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Mars is full of the organic molecules that life needs to thrive. Two studies from the Curiosity rover found methane that seeps up from the ground in summer, and more complex organic molecules that have been preserved in the clay for 3.5 billion years.

Most molecules that contain carbon are classed as organic. We’ve found some organic molecules on Mars before, but none as complex as the ones that Curiosity found in a dried lake bed.

“These are complex organic molecules that we think were floating around in a lake on Mars over 3 billion years ago and that we can pick up and examine today,” says Kirsten Siebach at Rice University in Texas. “Mars has shown the ability to preserve organics, and we have shown the ability to find them.”

These molecules could have arrived on meteorites or arisen from volcanic processes – and there’s a chance they came from living organisms. Even if they didn’t come from life, they could have acted as food for microbes on ancient Mars. And the fact that they were preserved for such a long time means that any such microbes, if they ever existed, could potentially be similarly conserved for us to find.

“Because there is a possible connection between organic molecules and life, understanding where there are organic molecules and how they are conserved is very, very important as we go into figuring out how to search for life,” says Jennifer Eigenbrode at NASA’s Goddard Space Flight Center in Maryland, who led the team that found these organics.

Underground deposits

Another team also used Curiosity to examine methane, a simpler organic molecule, in the Red Planet’s atmosphere. We knew methane existed there, but when and where its abundances peaked seemed random. Chris Webster at NASA’s Jet Propulsion Laboratory in Pasadena and his colleagues found that there is a seasonal cycle, with more methane in the Martian skies in summer.

These cycles could help us figure out where the methane is coming from. “It’s a bit like having a problem with your car that’s intermittent. You can’t figure out what it is if it comes and goes,” Webster says. “You want it to be repeatable so you can track it down.”

The fluctuations they observed best fit a model where Mars’s methane is coming from deep underground and wending its way to the surface through pores and fissures. When it’s cold, the methane would stick to the surface dust, and a temperature change of a few degrees would release it up into the atmosphere.

How the underground methane may have got there is still a mystery. It’s even possible that the methane and the other organic material could be related. “You might have this organic material at depth, going through various processes and forming methane which vents to the surface,” says Eigenbrode. “Are there organisms that are doing those processes? Is it life?”

Like the more complex organics, the methane could be produced either biologically or geologically – it’s almost impossible to tell the difference, even on Earth. There is a laundry list of non-biological processes that could produce organic molecules on Mars. But life isn’t off that list yet.

Read more: We could detect alien life by finding complex molecules