Sign of life: Tube-like structures on the ALH-84001 meteorite from Mars (Image: NASA/Science Photo Library)

Tiny carbon nuggets in meteorites from Mars were formed by cooling magma, not left by ancient alien microbes. That’s both good news and bad news for astrobiologists.

The 1996 discovery of carbonate structures in meteorite ALH-84001 – which travelled to Earth from Mars more than 13,000 years ago – was hailed at the time as evidence that alien microbes once lived on the red planet. However, subsequent studies of both the carbonate structures and tiny nuggets of macromolecular carbon (MMC) in the meteorite cast doubt on the claims.

To better understand where Martian carbon might have come from, Andrew Steele and colleagues at the Carnegie Institution of Washington in Washington DC examined samples from 11 Martian meteorites – including the famed ALH-84001.


Previous studies used pulverised samples of the rocks to analyse bulk chemical composition. As a result, they could not reveal where in the meteorites the carbon had originally been located. Steele and colleagues used Raman spectroscopy – a technique that scatters laser light off a substance to identify its structure and chemical make-up – to pinpoint the MMC in the rocks with a precision of around 360 nanometres.

They found that the carbon nuggets are encased in crystal that could only have formed when the rock initially cooled from magma. Because they are sealed tight, these carbon-filled “mineral bottles” exclude the possibility that the MMC came from ancient life.

Paradoxically, the find actually boosts prospects for finding signs of ancient life in Martian rocks. The carbon in MMC was originally chemically reduced – meaning it carries extra electrons and is quick to react. Such readily available and reactive carbon could have joined with other elements to create complex chemical molecules, perhaps even life.

“The presence of organic carbon at or near the Martian surface provides a potential nutrient source for putative life,” says co-author Francis McCubbin.

“Perhaps the formation of prebiotic chemistry on Mars was as simple as cooling of Martian lavas,” says Marc Hirschmann, a planetary scientist at the University of Minnesota in Minneapolis and St. Paul, who was not involved in the research. “It reinforces the idea that early Mars may have been ripe for the development of life.”

Steele says that the findings could help provide a baseline for carbon measurements to be carried out on the upcoming Mars Science Laboratory mission, scheduled to land on Mars in August of this year. “It’s helping to give context to the measurements to be made to detect life.”

Journal reference: Science, DOI: 10.1126/science.1220715