METHANE has been detected on Mars by three independent groups of scientists. One team is even claiming that the most likely source of the methane is bacteria, making this discovery the strongest signal yet of possible life on Mars.
On Earth almost all of the methane in the atmosphere comes from bacteria that digest organic matter and produce the gas as a by-product. Fossil methane also seeps out of oil and gas deposits through vents and fissures. The only known way of forming the gas inorganically is in volcanoes and geothermal reservoirs. But spacecraft orbiting the planet have found no evidence of volcanic activity.
“It’s an extremely important discovery, whatever its origin,” says Stephen Clifford of the Lunar and Planetary Institute, Houston, Texas. “If it’s of geological origin then the implication that Mars is still volcanically active is extremely important for our understanding of the evolution of Mars as a planet. On the other hand, if the methane is of biological origin, then the implication that life survives today within the planet’s interior is enormous.”
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The discovery of methane was announced with little fanfare by the European Space Agency last month. A team led by Vittorio Formisano of the Institute of Physics and Interplanetary Space in Rome made the find using the Planetary Fourier Spectrometer (PFS) aboard the Mars Express spacecraft which is orbiting the planet. The instrument maps the infrared radiation from the Martian environment. Elements in the atmosphere absorb radiation at characteristic wavelengths, leaving telltale dark lines in the spectra.
The researchers averaged data from nearly 1700 spectral samples taken by Mars Express in January and February and found a line corresponding to methane. “We have been able to detect a very small quantity of methane,” says Formisano. “It’s around 10.5 parts per billion.”
“This report is very exciting,” says Michael Mumma, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Although the result is preliminary, it appears to confirm Mumma’s own finding, reported without public announcements at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in September last year.
His team detected a strong spectral line for methane on Mars using highly sensitive infrared spectrometers attached to the 8-metre Gemini South telescope on Cerro Pachón, Chile, and the 10-metre Keck II telescope on Mauna Kea, Hawaii. “These are very high-resolution spectra compared with the spectra that the PFS is returning from Mars Express,” says Mumma.
And the results are supported by yet another group. Vladimir Krasnopolsky of the Catholic University of America in Washington DC and his colleagues have also found a spectral line for methane using the Canada-France-Hawaii telescope on Mauna Kea. They will present their results at the European Geophysical Union’s meeting in Nice, France, later this month. Krasnopolsky’s team also found concentrations of around 10 parts per billion.
However, while each team says it is confident of its findings, Mumma insists that for a discovery with such huge implications, more confirmation is needed. Methane produces other characteristic lines in the spectrum, but they are not so easy to detect. Ground-based spectrometers have to contend with interference from the methane in Earth’s atmosphere.
The only way to see the feeble Martian spectral lines is to capture data when Mars is moving relatively fast away or towards the Earth. This shifts the frequencies of spectral lines from the planet, so that they appear at different places on the spectra from Earth’s methane lines. The spectrometer on Mars Express does not have this problem, but methane spectral lines overlap with lines from water vapour and reflected sunlight. Mumma contends that the resolution of the PFS probe on Mars Express is not good enough to separate out these lines conclusively.
Despite these concerns, the fact that three teams have independently made the same discovery is already fuelling speculation about where the methane is coming from. Methane cannot last more than a few hundred years in the intense sunlight of the Martian atmosphere because it reacts with hydroxyl ions, forming water and carbon dioxide. So the methane has to be forming now. “The fact that methane is present on Mars means that there must be a source,” says Formisano.
There are various possible sources of methane on Mars. First, it could simply be primordial methane trapped in underground reservoirs, which is slowly seeping out. “[But] it’s a stretch, because there would have to be reservoirs that weren’t depleted in four billion years,” says Mumma. Alternatively, volcanoes and geothermal reservoirs might be forming methane inorganically. But the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey spacecraft has found no evidence of the heat that such volcanic activity would be expected to produce. Others have suggested that comets, which contain about 1 per cent methane, could be bringing the gas to the planet. But again, there is no evidence that comets are crashing into Mars regularly enough to keep the methane levels replenished.
