Two new studies are challenging the notion that the desolate Martian plains once brimmed with salty pools of water that could have supported some form of life.
Instead, the studies argue, the layered rock outcrops probed by NASA's robot rover Opportunity and interpreted as signs of ancient water could have been left by explosive volcanic ash or a meteorite impact eons ago.
That would suggest a far more violent and dry history than proposed by the scientists operating Opportunity and its twin rover, Spirit, on the other side of the planet.
The new scenarios, published in Thursday's journal Nature, paint a rather pessimistic view of whether the ancient Martian environment could have supported life.
In 2004, the six-wheeled Opportunity parachuted to Mars three weeks after Spirit landed on the opposite side of the planet. Opportunity touched down on Endurance Crater in the Meridiani Planum region and began examining numerous rocks and minerals for geologic evidence of past water.
After two months of surveying, scientists announced that chemical and geological clues gathered by the rover showed liquid water once coursed over the rocks and soils at that spot on Mars. Scientists suggested the rocks were deposited there by wind and water.
But the new studies reached different conclusions from the same data.
The sediment deposits appear to have formed from volcanic ash that reacted with small traces of acidic water and sulfur dioxide gas, said geochemist Thomas McCollom of the University of Colorado at Boulder.
McCollom hesitated to say whether the Mars rover science team was incorrect, but rather, he said their interpretation was "less likely."
"It's tough to put together a story to fit the geochemistry in the kind of scenario that they proposed," McCollom said.
In a second paper, geologist Paul Knauth of Arizona State University proposed another alternative. The rock patterns studied by Opportunity suggest the deposits were produced by a sudden surge of rock fragments, salts and sulfides from a meteorite impact, Knauth said.
In response, the Mars mission's principal investigator, Steven Squyres of Cornell University, said his team stands by its original interpretation. Squyres said Opportunity has since examined other layered outcrops that bolster their theory that the planet once was warm and wet.
The solar-powered rovers, managed by NASA's Jet Propulsion Laboratory in Pasadena, have long outlasted their primary, three-month missions and are operating on overtime.