Methane plume observations on 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 Trace Gas Orbiter might help 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. Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization. . However abiotic sources can sometimes have similar results, and the ratios can be modified in various ways after formation. The TGO can also measure concentrations of ethane which may help with the analysis (very low in most microbial gases), but this also is not conclusive. It's ability to localize the measurements of plumes to regions of Mars may help but it is likely any results are preliminary and to need later work to interpret them.

Here carbon 12 is the light stable isotope of carbon which gets taken up preferentially by biological processes through. 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.

Challenges for interpreting carbon 12 /13 ratios from TGO
The carbon 13 can be depleted by abiotic proceses. It can also be depleted in methane produced by heating organics (thermogenic methane) and meanwhile, sometimes methane produced by life is not depleted in carbon 13. It also depends on the isotopic composition of its precursor. Magmatic carbon, for instance, might be depleted in carbon 13. Also processes that alter the methane such as oxidation by hydrogen peroxide can deplete the carbon 12 turning a possible microbial signature into one that looks abiotic. Meanwhile diffusion through permeable rock can increase the carbon 12 levels to mimic a biogenic signature - this could be the dominant process if the methane has to pass through an almost impenetrable "cap" on its way to the surface.



The NOMAD instrument on Mars can measure the concentrations of other gases such as ethane. This could help as microbial gases typically have ethane concentrations of a thousandth or less.

So high concentrations of ethane could suggest abiotic origins. However the ratios could be different for Martian microbes, and it could be due to ancient organics degraded by temperature. Meanwhile there are also processes that can deplete the ethane on its way to the surface, through molecular fractionation and through oxidizing of the gas once it reaches the atmosphere.

TGO can localize the measurements to regions of Mars, so if the gas has higher concentration in plumes, locally then the geology may also help with interpretation.

They conclude:

"In conclusion, there  will  be  a  considerable  degree  of  uncertainty  regarding  the  origin  of  any  methane   detected   by   NOMAD.   Interpreting   methane-ethane  data  will  not  be  easy,  and  probably  there  will  be  more  questions  than  answers. Atmospheric and geological analysis can add  insight  into  gas  origins,  but  in  future  missions,  direct  gas  detection  in  the  Martian  sub-soil, coupled with a better knowledge of subsurface  geology  (type  of  rocks,  permeability,  temperatures)  should  reduce  the  interpretative  uncertainties."