On Earth, we take methane for granted. It’s produced by living creatures – such as cattle’s digestive systems and bacteria munching through our refuse in landfills – and geological activity (such as volcanoes and other geothermal processes). In fact, it’s also a rather undesirable byproduct from industry and, if global warming trends continue, we could see vast amounts of the greenhouse gas belching out from the seabed and Arctic permafrost before this century is out. In short, too much methane is very bad for the future health of our planet.
On Mars, however, the presence of methane in the atmosphere is puzzling at best. In 2003, scientists using ground-based observatories to zoom in on the Red Planet detected small quantities (in parts per billion quantities) of methane in the carbon dioxide-rich atmosphere. Not only was the gas found to be globally distributed, in 2009, observations showed that it changed with time.
Intriguingly, methane’s molecular bonds break down very quickly when exposed to ultraviolet light – a form of electromagnetic radiation that Mars’ atmosphere is bathed in. Therefore, Mars methane shouldn’t hang around for long. In fact, from production to chemical break-down, methane should last for approximately 300 years in the Martian atmosphere.
The researchers found the gas actually breaks down far quicker than that. The atmospheric methane appears to cycle within one year – meaning that not only does the planet have a voracious methane production mechanism; it also has a voracious methane destruction mechanism. This mechanism can’t only be caused by ultraviolet light, so there must be something else destroying the methane.
“Mars is not a dead planet,” declared Michael Mumma, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead scientist of the Mars methane findings, during a Discovery News interview in 2009. Indeed, there is little indication that Mars has anything going on inside – geologically or otherwise. There’s no major geological activity that we know of and its volcanoes were snuffed out hundreds of millions of years ago. But the presence of atmospheric methane, and its rapid replenishment, suggests something is going on below the surface.
Follow the methane
During follow-up studies in 2010, Italian researchers using six-years of data from NASA’s Mars Global Surveyor (that was lost in 2006) added some fascinating detail to the Mars methane mystery. They found that methane levels seemed to increase rapidly through the summer months, peak during the autumn and then drop drastically during the winter.
Also, there appears to be several key regions in the Martian northern hemisphere that produce concentrations of the gas: Tharsis and Elysium (locations of Mars’ massive volcanoes), Arabia Terrae (a large upland region that is known to contain large quantities of water ice) and Nili Fossae (a fracture partly filled with sediment). This seasonal variation and discrete regions of methane production is a quandary.
|The global summer distribution of Mars methane using NASA’s Mars Global Surveyor data [NASA]|
Complementary studies of methane in the Martian atmosphere supports the evidence, but it is unclear as to what could possibly be generating, and destroying, the gas.
In the hope of supporting the satellite measurements, NASA’s Mars Science Laboratory (MSL) landed inside Gale Crater near the planet’s equator on August 5 (PDT), 2012. The MSL rover, called Curiosity, has been showing us a never-before seen region of the Red Planet.
It landed on a plain called Aeolis Palus (which is now known to have been an ancient riverbed) near a 3-mile high mountain called Aeolis Mons (nicknamed Mt Sharp by the MSL team) located in the centre of the crater.
Its two-year primary mission is to investigate the geology of the region, making its way toward Aeolis Mons, uncovering potential habitats for past and present microbial life. So far, the science carried out by the one-tonne, nuclear powered robot has been nothing less than historic, but one tantalising question hangs over the mission: Can Curiosity “smell” methane in the air?
Taking a sniff
On November 2, mission scientists had a partial answer. Much to the disappointment of scientists and journalists listening into the NASA press conference, the answer (for now) is: no.
During preliminary tests of the mission’s Sample Analysis at Mars (SAM) Tunable Laser Spectrometer (TLS) instrument, the rover drew a blank. It detected no sign of the gas in a “gulp” of Mars air it allowed into the instrument. Before it could do this test, however, mission managers instructed the rover to “flush out” the instrument, dumping any residual Earth atmosphere from SAM – apparently it did detect trace amounts of the Earth methane contamination, but after the flushing exercise, with just Mars air, no methane (within experimental limits) was detected.
“Methane is clearly not an abundant gas at the Gale Crater site, if it is there at all. At this point in the mission we’re just excited to be searching for it,” said SAM TLS lead Chris Webster of NASA’s Jet Propulsion Laboratory in Pasadena, California. “While we determine upper limits on low values, atmospheric variability in the Martian atmosphere could yet hold surprises for us.”
Although the vast majority of questions from the media focused on methane, the first run of the SAM TLS instrument did turn up some other fascinating findings about the history of Mars’ atmosphere. While measuring the isotopic ratios of atmospheric gases, Curiosity corroborated our ground-based and satellite observations of massive atmospheric loss to space.
|Curiosty snaps a self-portrait of itself using its robotic arm-mounted MAHLI camera [NASA/JPL-Caltech]|
During Mars’ evolution, one theory suggests the small planet was whacked by a massive asteroid or planetary body. The impact had a deep effect, disrupting the planet’s core, “switching off” its planetary magnetic field.
On Earth, we have a global geomagnetic field (the magnetosphere) that not only deflects the Sun’s ionising radiation; it also prevents widespread erosion of our atmosphere to space. Mars isn’t so lucky.
Although it appears to have small pockets of magnetic structures over its surface, the Martian atmosphere is pretty much unprotected, allowing the solar wind to blast it into space. This is one of the reasons why scientists believe Mars has an atmosphere 100 times thinner than Earth’s – it’s leaking into space.
Curiosity has now provided (preliminary) surface results that support this atmospheric erosion theory. During the TLS’ test run, it detected larger quantities of heavier isotopes of the carbon held in carbon dioxide molecules and heavier argon isotopes.
During the formation of the Martian atmosphere, a higher fraction of lighter isotopes would have been present. Why is there an enrichment of heavier atmospheric isotopes at the current stage in Mars’ evolution?
The erosion of the upper atmosphere caused by the solar wind will “blow away” the lighter isotopes. The isotopic ratios will therefore start to favour the heavier isotopes as they preferentially exist at lower altitudes.
On the first run of the TLS, we’ve gained a huge insight to billions of years of Mars atmosphere evolutionary history, but what of the Mars methane mystery?
This is just the beginning. Curiosity mission scientists will fine-tune their instruments and continue to command the rover to sniff the air. If the methane distribution of the planet is as variable and discrete as other observations suggest, it could just be that the rover is exploring a tiny region of Mars deplete of methane.
Could it be that the methane destruction mechanisms in this particular location are more efficient? It is now known that the Martian soil contains oxidising agents (such as perchlorates and hydrogen peroxide), perhaps these oxidising chemicals formed in the atmosphere and diffused into the soil. This would help explain why the molecular bonds of methane break down so rapidly in the atmosphere.
Also, during Friday’s NASA press conference, it was suggested that the massive electric field generated by Martian dust storms and dust devils could break down atmospheric methane (although others suggest methane could be generated from such phenomena). Or perhaps the wind is blowing in the wrong direction? Or perhaps the mystery methane production mechanism is “switched off” at this time of year? Any number of unknowns will affect the results of this study.
But it’s difficult not to ponder: Is there a colony of bacteria holed up below the surface, under the rover’s six wheels, slowly metabolising minerals and potential reservoirs of liquid water? Will their gaseous waste products (aka microbial farts) reveal their presence? Are methane-eating bacteria complementing their microbial cousin’s metabolisms, removing this methane byproduct from the soil altogether?
Who knows, over the coming months and years of this exciting, long duration geological mission to Gale Crater, Curiosity may begin to unravel not only the past biological potential, but also the current biological potential of the Red Planet.
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