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For decades, Mars was the default answer to the question of where life might exist beyond Earth. Its dry riverbeds and ancient lake basins made it an obvious target. Yet the strongest case for liquid water today lies much farther out, beneath the frozen shells of moons around Jupiter and Saturn.
The ice on these worlds is thick. Estimates vary by moon, from a few miles in places to well over 20 miles in others. Beneath it, several moons appear to hold oceans that may have been stable for billions of years, and that’s a better setup than a cold, dry planet whose surface water vanished long ago.
Europa: The Ocean With a Spacecraft on the Way
Jupiter’s moon Europa is thought to hold a global saltwater ocean under an ice shell roughly 10 to 15 miles thick, though estimates differ. The ocean itself may be dozens of miles deep, which could mean it holds more water than all of Earth’s oceans combined. Hubble observations have also hinted at occasional water vapor plumes, though those detections remain debated.
NASA’s Europa Clipper launched in October 2024 and is expected to reach Jupiter in 2030. It won’t land or drill. Instead, it will make dozens of close flybys to measure the ice thickness, the ocean’s salinity and the chemistry of the surface.
Enceladus: Samples Already Flying Out Into Space
Saturn’s small moon Enceladus has an unusual advantage, because it sprays its ocean into space. Cassini flew through those plumes and found water, salts, and organic molecules. Later analysis of the ice grains also detected phosphates, an element essential to life as we know it.
The ice shell is thicker than 12 miles in many regions but far thinner near the south pole, where the jets erupt. Newer work published in 2025 reported fresh complex organic compounds in ice grains from a close Cassini pass. That doesn’t prove biology, yet it shows the ocean’s chemistry is rich.
Ganymede: A Giant With Layers of Ice and Water
Ganymede is the largest moon in the solar system, bigger than Mercury. It’s the only moon known to generate its own magnetic field. Magnetic measurements and aurora observations suggest a salty ocean buried deep inside.
Models point to a very thick crust, possibly over 100 miles, with ocean layers sandwiched between different forms of ice. Some scientists worry that such layering could limit the exchange of nutrients with rock. The European Space Agency’s JUICE probe launched in 2023 and is scheduled to arrive at Jupiter in 2031, with Ganymede as its main focus.
Titan: Lakes of Methane and a Hidden Water Layer
Titan stands apart because it has a thick atmosphere, rain, rivers, and seas made of liquid methane and ethane. Beneath this strange surface, Cassini gravity data indicated an internal water layer. That has long made Titan a candidate for two kinds of chemistry, one at the surface and one underground.
The picture has grown less certain. A 2025 reanalysis of Cassini measurements suggested the interior may be slushy rather than a wide open ocean, though pockets of liquid water could still exist. NASA’s Dragonfly rotorcraft, planned for launch in 2028, is meant to explore Titan’s surface chemistry after arriving in the mid 2030s.
Callisto: The Quiet Candidate Far From Jupiter’s Radiation
Callisto is the outermost of Jupiter’s large moons, and it’s heavily cratered and geologically quiet. Magnetic readings from the Galileo spacecraft hinted at a conductive layer below the surface, which many researchers interpret as a salty ocean. If it exists, it likely lies under a crust more than 60 miles thick, making it the deepest of this group.
Callisto’s distance from Jupiter means it receives much less radiation than Europa does, which makes it easier for future missions to study. Its ocean is less certain, however, and it probably gets less heat from tidal flexing. For that reason, it ranks as a longer shot than the others, though still a plausible one.
Why Mars Falls Behind
Mars certainly had liquid water in the distant past, and that’s the main reason it remains an exciting target. NASA’s Perseverance rover reported in 2025 on unusual spotted rock features in an ancient riverbed called Cheyava Falls. The team called them a potential biosignature, but the chemistry could also have non biological explanations.
Today, however, Mars’s surface is cold and dry, with a thin atmosphere and strong radiation. Any life would probably need to be underground, and no liquid water has been confirmed there. The icy moons, by contrast, are thought to have oceans right now, which is why many researchers rank them higher for present day habitability.
Where the Search Stands
No life has been found on any of these worlds, and none of the evidence above counts as proof. Europa Clipper, JUICE, and Dragonfly are all still en route or awaiting launch as of late 2026. Their first major results are expected between 2030 and the mid 2030s.
