: Kevin M. Gill/flickr)

For as long as humanity has been watching the skies, we’ve been fascinated with the possibility that other worlds — much like Earth — might contain living organisms. While our visits to the Moon taught us that it’s completely barren and uninhabited, other worlds within our Solar System remain full of potential. . Europa and Enceladus might have life teeming in a sub-surface ocean of liquid water. Even Titan’s liquid hydrocarbon lakes provide a fascinating place to search for exotic living organisms.

But by far, the most fascinating possibility is the red planet: Mars. This smaller, colder, more distant cousin of Earth most certainly had a wet past, where liquid water clearly flowed on the surface for more than a billion years. Circumstantial evidence has pointed to the plausibility of life on Mars, not only in the ancient past, but possibly still living, and perhaps occasionally active, even today. There are five possibilities for life on Mars. Here’s what we know so far.