For NASA's Spirit rover, the days of roaming the Red Planet may now be in the past, but the observations the wheeled bot made in its travels are still paying scientific dividends. A new analysis of geologic data gathered by the rover nearly five years ago finds that a rock outcrop on Mars is rich in carbonates, which are minerals that form readily in watery, carbon-rich environments. According to the study, the finding lends more credence to the hypothesis that Mars may have once had a wetter, warmer climate thanks to a dense carbon dioxide atmosphere. What is more, the aqueous processes implicated in the carbonate formation point to a neutral environment more hospitable to life than the acidic waters thought to have existed elsewhere on Mars.

Multiple lines of evidence point to past flows of water across the Martian surface, but conditions on the planet now preclude the existence of liquid water. In the past Mars's surface could have been much warmer, particularly if a robust atmosphere had provided a significant greenhouse effect on the planet. The dense carbon dioxide atmosphere often invoked to explain the warm era should have left its mark on the planet's geology, as carbon and oxygen sequestered in carbonate minerals. But prior to the new study, carbonates had only been found in small amounts on Mars, creating an evidentiary problem for the carbon dioxide hypothesis.

In January NASA declared Spirit a stationary science platform after months of efforts to free the rover from a patch of soft soil came up short. But in late 2005, when the rover was still mobile, it had investigated a group of rock outcrops in a region of Gusev Crater known as the Columbia Hills. Equipped with a rock-abrasion tool and a suite of spectrometers, the robotic geologist poked around the formation known as the Comanche outcrops, but its findings were not conclusive at the time. Now, however, evidence from Spirit's three spectrometers points to a large carbonate component in Comanche, according to the study published online June 3 by Science.

Part of the delay between data acquisition and analysis, says Arizona State University in Tempe geologist Steven Ruff, a study co-author, is that the instrument he works on, the Miniature Thermal Emission Spectrometer (Mini-TES), had been dusted by a windstorm prior to arrival at the Comanche outcrops. That storm cleaned the rover's solar panels and boosted its energy but threw a confounding contaminant in the spectrometer's optical path. "Something that was good for the rover was bad for our instrument," Ruff says.

Over the years the Mini-TES team learned how to correct for the dust contamination. At the urging of planetary scientist Richard Morris of the NASA Johnson Space Center in Houston, who works on Spirit's Mössbauer gamma-ray spectrometer, Ruff's group went back to review its data from the Comanche outcrops. "It was very clear that there was carbonate in that spectral signature," Ruff says. The data from the gamma-ray spectrometer and from a third spectrometer agreed with the Mini-TES spectra.

Morris, the lead author of the new study, says that having data from multiple instruments on Spirit makes a much more complete case for carbonates at Comanche. "They really interlock very nicely to not only tell us what carbonate it is, but also how much is there," Morris says. The minerals appear to be iron–magnesium carbonates that constitute as much as a third of the outcrop rocks.

The volume of carbonate present at Comanche is unique, Ruff says. "That's a huge number," he says. "What's been identified before is always a percent or two—it's a little bit....Here we are with outcrops that not only have carbonate but have a lot of it."

The question is whether the Comanche outcrops are an anomaly or are representative of the vast stores of carbon and oxygen that would be predicted by warm, wet greenhouse conditions on Mars. "If we just take this as one example of many across Mars, that there are outcrops on Mars that have this much carbonate, it does tend to support the idea that Mars may have had a denser carbon dioxide atmosphere in the past," Ruff says. But he acknowledges that there are other explanations for the carbonate finding—volcanic activity in the region, for instance, could have contributed localized carbon dioxide to form the Comanche minerals. "It's not a slam dunk that we've found evidence for a denser carbon dioxide atmosphere, but it's at least consistent with that idea."

Planetary scientist David Catling of the University of Washington in Seattle says that the Spirit data, in concert with other detections, such as that from the Mars Reconnaissance Orbiter observed in the Nili Fossae region of Mars, present a strong case for carbonates on Mars. "The old story that carbonates are absent on Mars is not entirely true," Catling says.

The amount of carbonate found in the new analysis is significant, Catling says, but its origin remains unclear. "That's quite a lot of carbonate," he notes, "but it's found in association with rocks that are primarily volcanic." If the carbon dioxide sprang from a geothermal source, it would not provide much information about Mars's early atmosphere as claimed by the new study, Catling says. "I would part company with the conclusions of the paper on that point," he adds.

"I think what it does show is [that] it provides more evidence that there were areas of liquid water on early Mars," Catling says. "Whether those were extensive globally or more localized in these hydrothermal systems is not clear in the data that we have."