Four of Uranus’ Moons Might Have Liquid Oceans, Too

The study of ocean worlds, planetary bodies with potential interior reservoirs of liquid water, has come to the forefront in terms of astrobiology and the search for life beyond Earth. From Jupiter’s Galilean Moons to Saturn’s Titan and Mimas to Neptune’s Triton and even Pluto, scientists are craving to better understand if these worlds truly possess interior bodies of liquid water. But what about Uranus and its more than two dozen moons? Could they harbor interior oceans, as well?

This is what a recent study published in the Journal of Geophysical Research hopes to reveal as a team of researchers examined the likelihood that interior liquid oceans might currently exist on the five largest of Uranus’ 27 moons—Ariel, Umbriel, Titania, and Oberon, and Miranda. This study comes as the future exploration of Uranus, specifically the Uranus Orbiter and Probe (UOP) mission, was emphasized in the National Academies’ “Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032″, which was published in 2022.

“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,” said Dr. Julia Castillo-Rogez, who is a planetary scientist at NASA’s Jet Propulsion Laboratory and lead author of the study. “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.”

For the study, the researchers built computer models to calculate the potential for interior oceans on the five moons based on data from a myriad of sources, one of which is Voyager 2, which remains the only spacecraft to explore Uranus and its moons, having accomplished this in January 1986. The remaining sources included ground-based observations along with several other spacecraft missions, such NASA’s Galileo, Dawn, Cassini, and New Horizons, all of which discovered at least one ocean world during their respective missions.

The study’s findings indicated potential interior liquid oceans for Titania (diameter: 1,580 km/980 mi), Oberon (1,520 km/950 mi), Umbriel (1,170 km/726 mi), and Ariel (1,160 km/720 mi) due to their abilities to sustain interior heat “with little or no tidal heating at present”, as the study notes. Unfortunately, Miranda (470 km/270 mi) was deemed as too small to sustain enough interior heat to possess a liquid ocean.

The five major moons of Uranus: Titania, Oberon, Umbriel, Ariel, and Miranda. (Credit: NASA/JPL/Jason Major)

The researchers determined that interior heat wasn’t the only component necessary for these moons to maintain interior oceans, as antifreezes such as chlorides, ammonia, and salt could also be responsible for the presence of interior liquid oceans, as well.

As noted, the UOP mission was a key topic of the most recent Decadal Survey, and having a better understanding of both the surface and interior compositions of these moons will allow scientists and engineers to make the most informed decisions pertaining to what science instruments would be used on a space mission. This includes instruments meant for surveying the surface and probing their interiors for liquid.

A January 2023 study published in Acta Astronautica proposed using aerocapture for the UOP mission, which involves using atmospheric drag and only a single pass through a planet’s atmosphere to slow down the spacecraft enough for orbit insertion without expending any fuel during the process. While aerocapture has been long-theorized as a potential method for planetary missions, it has yet to be used on a real-world space mission.

Artist rendition of a spacecraft using aerocapture to enter Mars orbit. (Credit: NASA)

The study notes that current baseline mission estimates put the flight time and orbital mission for UOP at 13 years and 5 years, respectively. In contrast, using the aerocapture approach, along with specific masses for the orbiter and probe and a Falcon Heavy Expendable as the launch vehicle, the study projects a flight time and orbital mission for UOP at 8 years and 10 years, respectively, with an anticipated launch date between 2028 and 2038 to meet missions schedule requirements.

What new discoveries will scientists make about Uranus’ moons in the coming years and decades, and how will UOP change our understandings of these unique worlds? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!