Mystery on Mars: Why Methane Fades Away So Fast

Methane on Mars can disappear from the planet's atmosphere mysteriously fast, fading in less than a Martian year (or about 22 Earth months), a new study finds.

Researchers mapping out the Red Planet's methane cycle have discovered that concentrations of the gas vary by season, by year and by location, peaking when it's warm in regions home to underground water ice and past witnessed volcanic activity.

The findings add yet another layer to the tricky debate over whether Mars' methane is created by biological processes or has a more mundane geochemical origin. [Image: New Mars methane map.]

"The source of the methane could be geological activity or it could be biological — we can't tell at this point," said Sergio Fonti of the Universita del Salento. "However, it appears that the upper limit for methane lifetime is less than a year in the Martian atmosphere."

Scanning Martian skies

Methane was first detected on Mars in 2003. The finding intrigued scientists, since the gas can be a sign of life. On Earth, methane bubbles up from the bottom of swamps as organic matter decays, and it is emitted by cows, goats and other animals.

But methane also forms from chemical and geophysical processes. For example, it's common in the atmospheres of Jupiter, Saturn, Neptune and Uranus. The Martian atmosphere, which is 95 percent carbon dioxide, has only trace amounts of methane.

Fonti and Giuseppe Marzo of NASA's Ames Research Center in Moffett Field, Calif., performed a comprehensive survey of the methane cycle on Mars. They compiled nearly 3 million observations taken by NASA's Mars Global Surveyor spacecraft to track the amounts of methane in the Mars atmosphere between July 1999 and October 2004 (about three Martian years).

The researchers found that methane, once emitted, sticks around in the Martian atmosphere for less than a single Martian year.

Levels of the gas are highest in the autumn in Mars' northern hemisphere, with localized peaks of 70 parts per billion — about 4 percent of the average methane concentration on Earth — though the gas can still be detected across most of the Red Planet at this time of year. There is a sharp decrease of methane in the northern Martian winter, the researchers found. Then concentrations build again in spring, rise rapidly in summer and spread across the planet, they added.

Three regions in the northern hemisphere had higher-than-normal methane concentrations, the researchers discovered. These were Tharsis and Elysium, the two main volcano provinces, and Arabia Terrae, which has a substantial cache of underground water ice.

"It's evident that the highest concentrations are associated with the warmest seasons and locations where there are favorable geological — and hence biological — conditions such as geothermal activity and strong hydration," Fonti said. "The higher energy available in summer could trigger the release of gases from geological processes or outbreaks of biological activity."

Wind factor

Researchers aren't sure what's injecting methane into the Martian atmosphere, and they're equally puzzled about why it fades so fast. Photochemical processes — destruction of the gas by sunlight — should not happen so quickly, the scientists said.

However, the winds of Mars may play a role, they added.

High winds can mix strong, reactive chemicals into the Martian atmosphere, quickly breaking down methane. One such destructive compound, perchlorate, has been detected in Martian dirt.

The new study should help scientists get to the bottom of such questions, the researchers said.

"Our observations will be very useful in constraining the origins and significance of Martian methane," Fonti added.

Fonti and Marzo planned to present their results at the European Planetary Science Congress in Rome tomorrow (Sept. 21).

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