The tiny moon of Uranus, Ariel, may have once harbored a massive subsurface ocean, constituting around 55% of its total volume. By meticulously mapping craters, crags, and ridges on the moon's surface, planetary scientists constructed models to determine the thickness of Ariel's crust before it cracked under tidal stress, giving rise to the geological features observed today. Subtracting the size of the crust and core, the researchers concluded that the Arielian ocean could have been approximately 170 kilometers thick as recently as 1 billion years ago. This discovery has profound implications, suggesting that other small icy moons could also have had subsurface oceans. Caleb Strom, a planetary geologist, emphasizes that this finding challenges previous assumptions, potentially indicating that creating ocean worlds is more feasible than previously thought.
Ariel, the second-closest of Uranus's five large moons, is surprisingly small, measuring only about 1,160 kilometers across, or a third the size of Earth's Moon. When Voyager 2 passed through the Uranus system in 1986, scientists were intrigued by the relatively young and geologically complex icy surface of Ariel, which displayed signs of cryovolcanism. These features bear resemblance to those found on Europa, Enceladus, and Triton, all confirmed ocean worlds. However, Strom notes that the expectation of finding an ocean world on Ariel was not universal.
Subsequent studies have identified ammonia and carbon oxide compounds on Ariel's surface, indicative of subsurface liquid. These molecules are short-lived unless frequently replenished, further supporting the presence of a subsurface ocean. However, the moon's small size and limited heat retention capacity led scientists to believe that any subsurface ocean would be thin and short-lived. Strom and his team initially aimed to understand the forces shaping Ariel's geological features, mapping the moon's surface and modeling its internal structure.
Their models revealed that a crust less than 30 kilometers thick would fracture under moderate tidal stress, creating the observed geological features. The researchers propose that an orbital resonance with nearby moon Miranda in the past 1-2 billion years stretched Ariel's orbit by 4%, causing surface fractures. This led to the conclusion that Ariel's subsurface ocean was approximately 170 kilometers thick and constituted 55% of its total volume. The findings were published in the journal Icarus in September.
Richard Cartwright, an ocean world scientist, finds the prospect of an ocean in Ariel in the past 1 billion years exciting, aligning with other studies suggesting recent activity on the moon's surface. However, Strom cautions that the moon's small size and poor heat retention may mean that any remaining ocean is thin and not conducive to life. Chloe Beddingfield, a planetary scientist, emphasizes that while Ariel's case demonstrates the potential for significant internal oceans on small moons, the likelihood of similar bodies developing oceans depends on their unique heat sources, chemistry, and orbital evolution.
The discovery of a 55% ocean volume on a moon like Ariel might seem substantial, but Beddingfield notes that it could be within the normal range for ocean worlds. The limited imaging of Voyager 2, covering only 35% of Ariel's surface, underscores the need for future long-term missions to the Uranus system to provide higher-resolution global maps and refine calculations of crustal thickness and subsurface ocean existence. Strom and his team plan to expand their research to other Uranus moons and possibly icy moons around other planets, aiming to enhance our understanding of these celestial bodies.
