Mars Methane Mystery Still Beckons

[/caption]
We’ve known about the methane in Mars’ atmosphere for over four years now. But we don’t know where it is coming from. On Earth, methane is produced from biological agents: rotting vegetation or flatulence from large animals like cows. But, of course, with our extensive explorations of Mars with rovers and high-resolution orbiting cameras, we’re fairly sure there are no Martian bovine equivalents chewing cud from the foliage on the Red Planet. Even if life existed in the past on Mars, methane is broken down quite quickly by sunlight, and scientists have calculated that methane should only exist for a few hundred years in the Martian atmosphere. The only possibility is that somehow, either chemically or biologically, the methane is being replaced on a regular basis. And now, two recent reports outlining separate discoveries on Mars make this methane mystery even more intriguing.

Methane was discovered on Mars by three independent groups in 2003 – 2004. One detection was made using the Mars Express spacecraft, another used observations from the Keck II and Gemini South telescopes, and the third used the Canada-France-Hawaii telescope.

And the mystery of how methane on Mars is being replenished has scientists continuing their observations in an effort to understand what’s happening on Mars. Michael Mumma of NASA’s Goddard Space Flight Center in Greenbelt, Maryland was one of the original methane discoverers. Observations he and his team have made over the last four years show methane is not spread evenly around Mars, but concentrated in a few “hotspots.” They have seen that methane clouds spanning hundreds of kilometers form over these hotspots and dissipate within a year – much shorter than the 300 – 600 years it was thought to take for atmospheric methane to be destroyed by sunlight. If methane is being destroyed so quickly, it also must be created at far higher rates than previously thought. Mumma reported these results at a planetary science conference last month.

One of the hotspots is Nili Fossae a fissure that has been eroded and partly filled in by sediments and clay-rich ejecta from a nearby crater. Could a living ecosystem be hidden here under the Martian surface? On Earth, subterranean microbes survive without sunlight, free oxygen, or contact with the surface. Additionally, the prospect becomes more intriguing when it is known on Earth, most deep-surface microbes are primitive, single-celled organisms that power their metabolism with chemical energy from their environment. These microbes are called “methanogens” because they make methane as a waste product.

Nili Fossae is one of the possible landing sites for the Mars Science Laboratory, the next generation of rover currently set to head off the Red Planet next year.

But astrobiologists aren’t ruling out the possibility of some type of ongoing chemical process on Mars, which could be producing the methane. But even this is intriguing, because it means there are active processes going on inside Mars. One idea proposed in a recent paper is that methane clathrates are near the Martian surface, and are constantly releasing small amounts of methane as temperatures and pressure near the surface change.
Methane clathrates are solid forms of water that contain a large amount of methane within its crystal structure.

Caroline Thomas and her colleagues at the Universite de Franche-Comte say the clathrates could only exist near the surface of Mars if the atmosphere had once been methane rich. Otherwise the clathrates could never have formed. One possibility is that the atmosphere was once temporarily enriched by a comet impact. Also, the discovery of gray crystalline hematite deposits on the surface could be a proof of an early methane-rich Martian atmosphere.

Otherwise, the researchers say, the only other possibility is a biological source.

“Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH4-rich atmosphere has existed or if a subsurface source of CH4 has been (or is still) present,” the researchers write.

So what does all this mean? The Mars Science Laboratory rover might have the ability to find out, or at least bring us closer to solving this mystery. Otherwise it will take a fairly large breakthrough from the other spacecraft and telescopes observing Mars. But it’s possible we might not fully understand why Mars has methane until humans actually go there themselves to find out.

Sources: arXiv, arXiv blog, New Scientist, Nature