A re-examination of data from the Voyager spacecraft, coupled with advanced computer modeling, has led to a groundbreaking discovery by NASA scientists. They now believe that four of Uranus’ largest moons – Ariel, Umbriel, Titania, and Oberon – likely contain an ocean layer sandwiched between their cores and icy crusts (see image here).
This new research is the first to delve into the evolution of the interior makeup and structure of all five major moons of Uranus, including Miranda, and suggests that four of these moons may harbor oceans that could be tens of miles deep.
Uranus boasts at least 27 moons in total, with the four largest ranging in size from Ariel, which measures 720 miles (1,160 kilometers) across, to Titania, which spans 980 miles (1,580 kilometers) in diameter. Scientists have long speculated that Titania, due to its larger size, would be more likely to retain internal heat generated by radioactive decay.
The other moons were previously thought to be too small to retain enough heat to prevent an internal ocean from freezing, particularly given that heating caused by the gravitational pull of Uranus is only a minor heat source.
The 2023 Planetary Science and Astrobiology Decadal Survey conducted by the National Academies prioritized the exploration of Uranus. In anticipation of a potential mission, planetary scientists are concentrating their efforts on studying the ice giant and its mysterious system.
The recent study, published in the Journal of Geophysical Research, not only provides insight into how a future mission might explore the moons of Uranus, but also has broader implications for understanding other celestial bodies, according to lead author Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Southern California.
“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 Castillo-Rogez.
“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.”
To conduct their research, the scientists revisited data from the 1980s Voyager 2 flybys of Uranus and combined it with findings from ground-based observations. They then developed computer models that incorporated additional discoveries from NASA’s Galileo, Cassini, Dawn, and New Horizons missions, each of which unveiled ocean worlds.
These models also included insights into the chemistry and geology of Saturn’s moon Enceladus, Pluto and its moon Charon, and Ceres – all icy bodies similar in size to the moons of Uranus.
The research team made use of advanced computer modeling to analyze the porosity of their surfaces. They discovered that the moons’ surfaces are likely insulated enough to retain the internal heat necessary for hosting an ocean.
Moreover, they identified a potential heat source within the moons’ rocky mantles, which release hot liquid and could help maintain a warm environment for an ocean—particularly in Titania and Oberon, where the oceans may even be warm enough to potentially support habitability.
By investigating the composition of the oceans, scientists can also gain insights into the materials that may be present on the moons’ icy surfaces, depending on whether substances from the subsurface were pushed up through geological activity.
Observations from telescopes have revealed evidence that at least one of the moons, Ariel, has material that flowed onto its surface, possibly from icy volcanoes, relatively recently.
Miranda, the innermost and fifth largest moon, also exhibits surface features that appear to be of recent origin, suggesting it may have held enough heat to maintain an ocean at some point. However, recent thermal modeling indicates that Miranda is unlikely to have hosted water for long, as it loses heat too rapidly and is probably frozen now.
Castillo-Rogez explained that internal heat isn’t the only factor contributing to a moon’s subsurface ocean. The study’s key finding reveals that chlorides, along with ammonia, are likely abundant in the oceans of Uranus’ largest moons.
Ammonia is known to act as an antifreeze, and the modeling also suggests that salts present in the water would provide an additional antifreeze source, helping to maintain the moons’ internal oceans.
Despite these groundbreaking discoveries, many questions about Uranus’ large moons remain. Castillo-Rogez acknowledges the need for further research: “We need to develop new models for different assumptions on the origin of the moons in order to guide planning for future observations.”
Unraveling the mysteries of these moons will help scientists and engineers select the most suitable instruments for studying them. For instance, knowing that ammonia and chlorides may be present would mean that spectrometers, which identify compounds by their reflected light, would need to use a wavelength range covering both types of compounds.
Furthermore, this knowledge can aid in designing instruments capable of probing the deep interior for liquid. Generally, searching for electrical currents contributing to a moon’s magnetic field is the best way to detect a deep ocean, as demonstrated by the Galileo mission scientists at Jupiter’s moon Europa.
However, the cold water in the interior oceans of moons like Ariel and Umbriel could hinder the oceans’ ability to carry these electrical currents, presenting a new challenge for scientists working to uncover what lies beneath the surface of these enigmatic celestial bodies.
The integration of this wealth of data has allowed scientists to deepen our understanding of the moons of Uranus and the potential for water-rich bodies throughout our solar system.
Uranus is the seventh planet from the Sun and the third largest in our solar system, with a diameter of about 31,518 miles (50,724 kilometers). It is an ice giant, like Neptune, and consists primarily of hydrogen and helium along with various ices such as water, ammonia, and methane.
It has a unique pale blue-green color due to the presence of methane in its atmosphere, which absorbs red light and reflects the blue-green hues. Uranus also has a distinct feature: its axis is tilted at an extreme angle of 98 degrees relative to its orbit, causing the planet to essentially rotate on its side.
Uranus has a ring system composed of 13 faint rings that are predominantly made up of dust and small chunks of rock. It also has a complex system of at least 27 known moons, named after characters from the works of William Shakespeare and Alexander Pope.
The five largest moons are Miranda, Ariel, Umbriel, Titania, and Oberon, each displaying distinct geological features and characteristics.
The innermost and smallest of the five main moons, Miranda has a diameter of about 290 miles (466 kilometers) and features a bizarre landscape with giant canyons, ridges, and icy cliffs. Some theories suggest that the moon might have experienced a massive impact or tectonic activity in the past.
With a diameter of approximately 720 miles (1,160 kilometers), Ariel is the fourth largest moon of Uranus. It has a relatively smooth surface with some large impact craters and a network of fault valleys and ridges, indicating signs of past geological activity.
Umbriel is the third largest moon, with a diameter of around 727 miles (1,169 kilometers). It has a dark and heavily cratered surface, indicating that it has been relatively inactive geologically.
The largest moon of Uranus, Titania has a diameter of approximately 980 miles (1,580 kilometers). Its surface features deep canyons, vast plains, and large impact craters, suggesting a mix of geological processes.
With a diameter of about 945 miles (1,520 kilometers), Oberon is the second largest moon of Uranus. It has a heavily cratered surface with some large impact basins and mountainous terrain.
Recent studies have revealed that four of these major moons—Ariel, Umbriel, Titania, and Oberon—likely contain subsurface oceans between their cores and icy crusts.
This discovery has sparked interest in understanding their potential habitability and the mechanisms that allow them to maintain liquid water beneath their surfaces.
While there is still much to learn about Uranus and its moons, these celestial bodies provide a fascinating glimpse into the diverse range of planetary systems and environments that exist within our solar system.