NASA’s Curiosity rover has collected and analyzed samples of Martian sediments, revealing a chemical signature that hints at ancient life on the Red Planet. These findings, published in the January 25th Proceedings of the National Academy of Sciences, are some of the best evidence yet for biological processes on Mars. Other mechanisms may account for the intriguing chemistry, however.

The Details Are in the Carbon

Since 2012 Curiosity has been exploring Gale Crater, an impact basin targeted because it hosted a river delta and a lake 3 billion years ago, when Mars had a far more hospitable climate. In the search for evidence of past life on Mars, the Curiosity team has subjected ancient deposits of waterborne sediments exposed by eons of weathering to a battery of tests that include carbon isotope analysis.

Two stable isotopes of carbon occur in nature: carbon-12 and carbon-13.  98.9% of Earth’s carbon is carbon-12, which has 6 protons and 6 neutrons in its nucleus. Almost all remaining terrestrial carbon is the carbon-13 isotope, which has 7 neutrons. One in every trillion carbon atoms are a third, unstable isotope, carbon-14, which has 8 neutrons. Its radioactive half-life of 5,730 years is the basis of the dating method that archeologists use to determine the age of organic objects.

The relative quantities of carbon-12 and carbon-13 throughout the solar system are essentially the same as those in the primordial solar nebula, the cloud of gas and dust that condensed to form the Sun and its retinue of planets 4.6 billion years ago. On Earth, variations in the carbon isotope ratio arise here and there as a variety of geological and biological processes, known as the carbon cycle, exchange carbon between the atmosphere, rocks, soils, and bodies of water.

The lighter carbon-12 isotope, which forms weaker chemical bonds, undergoes chemical reactions at a slightly faster rate than carbon-13, so it’s more likely to be incorporated into biological materials. As a result, biogenic carbon compounds contain comparatively less carbon-13 than do carbon compounds in the atmosphere or rocks.

Carbon . . . from Life?

The Curiosity team, led by Christopher H. House (Penn State), found that several Gale Crater sediment samples have far less carbon-13 compared to carbon-12 than the carbon dioxide in the Martian atmosphere or the Martian meteorites that have landed on Earth.

Such extreme carbon-13 depletion also occurs on Earth in layered sedimentary formations known as stromatolites. These 2.7 billion year–old remains of some of the earliest life forms contain fossilized mats produced by microorganisms that consumed methane (CH₄).

At the present time, there are only tiny traces of methane in the Martian atmosphere, measured in parts per billion. But Curiosity and the Mars Express orbiter have detected several sudden spikes in methane concentrations, perhaps gas released from subsurface reservoirs that disappears rapidly.

Controversy surrounds the origin of Martian methane. Chemical reactions between water, carbon dioxide, and minerals like olivine may be responsible, but production by microbes remains an intriguing possibility.

To House, the extreme carbon-13 depletion has several possible explanations, including two chemical reactions not related to biology and one that could be related to microbial life.  At this time, though, all three possibilities require further investigation.

Alternative Scenarios

It’s possible methane-consuming microbes on the planet’s surface metabolized the methane and sequestered its carbon in sediments. But there’s no “smoking gun” evidence (like fossilized microbial mats) to indicate that methane-consuming microbes were ever present on Mars, and solar ultraviolet radiation and wind-driven erosion are also capable of binding methane with Martian surface materials.

In fact, there may be ways to explain carbon-13 depleted sediments that don’t involve methane at all. The investigators cautiously cite two alternative mechanisms.

As the solar system orbits the center of the Milky Way Galaxy every 230 million years, it passes through tenuous clouds of gas and dust containing material with different proportions of carbon-12 to carbon-13 than the primordial solar nebula. These encounters deposit minute amounts of material on Earth, but geochemists have detected traces of past passages. To concentrate in sedimentary layers on Mars, the Curiosity team envisions cosmic dust accumulating on the surface of glaciers until they melted, leaving layers of sediment with anomalous carbon isotope ratios. While this explanation is plausible, there’s no evidence of past glaciers in Gale Crater.

A second way to reproduce the carbon isotope ratio may occur when ultraviolet radiation from the Sun converts carbon dioxide and water vapor in the Martian atmosphere into formaldehyde and other organic compounds. In the laboratory this transformation requires exotic catalysts, so it’s by no means certain that it happens on Mars.

“[The paper] is impressive in the breadth of possibilities,” says Michael Mumma (Goddard Space Flight Center), who was not involved in the study.

All three possibilities point to a Martian carbon cycle unlike anything on Earth today, but we’ll need more data to determine which scenario is correct. Curiosity continues to collect and analyze samples in Gale Crater, and investigators are especially keen to use the rover’s mass spectrometer on a future methane plume to see if its carbon isotope ratio indicates a biological origin.