Scientists think at least two of Uranus's moons — Titania and Oberon — may harbour liquid water oceans beneath their icy crusts, kept warm not by the sun but by radioactive decay deep in their interiors. If true, they join a growing list of ocean worlds in the solar system

At nearly three billion kilometres from the Sun, the moons of Uranus receive little of the solar energy that warms Earth. Their surfaces sit at temperatures around minus 200 degrees Celsius, and neither Titania nor Oberon displays the dramatic plumes that made Saturn’s Enceladus an obvious ocean world.

Yet both moons may still contain layers of liquid water beneath their frozen crusts. Thermal models suggest that heat released by radioactive elements in their rocky interiors, assisted by insulating ice and dissolved antifreezes, could have prevented their deepest water from freezing completely.

If those models are right, Titania and Oberon would expand the solar system’s population of ocean worlds into the remote domain of the ice giants.

The strongest cases are the two largest moons

Titania and Oberon are Uranus’s largest moons, measuring about 1,580 and 1,520 kilometres across respectively. Each appears to contain a substantial fraction of rock as well as water ice. That matters because the rock can carry long-lived radioactive isotopes capable of producing heat for billions of years.

A conservative 2022 modelling study that deliberately excluded tidal heating found that Titania and Oberon could retain significant subsurface oceans to the present if heat escaped slowly enough through their ice shells. Smaller Uranian moons did not preserve long-lived oceans as readily under the same assumptions.

The result does not amount to a detection. No spacecraft has measured the moons’ gravity fields or searched for the electromagnetic signature of salty water. It shows that present oceans are physically plausible even at Uranus’s great distance from the Sun.

Radioactivity can warm a world from within

Rock naturally contains isotopes such as uranium, thorium and potassium-40. As their unstable nuclei decay, they release energy that becomes heat. The same process contributes to Earth’s internal warmth, although a small moon has less material and loses heat more readily.

Titania and Oberon may have enough rock to make the balance work. A thick outer shell can slow heat loss, while ammonia and salts lower water’s freezing point. The surviving liquid would probably be a cold, chemically rich brine rather than an Earth-like sea close to zero degrees Celsius.

A 2023 reconstruction of all five large Uranian moons concluded that Titania and Oberon could preserve oceans tens of kilometres thick. The study suggested possible residual oceans under 50 kilometres thick in the two largest moons, although the answer depends on uncertain composition, porosity and thermal history.

Voyager saw surfaces with complicated histories

Nearly everything known directly about these moons comes from Voyager 2’s brief Uranus fly-by in January 1986. The spacecraft observed only part of each world and returned images far less detailed than those later obtained at Jupiter and Saturn.

Titania showed immense faults and canyons, including structures that may have formed as its interior cooled and water froze, forcing the crust to expand. Oberon’s old, heavily cratered surface also contains fractures and dark material on some crater floors. These features record internal change, but they do not prove liquid survives today.

NASA’s current interior models depict possible ocean layers in Titania and Oberon, as well as Ariel and Umbriel. Miranda is generally considered too small to retain enough internal heat under the same model assumptions.

As many as four Uranian moons may hold water

The broader 2023 study moved Ariel and Umbriel onto the candidate list because improved laboratory data and thermal calculations allowed liquids to persist under more conditions than older models suggested. Both could have residual oceans less than about 30 kilometres thick.

Titania and Oberon remain the most robust candidates because their greater size helps them store heat. Later work has also explored whether tidal forces could add energy to several Uranian moons. A 2025 analysis found that ocean tides may provide additional heating under some interior and orbital conditions, so radioactive decay need not be the only source.

This uncertainty is important. Scientists are not claiming that four new oceans have been discovered. They are identifying interiors that remain consistent with the limited measurements available and defining observations that could distinguish frozen worlds from living ocean systems.

A Uranus orbiter could settle the question

An orbiter making repeated close passes could search for induced magnetic fields created when Uranus’s unusual magnetic environment moves across a conductive salty ocean. Precise tracking could reveal each moon’s gravity field, while cameras and spectrometers could map fractures, young deposits and compounds brought up from below.

Those measurements would also show whether the oceans contact rock, a key issue for chemistry and possible habitability. Liquid water alone does not establish that life could exist. Long-term energy sources, useful chemical ingredients and exchange between water and minerals would matter as well.

NASA’s science assessment for exploring the Uranian satellites therefore treats all five major moons as candidate ocean worlds worthy of close investigation. That category includes bodies with credible but unconfirmed liquid reservoirs, not only worlds where an ocean has been securely demonstrated.

Titania and Oberon illustrate why the catalogue keeps growing. Sunlight is not the only way to sustain liquid water. A moon that appears frozen and inert can carry its heat source inside, allowing radioactive atoms in ancient rock to preserve a hidden sea long after its surface has become unimaginably cold.