Our solar system is filled with oceans. But only a few of those have captivated our attention.

During its 1979 Jupiter flyby, NASA’s Voyager 1 spacecraft found the surface of the moon Europa to be a cracked-up jumble of water ice, as if composed of icebergs floating atop some hidden sea. As the craft moved on to Saturn, it took measurements of that planet’s massive moon, Titan, and revealed the frigid world bore a thick atmosphere that could sustain lakes or seas of liquid hydrocarbons on the veiled, cryogenic surface below.

It took follow-ups by NASA’s Galileo mission that arrived at Jupiter in 1995, and later by the Saturn orbiter Cassini—a joint project between NASA and the European Space Agency (ESA)—to confirm this early evidence for extraterrestrial oceans. Galileo also hinted that two other Jovian moons, Callisto and Ganymede, perhaps harbored oceans as well. Cassini found abundant evidence of multiple ocean-bearing moons during its 13 years of studying the Saturnian system. In part because of these discoveries, both Galileo and Cassini were deliberately crashed into their respective gas-giant subjects, burning up in their atmospheres to avoid any chance of biocontaminating each planet’s promising moons. Galileo’s self-immolation occurred in 2003, and Cassini’s fiery end just unfolded on September 15.

When it reached Saturn in 2004 Cassini deployed a European-built lander, Huygens, to land on Titan’s frozen surface, where it found a bizarre landscape of methane–ethane lakes and hydrocarbon snows. In later observations Cassini revealed Titan possesses a watery ocean beneath its surface of hydrocarbon slush. The breakout star of Cassini’s investigations, however, was unquestionably Enceladus, an icy moon about as big as England is wide—too small, researchers had thought, to sustain much interesting geologic activity.

Against all odds Cassini found the moon jetting plumes of salty water vapor from its south pole—an unmistakable signpost of some kind of mysterious ocean hidden beneath its icy crust. Where there is liquid water, the thinking goes, there might well be life—just as there is on Earth, where water forms the cornerstone of biology as we know it. And unlike Jupiter’s moon Europa, which Galileo had shown contains a sunless sea perhaps impenetrably sealed beneath a thick crust of ice, Enceladus’s plumes offered a way to obtain direct samples of its dark waters. Starry-eyed astrobiologists were instantly enamored with this wee moon—and they still are.

But other astrobiologically interesting moons circle Saturn, too. A bit more than 400,000 kilometers from Enceladus spins another icy satellite, named Dione. Twice as big and similarly coated in ice and snow, it has something else in common with Enceladus: it is probably an ocean world. “There’s really good evidence” for Dione’s ocean, says Bonnie Buratti, a senior research scientist at the Jet Propulsion Laboratory who worked on the Cassini mission. “I kind of feel this is one of the things we’ve left hanging as we leave the Saturn system, that we just haven’t answered.”

Ocean worlds are bodies in the solar system that manage (or managed in the past, like Mars) to hold onto enough liquid water to form seas, lakes and other potential aquatic abodes for life. The newly discovered ones in the outer solar system still harboring oceans today all possess some internal energy source that keeps them from freezing solid like some of their siblings. Scientists argue over the details but most agree those worlds are warmed from within by a combination of radioactive decay and tidal heating (the friction-inducing flexure of their crusts from the gravitational pull of the parent planet).

Buratti lays out the case for Dione’s membership in this elite club: Its surface is fairly smooth, meaning something has been filling in and covering up the spots where craters and fissures might otherwise form. Liquid water rising from a moon-girdling subsurface reservoir, then freezing, would work nicely for that. Tentative evidence from Cassini’s instruments also hinted at plume activity—albeit much weaker than at Enceladus—as if a similar but subtler mechanism is at play inside Dione. Buratti and others suspect that Janiculum Dorsa, one of the moon’s few mountains, may be somehow responsible, but the scientists were unable to gather definitive proof before Cassini’s termination.

Janiculum Dorsa may be Dione’s equivalent of Enceladus’s south pole, where water gushes more than 60 kilometers into space. On the latter this mechanism (called cryovolcanism) is fed by an underwater ocean—and there is similar albeit more circumstantial evidence for Janiculum Dorsa as the source of similar “cryovolcanic” plume activity on Dione.

