To understand why methane matters so much, you first have to understand what it is.

Methane is a simple molecule — a so-called hydrocarbon — composed of four hydrogen atoms stuck to one carbon atom. It has no natural smell or color. And it’s also common because hydrogen is the most abundant element in the cosmos, and carbon is the third most abundant.However, it’s also fragile. It can’t handle things too hot. And the oxygen and carbon dioxide in Earth’s atmosphere can break its bonds. So, on Earth, methane doesn’t last long in our atmosphere.And most of the methane that we do have is produced by biology. Things die and their hydrocarbons get trapped in stores deep underground or in permafrost, where it’s known as a clathrate.Living things also churn out methane, too. Cows and other livestock produce enormous amounts of the greenhouse gas. And simple lifeforms, known as methanogens, also produce methane.All this means that, on Earth, methane is a sign of life. That’s given astronomers good reason to see methane as a potential signal of microbes on Mars.Like Earth, it also has methane-destroying conditions. The Red Planet’s atmosphere is almost completely made of carbon dioxide. And even the ultraviolet light that penetrates Mars’ weak atmosphere could destroy it.So, any methane we do see must have been released into the atmosphere very recently.But life isn’t the only process that makes methane. We know that because it’s abundant on Uranus and Neptune. And there’s enough of the stuff to create bizarre landscapes on the surfaces of Pluto and Titan. And even on Earth, a small amount of methane is made in specific sorts of volcanic reactions, even if it doesn’t stick around long.But Mars has no active volcanoes. And it doesn’t have ways of replenishing methane like those outer solar system worlds.To truly find out what’s causing these seasonal surges of methane, we’ll need new Mars missions capable of better searching for definitive signs of life. And, thankfully, those spacecraft are already in the works. NASA’s Mars 2020 rover, which will launch in a couple years, is custom-built for this purpose. And the European Space Agency’s ExoMars rover should soon follow with similar aims.Whatever the final cause — whether microbes or natural chemistry — we’re now closer to finding the source of Mars’ methane than ever before.

This article originally appeared on discovermagazine.com.