NASA’S laser-armed, nuclear-powered, car-sized rover Curiosity is not on Mars to look for signs of life. If modern Martians exist at all—and there is no evidence that they do—then it will probably be in the form of tough, slow-growing microbes that dwell under the ground, sheltering from the solar radiation that bombards a surface only lightly shielded by a thin, bone-dry atmosphere. But Curiosity is, nevertheless, an astrobiological mission. One of its main tasks is to determine whether, at some point in its past, Mars might have had a more accommodating environment than it does today.
On March 12th a team of NASA scientists announced some fairly strong evidence that it may have done. A few weeks before, Curiosity had drilled a small hole in the Martian surface and had deposited the resulting regolith into its on-board chemistry labs. The rover found traces of carbon, hydrogen, nitrogen, oxygen, phosphorous and sulphur (CHNOPS, in the lingo), the six chemical elements that make up the majority of biological molecules on Earth, and which are thought to be essential for life. “This is what we call paydirt,” said David Blake, one of the scientists.The dirt in question comes from a sedimentary rock called mudstone. Sedimentary rocks form in the presence of water, which, thanks to its solvent properties, is thought to be another vital ingredient for life. Chemical analysis of the rock indicates that the ancient aquatic environment in which it formed was relatively benign—not unduly salty, acidic or oxidising, as previous traces of water found by Curiosity’s predecessors, the twin rovers Spirit and Opportunity, are thought to have been. “If you had been around when this water was around, you would have been able to drink it,” said John Grotzinger, Curiosity’s chief boffin. Most intriguing of all, the powdered rock contained both oxidised and non-oxidised chemicals. Converting one into the other can release energy, which could have formed the basis of an ancient Martian microbial metabolism.Curiosity’s sample was drilled from the bottom of what seems to be an ancient, dried-up lake or river bed. Indeed there is plenty of other evidence, gathered before Curiosity touched down, that indicated that Mars had a warmer, wetter past, in which large bodies of open water existed on the surface. The new results build on such general observations by proving that, at least on some parts of the surface, the chemical precursors of life seem to have been gathered together in one relatively salubrious place.
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