By Orbital Team
Mars had liquid water on its surface billions of years ago, which is why it became the default target in the search for past life. But past is the key word: any Martian surface life would have to have survived the planet losing most of its atmosphere and nearly all of its surface water.
Europa and Enceladus offer something different: liquid water right now, in enormous quantities, kept liquid not by sunlight but by tidal flexing as they orbit their much larger host planets. Europa is thought to hold roughly twice the liquid water of all of Earth's oceans combined, sealed beneath an ice shell.
Enceladus goes further, actively venting plumes of water vapor and ice grains into space through cracks at its south pole, plumes that spacecraft have already flown through and sampled directly, without needing to drill through any ice at all.
Those plume samples have already shown the ingredients that matter: organic molecules, and molecular hydrogen consistent with ongoing hydrothermal activity on the ocean floor, the same kind of chemical environment that supports life around hydrothermal vents on Earth's own sea floor.
Neither moon has provided evidence of life itself, and none is expected soon. What has changed is the ranking of candidates: a subsurface ocean in continuous contact with a rocky, chemically active floor is now considered a more promising habitability signature than a dry, cold surface that was briefly wet billions of years ago.