Methane plume observations on Mars

This image illustrates possible ways methane might be added to Mars' atmosphere (sources) and removed from the atmosphere (sinks). NASA's Curiosity Mars rover has detected fluctuations in methane concentration in the atmosphere, implying both types of activity occur on modern Mars.

Methane was detected in the Mars atmosphere for the first time in 2004. This stimulated follow up measurements, and research into possible biological or geological origins for methane on Mars.

If these measurements are valid, then there must be some source continually producing methane. Methane dissociates in the atmosphere through photochemical reactions - for instance it reacts with hydroxyl ions forming water and CO₂ in the presence of sunlight. It can only survive for a few hundred years in the Mars atmosphere.. The measurements were confirmed by theree separate teams at the time, and more recently Curiosity also detected short lived possibly local methane plumes on many occasions.

Hyptheses for the methane sources

There are three main hypotheses for sources for the methane

• 'Life in the form of methanogens (methane producing bacteria). These are autotrophs which require little more than hydrogen and carbon dioxide to metabolize. For the hydrogen source they could use a geothermal source of hydrogen, possibly due to volcanic or hydrothermal activity, or they could use the reaction of basalt and water. Methanogens have been found to be able to grow in Mars soil simulant in these conditions of water, CO₂ and hydrogen., and to be able to withstand the Martian freeze / thaw cycles.
• Subsurface rocks such as olivine chemically reacting with water in presence of geothermal heat in the process known as serpentization.
• Ancient underground reservoirs, or methane trapped in ice as clathrates (with the methane originally created by either of the other two methods)

Confirmation by Curiosity

The original remote observations from Earth needed confirmation by close up inspection on Mars. When Curiosity first landed, no methane was detected to the limits of its sensitivity (implying none is present at levels of the order of parts per billion).

However around eight months later, in November 2013, Curiosity detected Methane spikes up to 9 ppb. These spikes were observed in only one measurement (the measurements were taken roughly every month) and then dropped down to 0.7 ppb again. This happened again in early 2014.

This suggests a localized source to the researchers, since there is no mechanism known that could boost the global atmospheric levels of methane so quickly for such a short time. The leading hypothesis therefore is that a plume of methane gas escaped from some location not far from Curiosity and drifted over the rover, where it detected it.

However the nature of that source is currently unknown. It could as easily be due to inorganic sources as due to life.

Trace Gas Orbiter science mission will give more information

The ExoMars Trace Gas Orbiter may help to answer this question, as it will be able to detect trace gases such as methane in the Mars atmosphere using techniques that are about a thousand times more sensitive than any previous measurements. It started its science phase in April 2018.

Once it does these measurements, then the hope is that the results would have the resolution necessary to pinpoint the geographical locations of the sources on the ground. This could then be used to target rovers for later surface missions.

Carbon 12 / 13 ratios

One way to distinguish between biogenic and abiogenic sources of methane might be to measure the carbon-12 to carbon-13 ratio, which is expressed as a percentage relative to a reference standard δ13C (Wikipedia). Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization.

Here carbon 12 is the light stable isotope of carbon which gets taken up preferentially by biological processes through kinetic fractionation (Wikipedia). The energy costs are lower if the carbon in the organism uses the lighter isotope. Carbon 13 is also stable but not so much favoured by biology. (Techy note, this is not to be confused with carbon 14 dating - carbon 14 is radioactive and unstable. Carbon 12 and 13 are both stable and don't decay at all.)

Plants have values of from -10% or so down to -30% or less, clustering at around -13% and -28%. Algae have a similar range of values, from higher than -10% down to -30% or less. Coal and marine petroleum typically has values around -25%, terrestrial petroleum around -30%, and land plants average around -25%, but with a fair bit of variation around those figures. See figure 1 in this article.Park, Roderic, and Samuel Epstein. "Metabolic fractionation of C13 & C12 in plants." Plant Physiology 36, no. 2 (1961): 133.

However, sometimes abiotic methane can have carbon 13 depleted to as low as -50%, for instance in hydrothermal vents.