Considering the audience of this site, many of you are likely aware that NASA has found methane on Mars. I've heard many people saying that this indicates that life is significantly more likely to have occurred on Mars. However, we also know that many terrestrial bodies have methane, sometimes even in considerable amounts in their atmosphere. My question is by how much does this finding increase the probability of finding life or remnants of life on Mars if at all and what other significance does this hold for future space exploration?
On Earth, pre-anthropogenic biological activity accounted for some ~85% of methane emissions, the majority of which was from the decomposition of dead plant material in wetland habitats by anaerobic bacteria.
In modern times, an even higher proportion of emissions comes from biological sources, due to our extensive use of agriculture and fossil fuels.
Detection of traces of methane in extraterrestrial atmospheres is not on its own clear proof of biological activity as relatively large amounts of methane can also be produced by geological processes and later released into the atmosphere through volcanoes, marine vents etc. Another methane source on Mars may be impacting meteors.
However, due to Mars's lower gravity and weak magnetosphere, it is believed that any atmospheric methane would be short lived, being quickly destroyed by photochemical processes.
Since it appears that Mars currently has very little geological activity, we can infer that any geologically-produced methane has long since dispersed and therefore any detected methane has a higher likelihood of being from biological sources - at least that's the hypothesis.
It's worth noting that the presence of biologically-produced methane doesn't imply that life currently exists on Mars, just that it may have done in the past.
Furthermore, an important distinction to make is that any observation does not change the likelihood of finding life on Mars - it's either there or it's not! All it can do is change how confident we are that we will or won't find life there in the future - it affects our expectations, but not the outcome.
This answer has a lot of good detail and discussion on the specifics of the most recent (2018) publications.
If atmospheric methane on Mars is found to be a strong indicator of extinct or extant life, it will directly affect our procedures with regard to future exploration. Protection of potential extraterrestrial life is a high priority.
If found in large enough quantities or from a reliable source, atmospheric methane could be used as propellant in future missions. See SpaceX's in-development Raptor engine as an example. (Note: it's more likely that methane for this purpose would be synthesised from other naturally occuring chemicals, see Sabatier Process)
The recent (June 2018) "Background levels of methane in Mars’ atmosphere show strong seasonal variations", Webster et al., Science 360, 1093–1096 (2018) paper (might be paywalled, sorry) found that the background level of methane in Mars' atmosphere is very small:
The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv).
0.4 ppbv corresponds to just a couple of grams of methane per cubic kilometer of atmosphere. If correct, that means nobody is getting rocket-fuel methane directly from the atmosphere of Mars anytime soon; some process will be needed to generate it from other resources.
As to evidence of life: Earth's pre-industrial level was about 640 ppb in a much denser atomsphere. If this is from life on Mars, it's not much of a life. The paper goes to some trouble to consider other causes.
They also found much-larger pulses of methane that are harder to explain, either biologically or via other means: Geology, surface chemistry, even impact by methane-being meteors have been considered.
The conclusion paragraphs show how complicated this is, and how little is really completely known yet:
With ancient atmospheric pressures of several hundred millibars (40), large amounts of methane may be stored in the cold martian subsurface as clathrates in a stability zone several times thicker than that of Earth (41–43). Although the seasonal signature of the TLS-SAM measurements is not consistent with direct clathrate release, clathrates may provide a source of surface microseepage (diffuse exhalations without any specific morphological structure that may vent from outcropping of rocks or river or lake beds) (43–45). On Mars, such seepage would occur preferentially through permeable pathways, such as faults, fractures, or in breaches in sealing lithologies; this would not require identifiable geomorphological structures on the surface. Weak microseepage exhalations could explain back- ground and plume methane anomalies observed on Mars (43), perhaps near the dichotomy boundary and at Gale crater, where there is fractured sedimentary rock. Microseepage flux may vary over time, depending on variations of gas pressures along the subsurface migration pathway or on seasonal changes in the soil, or even where microbial activity may consume methane.
Regardless of the subsurface origin, methane that finds its way to surface layers over long time periods (42, 43) may be expected to show seasonal variation. We consider a process that retains methane at the surface temporarily before releasing it through a process linked to the surface temperature. That process could be adsorption on a surface with a high surface area– to-volume ratio, such as dust or soil. Although mineral dust cannot serve as a methane sink, it can moderate the release (11, 12). Adopting an energy barrier of ~20 to 35 kJ/mol—which is somewhat higher than that reported for the physical adsorption of methane into clays (46), zeolites (47), and Mars analog soil (12)—we found that large seasonal variations are expected (fig. S41). Plausible correlations of the background methane values with atmospheric water vapor and with surface temperatures point to physical or chemical surface (or dust) processes, or micro- seepage release. The amplitude of the seasonal cycle indicates that there remain unknown atmospheric or surface processes occurring in present-day Mars.