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Mars’ vanishing carbon

How the Martian atmosphere turned to stone, by Helen Maynard-Casely.

Rover Opportunity surveys the red planet – Getty Images

Mars today is a cold, dry and probably dead world but increasingly we’re discovering that this hasn’t always been the case. The Mars Science Laboratory, currently roving about the planet’s surface, is finding lots of evidence that the red planet was once warmer and wetter.

The answer as to why it changed seems to lie with Mars’ atmosphere – or rather its lack of one dense enough to stop liquid water on the surface from evaporating into space. What atmosphere there is, is predominately carbon dioxide and it seems likely that Mars once had a lot more of it. So what happened to all the carbon dioxide? The rocks of Mars may have sucked it up. That’s the suggestion of an October publication in Nature Communications by Tim Tomkinson’s team at Glasgow University.

On Earth, some rocks certainly do suck up carbon dioxide and become chemically altered in the process. Known as “carbon sequestration”, it is an ongoing process that keeps the gas locked up over geological time scales. For example, in Oman, a slab of peridotite rock that originally formed deep in the Earth is currently sequestering more than 10,000 tonnes of carbon dioxide every year. The reaction of water and atmospheric CO2 with the rock also generates heat that drives the self-perpetuating chemical reaction responsible for the sequestration.

Tomkinson’s team has shown for the first time that Mars rocks can undergo the same process. The findings were based on a study of the Lafayette meteorite, a 1.3 billion-year-old, 800 g piece of Martian rock discovered in Indiana in 1931. Tomkinson’s team discovered that this rock, much like the ones in Oman, was rich in carbonates, suggesting Mars rocks have also been sequestering CO2 from the red planet’s atmosphere.

The team used techniques such as electron microscopy to examine what minerals have formed and how they are arranged inside the meteorite. The researchers concluded that the carbonisation occurred whilst the rock was still on Mars. The process could only have been kick-started by interaction between water and CO2. In doing so, this rock sucked up a substantial amount of carbon.

“Weathering of rocks like these plays a significant role in regulating the carbon dioxide in our atmosphere on Earth; it is interesting to see it playing a similar role on Mars,” says Siobhan Wilson, a geologist at Monash University.

Could this have implications for dealing with our own excess carbon dioxide? “Carbonation of volcanic rocks is the most efficient way to irreversibly sequester CO2,” says Tomkinson. “We have found that carbonate replaces the volcanic mineral olivine in a specific way on Mars, which if utilised by industry could encourage carbon dioxide storage on Earth.”

The Lafayette meteorite not only reveals how Mars’s climate met its demise; it may also point the way to save our own climate.

Helen Maynard-Casely is a physicist and science writer based in Sydney.
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