Climate Change or Crater Impacts?
In less equivocal findings, the HiRISE images showed ponds of material in and around the 60-kilometer- (35-mile-) wide Mojave Crater that lies near channels and fan shapes extending downhill, suggesting that the Mojave impact melted subsurface ice to water and splashed the resulting mud around.
Similar patterns appear at 10 other craters, the team reports. "We think this is evidence that the crust of Mars is icy," McEwen says.
More than that, he says, impacts could also explain channels and other features that provide evidence for extensive water flow 3.8 billion years ago could also be explained as the product of numerous impacts. Although the craters imaged by HiRISE are a mere 10 million to 100 million years old, "the impact flux was much higher" in the planet's early history, he says.
Christensen says estimates show that a massive impact leaving a 400- to 500-kilometer (250- to 300-mile) diameter crater could have warmed Mars for a few hundred or thousand years.
Rounding out the dry results, MRO revealed large boulders in the northern Vastitas Borealis Formation, once thought to have been covered in silt from a dried-out ocean, and signatures of pooled lava in Athabasca Valles, a proposed floodplain, suggesting that neither region was ever wet for long.