Turning a greenhouse gas into stone is a climate change technofix some have suggested for Earth. Now it seems it may have dramatically cooled the Red Planet 3 billion years ago.
The conclusion comes from a study of minerals in a Martian meteorite. "It has big implications for global warming and CO2 reduction in our own atmosphere," says Tim Tomkinson, who studied the rock. "By understanding how this occurred on Mars we can gain insights into how we can do it on Earth."
These days the Martian atmosphere is thin and about 95 per cent CO2, but it is thought that around 3 or 4 billion years ago the planet's gassy envelope was much thicker and even richer in carbon, making its surface warm enough to support liquid water - and therefore, possibly, life.
Just what happened to all the CO2 is a bit of a mystery, says Tomkinson, who works at the Scottish Universities Environmental Research Centre in East Kilbride. It could have been blown into space by the solar wind or frozen in the dry ice caps at the poles, but that wouldn't account for all the carbon.
Another possibility is that the CO2 was sucked into rocks, in a process called carbonation which also occurs naturally on Earth. Tomkinson and colleagues studied a meteorite known as Lafayette, thought to have landed on Earth roughly 3000 years ago. Using a scanning electron microscope they found veins of carbonate minerals. These form when carbon dissolved in liquid water seeps into rocks containing the mineral olivine. The carbon replaces the olivine, locking it away (Nature Communications , DOI: 10.1038/ncomms3662).
NASA's Mars Reconnaissance Orbiter detected signs of carbonate in 2008, and traces have been found in other meteorites. By looking at patterns in the Lafayette meteorite to see how the mineral was laid down, the new study demonstrates that it replaced olivine in a natural geoengineering event, says Tomkinson.
It is not yet clear how much of Mars's ancient atmosphere might have ended up trapped in rocks. Studies suggest that using olivine to transform Earth's climate would require more mineral than can fit on its surface, so carbonation is unlikely to be solely responsible on Mars.
Some answers may come from a pair of new probes: India is launching its first Martian orbiter next month to study how the planet's atmosphere has changed over time, and the US is launching a similar one later this year.
For now, however, the new findings shows that olivine can play a significant role in moulding a planet's climate, says Bethany Ehlmann of the California Institute of Technology in Pasadena, California, who led the MRO discovery. "People don't usually think about it but alteration of the rocks really influences the air we breathe over geologic time."
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