BioEd Online

Soon after NASA's robotic rover Opportunity began exploring Mars, it found minerals and rocks that its handlers said were evidence of a warm, wet history. But two groups of scientists have now questioned this interpretation.

Most scientists agree that water flowed on Mars's surface at some point. The planet has deep valleys similar to Earth's own Grand Canyon, which are thought to have been carved by rushing water.

But it is unclear whether all the water on Mars came in sudden bursts a long time ago, when meteorites battered the ice deposits of the young planet, or whether some stood about in warmish puddles later in Mars' life, which might have given life time to evolve.

When Opportunity touched down on a part of Mars called Meridiani Planum, it came across geology that looked tantalizingly like the product of standing water. The researchers running the mission wrote that "surface conditions at Meridiani may have been habitable for some period of time in martian history"¹.

Opportunity found a scattering of tiny round pebbles that looked as if they had formed in water, and rectangular holes in the crater wall that could have been left by dissolved mineral crystals. The team also saw ripple patterns in the rock, and a layering that looked like sediments that had settled out of water.

But in this week's Nature²,³, other researchers suggest an alternative, dry, explanation.

Meteors and volcanoes

Paul Knauth, a geologist from Arizona State University, Tempe, and his colleagues argue that all these features were produced by a surge of rock, minerals and brine caused by a meteorite impact, along with chemical changes from groundwater - but no pools².

"Opportunity's observations themselves are valid," says Knauth's Arizona colleague Donald Burt. "But the current interpretations are not supported by the evidence."

Burt points out that any water in the area would have left channels, deltas and other shoreline features that Opportunity has not found. He adds that briny, acidic water would dissolve through alkaline basalt rock, not create puddles. "It would be like trying to dump acid in a marble bathtub," he says.

The layered features found by Opportunity, interpreted as sedimentation from water, are also seen on Earth. But in some cases they are attributed to surges of material from meteorite impacts.

Thomas McCollom and Brian Hynek, both planetary scientists at the University of Colorado, Boulder, provide another arid explanation. They suggest that the mineral deposits are volcanic ash that has reacted with small amounts of acidic water and the volcanic gas sulphur dioxide³.

The watery interpretation fails, they argue, because the sediments do not contain enough of some metals - such as calcium, magnesium and iron - that ought to be in rocks laid down by water.

Slim chance

If either explanation is true, it could reduce the chances that life ever existed on Mars.

But Steve Squyres, the planetary scientist from Cornell University, Ithaca, New York, who leads Opportunity's science team, sticks by his original conclusions. "There's no question there were standing bodies of water at the surface," he maintains.

The rocks in a larger crater, called Endurance, look as if they were shaped by water but then altered by strong winds, says Squyres. This can explain some of the objections raised by the Nature papers. As the results from Endurance were published only recently⁴, the two rival explanations have missed the full story, Squyres suggests.

Opportunity is now heading for an even larger crater called Victoria. "Victoria is a potential time tunnel, allowing access to ancient martian material that otherwise would be buried deep beneath the surface," says John Callas, deputy manager of the rover mission at the Jet Propulsion Laboratory in Pasadena, California. If the robot can hold out long enough, it may gather enough data to settle the debate.