THE footprint of life on Mars may have been plain to see all along in the sulphurous minerals that litter the planet’s surface. What’s more, the next Mars lander should be able to detect the evidence.
No mission to Mars has ever found complex carbon-based molecules, from which life as we know it is built. But sulphur is everywhere on Mars – it is more abundant there than on Earth – and it could contain one of the signatures of life. On Earth, the activity of some microbes converts one class of sulphur-containing compounds, the sulphates, into another, the sulphides. The microbes prefer to work with the lighter sulphur-32 isotope, so the sulphides they produce are relatively deficient in the heavier isotope, sulphur-34. Planetary scientists have long wondered whether we could use this pattern to discern signs of life on Mars. Now the prospects for this technique look better than ever.
John Parnell of the University of Aberdeen, UK, and his colleagues found sulphides, apparently formed through microbial activity, permeating the rocks of Haughton crater in the Canadian Arctic (Geology, DOI: 10.1130/G30615.1). “It was amazing – it was everywhere,” says Parnell, who pressed the case for investigating sulphur isotopes on Mars at the Lunar and Planetary Science Conference in Houston, Texas, last week.
An analysis of the crater’s rocks indicates the sulphides were produced at temperatures above 70 °C. That suggests they formed shortly after the crater itself was created by a meteorite impact 39 million years ago, when water warmed by the impact would have circulated through the crater rocks.
Despite the passage of time, the signature of life at Haughton crater remains clear, with sulphur-34 depleted by 7 per cent in the sulphides compared with the sulphates. This suggests that such a signature is not easily erased, bolstering the chances that Martian rocks that were moist enough to harbour life long ago could still carry a detectable signature of life, says Parnell.
NASA’s Mars Science Laboratory (MSL) rover will land on the Martian surface in 2012. It will carry a mass spectrometer that should be sensitive enough to see variations as small as 2 per cent in sulphur isotope abundances, says John Grotzinger of the California Institute of Technology in Pasadena, the lead scientist for the mission.
Sulphur is “definitely a promising candidate” to reveal signs of life on Mars, says David Des Marais of NASA’s Ames Research Center in Moffett Field, California, who is also involved in the mission. “If there are big isotopic differences that would be very suspicious. The only way we know how to do that on Earth is with life.” …