Uranus is encircled by at least 27 moons, with the four largest ranging from Ariel at 720 miles (1,160 kilometers) in diameter to Titania at 980 miles (1,580 kilometers) across [1]. Scientists have long suspected that Titania, due to its size, would be most likely to retain internal heat generated by radioactive decay [1]. The other moons were previously thought to be too small to maintain the necessary heat for preventing an internal ocean from freezing, particularly since heating caused by Uranus' gravitational pull only provides a minor heat source [1].
As the National Academies' 2023 Planetary Science and Astrobiology Decadal Survey prioritizes exploring Uranus, planetary scientists are focusing on the ice giant to enhance their understanding of the enigmatic Uranus system [1]. Published in the Journal of Geophysical Research, the new findings could guide how a future mission might study these moons while also having implications beyond Uranus, according to lead author Julie Castillo-Rogez from NASA's Jet Propulsion Laboratory in Southern California [1].
The researchers utilized advanced computer models, incorporating additional findings from NASA's Galileo, Cassini, Dawn, and New Horizons missions, to assess the porosity of Uranian moons' surfaces, discovering that they are likely insulated enough to retain the internal heat required for hosting an ocean [1]. Furthermore, they identified a potential heat source in the moons' rocky mantles, which release hot liquid and could help maintain a warm ocean environment – a scenario particularly likely for Titania and Oberon, where the oceans may even be warm enough to support habitability [1].
By examining the composition of these possible oceans, scientists can also gain insights into materials found on the moons' icy surfaces, depending on whether substances from below were pushed upwards by geological activity [1]. There is evidence from telescopes that Ariel, at least, has material that flowed onto its surface, possibly from icy volcanoes, in relatively recent times [1]. Interestingly, Miranda, the innermost and fifth-largest moon, also exhibits surface features that appear to be of recent origin, suggesting it may have once harbored enough heat to sustain an ocean, although recent thermal modeling indicates that Miranda likely lost heat too quickly and is now frozen [1].
Nonetheless, internal heat is not the sole factor affecting a moon's subsurface ocean. A crucial discovery in the study indicates that chlorides, along with ammonia, are likely plentiful in the oceans of Uranus' largest moons. Ammonia has long been recognized for its antifreeze properties. Moreover, the models suggest that the presence of salts in the water could act as an additional antifreeze source, sustaining the internal oceans of these celestial bodies.
Naturally, numerous questions remain about Uranus' large moons, as Castillo-Rogez acknowledged, stating that there is still much work to be done. "We need to create new models based on different assumptions about the moons' origins to inform future observation planning."
Understanding the composition of these moons' surfaces and interiors will aid scientists and engineers in selecting the most appropriate instruments to study them. For example, realizing that ammonia and chlorides might be present implies that spectrometers, which identify compounds through reflected light, should cover a wavelength range that includes both types of compounds.
Similarly, this knowledge can be used to design instruments capable of probing the deep interior for liquid. Detecting electrical currents contributing to a moon's magnetic field is generally the most effective method for finding a deep ocean, as demonstrated by Galileo mission scientists with Jupiter's moon Europa. However, the cold water in the interior oceans of moons like Ariel and Umbriel may reduce the oceans' ability to conduct these electrical currents, posing a new challenge for scientists seeking to uncover what lies beneath.
Credit: NASA's Jet Propulsion Laboratory (JPL)
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