Athabasca Valles is an intriguing Martian landscape of channels and islands that resembles Washington State's Channeled Scablands—modern features thought to be the result of a massive flood thousands of years ago.
Some scientists had theorized that Martian flooding created Athabasca Valles within the past few million years.
But MRO data suggest that the entire region is covered with a thin layer of lava, which likely came from a fissure in the planet's surface.
This means that if the channels were made by liquid water, the wet stuff would have to have been present well before the lava flows started.
Roger Phillips, a professor of earth and planetary sciences at Washington University in St. Louis who was unaffiliated with the study, notes that the lastest finding does not disprove that water played a part in creating the channels.
"The [researchers] are not saying that the valley was not originally carved by water, it's just that it was subsequently covered by lava," Phillips said.
"As the observable lava fill is not tied to water-related processes that carved the channels, the age of the channel formation is now uncertain. This is a surprise."
No Squirting Gun?
The MRO images also stirred up doubts about the presence of an ancient Martian ocean covering the planet's northern lowlands (see a map of Mars).
Based on previous images, some experts had theorized that the region is a sandy, sedimentary seabed.
But the new data reveal that the lowlands are peppered with large boulders that are inconsistent with a leading model of the ancient ocean floor.
"The whole ocean hypothesis is extremely controversial in the first place," Phillips said.
Explaining why the boulders would exist if the region was once an ocean would require a new and probably less-likely hypothesis, he said.
MRO also offered a new look at Martian gullies that last year showed bright deposits that some scientists thought were left behind by recently flowing water.
(Read: "Mars Has Liquid Water, New Photos Suggest" [December 6, 2006].)
But the MRO images show just as much evidence to suggest that the deposits were made by landslides.
In addition, data collected about the material inside the deposits suggests that they are not brightly colored because of the presence of water or ice.
The deposits are therefore no longer seen as the "squirting gun" that offers definite evidence of modern water flows.
"[The new data] doesn't prove that it wasn't water" that created the deposits, lead study author McEwen said.
"But given the difficulties of explaining how liquid water could be on the Martian surface in the present climate, [the landslide theory] is a more conservative hypothesis."
On the other hand, MRO has returned some promising evidence for running water elsewhere on the red planet within the past several million years.
The 37-mile-wide (60-kilometer-wide) Mojave Crater, for example, has runoff patterns that resemble alluvial fans—geologic evidence of flowing water—in Earth's deserts.
The fans on Mars may have resulted when heat from the collision that created the crater melted subsurface ice and caused some of it to flow, briefly, as water.
"They are basically saying that there is good evidence that there is ice in the crust even today and even at low latitudes," Washington University's Phillips said.
"That, I think, is the most interesting result."
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