Habitability Factors
What a world needs to host life — liquid water, energy, chemical building blocks, and the extremophiles that widen the possibilities.
- Chemical Building BlocksHabitability factor
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.
- Energy SourceHabitability factor
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.
- ExtremophilesHabitability factor
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.
- Liquid WaterHabitability factor
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.
- Subsurface OceanHabitability factor
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.