Water is everywhere. We think of it as a liquid, but throughout much of the Solar System, water has frozen like stone, forming the crystalline surfaces of moons, comets, and other stray bodies. Some icy moons, such as Europa and Enceladus, are known to contain layers of spherical liquid oceans deep within their frozen outer surfaces. Probably Ganymede, Callisto and Titan as well. How many other satellites are in the ocean club?

A new paper reanalyzing Voyager observations shows that all four of Uranus’s five icy moons contain oceans: Ariel, Umbriel, Titania, and Oberon. (Little Miranda, only medium-sized between Mimas and Saturn’s Enceladus, doesn’t seem like it.) The oceans are extremely thin: less than 30 kilometers (20 miles) inside Ariel and Umbriel (both about 1,000 kilometers wide, in size) Tethys) and Saturn’s Dione) and less than 50 kilometers (about 1,500 kilometers larger, similar to Saturn’s Rhea and Iapetus) on Titania and Oberon.

If there were oceans, they were the remnants of much larger layers of liquid that formed when the moons first formed. The remaining liquid will be quenched against the weakened heat of the moons’ rocky cores, protected by hundreds of kilometers of solid ice. They would be overly salty, overconcentrated with all the solutes helping to lower the temperature at which the water would otherwise freeze. There are two candidates: salt and ammonia.

We will need to send a spacecraft to validate these models. A salty ocean could be detected by a spacecraft equipped with a magnetometer, while an ocean with ammonia water could not be detected. But even the ammonia-water ocean is visible because the spherical liquid layer mechanically separates the ice sheet from the rocky core. Careful monitoring of the movement of the surface elements as the moons rotate in their elliptical orbits around Uranus can reveal that the outer layers of icy moons are separating from their cores.

We hope that we can confirm the results of this article. A ten-year survey of the scientific community by the National Academy of Sciences has revealed that an orbiter and probe for the Uranus system is the top science priority for the next new flagship mission, now returning to Mars and Europa with a mission to Jupiter continues. Read about the potential of the Uranus Orbiter and Probe mission in the July issue of the journal. Sky and Telescope will hit newsstands in a few weeks. How many icy moons contain oceans? The only way to know for sure is to visit them!