NASA unveiled data nine months ago showing bright, riverlike shapes suddenly appearing on two sloping Martian gullies, which could be interpreted as trails left by melted ice springing from below the surface.
The new images, taken by NASA's Mars Reconnaissance Orbiter (MRO), reveal four additional new tracks but also show bright material in unchanged gullies, suggesting that the flows, which all occur on steep slopes, may have been dust avalanches.
The scans do expose traces of liquid water in and around relatively young impact craters, but not in several other regions thought to have once been flooded.
Researchers say the results, described in a pair of reports in Science, support the view that liquid water was likely the exception rather than the rule on Mars for most of its history. This contrasts with the belief that the planet once had a warm, wet climate.
"I think most of Mars for most of its history has been very dry," says Phil Christensen, a planetary scientist at Arizona State University in Tempe, who was not part of the study.
Liquid water does seem to have left its mark on some spots, such as Endurance Crater in Meridiani Planum, where the Mars rover Opportunity three years ago discovered minerals that form on Earth in salty water.
"It's got these few gems that [show], wow, something cool went on there," Christensen says. "It's sort of Yellowstone in this sea of the Midwest."
Such "Yellowstones" make stargazers drool, because liquid water is the best indicator that life could have bloomed elsewhere in the universe. NASA planners are also eager to learn where Mars might contain liquid water to help plan a possible trip to the Red Planet in coming decades.
A primary goal of the MRO is to understand the history of Martian water, which is locked up as ice in the planet's poles and perhaps in lesser quantities across the surface. The high-resolution science imaging experiment (HiRISE), a camera on board the MRO, allows researchers to map the surface for signs of past water.
Dry Me a River
Researchers quickly homed in on Centauri-Hellas Montes and Terra Sirenum, where brilliant flowing deposits appeared in images taken by an earlier orbiter, the Mars Global Surveyor, in 2004 and 2005 that were absent in earlier snapshots.
The deposits fanned like a fluid around rocks, the group confirms, but they had not changed shape since last observed 15 months ago, suggesting they are something other than frost or ice, which would have evaporated. In addition, the average slope of the six total deposits was more than 20 degrees.
"They still could have been moved by water, but we also found the slopes were steep enough that they could have been caused by dry fluids," says Alfred McEwen, professor of planetary sciences at the University of Arizona in Tucson, and HiRISE team leader. "That would have to be considered the more conservative hypothesis."
Christensen says he still believes the gullies were carved by running water, but "there's no conclusive smoking gun that, gosh, water came roaring out the surface within the last year," he says.
"What everyone's been hoping for is that you could actually see one of these gullies discharging water. I think we're going to have to actually land near one of these sites."
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.