A burst of methane detected on Mars has raised hopes we'll one day find life there.
The New York Times reports a measurement taken by the Curiosity rover on Wednesday showed "startlingly high" levels of the gas. On Earth, methane is usually is produced through biological processes.
"Given this surprising result, we've reorganised the weekend to run a follow-up experiment," scientist Ashwin Vasavada told the newspaper.
The reading was 21 parts per billion - three times higher than the previous record, set in 2013.
They hope to get more readings from Curiosity by Monday (US time) to confirm what's going on.
Until then, NASA is refusing to comment on whether the high methane reading could be a real sign of microbial life on the red planet.
"To maintain scientific integrity, the project science team will continue to analyze the data before confirming results," it said in a statement.
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Curiosity found no trace of methane when it arrived in 2012. The spike in 2013 lasted a couple of months before ebbing away, before there was another spike in December 2014.
Last year the rover found simple organic compounds in 3 billion-year-old rocks.