THE first hard evidence that methane exists on Mars is forcing scientists to confront a crucial question: is it inorganic in origin or a sign that Martian microbes are at work?
Methane is broken down by sunlight within a few hundred years on Mars, so whatever process is creating it must be active today. The discovery of the gas, which occurs mostly in patches above icy ground, is arousing interest because on Earth almost all methane comes from living things, ranging from rotting plants to flatulent cattle.
Initial reports last year that low levels of methane had been detected on Mars were greeted with caution. Now Michael Mumma of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, has confirmed the discovery. Using the new 8-metre Gemini South telescope in Chile, he found two characteristic optical signatures of methane in the Martian atmosphere. 鈥淚t鈥檚 real,鈥� says Stephen Squyres, chief scientist for NASA鈥檚 Mars rovers. 鈥淭here鈥檚 something there. It really warrants a close look.鈥�
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Mumma鈥檚 observation that the gas is not distributed evenly over the surface confirms earlier reports from Mars Express, the European Space Agency鈥檚 spacecraft that has been orbiting Mars since late last year. The methane seems to be concentrated in two or three areas above parts of the surface where there is water ice.
Sushil Atreya of the University of Michigan, Ann Arbor, says that the patchiness is evidence of a dynamic process at work 鈥� a methane cycle of some kind. He suggests that hydrogen peroxide, which he detected in the atmosphere last year, is breaking down the methane as fast as it forms, so there is no time for winds to spread it round the planet. If this is what is happening, the methane must be being produced far faster than previously believed.
Working out exactly what creates the methane will be tricky. On Earth, biogenic methane from deep-water habitats tends to be produced with almost equal amounts of ethane. Nobody has really looked for ethane on Mars, Mumma says, but if it鈥檚 there in the right quantities it might be a strong indicator of life. He plans to look for it during Mars鈥檚 next close approach using the Gemini telescope.
鈥淥n Earth, almost all methane comes from living things, ranging from rotting plants to flatulent cattle鈥�
Non-biological sources of methane are also possible. Vladimir Krasnopolsky of the Catholic University of America in Washington DC says that if the methane is volcanic in origin it should be accompanied by sulphur dioxide. This has not yet been seen. Another possibility is that the ice is the remains of old comet impacts and that methane which came in as part of the comet is now slowly leaking away.
鈥淭he methane seems to be concentrated in two or three areas above parts of Mars where there is water ice鈥�
