Ocean Worlds: The Most Likely Places to Find Life in Our Solar System

The search for life in our solar system used to focus almost entirely on Mars. The logic was simple: Mars has surface features that suggest ancient liquid water, a thin atmosphere, and a day length similar to Earth's. It remains a compelling target. But the discovery of subsurface liquid water oceans on multiple icy moons has dramatically expanded the list of potentially habitable worlds — and some of the new candidates may be more promising than Mars.

Europa: The Gold Standard

Jupiter's moon Europa has a global subsurface ocean containing roughly twice the volume of water as all Earth's oceans combined, kept liquid by tidal heating from Jupiter's immense gravitational influence. The ocean sits beneath a 10-30 km ice shell and may be in direct contact with a rocky seafloor — the same configuration that supports hydrothermal vent ecosystems on Earth.

NASA's Europa Clipper mission, launched in October 2024, will conduct dozens of close flybys of Europa to characterize the ice shell, the ocean, and plume activity. ESA's JUICE mission will also study Europa as part of its Jupiter system tour.

Enceladus: The Active Geysers

Saturn's small moon Enceladus is actively venting its ocean into space through geysers at its south pole. Cassini flew through these plumes and detected water ice, organic molecules, molecular hydrogen (a potential energy source for life), and silica particles consistent with high-temperature hydrothermal activity at the ocean floor. Enceladus is arguably the most compelling astrobiological target in the solar system precisely because its ocean is sampling itself for us.

Titan: The Alien Chemistry Lab

Titan is unique: the only moon with a thick atmosphere and the only world besides Earth with stable surface liquids. Its lakes and seas are liquid methane and ethane rather than water. Exotic chemistry — possibly including chemistry capable of supporting life using different solvents than water — may be occurring in this cold, complex environment. NASA's Dragonfly mission, a rotorcraft lander, will arrive at Titan in the 2030s to investigate.

Ganymede, Callisto, and Beyond

Ganymede (the solar system's largest moon) and Callisto both likely have subsurface liquid water oceans, though deeper and less accessible than Europa's. Subsurface oceans have also been proposed for Pluto, Ceres, and several other Kuiper Belt Objects. The solar system is far wetter than anyone suspected thirty years ago.