Once upon a time, the only world known to have an ocean of water was Earth. Now, planetary scientists think there are many ocean worlds – albeit with their oceans covered by deep layers of ice, rather than hanging out on the surface like ours.
Top on the list is Jupiter’s moon Europa, believed to have a 100-kilometre-deep ocean beneath perhaps 10–30 km of ice. But Saturn’s moons Enceladus, Titan, and Dione are also thought to have oceans, as is Pluto.
And those may just be the tip of the iceberg. Other moons in the outer solar system are also believed or suspected to have frozen-over oceans. Still more aren’t well studied enough for scientists to be sure, but could be capable of hosting water.
All of this means that even as two NASA rovers are searching Mars for relics of ancient surface water and possible life, other missions, both from NASA and the European Space Agency, are gearing up to probe the outer Solar System for subsurface oceans that might support life today.
Top of the list is NASA’s Europa Clipper mission, scheduled to lift off for Jupiter sometime in 2024. Once at Jupiter (in 2030) it will go into orbit around the planet and make dozens of flybys of Europa to study it in detail.
One goal, says Paul Schenk, a planetary scientist at the Lunar and Planetary Institute, Houston, Texas, will simply be to take better images of Europa’s surface than were provided by the Galileo mission, which orbited Jupiter from 1995 to 2003. Due to an antenna failure that reduced the amount of data Galileo could send back to Earth, he says, “all of the Galilean satellites are poorly mapped.”
“It’s kind of like trying to understand the geology of the United States when you’ve only mapped six of the 50 states,” he says. “You’ve got some interesting ideas, but you’re missing quite a lot.”
But the big prize is to learn more about Europa’s ocean.
One way Europa Clipper will do this is with a state-of-the-art magnetometer that will measure how that ocean interacts with Jupiter’s magnetic field. This instrument, he says, will not only be able to measure the ocean’s depth, but also determine its salinity, an important variable related to its potential habitability.
Other scientists will probe the ocean by measuring how Europa’s gravity field affects its radio signals, says Erwan Mazarico of NASA’s Goddard Space Flight Center.
As Europa orbits Jupiter, he says, the moon flexes in response to Jupiter’s gravity, thereby warping its own gravity field. “An ocean will affect this,” he adds.
With enough flybys, this should allow scientists to map variations in the thickness of the ice shell. They should also be able to spot upwellings from below, such as lakes trapped between layers of ice, and even places where water once erupted onto the surface then froze into layers akin to lava beds.
But Europa isn’t the only world Europa Clipper will visit. It will make flybys of two other Jovian moons suspected of having oceans, Ganymede and Callisto.
“Europa Clipper will spend most of its time at Europa,” Cochrane says, “but to get there it needs to lose energy, and the way it does that is that it cuts close to Ganymede and Callisto.”
Scientists are keen to see Ganymede in particular. Not only is it the largest moon in the Solar System (slightly larger than the planet Mercury), but it also has an atmosphere, and is the only moon known to have its own magnetic field. “We have a pretty good idea that it has a large rocky core and an icy mantle, and is an ocean world,” he says.
Ganymede is interesting enough, in fact, that the European Space Agency’s JUICE mission (JUpiter ICy moons Explorer), scheduled for launch in 2022 (and arrival at Jupiter in 2029) will fly by it, Europa, and Callisto for three years, and then, if all goes well, brake into orbit around Ganymede to study it in detail.
“Ganymede is a prime target of two missions,” Schenk says.
But there are also schemes to visit Neptune and Uranus, which haven’t been seen up close since the Voyager missions of the 1980s.
The most advanced of these is NASA’s Trident Mission, tentatively targeted for launch in 2025 or 2026, when it can take advantage of a Jupiter gravity assist to speed it on its way for a flyby of Neptune’s giant moon Titan. “If we don’t launch then, we can’t get there,” Cochrane says. “If we do, we will get there around 2038.”
It’s an exciting idea, says Noah Hammond of Collage of the Holy Cross, Worcester, Massachusetts, because Triton is high on the list of suspected ocean worlds.
To start with, he says, “it has active geysers.” It also has a young surface, possibly created by water erupting from its interior not all that long ago, geologically speaking.
Theory also suggests it might be an ocean world, because its exotic orbit (tilted and backward from most other orbits in the Solar System) implies that it is wasn’t formed along with the rest of the Neptune system, but is instead an interloper that was gravitationally captured, and quite possibly subjected to prolonged internal heating that might well have left a remnant ocean still beneath its surface.
And with a state-of-the-art magnetometer, Cochrane says, it should be possible to determine if such an ocean still exists, even on a single-pass flyby mission like Trident.
The moons of Uranus are more mysterious.
None of them are large: the two of most interest, Miranda and Ariel, are only 470 and 1120 kilometers in diameter, respectively.
But both, Cochrane says, show surface features a lot like Europa and Enceladus, suggesting that they might also be ocean worlds.
And, he says, it’s possible to visit both with a single spacecraft. “We designed a trajectory a couple years back [where] we’d orbit Uranus and on each [orbit] make a close pass to one of the moons – I think three for each,” he says.
Not that Miranda and Ariel are the only moons of Uranus that might have oceans. Three others, Umbriel, Titania, and Oberon are larger, and might also have oceans. But their oceans would be harder to detect than those at Miranda and Ariel, largely because they are farther from Uranus and therefore less subject to its magnetic field.
Either way, if we could find ocean worlds among the moons of Uranus, that would mean that they are not only common, but—in the supposedly cold, dark reaches of the outer Solar System—nearly ubiquitous.
For now, we have a count of at least 14 potential ocean worlds in the Solar System: Earth, Europa, Callisto, Ganymede, Enceladus, Dione, Titan, Miranda, Ariel, Umbriel, Titania, Oberon, Triton, and Pluto. Not to mention the asteroid Ceres, which most likely also once had liquid water.
And if our Solar System has them aplenty, how many others are there, circling other stars?
Richard A Lovett
Richard A Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos.
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