In December exciting new data, including two new papers, from the Curiosity Rover team were released. The update focussed mainly on the detection of methane and other organic compounds by the rover. I’ll cover the other compounds in a more comprehensive post in February but in this article I’ll focus on methane and its importance in trying to understand Mars.
Methane is an exciting compound to detect because on Earth the primary source of this simple molecule is life. However, methane can also be produced by non-biological processes. Regular impacts from asteroids, comets and interplanetary dust particles are known to add organic compounds to the Martian surface. The atmosphere of Mars offers no protection from ultraviolet radiation and the breakdown of these organics by UV could cause methane release. In addition, reactions between water, certain minerals and dissolved carbon dioxide can produce methane. Ice can trap methane in its structure, store it and then release it upon melting, with ice at depth capable of being stable for billions of years. Therefore, if we detect methane on Mars it is indicative of some sort of active process occurring on the planet, whether it be biological, the melting of ice, water-rock reactions, volcanic activity, or as simple as degradation of meteoritic organic compounds by UV radiation. Life would certainly be the most exciting candidate on the list but the detection of methane does not necessarily mean the detection of life.
Mars has been scoured for methane many times before. Earth based telescopes and orbiting satellites have all suggested methane levels of around 10 parts per billion (ppb), some measurements even suggested localised sources and highly variable methane concentrations. However, there were many questions about whether the measurements were correct and many of the observations contradicted each other. When Curiosity Rover first landed and couldn’t measure methane at levels above its instrument’s detection limits it was a significant blow to those scientists that had argued that methane was present at detectable levels on Mars.
Curiosity is equipped with a laser spectrometer that can make unambiguous detections of methane on Mars, it was therefore enormously disappointing when the instrument at first could only set an upper limit for Martian methane (1.3 ppb). Scientists reprocessed all of the data Curiosity had collected and reduced the susceptibility of the processing algorithm to the temperature within the spectrometer. If the temperature change was too quick then the smaller the characteristic methane spectral peaks would be, and therefore the harder to analyse. When the team started the reprocessing there were now 605 Martian days (sols) of data to work through. During this time new experiments had been tested, including some which enriched any potential methane in the gas being sampled by removing the carbon dioxide that made up most of Mars’ atmosphere using a scrubber material within the rover.
With the team able to analyse data more sensitively they found methane at a low level of 0.7 ppb, a value likely representative of the mean background level for methane abundance on Mars. Most excitingly, during these 605 sols the rover also detected high methane levels of 7 ppb over a 60 sol period. A value that did not correlate with relative humidity, atmospheric pressure, ground or air temperature, inlet pointing or radiation levels recorded at the same time as atmospheric sampling. The methane levels than rapidly decreased back to background levels. The prevailing winds in Gale Crater at the time suggest a source to the Rover’s north.
Mars has an atmosphere that destroys methane so for it to be measured at all means there must be something continuously replenishing it. The fact that a stable background level and a transient high level of methane have been detected indicates there must be at least two sources. One long lasting and stable but producing low possibly global concentrations and one very brief and local but much stronger. The obvious question is what the likely sources are, the breakdown of meteoritic organic compounds by UV light is a realistic explanation for the stable low background levels. The high level is much more debatable, it could be melting of some ancient subsurface ice or water-rock reactions or perhaps life. The rover is now on the lookout for further methane spikes and if one is detected more sophisticated analyses may be able to pinpoint a source. What is certain is that Mars continues to prove that it is far from being a dead world, something active and unknown is occurring on the red planet, stay tuned and maybe we’ll find out soon what it is.
You can read the new paper here.