Scientists have many questions about Mars, the planet most like Earth in the solar system, but few are as vexing as why atmospheric methane levels occasionally spike and how the gas manages to dissipate so quickly.
Most methane on Earth comes from living organisms, though small amounts are produced by the geological interaction of rock with hot water. Scientists don't know what causes methane on Mars, but they intend to spend the next 10 years compiling detailed maps on when, where and how often the gas appears.
The data will come from a newly arrived European satellite, Trace Gas Orbiter, or TGO, which this week began testing its science instruments. It will take TGO about a year to circularize its orbit and shift into the near-polar inclination considered ideal for its mapping mission.
"I believe TGO is going to play a central role in expanding our knowledge of what's going on here," said NASA senior scientist Michael Mumma, with the Goddard Space Flight Center in Greenbelt, Maryland.
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In 2003, Mumma led a team that made the first definitive measurements of methane on Mars using an infrared telescope in Hawaii. The methane appeared in clouds, or plumes, over specific regions of Mars and reached a maximum density of about 60 parts per billion.
More reports followed a year later, though the methane appeared over a different part of Mars. In 2006, Mumma and colleagues scanned the location of the original plumes and to their surprise discovered that the methane had vanished, suggesting a seasonal phenomenon was at work.
The methane's disappearance opened another mystery: Computer models show that methane on Mars should survive about 300 years before ultraviolet light from the sun breaks down the molecules. Instead, analysis showed the plume likely vanished in a matter of months or even weeks.
NASA's rover Curiosity picked up the methane hunt after its arrival in August 2012. It too found a large spike during the southern hemisphere's autumn season, but the process did not repeat the following year. Since then, scientists believe they have identified slight seasonal variations in the background levels of methane, said Ashwin Vasavada, lead scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Those studies are still underway.
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With a sensitivity that rivals Curiosity's instrument, TGO will attempt to map the planet's methane on a global scale.
"This is going to be the crème de la crème of methane measurements," Mumma told Seeker.
With luck, TGO may even be able to make related measurements of carbon and hydrogen isotopes so scientists can compare the chemical makeup of Mars' methane with its terrestrial counterpart. That could provide more clues about whether Mars' methane came from biology or geology.
"This is a story that's going to continue for a long time to come," said Mumma.
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