Perhaps none of Mars’s methane is actually fresh. Methane can get trapped in permafrost and ice on Earth, and Mars has plenty of ice old enough to contain methane produced billions of years ago. “All you have to do,” says Oehler, “is stress the planet or change local conditions in some way that could destabilize the ice” and release the gas out to the surface. These rumbles could be caused by seismic activity, latent volcanic activity, meteor impacts—anything that can open up faults and breach the permafrost seal. Håkan Svedhem, project scientist for the TGO mission at ESA, also adds that shifts in temperatures triggered by seasonal or diurnal effects could open up a path for discrete concentrations of gas to move to the surface.