Hidden oceans could be lurking deep within Uranus’ moons

Oceans may be dwelling in some of our solar system’s unlikely spots.
A NASA image of the planet Uranus with six of its 27 known moons, Ariel, Puck, Miranda, Umbriel, Titania, and Oberon, labeled.
This wider view of the Uranian system released on April 06, 2023 was taken with the James Webb Space Telescope’s NIRCam instrument features the planet Uranus as well as six of its 27 known moons (most of which are too small and faint to be seen in this short exposure). A handful of background objects, including many galaxies, are also seen. SCIENCE: NASA, ESA, CSA, STScI. IMAGE PROCESSING: Joseph DePasquale (STScI)

Uranus’ four largest moons could very likely be home to an ocean layer dozens of miles deep between their icy crusts and deep cores. A new analysis from NASA published in the Journal of Geophysical Research, could help determine how a future mission to Uranus might investigate the seventh planet from the sun’s moons, but also has implications that go beyond Uranus.

[Related: Expect NASA to probe Uranus within the next 10 years.]

At least 27 moons circle Uranus. The four largest are about two to three times smaller than  Earth’s moon, with Ariel at about 720 miles across and the largest, Titania, at 980 miles across. Titania’s size has long led scientists to believe that it is the most likely satellite to retain internal heat that is caused by radioactive decay. Uranus’ other moons were believed to be too small to retain the head that is necessary to keep an internal ocean from freezing since the heating created by Uranus’ gravitational pull is only a minor source of heat.  

This new analysis uses data from the Voyager 2 spacecraft and some new computer modeling looked at all of the planet’s five large moons: Ariel, Umbriel, Titania, Oberon, and Miranda. Of these large moons, Titania and Oberon orbit the farthest from Uranus, and these possible oceans could be dwelling 30 miles below the surface. Ariel and Umbriel may have oceans 19 miles deep. 

New modeling shows that there likely is an ocean layer in four of Uranus’ major moons: Ariel, Umbriel, Titania, and Oberon. Salty – or briny – oceans lie under the ice and atop layers of water-rich rock and dry rock. Miranda is too small to retain enough heat for an ocean layer. CREDITS: NASA/JPL-Caltech

“When it comes to small bodies – dwarf planets and moons – planetary scientists previously have found evidence of oceans in several unlikely places, including the dwarf planets Ceres and Pluto, and Saturn’s moon Mimas,” co-author and planetary scientist at NASA’s Jet Propulsion Laboratory Julie Castillo-Rogez said in a statement.  “So there are mechanisms at play that we don’t fully understand. This paper investigates what those could be and how they are relevant to the many bodies in the solar system that could be rich in water but have limited internal heat.”

The new study revisited the data from Voyager 2 flybys of Uranus during the 1980s and from more recent ground-based observations. The authors then built computer models using additional findings from NASA’s Galileo, Cassini, Dawn, and New Horizons missions (which all discovered ocean worlds), and insights into the chemistry and the geology of Saturn’s moon Enceladus, Pluto and its moon Charon, and Ceres. These Plutonian and Saturnian moons are all icy bodies about the same size as the Uranian moons.

The team used the modeling to gauge how porous the surface of the Uranian moons are, and found that they are likely insulated enough to retain that internal heat needed to host an ocean. Additionally, the models found a potential heat source in the moons’ rocky mantles. These sources release hot liquid that would help an ocean maintain a warm environment. This warming scenario is especially likely in the moons Titania and Oberon, where the oceans could  even be warm enough to support some sort of lifeforms. 

[Related: Ice giant Uranus shows off its many rings in new JWST image.]

Investigating the composition of these oceans can help scientists learn about the materials that may be found on the icy surfaces of the moons as well, depending on whether or not the substances underneath were pushed up from below by internal geological activity. Evidence from telescopes shows that at least one of the moons (Ariel) has material on it that flowed onto its surface relatively recently, possibly from icy volcanoes. 

Miranda, the innermost and fifth largest Uranian moon, also hosts surface features that may be of recent origin, which suggests it may have held enough heat to maintain an ocean at some points. However, recent thermal modeling found that Miranda likely didn’t host that water for very long, since the moon loses heat too quickly and the ocean is probably frozen now.

Another key finding in the new study suggests that chlorides and ammonia are likely abundant in the oceans. Ammonia can act as an antifreeze, and the author’s modeling suggests that the salts that are likely present in the water would be another source of temperature regulation  that maintains the bodies’ internal oceans.

Digging down into the inner workings of a moon’s surface could help scientists and engineers choose the best instruments to survey them in future missions, but there are still many questions about Uranus’ large moons and work to be done.

“We need to develop new models for different assumptions on the origin of the moons in order to guide planning for future observations,” Castillo-Rogez said.