The five large moons of Uranus — Miranda, Ariel, Umbriel, Titania, and Oberon — are important targets for future spacecraft missions. Studying these bodies would help address the extent of habitable environments in the outer Solar System. To motivate and inform the exploration of these moons, planetary researchers modeled their internal evolution, present-day physical structures, and geochemical and geophysical signatures that may be measured by spacecraft. They predicted that if the moons preserved liquid until present, it is likely in the form of residual oceans less than 30 km (18.6 miles) thick in Ariel, Umbriel, and less than 50 km (31 miles) in Titania, and Oberon.
Castillo-Rogez et al. show 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. Image credit: NASA / JPL-Caltech.
At least 27 moons circle Uranus, with the four largest ranging from Ariel, at 1,160 km (720 miles) across, to Titania, which is 1,580 km (980 miles) across.
Planetary scientists have long thought that Titania, given its size, would be most likely to retain internal heat, caused by radioactive decay.
The other moons had previously been widely considered too small to retain the heat necessary to keep an internal ocean from freezing, especially because heating created by the gravitational pull of Uranus is only a minor source of heat.
“Our work could inform how a future mission might investigate the moons, but the paper also has implications that go beyond Uranus,” said Dr. Julie Castillo-Rogez, a researcher at NASA’s Jet Propulsion Laboratory.
“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.”
“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.”
This Webb image shows Uranus and six of its 27 known moons (most of which are too small and faint to be seen in this short exposure). Image credit: NASA / ESA / CSA / STScI / J. DePasquale, STScI.
In their study, the researchers revisited findings from NASA’s Voyager 2 flybys of Uranus in the 1980s and from ground-based observations.
They built computer models infused with additional findings from NASA’s Galileo, Cassini, Dawn, and New Horizons missions, including insights into the chemistry and the geology of Saturn’s moon Enceladus, Pluto and its moon Charon, and Ceres — all icy bodies around the same size as the Uranian moons.
They used that modeling to gauge how porous the Uranian moons’ surfaces are, finding that they’re likely insulated enough to retain the internal heat that would be needed to host an ocean.
In addition, they found what could be a potential heat source in the moons’ rocky mantles, which release hot liquid, and would help an ocean maintain a warm environment — a scenario that is especially likely for Titania and Oberon, where the oceans may even be warm enough to potentially support habitability.
Miranda, the innermost and fifth largest moon, hosts surface features that appear to be of recent origin, suggesting it may have held enough heat to maintain an ocean at some point.
The thermal modeling found that Miranda is unlikely to have hosted water for long: it loses heat too quickly and is probably frozen now.
But internal heat wouldn’t be the only factor contributing to a moon’s subsurface ocean.
A key finding in the study suggests that chlorides, as well as ammonia, are likely abundant in the oceans of the icy giant’s largest moons. Ammonia has been long known to act as antifreeze.
In addition, the modeling suggests that salts likely present in the water would be another source of antifreeze, maintaining the bodies’ internal oceans.
“Of course, there still are a lot of questions about the large moons of Uranus,” Dr. Castillo-Rogez said.
“There is plenty more 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.”
The study was published in the Journal of Geophysical Research.
_____
Julie Castillo-Rogez et al. 2023. Compositions and Interior Structures of the Large Moons of Uranus and Implications for Future Spacecraft Observations. Journal of Geophysical Research 128 (1): e2022JE007432; doi: 10.1029/2022JE007432
