Second most distant planet in the solar systemUranus, 27 satellites have been discovered so far. Among them, the large sizeMiranda“”Ariel“”Umbriel“”titania“”Oberonare known as the five largest satellites of Uranus.
[▲ Figure 1: Uranus and its moons taken by the James Webb Space Telescope. The 5 largest satellites Miranda, Ariel, Umbriel, Titania, and Oberon are shown, as well as the 6th largest satellite pack (Credit: NASA, ESA, CSA, STScI / Japanese added by the author)]
These moons are called icy moons because they are mainly composed of ice. For Titania and Oberon, which are particularly large, if ammonia, which acts as an antifreeze agent (a substance that prevents water from freezing even at temperatures below 0°C), is dissolved in the seawater, the inner sea would not be maintained even now. There was also an expectation that it would. In particular, Ariel is an interesting target for observation, as we have obtained data that seems to be traces of recent volcanic activity. However, in recent years, it has been found that such seawater is unstable as a substance, and at least the presence of antifreeze alone cannot maintain the internal sea.
On the other hand, the recent activity of planetary probes has raised the possibility that a much larger number of celestial bodies than previously thought have internal oceans. Examples include Saturn’s moon Enceladus, which is actively plumeing, quasi-moon Ceres in the asteroid belt, Pluto, and moon Charon. All of these are ice-based bodies that have been probed in recent years, and the five largest moons of Uranus are about the same size as these bodies.
A research team led by Julie Castillo-Rogez of JPL (Jet Propulsion Laboratory) has investigated the existence of internal oceans in the five largest moons of Uranus based on previous exploration data of celestial bodies whose main components are ice. We considered. The data considered included surface chemistry, geology, mechanical models and geometry data.
The only close-up observation data of Uranus was obtained by NASA’s (National Aeronautics and Space Administration) planetary probe “Voyager 2” that conducted a flyby exploration in 1986, and about 40% of the surface of the five major satellites. There is only data for Observational data is limited, but by comparing it with data from other celestial bodies, it is possible to estimate the internal structure of the celestial bodies and to study in detail the movement of heat inside the celestial body, which is particularly important.
As a result of the analysis, it was found that the rocks near the surface of the five largest satellites are porous and have high heat insulating properties that retain heat inside. It was also found that the decay of short-lived radioactive materials generates heat for several hundred years immediately after the birth of the satellite, and that residual heat is the main source of heat that melts the ice inside.
On the other hand, it was also revealed that the tidal force (the force by which the body is stretched due to gravitational interaction with other celestial bodies), which is considered for other icy bodies, is almost non-existent for the five major moons. Previous studies have shown that tidal forces hardly act on the five major satellites, but in this study, tidal forces were generated only when the orbits were unstable immediately after the satellites were born. I understand.
Frictional heat is generated in celestial bodies where tidal forces act, and a phenomenon called tidal heating may occur, which warms the interior. However, among the five largest moons of Uranus, even Miranda and Ariel, which were most affected by tidal forces, were found to have much less internal heating than the decay heat of radioactive material. It was also found that if the tidal force is too strong, the surface rock loses its porosity and has the negative effect of losing its insulating performance to retain residual heat.
[▲ Figure 2: The internal structures of the five major satellites estimated in this study. Ariel and Umbriel are estimated to have subsurface oceans less than 25 km deep, and Titania and Oberon have subsurface oceans less than 50 km deep, while Miranda is estimated to be completely frozen (Credit: NASA). /JPL-Caltech / Translation added by the author)]
Considering theseAriel, Umbriel, Titania, and Oberon are large enough to maintain internal oceans even today.It turns out.On the other hand, it is the smallest of the five large satellites, and loses internal heat faster.Miranda’s internal ocean likely froze within 1 billion years after its birthis estimated to be
If internal oceans exist on the four moons, the mean internal sea temperature is estimated to be between -5°C and -30°C, with chlorides acting as antifreeze agents in addition to ammonia.The size of the internal sea isless than 50 km deep in Titania and Oberon,Depths up to 25 km in Ariel and Umbrielis presumed to be
As other results, Miranda does not have a clear core, Ariel and Umbriel have a watery rock core, Titania and Oberon have a watery rock core and an inner dry rock core. It is also presumed that there is no metal-based core in any object.
However, it is unclear whether the inner sea has actually remained unfrozen until now. Although the 50 km depth of the internal oceans at Titania and Oberon is a best estimate, it is considerably smaller for an ice body’s internal ocean. In addition, the model calculations this time point out the possibility that instead of being an ocean composed of liquid water, the rocks with liquid water filling the gaps continue to exist.
By measuring the magnetic field of each satellite, we may be able to determine whether there is an internal ocean, and if so, what it is like. On the other hand, if the seawater contains a lot of ammonia and chlorides, almost no magnetic field is generated by the seawater, so it may be difficult to prove it by observation.
Every ten years, NASA and NSF (National Science Foundation of the United States) publish a plan called the “Planetary Science Decadal Survey,” which explores the mysteries and challenges in planetary science at that time. It covers exploration and observation plans recommended for resolving . 2023 is the time when the third plan will start, and the top priority among them is the orbital exploration plan for Uranus. Of course, the plan also includes observations of the five largest moons, so in the future, we may be able to learn more about the presence or absence of the internal oceans of the five largest moons through Uranus exploration missions.
Text: Riri Ayae