The rover Curiosity has detected simple organic molecules in samples of Martian sand, but they're almost certainly byproducts of the testing process.It's been four months since the Mars Science Laboratory, a.k.a. Curiosity, landed inside Gale crater on Mars. Although the craft's science team has urged patience as they put the rover through ever-more-complex activities, the public is growing increasingly eager to see some boffo results from NASA's $2½ billion investment. So the Web was understandably abuzz this past week with hints that mission scientists would announce Something Big at this week's American Geophysical Union meeting in San Francisco.
They did, and yet they didn't.
The good news: Curiosity is working exactly as designed. For several weeks, it's lingered at a site called Rocknest to make sure its sample-handling mechanisms are working and to analyze a few dabs of carefully prepared sand. The grains that go down the chute can't be any larger than 150 microns across, finer than granulated sugar but coarser than ground flour.A few weeks ago the CheMin instrument used X-ray diffraction to determine that its first delivery of this fine-grain material consists of standard-issue rock-forming minerals such as feldspar, pyroxene and olivine, along with abundant volatile elements like sulfur.
Now attention has turned to the rover's most sophisticated instrument, called Sample Analysis at Mars, which actually has three parts. One uses a mass spectrometer to determine the molecular weight of gases — either in the Martian atmosphere or the ones releases as a solid sample is gradually heated to higher and higher temperatures. A second component assays how much water, carbon dioxide, and methane are in a sample and the ratios of isotopes in those molecules. The third section, a gas chromatograph, will look for various organic molecules.SAM found that the powder from Rocknest released water vapor, oxygen, and sulfur dioxide when heated. But that's nothing new — these results match the compositions of the dust found elsewhere on Mars by previous rovers and landers. Still, notes Paul Mahaffy, principal investigator for SAM, "We consider this a terrific milestone." The water vapor, for example, probably came from hydrated minerals like clays.
The instrument did identify some simple organic molecules, variations of methane (CH4) in which chlorine atoms have substituted for one or more hydrogen atoms. But there's a huge caveat: it's very unlikely that chlorinated methane actually exists on Mars. Instead, SAM's tiny ovens probably caused perchlorate molecules (found commonly in martian dirt) to release lots of chlorine and oxygen as they broke down."We have no definitive detection of Martian organics at this point," Mahaffy says. It's not clear yet where the carbon comes from — perhaps it's derived from carbonates in the Rocknest sample, or it could be traces of carbon-based contamination carried from Earth.
Potentially more interesting is SAM's determination that the derived water vapor is highly enriched in hydrogen's "heavy" isotope, deuterium. The D:H ratio is more than five times that in Earth's seawater. The thinking goes that, over the eons of history, molecules of water vapor have been broken down by ultraviolet sunlight high in the Martian atmosphere, and the lighter hydrogen atoms have preferentially escaped to space. In principle, this five-fold deuterium enrichment can be used to figure out just much water has been lost. It's also possible that Curiosity will eventually unearth a sample of "old" water — locked up in an ancient rock layer — to reveal how the atmosphere has evolved over time.
All this scratching and sniffing takes time, cautions project scientist John Grotzinger. Plans call for Curiosity to be maneuvered to an overlook called Point Lake before heading for a tantalizing outcrop called Yellowknife. Getting the rover to its primary objective — the ancient sediments at the base of "Mount Sharp" (officially Aeolis Mons) — would appear to be many weeks away.