Scientific Method / Science & Exploration


Team not yet ready to rule out contamination from Earth either.

The soil sample where it was taken (left) and inside Curiosity's sample-halding machinery (right).

If you've been following the news from the Curiosity rover on Mars, you can be forgiven if you've had bad flashbacks to the Mars Phoenix lander. Back in 2008, rumors started circulating that Phoenix found evidence of life on the red planet, forcing NASA to hastily schedule a press conference in which they shot the rumors down. This time around, NASA has only itself to blame. About two weeks ago, one of its scientists told NPR the rover had made a discovery that would be "one for the history books." Naturally, he neglected to say what it was.

Speculation quickly focused on Curiosity's on-board chemistry lab, which has the ability to detect organic molecules. Although a variety of natural processes can lead to their formation, the presence of organic compounds could provide some indication of whether life was ever possible on Mars. When NASA announced a press conference on the latest Mars results, to be held at the American Geophysical Union meeting, it was careful to manage expectations. The organization noted, "Rumors of major new findings at this early stage are incorrect."

In fact, as scientist Ralf Gellert described it, the goal of the initial chemistry work was to find something completely un-special. This way, NASA could both get a picture of the average environment on the red planet and then confirm that its instruments weren't doing anything odd.

With those caveats in mind, Curiosity's instruments have seen some organic compounds, but the team running the experiment (represented by Paul Mahaffy) isn't yet ready to say that they originated on Mars. There's still a chance they represent leftover contaminants from Earth.

Ken Edgett of the Hand Lens Imager team described a bit about the sample gathering and processing. To get a sample that's typical of the surface of Mars, the team obtained some sandy material of the sort that's shifted around during the planet's epic dust storms. Curiosity took five scoops of soil at a site called Rock Nest. Edgett described the site in culinary terms: a coating of thick grains "like the salt grains on those big hot pretzels you can get" covered much finer material with grain sizes "kind of like those artificial sweeteners." After using a few scoops to clear out the chemistry system, a sample was sent through for analysis.

The new analysis, described by Mahaffy, involved heating one of the samples up to about 500°C while tracking the gas that comes off it. The most abundant gas was water, but even that had a little surprise. On Earth, one of about every 6,500 hydrogen atoms in water is a heavier isotope of hydrogen called deuterium. On Mars, the ratio of deuterium to regular hydrogen appears to be five times higher. This could be a result of Mars' gradual loss of atmospheric material, in which lighter isotopes were preferentially lost.

At higher temperatures, carbon dioxide came off the sample, and sulfur dioxide appeared at even higher temperatures. Although the soil appears to be oxidizing (more on that below), some hydrogen sulfide was present, suggesting the oxidation in the soil isn't complete.

The oxidizing material probably comes from chlorate compounds, which were also found by the Phoenix lander (in this case, they suspect calcium chlorite). A variety of chlorine compounds were detected in the gasses given off by the heated samples, probably generated as the chlorates decayed and reacted with other materials in the sample. These reactions appear to have generated a variety of methane (CH4) derivatives, with chlorine replacing a variable number of hydrogens. They've also found a four-carbon chlorinated compound in the material given off.

They aren't yet convinced that these carbon compounds aren't the result of contamination left over from the rover's time on Earth. And, if they are local, they're not sure what the chlorate compounds reacted with to produce what they see. It could potentially be something as simple as the CO2 being given off at the same time. Even if it's from the soil, there's no way of telling whether it was native to the soil or had arrived as Mars has been struck by material from comets and asteroids.

Although there were a lot of caveats, the researchers have a clear plan. Modern isotopes and chemistry can be compared to the samples they get by drilling into older rocks, and the rover carries some standardized materials to analyze for comparison. Researchers will also track how results do (or don't) change over time and as they look at different materials to get a sense of whether contamination is a problem. The Internet may want to "move at Instagram speed," as scientist John Grotzinger put it, but the team will continue to work at the cautious pace of science.

But one slide from the press conference hinted that the results were worth waiting for. To confirm that their soil sample was typical, the Curiosity team compared their results of mineral compositions to those obtained by the Spirit and Opportunity rovers. The data matched extremely well. That tells us that the hardware is working well, and should be able to analyze a variety of samples as Curiosity crawls further up Mount Sharp.

Editor's note: a reader has pointed out that our original concluding paragraph contained two significant errors. We have corrected them.

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