When Voyager 2 whizzed past Uranus in January 1986, it encountered a system tilted by 97.7 degrees, as if lying on its side. Such a configuration made the Uranus system set up like a bullseye for the space probe, and it had to fly perpendicular through the system, unlike its previous encounters with the Jovian and Saturnian systems. Such a trajectory reduced the time spent studying up-close the icy moons (27) that form Uranus' satellite system.
Nevertheless, of the four mid-size icy moons: Titania, Oberon, Umbriel, and Ariel, it was the latter that raised many eyebrows when the images of its surface, only mapped at 35%, were returned.
These show resurfaced areas of smooth plains with few large craters present. Flow-like features forming complex channel networks are also visible, hinting at large cryovolcanic events in the moon's past, where water (rich in ammonia) must have poured out on the surface multiple times. To everyone's surprise, Ariel displays the most active surface of any icy moon in the Saturnian and Uranian systems after Enceladus.
In addition to the surface images, Ariel's active past is also implied by studies showing that radiogenic heating from the moon's big rocky core coupled with the tidal heating it experienced as it underwent several resonances with Umbriel and Titania was sufficient to melt a substantial part of its icy mantle*. This created a warm subsurface ocean for hundreds of millions of years.
Ever since Voyager 2 returned such a treasure trove of data over thirty years ago, planetary scientists have wondered if the moon could still hold a subsurface ocean today. Ariel's orbit has a slight eccentricity (0.0012) and is tidally locked with Uranus (always showing the same face to the planet), subjecting it to tidal heating. Yet, with Voyager 2's only flyby to work on, scientists were constrained in their abilities to understand this peculiar moon.
Nevertheless, scientists are a determined bunch, and last week, a fantastic piece of news surfaced: a research paper submitted to the Geophysical Research Letters titled 'A localized and surprising source of energetic ions in the Uranian magnetosphere between Miranda and Ariel' by Cohen et al., revisits the Voyager 2 data and makes a startling revelation.
Here's how it goes. In 1986, energetic ions were observed by Voyager 2's Low Energy Charged Particle (LECP) instrument in the region between Miranda and Ariel. Our understanding of Uranus' magnetosphere at the time interpreted this signature as a result of the system's dynamics. New models presented in this paper show that this is not the case and suggests that a source of energetic ions is present either on Ariel or her much smaller sibling, Miranda. In other words, material from one of these icy moons is being vented off into space!
The possibility of Ariel venting off plumes similarly to Enceladus has been considered for many years now, with astronomers even trying to detect a hypothetical 'e-ring' around Uranus without success. This new research paper brings back the idea of vents and might even be its first line of evidence! I can hardly contain my excitement.
On the other hand, Miranda, mentioned in the paper, is a confused planetary body. It seems to have experienced traumatic events as well as some forms of tidal heating in its past. Still, it is just too small to have been able to keep the heat, and it is doubtful, given our current understanding of icy worlds, that a subsurface ocean or body of water lurks under its icy shell, which is why I'm putting all my money on Ariel.
So, there it is, dear reader. Thanks to Cohen et al., Ariel will now join the planetary club of the very-likely-an-Ocean-World-but-not-yet-confirmed. It will be next to Pluto, Dione, Triton and Ceres. One day, with enough data, these will then move to the prestigious club of confirmed Ocean Worlds with the likes of Europa, Enceladus, Callisto, Ganymede and Titan. Let’s hope this day comes soon.
In the meantime, there’s liquid water in the Uranus system! We need to go back.
Onwards and upwards!
Bernard
*Ariel's density, calculated at 1.66 g/cm3, is similar to its bigger siblings, Titania and Oberon, implying a composition of half water-ice and half rocks (as well as other non-ice constituents such as carbonaceous material). Image credit: NASA