One-armed robots roaming another planet have done something no human has ever managed: They have discovered another place in the universe where life could once have existed. Aided by other robots in orbit and a modicum of luck, the two Mars rovers earlier this year homed in on locales once rich with water. Their finds mark a milestone in humankind's search for life elsewhere in the universe.
The two Mars rovers, Spirit and Opportunity, confirmed what many Mars scientists have long suspected: eons ago, enough water pooled on the surface of Earth's neighbor long enough to allow the possibility of life. Despite tantalizing hints starting with the Viking missions almost 30 years ago, Mars scientists could never be sure whether the water-carved valleys, channels, and gullies that they saw through orbiting cameras implied the prolonged presence of surface waters.
The Mars rovers have now put a bound on the water debate. Thanks to the hardy little robots, we know that Mars of several billion years ago was warm enough and wet enough to have a shallow, salty sea (ScienceNOW, 23 March). This sea probably came and went, turning into wind-blown salt flats from time to time, but the puddles spanned an area the size of Oklahoma. Enough water passed through it to leave behind at least 300 meters of salt. And the dirty salt buried beneath its floor remained wet long enough to grow marble-size iron minerals.On the opposite side of the planet, shallow groundwater also lingered long enough to transform hundreds of meters of what appears to have been volcanic ash into soft, iron-rich rock. And the latest spectroscopy from the newly arrived Mars Express orbiter shows that the salt from all this water-weathering of martian rock lingers in depressions elsewhere, sometimes in intriguing layered deposits that fill craters around the planet. For a time, it seems, early Mars was a watery, habitable place.