What is an ‘ocean world’?

By Fred Watson, Australia's Astronomer-at-Large 25 June 2024
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One of the big surprises that’s come in recent years from our exploration of the solar system is the existence of icy ocean worlds in its outer reaches. Most are moons of the giant planets, although some dwarf planets such as Pluto may also have a similar structure.

So, what is an ocean world?

Basically, it’s a ball of rock enveloped in water, which is in the form of ice unless it’s within the “habitable zone” of its parent star – in which case it becomes liquid water.

Beneath the frozen surface of Europa, one of Jupiter’s moons, lies a vast saltwater sub-ocean. Image credit: courtesy NASA/University of Arizona

Earth holds that privileged position in our solar system, but further out, ice predominates, with the subtle twist that internal heating from beneath the rocky surface can melt the lower levels of the ice mantle to create a sub-ice ocean. 

Perhaps the best-known examples of this structure are Jupiter’s moon Europa and Saturn’s moon Enceladus. 

Europa, with a diameter of 3122km, is six times the size of Enceladus, but they both have smooth, icy surfaces with prominent fractures and few craters. Their surfaces are kept fresh by fountains of ice crystals erupting from the fractures.

In the case of Enceladus, the ice-plumes have been sampled directly by the extraordinarily productive Cassini spacecraft, which orbited Saturn from 2004 to 2017.

Now that cohort of ocean worlds may have been joined by an unexpected newcomer.

With a heavily cratered surface, Saturn’s small moon Mimas could hardly appear more different from a conventional ice-world.

One very large crater, Herschel, gives rise to Mimas’s nickname – “Death Star”– because it resembles the fictional space station in Star Wars.

But it’s the subtleties of Mimas’s orbit and rotation that led an international team of astronomers based at the Paris Observatory to conclude that a large volume of liquid water exists some 20–30km beneath the icy surface. 

Shadows cast across Mimas' defining feature, Herschel Crater, provide an indication of the size of the crater's towering walls and central peak.
Shadows cast across Mimas’ defining feature, Herschel Crater, provide an indication of the size of the crater’s towering walls and central peak. Image credit: NASA/JPL-Caltech/Space Science Institute

If that’s the case, why hasn’t the pressure of the ice forced water through cracks to erupt as ice geysers, as in the case of Enceladus and Europa?

Further analysis of Mimas’s orbit led researchers to conclude that its ocean is young, perhaps less than 25 million years old.

Moreover, it’s still growing upwards, with the likelihood it will eventually crack the surface ice and allow geysers to erupt through.

This will coat it with a fresh layer of ice and smooth its pockmarked surface to look more like Enceladus or Europa.