WASHINGTON • Mars, it appears, is belching a large amount of a gas that could be a sign of microbes living on the planet today.
In a measurement taken last Wednesday, Nasa's Curiosity rover discovered startlingly high amounts of methane in the Martian air, a gas that on Earth is usually produced by living things.
The data arrived on Earth last Thursday and was greeted with much excitement by scientists working on the mission.
"Given this surprising result, we've reorganised the weekend to run a follow-up experiment," Dr Ashwin Vasavada, project scientist for the mission, wrote to the science team in an e-mail.
The mission's controllers on Earth sent new instructions to the rover last Friday to follow up on the readings, bumping previously planned science work. The results of these observations are expected back on the ground today.
People have long been fascinated by the possibility of aliens on Mars. But Nasa's Viking landers in the 1970s photographed a desolate landscape.
Two decades later, planetary scientists thought Mars might have been warmer, wetter and more habitable in its youth some four billion years ago. Now, they are entertaining the notion that if life ever did arise on Mars, its microbial descendants could have migrated underground and persisted.
Curiosity arrived on Mars, looked for methane and found nothing.
It detected a sudden spike, up to 7 parts per billion, that lasted at least a couple of months.
The latest measurement was 21 parts per billion of methane, or three times the 2013 spike.
Methane, if it is there in the thin Martian air, is significant because sunlight and chemical reactions would break up the molecules within a few centuries. Thus, any methane detected now must have been released recently.
On Earth, microbes known as methanogens thrive in places lacking oxygen, such as rocks deep underground and the digestive tracts of animals, and they release methane as a waste product. However, geothermal reactions devoid of biology can also generate methane.
It is also possible that the methane is ancient, trapped inside Mars for millions of years but escaping intermittently through cracks.
Scientists first reported the detections of methane on Mars 11/2 decades ago using measurements from Mars Express, an orbiting spacecraft built by the European Space Agency and still in operation, as well as from telescopes on Earth.
However, those findings were at the edge of the detection power of these tools and many researchers thought the methane might just be a mirage of mistaken data.
When Curiosity arrived on Mars in 2012, it looked for methane and found nothing, or at least less than one part per billion (ppb) in the atmosphere. In 2013, it detected a sudden spike, up to seven ppb, that lasted at least a couple of months.
The measurement this past week found 21 ppb of methane, or three times the 2013 spike.
Nasa acknowledged the methane detection in a statement last Saturday but called it an "early science result". Its spokesman added: "To maintain scientific integrity, the project science team will continue to analyse the data before confirming results."
Meanwhile, rovers scheduled for launch next year will carry instruments designed to search for the building blocks of life, although neither is designed to answer the question of whether there is life on Mars today.