https://www.theregister.co.uk/2019/04/03/methane_on_mars/

The European Space Agency’s Mars Express spacecraft caught a massive whiff of methane from orbit, confirming an earlier gas detection on the Red Planet by America's Curiosity rover.

Signs of the hydrocarbon molecule rouses excitement because there is a possibility that it may be produced by biological processes, and therefore, some sort of alien life is out there. On Earth, the gas is created as a byproduct of anaerobic respiration by a group of ancient simple unicellular microbes, but it can also be created by geological processes too.