit in recent years, and carbonates have been reported in martian dust using spectroscopy. These minerals could conceivably have been blown to the Phoenix site, although Smith sees few signs of such transport. "I'm not sure what they could do to test that," says Tosca.

There's not much time left to find out. Martian winter is coming on, with the sun spending more and more time each day below the horizon, starving the spacecraft's solar panels for energy. Mission managers are rushing to fill TEGA's last four sample cells before falling power levels preclude operation of the sampling arm. "We have not finished," declares Smith. Near the top of the team's to-do list will be looking for organic matter possibly lingering from past life. "If there's any there, it's not very much," said Boynton. "We are still looking."

By late November, the crushing cold should shut down the lander as carbon dioxide frost begins to encase it. Phoenix's skyward-pointing laser has already detected water-ice snow beginning to fall from passing clouds.

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