With inorganic sources looking unlikely, there remains the intriguing possibility of fossil methane produced by past life forms bubbling out from under the Martian crust. If this is the case, “it would be a big discovery, because it means that in the past there was organic matter on the planet”, says Giuseppe Etiope, an expert on geological emissions of methane at the National Institute of Geophysics and Volcanology in Rome, Italy. “A thing to look for is morphological evidence of methane-venting structures on the Martian surface, such as pockmarks, small mud volcanoes, and mud lavas.”
However, even if all these sources of methane were active, the concentration of methane would still be 30 times lower than that detected, according to calculations performed by Krasnopolsky’s team. “Our conclusion is that the methane is formed on Mars by methanogenic bacteria,” he told New Scientist. “We consider it as evidence in favour of the existence of life on Mars.”
That’s a controversial view, but not an impossible one. While the present Martian surface environment is hostile to life as we know it, Clifford has argued that given the past presence of abundant water on Mars (see “Alien oceans rippled across mars”), early Martian life may still survive in deep sub-permafrost aquifers, and that methane would be a likely by-product of their metabolism. “There’s reason to believe that water and temperatures above freezing could well persist to the present day at depths from a kilometre to several kilometres beneath the surface,” he says. “If that were the case, and life did adapt to that kind of environment, it would be enormously stable.”
The next step is to work out where on the planet the methane is coming from. The teams are now trying to determine variations in the concentration of the gas, to nail down any obvious sources.
It is also crucial to distinguish between organic and inorganic sources. One way to do this is to measure the ratio of the isotopes carbon-12 and carbon-13 in the methane. On Earth, life forms preferentially use carbon-12, so there is slightly more carbon-12 to carbon-13 in organic compounds than in inorganic ones.
Unfortunately, while the Beagle 2 craft which crashed in December had instruments designed to test for such variations in the isotopes, the rovers do not, and neither the ground-based telescopes nor the Mars Express probes are sensitive enough for this task. But the results provide an extremely good reason to include such instruments on future Mars missions. “These are very, very big questions,” says Etiope.
Alien oceans rippled across mars
THE dreamers were right. Mars once had waves and ripples shimmering across its surface, lapping at the shores of a vast sea or ocean. While the methane findings are still controversial, this aquatic view of Mars is now the official verdict from NASA.
The idea that Mars once had a vast northern ocean, or at least a network of large salty seas, was considered wildly speculative when it was first discussed in the late 1970s and 1980s. Even a month ago, the idea was still far from mainstream. But with a set of observations from NASA’s Opportunity rover revealed last week, the heretical idea suddenly became the official line.
When the NASA team announced evidence of past liquid water in the soil three weeks ago (New Scientist, 13 March, p 13), they were careful to say this did not necessarily mean large bodies of water. But in a carefully orchestrated press conference in Washington DC on 23 March, the rover mission’s chief scientist, Steve Squyres of Cornell University, said he is confident the evidence written in smile-like curves in the layered bedrock of Meridiani Planum does prove these sediments were laid down by flowing water.
Opportunity’s landing site lies in what was once “a salty sea on Mars”, Squyres said. Although many planetary scientists have been building such a case for years, this evidence finally confirms it, showing that the river-like features seen from orbit really were carved by the action of liquid water long ago. “We have never before had this definitive class of evidence from the Martian rocks themselves,” says James Garvin, NASA’s chief of Mars science.
Squyres and others on the science team have resisted making any guesses about the possible extent of the sea. But deposits of haematite seen from orbit suggest that, at the very least, the water must have formed a lake larger than the UK (New Scientist, 27 March, p 14). And detailed topographical information compiled by Mars Global Surveyor raises the more exciting possibility of a vast ocean that would have covered much of the northern hemisphere – perhaps the ideal habitat for life to start. The Meridiani site is right on the shoreline of that ocean.
David L. Chandler
