As far as we can tell, the surface of Mars is lifeless. But there are hints that microscopic organisms might live below the surface. One of the strongest hints is occasional puffs of methane gas detected by the Curiosity rover, which has been scooting around Mars for more than a decade.

On Earth, methane is mainly produced by life. It can come from volcanoes and from chemical reactions in the rocks. But there are no active volcanoes on Mars, and chemical reactions seem unlikely to produce big puffs of the gas.

Curiosity’s instruments have detected several outbursts of methane. They’ve all happened at night, and most have taken place in late spring or early summer, when the ground is warming up. A few of the outbursts have been especially potent.

The methane could come from microbes far below the surface, perhaps living in small pockets of water. Such organisms have been found on Earth. They combine hydrogen and carbon molecules to make methane. The methane could be trapped in bubbles that are freed when the ground gets warmer, or when the rover cracks the surface above them – releasing methane into the atmosphere.

That’s only one possible explanation for the methane. But it provides a hint that Mars could be inhabited by “gassy” microbes.

Mars is shining at its best this month. It looks like a brilliant orange star. It’s low in the eastern sky at nightfall, and remains in view all night. More about Mars tomorrow.

Script by Damond Benningfield