NASA's "follow the water" approach to finding life--or evidence of past life--on Mars has finally hit pay dirt. Three weeks into its 90-day mission, the Phoenix lander has scraped a few centimeters down to an irrefutable layer of water ice in the martian arctic. The first robotic contact with water on Mars promises a score of chemical analyses in the next few months that could reveal whether this ice ever melted to liquid water that could have supported living organisms. And the discovery has already revealed some new mysteries beyond the question of life.
"It is with great joy that I report we have found the proof that this material really is water ice," Phoenix principal investigator Peter Smith of the University of Arizona, Tucson, said during a media teleconference today. The kilometers-thick ice cap to the north of Phoenix's landing site is exposed water ice, but orbital observations of the past decade had implied that ice must also lurk beneath just a few centimeters of loose soil well south of the ice cap. So NASA sent Phoenix to a safe-looking spot where, if past climates were warm enough, that ice might have melted to form a cozy zone for life between the ice and the soil surface (Science, 9 May, p. 738).
Phoenix's first digging exposed a thin layer of white material, but team members couldn't tell whether it was ice or perhaps salts. So they waited and watched the trench. In 4 days, at least eight crouton-sized white chunks formed by the digging disappeared, vaporized into the cold, dry air. "Salt does not behave like that," said Smith, "so we are confident now that this is ice." "The big story is that we can reach down and touch it now," said team member Mark Lemmon of Texas A&M University in College Station.
"I'd have to say this is quite definitive," says David Paige of the University of California, Los Angeles, who works on remote sensing of planetary ice. "There is indeed quite a bit of ice beneath the surface. It is wonderful to actually see this stuff instead of inferring it." Paige wonders, however, how the ice chunks disappeared so thoroughly, leaving no sign of any salt or dirt that they may have been carrying. "That's puzzling," he says, because experts think this near-surface ice arrives as water vapor from the atmosphere and then freezes, filling the air spaces between soil particles. That should leave an ice layer containing at least half dirt, not nearly pure water. Following the water may not be all that straightforward.