Meteorites from Mars contain specks of graphite-like carbon trapped within them. There have been a variety of explanations for this reduced form of carbon over the years, with some even viewing it as an indication of life on the Red Planet. But new evidence indicates that the rock holding the carbon is volcanic in origin and originated from deep inside the planet, not from living organisms. This helps scientists narrow down possible chemical reactions on the planet that could create building blocks for life, the researchers say.

Scientists have found reduced carbon compounds, like polycyclic aromatic hydrocarbons, in various meteorites with a Martian origin over the past 15 years. They’re particularly interested in this type of carbon—as opposed to the oxidized carbon in carbon dioxide and carbonate minerals—because it contains carbon-carbon bonds critical to life as we know it.

There are many different ways that these compounds can form in space, so finding reduced carbon in the meteorites tells scientists very little about how it got there in the first place. Still, attempts to explain its origin abound. Using clues from the rocks themselves and the Mars environment, ideas include the possibility of living organisms, condensation from volcanic gases, to contamination once the rocks fell to Earth.

Scientists haven’t been able to pinpoint the precise location of these carbon compounds in the meteorites—until now. “That’s like suddenly having material evidence for a robbery,” says geochemist Everett Shock, of Arizona State University, who was not involved with the current study. Knowing the composition of the surrounding minerals helps scientists eliminate some possibilities for the origin of that carbon, just like forensic evidence narrows down a field of suspects for a crime.

Andrew Steele, of the Carnegie Institute of Washington, and his colleagues peered inside mineral grains in thin slices of eleven Martian meteorites using lasers. Light shining on the grains triggers emissions with patterns unique to the minerals and carbon-containing molecules inside.

The researchers found tiny clusters of graphite-like carbon, about one to ten microns wide, in 10 of the 11 meteorites. Analyzing one meteorite, Dar al Gani 476, more closely, they found pyrene, phenanthrene and other aromatic hydrocarbons mixed in with the graphite-like sheets. It’s likely that the other meteorites contain the same mixture of carbon too, Steele says.

These carbon specks were always trapped with clusters of iron, titanium, and aluminium oxide inside grains of two different minerals. These minerals indicate the carbon clusters probably solidified as molten volcanic rock cooled within the planet, the researchers say. That means the carbon in those clusters came from a source inside the planet—not a biological source.

Identifying which minerals are associated with these carbon clusters is important because the minerals provide clues to how the clusters formed, Shock says. Any future explanations for the origins of carbon on Mars will have to account for these observations, he adds.

There are several reasons to believe that this carbon did not come from a living organism, Steele says. First, Mars lacks the plate tectonics found on Earth. That means any carbon present on the surface of Mars, whether from biological life or not, would never be shoved down to the liquid mantle. And it would have had to reside in the mantle to come shooting out of a volcano mixed with molten rock later.

Also, the metal oxides encasing the carbon are among the first to solidify as the molten volcanic rock cools. That means the carbon clusters form when temperatures are around 1300 to 1400°C—far too high for life to exist nearby.

Because Steele only studied sections of rock that were a safe distance from any cracks contaminated after they fell to Earth, it’s unlikely that this carbon is due to microbes munching on the meteorite once it hit Earth.

The next question is to figure out what happens to the carbon on Mars, Steele says. The Curiosity rover, set to land on the planet in August as part of the Mars Science Laboratory mission, will analyze the elements in rocks and soil from the surface. The mobile robotic laboratory may find carbon similar to what is in these rocks, the researchers say.

But if it detects molecules completely different from what we know to be life, scientists can start to develop new theories for the formation of those molecules, knowing that the carbon came from the planet itself, Steele says.

Science, 2012. DOI:10.1126/science.1220715; (About DOIs).