— The ARES plane soars over the Martian surface (Illustration: NASA)
A rocket-powered plane may offer the best chance of finding the mysterious source of Mars's methane, which some scientists have speculated could come from living micro-organisms.
Europe's Mars Express orbiter and Earth-based telescopes have detected methane in the atmosphere of Mars in the last few years, which is a puzzle since the gas quickly breaks down when exposed to sunlight.
That suggested the gas has been released recently perhaps by lava welling up close to the planet's surface, or, more intriguingly, by micro-organisms like those that produce methane on Earth (see Methane on Mars: the plot thickens).
Now, a NASA scientist says an aeroplane that can suck in and analyse gas directly from the Martian atmosphere has the best chance of locating sources of the methane.
Joel Levine of NASA's Langley Research Center in Hampton, Virginia, US, led research into a Mars aeroplane mission called ARES (Aerial Regional-scale Environmental Survey of Mars). Though Levine and his colleagues hoped the concept could launch in 2011 as part of NASA's Mars Scout programme, it recently failed to make the agency's shortlist of possible missions in that time frame.
But on Friday, Levine explained to scientists at the Lunar and Planetary Science Conference in Houston, Texas, US, why he thinks such a mission would offer the best hope of tracking down methane sources on Mars.
Mars Express has detected methane from orbit by looking for the characteristic way that it absorbs light. The spacecraft is able to detect some variation in the concentration of methane across the planet, but not with a resolution high enough to pin down a specific source.
Rovers fitted with the right equipment would be able to detect specific methane sources but only if the sources happened to be located within the limited driving range of such vehicles.
An aeroplane would be able to detect local concentrations of methane, as well, but would also be able to survey a path hundreds of kilometres long.
According to Levine's team, the ARES plane would enter Mars's atmosphere protected by a heat shield, drift close to the surface on a parachute, then cut loose at an altitude of 1.5 kilometres for a rocket-powered flight. Watch a simulation of ARES flying over the surface of Mars.
ARES would be able to identify different compounds in the atmosphere, including methane, by precisely weighing molecules sucked into a device called a mass spectrometer. Such a device could detect methane at levels of just a few parts per billion.
This would be enough to measure the way the abundance increases or decreases from place to place, which is called the concentration gradient. "You can actually measure gradients and identify the source of these gases," Levine says. "I think this is a way to do it."
Jay Bergstralh, a scientist at Langley who is not on the team, says there are advantages to an aerial vehicle. "It fills in a gap between a lander or rover and at the other extreme, an orbiter," he says. "It's a good way to go sniffing out point sources."
On the other hand, a plane would have a relatively short lifetime, he says, giving it just a few hours in which to observe a potential outgassing event. A balloon-borne mission could last longer, he adds but unlike a plane, which could be remotely steered along a desired path, a balloon would be blown around by the wind.