Previous research detected methane, the simplest organic molecule, in the martian atmosphere. While there are abiotic ways to produce methane — such as volcanic activity — much of this colorless, odorless, flammable gas in Earth’s atmosphere is produced by life, such as cattle digesting food.

“One of the exciting moments for me was the detection of methane in the martian atmosphere,” Mickol said. “On Earth, most methane is produced biologically by past or present organisms. The same could possibly be true for Mars. Of course, there are a lot of possible alternatives to the methane on Mars and it is still considered controversial. But that just adds to the excitement.”

On Earth, microbes known as methanogens produce methane, also known as natural gas. Methanogens typically live in swamps and marshes, but can also be found in the guts of cattle, termites and other herbivores, as well as in dead and decaying organic matter.

Methanogens are among the simplest and most ancient organisms on Earth. These microorganisms are anaerobes, meaning they do not require oxygen. Instead, they often rely on hydrogen for energy, and carbon dioxide is the main source of carbon atoms they use in creating organic molecules.

The fact that methanogens neither require oxygen nor photosynthesis means they could live just beneath the martian surface, shielded from harsh levels of ultraviolet radiation on the Red Planet. This could make them ideal candidates for life on Mars.

However, the area just below the surface of Mars is exposed to extremely low atmospheric pressures, normally considered inhospitable to life. The surface pressure on Mars on average ranges from one-hundredth to one-thousandth that of the surface pressure of Earth over the course of the martian year, too low for liquid water to last on the surface. In such thin air, water easily boils. (In contrast, the pressure at the highest point on Earth’s surface, the top of Mount Everest, is about one-third that of Earth’s surface pressure at sea level.)

To see if methanogens might survive such extremely thin air, Mickol and Timothy Kral, the senior author of the study and an astrobiologist at the University of Arkansas at Fayetteville, experimented with four species of methanogens. They included: Methanothermobacter wolfeii, Methanosarcina barkeri, Methanobacterium formicicum, and Methanococcus maripaludis. Previous experiments on these four species over the course of more than 20 years generated a lot of data on these organisms and their rates of survival in simulated martian conditions.