Methane Leaves its Mark
By Ashley Yeager
The surprising thing about finding methane on Mars is first that astronomers detect it. The gas is not stable in the planet’s atmosphere. It goes away rapidly which means the methane is recently generated and continually re-generated, says Michael Mumma of NASA’s Goddard Spaceflight Center.
He and his colleagues discovered traces of the gas using spectroscopy, and to their surprise only detected it in certain areas, which means it’s coming from discrete vents, Mumma explains.
Using spectroscopy instruments attached to the Keck and NASA infrared telescopes, the scientists spread the Martian light into its component colors or wavelengths just as a prism separates white light into the colors of the rainbow and created a light spectrum. Mumma and his colleagues then looked for dark areas in specific places along the spectrum.
At these points, methane, if present in the atmosphere, would absorb sunlight reflected from the Martian surface. The team found three of these features called absorption lines, which together are a definitive signature of methane. To verify their result and ensure that they were not detecting the Earth’s own methane, the scientists used the motion of the Red Planet to look for a shifted position of the Martian absorption lines. Now that methane has been detected, scientists next want to know whether the methane is produced biologically or geologically, or by both processes. Right now it’s too hard to tell, but the question is intriguing, Mumma says.
Methane is the main component of the natural gas found on Earth and is made of four atoms of hydrogen bound to a carbon atom. Terrestrial organisms actually release 90 percent of our planet’s methane as they digest nutrients. Other purely geological processes, like oxidation of iron, release the other ten percent. But, because such a large proportion of the Earth’s gas comes from living creatures, the discovery of methane on Mars is a tantalizing result, especially for astrobiologists.
Scientists looking for life on other planets are excited by the results because on Earth about two to three kilometers, or about mile and a half, beneath the Witwatersrand basin of South Africa, terrestrial microorganisms thrive. The microorganisms use hydrogen, which forms from natural radioactivity splitting water molecules into molecular hydrogen and oxygen, for energy. So, if microscopic Martian creatures are producing the methane, they too likely live, and could very well have survived for billions of years, below the permafrost layer on Mars.
An aquifer below the Red Planet’s permafrost layer would hold the necessary liquid water, radioactive decay would supply energy and carbon dioxide or carbonates could provide carbon for Martian microorganisms to manufacture methane as a bi-product of their metabolism. The gas and other substances, such as ethane and hydrogen sulfide (accumulating now or previously in such underground zones), could then seep into the atmosphere through the planet’s pores or fissures. Warm temperatures might cause the planet’s surface ice plugs to open seasonally at crater walls or canyons, and the gas could then escape from the deep zones into the atmosphere.
According to their observations, during mid-summer in the Martian northern hemisphere, one vent released 19,000 metric tons of methane, comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, Calif.The other observed plumes also burped up methane during the spring in the Martian southern hemisphere and were detected over regions that show either evidence of ancient ground ice or the flow of liquid water. But during the Martian winter the temperature drops, the ice returns and the cracks that release the gas are plugged up, so the methane signal disappears. Mumma adds that rather than being sealed as vapor inside ice-covered rock reserves, the methane could also be trapped inside molecular cages called clathrates. Connecting gas-filled voids in the rock with the atmosphere by clearing plugged pores and fissures would reduce the gas pressure and de-stabilize clathrates, perhaps releasing methane and water vapor.
“Of course, we can’t state with certitude that the methane is biologically produced, and so we also consider geochemical mechanisms,” Mumma says.
One way to create methane geochemically is through the combination of iron oxide, carbon dioxide and water under very high temperatures and pressures. The process, which occurs on Earth so it might also occur on Mars, transforms the water, gas and oxide into minerals called serpentines. But because the process takes place far below ground, the ensuing methane could be trapped for long periods, later venting into the atmosphere if seasonal warming opens pores and fissures at scarp faces.
“One of the most important consequences of our discovery is that we’ve identified certain signposts on Mars that are basically little flags that say ‘Come here. Here I am’,” he says. “But the big question is whether this methane was produced biologically or geochemically?”
Searching for other sites
To answer this question will take much follow-on research, Mumma explains. Not only will it require landers and rovers exploring the “little flags” but it will also require additional spectroscopic measurements. In a new initiative with the Keck telescope, he will use the adaptive optics system, AO, in combination with the NIRSPEC instrument to more precisely locate, down to a resolution of 50 kilometers, the regions that are leeching the methane.
With NIRSPEC and AO, Mumma also plans to search for other trace gases on Mars such as ethylene, formaldehyde and ethane. Finding them will give a better idea of whether the methane’s origin is geological or biological, and combining this data with information dug up by robot crafts sent to Mars will eventually tell us whether or not we have or have had living counterparts only a planet away.