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Planetary geology
No lake on Mars?

Ever since the Mars rover Opportunity landed in the Meridiani Planum crater in January 2004, scientists have been investigating whether the site was once drenched in water. NASA’s Mars Exploration Rover team says that the presence of standing water in Mars’ geologic past can account for the geologic features and chemistry found in some places there. A group of geologists, however, is now challenging that idea, saying that a meteorite impact is responsible for the observed features at the site.

When Paul Knauth, a geologist at Arizona State University in Tempe, and colleagues saw the images from the rover landing site, they immediately recognized the features’ similarity to impact craters observed on Earth, Knauth says. After a large explosion or impact, a cloud of debris spreads out and deposits layers in a “base surge.” Images taken by Opportunity as it further explored the landing site showed what “looked exactly like a base surge,” he says, but the real kicker for the team was when the rover sent back its chemistry analyses.

If the site was a lakebed or playa, then when the water evaporated, salts would have precipitated out into distinct layers, Knauth says. “But that doesn’t happen there,” he says. Instead, the salts are “all mixed up,” with the most soluble and least soluble together. A base surge from an impact, however, would account for the mixing, says Knauth, whose team published their findings in the Dec. 22 Nature.

Also, the presence of what the Mars rover team calls blueberries, or small round hematite nodules found in place at the site, support the impact hypothesis, Knauth says. Features similar to the blueberries have been found on Earth around volcanoes and impact craters such as Meteor Crater in Arizona. An impact from an iron meteorite could also explain the iron content found in the blueberries.

Steve Squyres, principal investigator of the Mars Exploration Rover mission and professor at Cornell University in Ithaca, N.Y., stands by the hypothesis that water accounts for the features at Meridiani. The blueberries, his team says, could have formed in an environment that alternated between a shallow body of water that saturated the rock layers below and a windblown, dry sandy desert, similar to regions of Earth with such nodules. Reporting at the December meeting of the American Geophysical Union, Squyres said that the Mars rovers have discovered more sulfates at outcrops that are further evidence of such evaporative settings.

When looking at 3.5-billion-year-old strata, Knauth says that it’s difficult to know exactly what happened. “It’s very interpretive, so what you try to do is come up with multiple hypotheses and then rank them according to simplicity,” Knauth says. “We’ve done that, and we put our money on the impact surge as the easiest hypothesis.”

In the same issue of Nature, another team of researchers suggests that volcanic activity could also account for the evidence of a shallow lake on Mars — and so the debate is not likely to dry up anytime soon.

Kathryn Hansen

This story was originally published online as a Web Extra on Dec. 23. Check out the Web Extra archive at www.geotimes.org/WebextraArchive.html for more Mars coverage.

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