Planetary Habitability

The elements and molecules life is built from — carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, and complex organic chemistry. Titan has a rich organic chemistry, and Enceladus's plumes carry organic molecules from its ocean.

Life needs a source of energy to drive its chemistry — sunlight for photosynthesis, or chemical and thermal energy where the Sun cannot reach. Tidal heating warms the interiors of the icy moons, powering hydrothermal vents like those that may nourish life on Earth.

Earth organisms that thrive in extremes — searing vents, acid, salt, cold, and intense radiation — that once seemed impossible for life. They vastly widen where life might survive, and are the closest analogs for what an ocean-world biosphere might be.

The one requirement every known form of life shares — a liquid solvent for its chemistry. The habitable zone is defined by where a planet could hold liquid water on its surface, but subsurface oceans extend the possibilities far beyond it.

A layer of liquid water beneath an icy shell, kept warm by tidal heating and in contact with a rocky seafloor. Europa and Enceladus have them; they are the leading places to look for life beyond Earth, and the target of the Europa Clipper mission.