"What we've done is thrown out an alternative that we think is simpler," Knauth said. "A controversy has developed now."

Knauth and Burt's hypothesis does not completely dismiss the idea that water once flowed on Mars. Rather, it implies NASA should search for signs of life in other Martian formations, Knauth said.

NASA scientists initially speculated that sedimentary layers seen in photos taken by the Opportunity and Spirit rovers were formed by evaporating water.

"NASA's Opportunity rover has demonstrated some rocks on Mars probably formed as deposits at the bottom of a body of gently flowing saltwater," a NASA press release announced in March 2004, three months after Opportunity's landing.

But when Knauth and Burt stumbled across these photos on the Internet, they noticed the Martian rocks resembled formations on Earth that were caused by surge impacts, not water.

If the sedimentary layers on Mars were formed by evaporating water, they would look similar to the Grand Canyon, Knauth said.

But the Martian sedimentary layers create what's called a cross-bedding pattern, a feature that occurs when a major impact creates a cloud of debris.

"The layering is a dead ringer for a base surge," Knauth said.

Also, if water had caused the layered patterns, then salts within the rock would have been grouped by solubility, similar to how a bathtub ring is formed.

That wasn't the case in the Martian sedimentary rock, which featured salts of differing solubilities mixed together.

"You don't get that in an evaporating lake," Knauth said.

The Opportunity landing site also featured small BB-like spheres that could have originated from a surge impact.

Burt said these formations occur when a meteorite hits and debris explodes into a cloud.

As both debris and steam condense within that cloud, small uniform spheres that rain down like hail stones are formed.

Knauth and Burt compared NASA's Mars findings and photos to their research on terrestrial surge impacts.

They also contrasted Martian features with those they've studied in Arizona, including the Grand Canyon, Meteor Crater and Tempe's Hayden Butte or "A" Mountain.

They said that while they can't be sure their hypothesis is correct, they believe it is the simplest explanation of the sedimentary layers.

Knauth and Burt suggested NASA search for life in fractures filled with mineral growth.

Those areas, Knauth said, should have trapped any microbes that survived meteorite impacts.

Getting a rover to these fractures, however, might be difficult because the terrain tends to be unsuitable for a gentle landing.

NASA landed both the Spirit and Opportunity rovers in flat areas.

"It's like trying to look at rocks in the middle of Iowa instead of the Grand Canyon," Burt said.

Knauth and Burt published their hypothesis in Nature magazine last month.

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