NASA will soon have some answers to a long-standing puzzle over what could be producing methane gas detected in the thin Martian air with the August 5th touchdown of the Mars Science Laboratory. It will deliver an S.U.V.-size rover named Curiosity that carries an instrument that can detect methane in the air, and if it does, it will unleash new excitement about the prospect of life on Mars. Methane molecules are easily blown apart by ultraviolet light from the Sun, so any methane around must have been released recently.
For the past few years it's been a hot debate between NASA's Ames Research Center vs the Goddard Space Fight Center. It's methane on Mars as a potential sign of biology vs methane on Mars as an Earth' based illusion.
The evidence for methane on Mars may actually be due to Earth-based methane, say researchers led by Kevin Zahnle of the NASA Ames Research Center in Moffett Field, California. "This is not a done deal," says Zahnle. While most researchers have assumed that this was due to an as-yet-unknown Martian process, Zahnle's team found that the dips in the spectrum are not due to Mars-based methane.
In 2009 from a team led by Michael Mumma at NASA's Goddard Space Flight Center in Greenbelt, Maryland used ground-based telescopes to look at light emitted by Mars and attributed dips in the spectrum to martian methane absorbing those frequencies --- a possible sign that life may have existed on the Red Planet sometime in the distant past. The readings, taken several years apart, suggested that methane's lifetime in the Martian atmosphere is unexpectedly –-- and inexplicably --- brief.
Mumma's team focused on readings taken when Mars was either moving toward or away from Earth, when any dips in its light would be shifted to higher or lower frequencies and so could be separated from dips caused by methane in Earth's atmosphere. But Zahnle's team notes that the frequency of a spectral dip attributed to Martian methane could also be caused by earth-based methane containing carbon-13 instead of the more common carbon-12 isotope.
Mumma, however, is confident his team measured Martian methane. "My group does not publish until we're certain we've exhausted all other possible explanations," he says.
An earlier study by Sergio Fonti (Universita del Salento) and Giuseppe Marzo (NASA Ames) have used observations from NASA’s Mars Global Surveyor spacecraft to track the evolution of the gas over three martian years. Their findings indicate that methane in the atmosphere of Mars lasts less than a year.
Methane is replenished from localized sources that show seasonal and annual variations. This pattern of methane production raises questions as to whether the methane comes from geological activity --- or biological processes. The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water, as well as methane.
“Only small amounts of methane are present in the martian atmosphere, coming from very localized sources. We’ve looked at changes in concentrations of the gas and found that there are seasonal and also annual variations. The source of the methane could be geological activity or it could be biological --- we can’t tell at this point. However, it appears that the upper limit for methane lifetime is less than a year in the martian atmosphere,” said Fonti.
They found that levels of methane are highest in autumn in the northern hemisphere, with localized peaks of 70 parts per billion, although methane can be detected across most of the planet at this time of year. There is a sharp decrease in winter, with only a faint band between 40 and 50 degrees north. Concentrations start to build again in spring and rise more rapidly in summer, spreading across the planet.
“One of the interesting things that we’ve found is that in summer, although the general distribution pattern is much the same as in autumn, there are actually higher levels of methane in the southern hemisphere. This could be because of the natural circulation occurring in the atmosphere, but has to be confirmed by appropriate computer simulations,” said Fonti.
There are three regions in the northern hemisphere where methane concentrations are systematically higher: Tharsis and Elysium, the two main volcano provinces, and Arabia Terrae, which has high levels of underground water ice. Levels are highest over Tharsis, where geological processes, including magmatism, hydrothermal and geothermal activity could be ongoing.
“It’s evident that the highest concentrations are associated with the warmest seasons and locations where there are favorable geological -- and hence biological -- conditions such as geothermal activity and strong hydration. The higher energy available in summer could trigger the release of gases from geological processes or outbreaks of biological activity,” said Fonti.
The mechanisms for removing methane from the atmosphere are also not clear. Photochemical processes would not break down the gas quickly enough to match observations. However, wind driven processes can add strong oxidizers to the atmosphere, such as the highly reactive salt perchlorate, which could soak up methane much more rapidly.
Martian years are nearly twice as long as Earth years. The team used observations from the Thermal Emission Spectrometer (TES) on Mars Global Surveyor between July 1999 and October 2004, which corresponds to three martian years the team studied one of the characteristic spectral features of methane in nearly 3 million TES observations, averaging data together to eliminate noise.
“Our study is the first time that data from an orbiting spectrometer has been used to monitor methane over an extended period. The huge TES dataset has allowed us to follow the methane cycle in the martian atmosphere with unprecedented accuracy and completeness. Our observations will be very useful in constraining the origins and significance of martian methane,” said Fonti.
Methane was first detected in the martian atmosphere by ground based telescopes in 2003 and confirmed a year later by ESA’s Mars Express spacecraft. Last year, observations using ground based telescopes showed the first evidence of a seasonal cycles.
All this and more will be answered beginning this August when Curiosity rover lands in the Gale Crater site. We're placing our bet on methane on Mars.
Because of its history, 96-mile wide Gale Crater crater with its strangely sculpted mountain --three times higher than the Grand Canyon is deep--is the ideal place for Curiosity to conduct its mission of exploration into the Red Planet's past. Joy Crisp, MSL Deputy Project Scientist from NASA's Jet Propulsion Laboratory, explains:
"This may be one of the thickest exposed sections of layered sedimentary rocks in the solar system. The rock record preserved in those layers holds stories that are billions of years old -- stories about whether, when, and for how long Mars might have been habitable."
The Daily Galaxy via New York Times and the European Planetary Science Congress Journal reference: Icarus, DOI: 10.1016/j.icarus.2010.11.027
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