When the Tissint meteorite crashed to Earth in Morocco last year, international drama ensued. The charred pieces of splintered rock were particularly coveted because they came from Mars, and local tribesmen, meteorite collectors, and scientists jockeyed for position. One of the first scientists to get his hands on Tissint was Andrew Steele, a senior staff scientist at the Carnegie Institution of Washington, and while he cryptically previewed his findings at the Conference on Life Detection in Extraterrestrial Samples earlier this year, some of his results were published online last week in Science.
Steele and his team examined 11 martian meteorites in all, using Raman imaging spectroscopy to search for carbon-containing molecules. Carbon, of course, is the central atom in life as we know it, forming the backbones of sugars, lipids, amino acids, and other cellular building blocks, so its detection is an important step in the search for life beyond Earth.
The biggest worry in any carbon-based study of meteorites is contamination from Earth’s pervasive biosphere. Most of our planet’s surface is teeming with microbes (the atmosphere, as well, is one giant microbial suspension) so it’s hard to keep a newly fallen meteorite truly isolated from terrestrial carbon.
But Steele’s use of rock-penetrating Raman spectroscopy helps allay these concerns by peering inside the meteorites. Steve Chemtob is a geochemist at the California Institute of Technology who regularly applies Raman techniques to environmental samples. “The distance that the laser penetrates depends on the material,” he says, “but most of the Raman excitation is happening at the focal plane, so by moving the sample up and down, you can analyze targets beneath the surface.” Under the best circumstances, Chemtob notes, it’s possible to get reliable spectra up to hundreds of micrometers inside a rock.
By making sure all of their spectra were taken 5 to 10 micrometers inside the meteorites and far away from any “visible cracks,” the researchers were confident that they measured native molecules and avoided the contamination bugaboo. The fact that Tissint, the most recently fallen meteorite, exhibited just as much carbon as the other nine samples gives the researchers additional confidence that the signal was real.
In what may be the first recorded application of the interplanetary “5-second rule,” Steele suggests that the minimal time Tissint spent on the Earth’s surface makes it exceedingly unlikely that the detected carbon is terrestrial.
The Raman spectra had a few telltale peaks, jolts of energy released at given wavelengths in response to laser excitation, that point to certain types of materials. The shapes of the peaks -- narrow or wide? smooth or noisy? -- indicate how closely the target resembles the spectral library’s reference samples. “The shifts in the position of a band in a Raman spectrum can indicate a gradual structural change or mineral transformation,” says Chemtob.
Steele used this principle to examine the most suggestive peaks at 1350 and 1590 wavenumbers. Based on the peaks’ positions and shapes, Steele believes he’s found “macromolecular carbon” (MMC), a catchall term that encompasses anything from an amorphous blob of linked carbon atoms to slightly more coherent sheets of regularly spaced carbons (i.e., graphite). The team used another mouthful of an analytical technique, laser desorption ionization mass spectroscopy, to identify polycyclic aromatic hydrocarbons (PAHs) in one particular meteorite. PAHs are more-structured carbon-based molecules whose potential role in the origin of life is perennially debated at astrobiology conferences.
Carbon is a necessary component of life, but its mere presence certainly doesn’t guarantee that anything has been slithering across the surface of Mars. The secret to establishing the carbon’s role in potential martian biology is knowing the company it keeps, so the team looked to nearby minerals for further clues about the MMC.
What they found was a disappointment to tabloid writers eager to proclaim the discovery of little green men: The metal oxides, pyroxenes, and olivines that hosted the MMC are consistent with the extensive volcanism that geologists believe has paved most of the planet’s surface. Steele pulls off the band-aid quickly: “Because MMC was always associated with igneous phases,” he writes, “we conclude that it crystallized from the host magma. This textural relationship negates any biological origin of the MMC and PAHs.”
The apparent pervasiveness of abiotic carbon in martian samples confirms a growing sense that the key questions of future astrobiological work will center around not just the abundance of carbon, but its structure and molecular form. After all, in the search for life beyond Earth, not all carbon is created equal.
