Thanks to more than a decade of robotic studies, the surface of Mars is becoming a place as familiar to some of us as similar garden spots on Earth such as the Atacama Desert in Chile, or Devon Island in Canada. But this rust-colored world still has some tricks up its sleave!
Back in 2003, NASA astronomer Michael Mumma and his team discovered traces of methane in the dilute atmosphere of Mars. The gas was localized to only a few geographic areas in the equatorial zone in the martian Northern Hemisphere, but this was enough to get astrobiologists excited about the prospects for sub-surface life. The amount being released in a seasonal pattern was about 20,000 tons during the local summer months.
The discovery using ground-based telescopes in 2003 was soon confirmed a year later by other astronomers and by the Mars Express Orbiter, but the amount is highly variable. Ten years later, the Curiosity rover also detected methane in the atmosphere from its location many hundreds of miles from the nearest ‘plume’ locations. It became clear that the hit-or-miss nature of these detections had to do with the source of the methane turning on and off over time, and it was not some steady seepage going on all the time. Why was this happening, and did it have anything to do with living systems?
On Earth, there are organisms that take water (H2O) and combine it with carbon dioxide in the air (CO2) to create methane (CH3) as a by-product, but there are also inorganic processes that create methane too. For instance, electrostatic discharges can ionize water and carbon dioxide and can produce trillions of methane molecules per discharge. There is plenty of atmospheric dust in the very dry Martian atmosphere, so this is not a bad explanation at all.
This diagram shows possible ways that methane might make it into Mars’ atmosphere (sources) and disappear from the atmosphere (sinks). (Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan)
Still, the search for conclusive evidence for methane production and removal is one of the high frontiers in Martian research these days. New mechanisms are being proposed every year that involve living or inorganic origins. There is even some speculation that the Curiosity rover’s chemical lab was responsible for the rover’s methane ‘discovery’. Time will tell if some or any of these ideas ultimately checks out. There seem to be far more geological ways to create a bit of methane compared to biotic mechanisms. This means the odds do not look so good that the fleeting traces of methane we do see are produced by living organisms.
What does remain very exciting is that Mars is a chemically active place that has more than inorganic molecules in play. In 2014, the Curiosity rover took samples of mudstone and tested them with its on-board spectrometer. The samples were rich in organic molecules that have chlorine atoms including chlorobenzene (C6H4Cl2) , dichloroethane (C2H4Cl2), dichloropropane (C3H6Cl2) and dichlorobutane (C4H8Cl2). Chlorobenzene is not a naturally occurring compound on Earth. It is used in the manufacturing process for pesticides, adhesives, paints and rubber. Dichloropropane is used as an industrial solvent to make paint strippers, varnishes and furniture finish removers, and is classified as a carcinogen. There is even some speculation that the abundant perchlorate molecules (ClO4) in the Martian soil, when heated inside the spectrometer with the mudstone samples, created these new organics.
Mars is a frustratingly interesting place to study because, emotionally, it holds out hope for ultimately finding something exciting that takes us nearer to the idea that life once flourished there, or may still be present below its inaccessible surface. But all we have access to for now is its surface geology and atmosphere. From this we seem to encounter traces of exotic chemistry and perhaps our own contaminants at a handful of parts-per-billion. At these levels, the boring chemistry of Mars comes alive in the statistical noise of our measurements, and our dreams of Martian life are temporarily re-ignited.
Meanwhile, we will not rest until we have given Mars a better shot at revealing traces of its biosphere either ancient or contemporary!
Check back here on Thursday, March 2 for the next essay!