NASA's Curiosity rover has detected unusually high levels of methane gas on the surface of Mars, indicating the possible presence of alien life on the Red Planet.
The rover detected about 21 parts per billion units by volume (ppbv) of methane, which is the largest amount ever measured during the mission, according to NASA.
One ppbv means that if you take a volume of air on Mars, one billionth of the volume of air is methane.
Scientists are excited by the discovery because microbial life is an important source of methane on Earth – and could be a sign of microbes living on Mars in the last few hundred years.
Sunlight and chemical reactions break up methane molecules within a few centuries, so any methane detected now must have been released relatively recently.
However, methane can also be created through geological processes, and Curiosity doesn't have instruments that can definitively say what the source of the methane is.
"With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern," said Paul Mahaffy of NASA's Goddard Spaceflight Center.
The Curiosity rover has detected methane many times over the course of the mission.
Background levels of the gas seem to rise and fall seasonally, and scientists have also noted sudden spikes of methane.
But the science team knows very little about how long these transient plumes last or why they're different from the seasonal patterns.
It has been suggested that the methane on the planet could simply be ancient gas that is now bursting through the cracks on the surface.
NASA will now run follow-up experiments to gather more information on the methane plume and allow scientists to analyse this data, before confirming the results.
They also need time to compare the results with the European Space Agency's Trace Gas Orbiter, which has been in orbit around Mars for over a year without detecting any methane.
Combining observations from the surface and from orbit could help scientists locate sources of the gas on the planet and understand how long it lasts in the Martian atmosphere.
"While increased methane levels measured by Mars Curiosity are exciting, as possible indicators for life, it's important to remember this is an early science result," said Thomas Zurbuchen of NASA's Science Mission Directorate in a tweet.
"To maintain scientific integrity, the science team will continue to analyse the data before confirming results."
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