July 25, 2005
Origins of Methane on Earth
On Earth, methane is mostly produced by life. The recent detection of methane in the martian atmosphere therefore has given rise to much speculation about the possibility for life on the Red Planet. In part two of this four-part series, the various ways nature produces methane are considered.
Astrobiology Magazine -- In trying to understand the Mars-shaking news about methane on the Red Planet, astrobiologists look, as usual, to the home planet for instruction. The 1700 parts per billion (ppb) of methane in Earth's atmosphere is almost entirely produced by biology. Less than 1 percent comes from non-biological (abiogenic) processes, such as volcanism.
In recent years, new information -- all of it relevant to the Mars debate -- has emerged about both biological and non-biological sources of Earth's methane.
Methanogens at work!
Almost all the methane on Earth is made, directly or indirectly, by organisms. A small proportion comes from buried, decomposing plants, whose insoluble parts become a material called kerogen. When kerogen breaks down through thermal "cracking," the result is methane, as well as longer-chain hydrocarbons like ethane, propane, and butane. [Methane, the simplest hydrocarbon, has one carbon and four hydrogens (CH4). Ethane has two carbons and six hydrogens (C2H6). The formula for propane is C3H8, and butane is C4H10.]
Much more methane comes from anaerobic microbes called methanogens. Some methanogens are called "extremophiles" because they can prosper under extreme acidity, alkalinity, or saltiness -- conditions once thought intolerable to life.
Methanogens can also tolerate extreme temperatures. Methanopyrus kandleri, for example, lives in the 80 to 100 degrees C water around black smokers in the Gulf of California. Other methanogens live below 0 degrees C in Antarctica.
Methanogens are "extremely widespread on Earth," says Stephen Zinder, a microbiologist at Cornell University in New York. "Anywhere there is a place that usually doesn't have oxygen, you find them. Whether it's in the gastrointestinal tract, the soil, or the deep subsurface, you find them." Although they are anaerobes, methanogens can sometimes survive -- if not reproduce -- when exposed to small concentrations of oxygen.
Methanogens living in wetlands produce about 21 percent of the methane in Earth's atmosphere, says Sushil Atreya of the University of Michigan (Atreya was a co-author of the Science paper on the methane results from Mars Express.). Methanogens in the guts of cows and other ruminant produce about 20 percent. Microbes in termites and similar organisms make 15 percent of atmospheric methane, and in rice paddies, about 12 percent. Other major sources include natural gas releases and biomass burning.
On Earth, a large amount of methane is locked inside ice crystals under permafrost and beneath the continental shelves. These deposits of methane hydrate, also called methane clathrate, are vast. They are thought to contain far more carbon than all fossil fuels put together.
If clathrates are so dominant as a methane storage on Earth, why not on Mars too? Clathrates form on Earth under certain combinations of pressure and temperature, and some scientists think these combinations could occur on Mars as well.
Making methane without biology
Although nearly all methane on Earth has a biological origin, scientists have recently begun to appreciate how many ways abiogenic methane can be generated. The essential precondition for abiogenic methane, says Juske Horita of the Chemical Sciences Division at Oak Ridge National Laboratory in Tennessee, is the presence of molecular hydrogen (H2) and carbon dioxide (CO2).
"If you put CO2 and hydrogen together, thermodynamics dictates that it has to go to methane," says Horita.
The reaction speed is dependent on pressure, temperature, and the presence of catalysts. Since carbon dioxide is common in so many environments, finding sources of abiogenic methane is largely a search for hydrogen and suitable catalysts for the reaction. Abiogenic methane does not form in Earth's atmosphere, even though CO2 is abundant, because molecular hydrogen is so rare.
Most abiogenic methane is generated by the "serpentinization" reaction, which forms the mineral serpentine. At mid-oceanic ridges, water heated by magma reacts with rocks like olivine, which contain high levels of the catalysts iron and magnesium. During serpentinization, hydrogen liberated from water reacts with carbon from carbon dioxide to form methane. The reaction creates heat and vast deposits of serpentine on the ocean floor.
Until recently, the abiotic water-mineral-carbon dioxide reactions, including serpentinization, were thought to require 200 degrees C water, and no one knows if water on Mars goes deep enough to get that hot. There are indications that similar methane-making reactions could take place in cooler conditions. Horita, for example, notes that serpentinization may be occurring in 50 to 70 degrees C water in Oman and the Philippines. And in 1999, Horita and Michael Berndt, a geochemist then at the University of Minnesota, published a recipe for a related reaction that makes methane in the presence of a nickel-iron mineral catalyst. While the reaction made methane in a few days at 200 degrees C, Horita suspects it would also work, although more slowly, at 50 to 70 degrees C. To his knowledge, that experiment has not been done.
Researchers have found other ways to make methane, using different catalysts and minerals. In May 2004, Dionysis Foustoukos and William Seyfried Jr. of the University of Minnesota made methane, ethane and propane at 390 degrees C and 400 times the atmospheric pressure at Earth's surface, using a chromium-bearing mineral as catalyst.
In September 2004, Henry Scott of Indiana University at South Bend published a study which found that, by subjecting iron oxide, calcite, and water to the intense heat and pressure of Earth's mantle, methane formed.
Yet despite the multiple discoveries of new pathways to abiogenic methane, most methane on Earth is biogenic. "So much methane is produced by bacteria on Earth, it's widespread, it's everywhere," says Horita. "As a part of the global methane budget, I don't think [abiogenic is] important. However, abiogenic may be locally important, possibly including Mars."
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