Four of Uranus’s five icy moons likely contain a thin layer of briny (or otherwise enriched) water, astronomers have concluded from Voyager 2 data.
Water is everywhere. We think of it as a liquid, but in most of the solar system, water is frozen as hard as rock, forming the crystalline surfaces of moons, comets, and other wandering bodies. Some icy moons, like Europa and Enceladus, famously host global layers of liquid ocean deep beneath their frozen exteriors. Ganymede, Callisto, and Titan likely do as well. How many other moons are in the ocean club?
A new paper re-analyzing Voyager observations suggests that four of Uranus’ five icy satellites also host oceans: Ariel, Umbriel, Titania, and Oberon. (Only small Miranda, intermediate in size between Saturn’s Mimas and Enceladus, appears not to.) The oceans are desperately thin: less than 30 kilometers (20 miles) thick inside Ariel and Umbriel (both of which are about 1,000 kilometers across, similar in size to Saturn’s Tethys and Dione), and less than 50 kilometers thick within Titania and Oberon (which are larger at about 1,500 kilometers, similar to Saturn’s Rhea and Iapetus).
If the oceans exist, they would be left over from much larger liquid layers that formed when the moons first formed. The remnant liquid would be snuggled close to the waning heat of the moons’ rocky cores, sheltered beneath hundreds of kilometers of solid ice. They’d be extremely briny, hyper-concentrated with whatever dissolved materials helped to lower the temperature at which water would otherwise freeze. There are two candidate materials: salt and ammonia.
To validate these models, we would need to send a spacecraft. A salty ocean would be detectable from a spacecraft equipped with a magnetometer, while an ammonia-water ocean would not. But even an ammonia-water ocean remains detectable, because a global layer of liquid would mechanically disconnect the icy mantle from the rocky core. Careful tracking of the motion of surface features as the moons nutate in their elliptical orbits around Uranus could reveal that the icy moons’ outer layers are decoupled from their cores.
There’s hope that we’ll get to test the conclusions of this paper. A once-a-decade survey of the scientific community conducted by the National Academy of Sciences determined that an orbiter and probe to the Uranus system is the top scientific priority for the next new flagship mission, now that the Mars Sample Return and a Europa-focused Jupiter mission are in process. Learn about the potential of the Uranus Orbiter and Probe mission in the July issue of Sky & Telescope magazine, on newsstands in a couple weeks. Do lots of icy moons host oceans? The only way to know for sure is to visit them!