Here on Earth, atmospheric methane is produced mostly by animals and plants, with the amount varying seasonally. Now measurements taken by the Curiosity rover indicate that methane concentrations on Mars also change with the seasons. The observation was first reported last month by Chris Webster of JPL and colleagues, and has already sparked heated discussions in the scientific community as to what might be the cause.
If extraterrestrial astronomers were to view our planet from afar, the presence of methane would likely be considered an indication of life, since it has to be replenished continuously to be detectable. The situation is similar on Mars. A methane molecule there has an average “life expectancy” of less than 400 years before being destroyed by ultraviolet radiation and chemicals in the atmosphere. To persist on Mars, methane is somehow being actively replenished.
Martian methane has been detected by telescopes on Earth, by spacecraft orbiting Mars, and by the Curiosity rover on the Martian surface. So there’s no doubt it exists—even though the concentrations are much smaller than in Earth’s atmosphere.
Several explanations have been advanced for these observations. Some scientists think the methane comes from biological sources, either from still living microbes, or as a remnant signature of life processes from the planet’s wetter ancient past. If they’re correct, that would mean life has existed on Mars, and probably still does today, in some environmental niches. But how to prove this, short of a dedicated life-detection mission? A variety of inorganic sources have also been proposed for the methane, but the challenge there is to reach the levels of concentration found in the Martian atmosphere, which translates to thousands of tons.
The new observation of seasonality supports the biological hypotheses. Life on Earth has a distinct seasonal signature, thriving in the warm summer months and going dormant in the winter months, particularly in the colder latitudes. The same should be true for Mars. There are also chemical processes that exhibit seasonal variations, but according to Webster and his colleagues, the observed signal is about three times greater than would be the combined effect of these inorganic processes (such as methane production from ultraviolet degradation of meteoritic material, or sublimation and freezing of carbon dioxide at the poles).
The previous discovery of methane plumes already hinted toward biology, and the seasonality of the methane concentrations observed over the last five years (2.5 Martian years) only strengthens that possibility. However, more evidence is needed. And that we might get with ESA’s ExoMars Trace Gas Orbiter which is expected to begin its study of the Martian atmosphere this spring.