08/06/2018

Seasonal variations of Martian methane unveiled


Record of methane concentration on Mars' atmosphere
Credits: NASA

A new study published by Science magazine and co-authored by Javier Martín-Torres and María Paz Zorzano, of the Group of Atmospheric Science, has established a clear seasonal pattern in the variations of methane concentrations within the Martian atmosphere. This discovery means a step forward to unveil the mystery of the presence of methane on Mars, one of the biggest conundrums to be faced in the planet’s exploration field.

The existence and variability of traces of methane on the Martian atmosphere has been a puzzling matter since its first remote detection from Earth-based telescopes. After almost 6 years of operation on Mars, the Curiosity rover has managed to get unambiguous measurements of the concentration and variability of methane in the Martian atmosphere, close to the surface of the planet. The measurements show a repetitive pattern over seasons, with a background of a low amount of methane that varies through seasons, and then some sporadic “plumes”, which are high emissions of methane.

This variability opens a new question: the need to describe the unknown mechanisms that determine its generation, and those that act as sinks favouring its later quick disappearance.

On Earth, up to 95% of the methane present in the atmosphere comes from metabolism, with the remaining 5% stemming from volcanic activity and other geochemical processes. Nevertheless, on Mars the source(s) and sink(s) of atmospheric methane are both unknown, and hypotheses including biological and non-biological mechanisms are taken into consideration to explain the observe behaviour of the gas.

Diagram of possible sources and sinks of methane
Credits: NASA / JPL / SAM-GSFC / U. Michigan

Until now the registered Curiosity observations of methane were fragmentary both spatially and temporarily, so it was not possible to fix an underlying trend in the variations. Now that Curiosity has collected sporadically values of atmospheric methane concentration during a span of nearly 3 Martian years, it has been possible to draw an overall depiction of the methane behaviour at the Gale crater.

The results show that there is a clear seasonal variation in the amount of atmospheric background amount of methane, measured in particles per billion in volume (ppbv), ranging from 0.24 ppbv on winter to 0.65 ppbv near the end of northern summer, what yields a mean concentration of 0.41 ppbv throughout the Martian year, with some unexplained peaks of up to 7 ppbv.

The study correlates the methane variations with other environmental parameters, what could give clues to identify the processes responsible for the release of methane. For instance, they are consistent with the existence of small localized sources on the surface or in the subsurface from which the methane is released. These sources could be clathrates producing micro-seepages when the environmental conditions are favourable along the sequence of seasons, constrained by some retaining process linked somehow to the surface temperature. Also, it has been concluded that the production of methane from exogenous materials (i.e. dust, micrometeorites, or cometary debris containing organics) would not be consistent with the seasonal background values variations.

Hence, some keys of the mystery have been cleared out, but there are still many questions to be answered. Neither a geological nor a biological origin has been discarded, so we will have to wait attentively for further investigations, that doubtlessly will bring amazing new discoveries.