Making Sense of Mars Methane
In a quest to understand the source of methane detected in the atmosphere of Mars, NASA scientists are looking at methane bubbling from the ground at an outdoor salt factory on Mexico’s Baja Peninsula. By measuring carbon isotopes in the Mexican methane, these scientists hope to help unravel the mystery of the martian methane. In particular, they want to know whether or not the martian methane, like most methane on Earth, is made by microbes.
The 2003-2004 observations of methane on Mars (ranging from 7 to 200 parts per billion) were made remotely by three teams working with separate data, and their accuracy is still under debate, says Brad Bebout, a microbiologist at NASA’s Ames Research Center. But if the methane is truly present in the atmosphere of Mars, then something must be producing it on the planet now, because the gas is broken down by sunlight within 300 years.
Most methane in Earth’s atmosphere is made by primitive microbes called archaea that reside in anaerobic locations like rice paddies and the guts of ruminants like cows. However, methane is also produced non-biogenically, by reactions between water and hot, carbon-bearing rocks, or by the natural decay of coal and petroleum.
Methane, composed of one carbon and four hydrogen atoms, attracts a lot of attention from astrobiologists “because it is a key biomarker,” says Bebout. “But methane can be produced by non-biological means, so when you see it in the atmosphere of a planet, biology was not necessarily involved, and that’s why we want to know about isotopes.”
Isotopes are key to understanding the origin of methane because organisms tend to use more of the lighter isotopes. Biogenic methane usually – but not always – contains a higher percentage of the lighter carbon-12 than non-biogenic methane, which contains relatively more of the heavier carbon-13. The two types of carbon atoms both have the same number of protons, but carbon-13 has one more neutron than carbon-12.
Isotopic measurements of the carbon in martian methane are a goal of the tunable laser spectrometer on NASA’s Mars Science Laboratory, a rover now scheduled for launch in Fall 2009. But to get context for interpreting that data, scientists want more information on how primitive life produces methane on Earth, and one of the best locations for that is the salt factory on Baja.
The salt flats of Guerrero Negro (the name, “black warrior,” is taken from a nearby shipwreck) are an excellent outdoor laboratory because pools of increasing salinity are created as ocean water is evaporated to concentrate the salt. In many of the pools, microbial mats dominated by salt-tolerant cyanobacteria generate small amounts of methane, which is made by archaea living in association with the photosynthetic cyanobacteria. Bebout has identified these archaea, and is working on measuring their individual contributions to the overall level of methane.
This microbial mat is absent in the hyper-saline Area 9, where methane is even more abundant: a stream of gas containing about 50 percent methane is bubbling up to the salty surface. “In that area, there is no mat; the bottom is basically solid gypsum,” says Bebout.
Apparently that gypsum is home to some archaea, because the carbon isotopes suggest that the methane bubbles have a biological origin. The isotopes, however, are not conclusive, because they are slightly outside the accepted range for biogenic methane. In cases like these, scientists can try to clarify the methane’s origin by looking not only at its carbon atoms, but also at its hydrogen atoms. Hydrogen has two stable (non-radioactive) isotopes: protium (containing one proton and no neutrons) and deuterium (one proton plus one neutron).
The combined isotopes of the hydrogen and carbon in the Area 9 methane are in a range “that is usually not considered biological,” says Bebout. However, when the geologic conditions are taken into account, “It’s very unlikely this is caused by water-rock interactions, or is thermogenic – resulting from the decay of organic matter – so we are pretty sure it is biological."
Carbon and hydrogen isotopes in the Area 9 methane comprise “a pretty unique signature,” which falls outside the accepted range of biogenic methane, and thus begs for a more complete explanation, says isotope expert Jeff Chanton, a professor of oceanography at Florida State University. For that reason alone, the salt ponds in Baja are worth studying.
The primitive bacteria that are making methane in Area 9 may resemble whatever is making methane on Mars, says Chanton. “If there was life on Mars, it would not be like little green men. It would be some sort of microbe, a prokaryote [a cell without a nucleus]. On Earth, bacteria were the first form of life.”
Finding evidence of ancient bacteria on Mars would not be too surprising, considering that early Mars was warm and wet, rather like Earth during the evolution of methane-making bacteria.
A better understanding of methanogens in the salt flat and further isotopic study of their methane will help interpret future measurements of the isotopic composition of methane on Mars. The recent detection of highly saline environments on Mars by Mikki Osterloo, of the University of Hawaii, using data from the THEMIS instrument on NASA’s Mars Odyssey orbiter makes the saline investigation all the more important, Bebout says. “We want to really go through the system and characterize all of these environments, which might be like something the MSL rover would stumble on, and figure out what’s going on. On Earth, we can do experiments we can’t do on the rover.”
Bebout’s research is supported by a grant from NASA’s Exobiology and Evolutionary Biology program.