Subsurface Oceans Possibly Lurking beneath the Five Largest Moons of Uranus

Friday - May 12 2023, 01:52 UTC - 1 year ago

The five large moons of Uranus are important targets for future spacecraft missions because they may host residual oceans a few tens of kilometers thick at present; however, Miranda is unlikely to host liquid at present. Thermal metamorphism could create a late, second generation ocean in Titania and Oberon which could be detectable by future spacecraft-based magnetometers if the ocean is maintained primarily by ammonia. These models represent a baseline for the formulation of observations with the Uranus Orbiter and Probe.

At least 27 moons circle Uranus, with the four largest ranging from Ariel, at 720 miles (1,160 kilometers) across, to Titania, which is 980 miles (1,580 kilometers) across. Scientists have long thought that Titania, given its size, would be most likely to retain internal heat, caused by radioactive decay. The other moons had previously been widely considered too small to retain the heat necessary to keep an internal ocean from freezing, especially because heating created by the gravitational pull of Uranus is only a minor source of heat.

There is evidence from telescopes that at least one of the moons, Ariel, has material that flowed onto its surface, perhaps from icy volcanoes, relatively recently.

In fact, Miranda, the innermost and fifth largest moon, also hosts surface features that appear to be of recent origin, suggesting it may have held enough heat to maintain an ocean at some point. The recent thermal modeling found that Miranda is unlikely to have hosted water for long: It loses heat too quickly and is probably frozen now.

But internal heat wouldn’t be the only factor contributing to a moon’s subsurface ocean. A key finding in the study suggests that chlorides, as well as ammonia, are likely abundant in the oceans of the icy giant’s largest moons. Ammonia has been long known to act as antifreeze. In addition, the modeling suggests that salts likely present in the water would be another source of antifreeze, maintaining the bodies’ internal oceans.

The thermal models suggested that the four largest moons — Ariel, Umbriel, Titania and Oberon — are most likely to contain shallow subsurface oceans; however, these would be very different from the subsurface oceans of the moons of Jupiter and Saturn. Each of the Uranian moons has its own physical characteristics, and those determine what type of ocean can possibly form and can be sustained over time. For example, because Ariel has the lowest thermal conductivity of the four moons, it is the best candidate for preserving an ocean of complex liquid. Umbriel has the highest thermal conductivity, suggesting that it would not be able to sustain an ocean of liquid any longer.

The five large moons of Uranus are important targets for future spacecraft missions. To motivate and inform the exploration of these moons, they model their internal evolution, present-day physical structures, and geochemical and geophysical signatures that may be measured by spacecraft. They predict that if the moons preserved liquid until present, it is likely in the form of residual oceans less than 30 km thick in Ariel, Umbriel, and less than 50 km in Titania, and Oberon. The preservation of liquid strongly depends on material properties and, potentially, on dynamical circumstances that are presently unknown. Miranda is unlikely to host liquid at present unless it experienced tidal heating a few tens of million years ago. They find that since the thin residual layers may be hypersaline, their induced magnetic fields could be detectable by future spacecraft-based magnetometers. However, if the ocean is maintained primarily by ammonia, and thus well below the water freezing point, then its electrical conductivity may be too small to be detectable by spacecraft. Lastly, our calculated tidal Love number (k2) and dissipation factor (Q) are consistent with the Q/k2 values previously inferred from dynamical evolution models. In particular, we find that the low Q/k2 estimated for Titania supports the hypothesis that Titania currently holds an ocean.

From the research, we can extract three main points: (1) Most of the major Uranian moons may host a residual ocean a few tens of kilometers thick at present, except for Miranda. (2) Thermal metamorphism could create a late, second generation ocean in Titania and Oberon. (3) These models represent a baseline for the formulation of observations with the Uranus Orbiter and Probe.