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.
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