The Curiosity rover has tasted Mars' air: It's made mostly of carbon dioxide with hints of other gases.
The measurements by the most advanced spacecraft to land on the red planet closely match what the twin Viking landers detected in the late 1970s and what scientists have gleaned from Martian meteorites - rock fragments that fell to Earth.
Mars' atmosphere is overwhelmingly dominated by carbon dioxide, unlike Earth's air, which is a mix of nitrogen and oxygen.
There was a small surprise: Viking found nitrogen to be the second most abundant gas in the Martian air, but Curiosity's measurements revealed a nearly equal abundance of nitrogen and argon, a stable noble gas.
Mission scientists are puzzled, but suspect it might have to do with the different tools used to sample the atmosphere.
"It's more or less an interesting observation" but doesn't change the notion that Mars lost most of its original atmosphere to space, transforming the planet into a cold desert, said Paul Mahaffy of Nasa's Goddard Space Flight Center, who is in charge of Curiosity's air sampling experiments.
The nuclear-powered, six-wheel rover set down in an ancient crater near the Martian equator almost a year ago. The atmospheric measurements were detailed in two studies appearing in the journal Science.
While "there's nothing profoundly different" between what Curiosity found and previous results, the new work is more detailed, said Michael Mumma of Goddard, who is not part of the mission.
Neither study dwelled on the issue of methane gas on Mars.
Several years ago, telescopes on Earth detected a surprising and mysterious belch of methane in three regions in the Martian western hemisphere.
On Earth, methane is mainly a byproduct of life - from animal digestion and decaying plants. The gas can also be produced by non-biological processes.
Last year, the Curiosity team reported no definitive whiff of methane near the landing site. Since then, the rover has taken several more air samples.
There are also plans to sniff the atmosphere for methane during the monthslong drive to Mount Sharp, a 5km-high mountain looming from the crater centre.
Nasa is set to launch in spring a Mars-orbiting spacecraft aimed at solving the methane mystery.
Called Maven, the craft will target the Martian atmosphere.
Scientists want to know if it actually exists, determine the abundance and whether that varies by year or location, said mission chief scientist Bruce Jakosky of the University of Colorado.
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