Cassini’s measurements of Dione’s gravitational field also indicate that something strange is going on beneath the surface. When a spacecraft flies by a completely solid body, the craft’s trajectory tends to be more or less “straight.” In simple terms, it flies by without a discernable difference in the amount of gravity attracting it. But if the body is less homogeneous—for instance, a liquid ocean beneath an icy crust—the spacecraft’s trajectory can exhibit faint but detectable deviations in the tug due to this liquid mass. This is precisely what Cassini experienced as it swooped by Dione. “If Dione has an ocean, it’s another example where there might be a habitable environment because we have liquid water—there’s a heat source and there might be organic molecules in there that contribute to primitive bacterial life,” Buratti says.

And Dione is not the only promising place passed over by many astrobiologists infatuated with Enceladus, Titan and Europa. There are at least a half dozen other ocean worlds that might merit inclusion in the quest to discover alien life. “Might” being the operative term here—these oft-overlooked oceans tend to be less accessible than their more popular peers. They are either locked beneath thicker ice crusts or just so far out in the solar system (in the cases of moons of Uranus and Neptune) that no one can yet say how promising their hidden seas really are. That is, save Dione—which appears to be a sleepier but no less interesting cross between Enceladus and Europa.

The Most Massive Moon of All

Ganymede is a truly giant moon. Larger than Mercury and not much smaller than Mars, it is a would-be planet forever demoted from that lofty status by its host Jupiter. Just as Dione is perennially overshadowed by Enceladus and Titan, Ganymede tends to take a back seat to its sister ocean world, Europa, which is slated to be studied up close by NASA’s Europa Clipper mission sometime in the 2020s. But Ganymede is deserving of study, too: Telltale cycles of auroral activity on the surface, witnessed from afar by the Hubble Space Telescope in Earth orbit, reveal oscillations in the moon’s magnetic field best explained by the internal sloshing of a huge ocean hundreds of kilometers below the surface.

How huge? Olivier Witasse, a project scientist working on ESA’s future Jupiter Icy Moon Explorer (JUICE), says Ganymede’s ocean is even bigger than Europa’s—and might be the largest in the entire solar system. “The Ganymede ocean is believed to contain more water than the Europan one,” he says. “Six times more water in Ganymede’s ocean than in Earth's ocean, and three times more than Europa.”

JUICE will orbit Ganymede, giving scientists a chance to study this second-fiddle moon in detail. On the way there the craft will make several sweeps past another potentially ocean-bearing Jovian moon, Callisto. “We think that Callisto also harbors a subsurface ocean, but the available data is unclear,” Witasse says. “What we hope to do is to check whether there is an ocean or not—and if yes, at which depth.”

Icy Moons around Icy Giants

Out past Jupiter and Saturn other ocean worlds may be found as well. The moon Ariel twirls in a two-day orbit just 190,000 kilometers from the gas-giant Uranus. All we know about the moon comes from a single encounter with the Voyager 2 spacecraft in 1986. That flyby revealed Ariel to be relatively smooth, as if its surface was being continually renewed by activity deep within. It is currently believed to be the only ocean world in the Uranian system.

The smoothness of Ariel’s surface may be a hallmark of “extrusive cryovolcanism,” or geyser activity that throws material onto a world’s surface. Imagine a flow of molten rock from a volcano on Earth, except with the flow being made of molten ice—in other words, liquid water. Zibi Turtle, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, says finding and studying instances of cryovolcanism would be a “holy grail in solar system evolution” that could explain how icy worlds, oceanic or not, change over time. She says there may also be a tentative connection between Ariel’s geology and the Uranian rings—but without more data, not much more can be said. (Enceladus’s plumes similarly interact with one of the rings of Saturn, steadily replenishing and maintaining the ring’s tenuous existence with injections of icy particles.) “Really, the best way to learn about the interiors and geologic history—the evolution of these satellites—is going to be another mission that goes through or in orbit around the Uranian system,” she notes.

Farther out, the gas-giant planet Neptune has only one moon of any substantial size, Triton. The satellite appears to be a captured interloper from the Kuiper Belt, a diffuse ring of icy bodies at the outer reaches of the solar system. That would make it kin to Pluto, the Kuiper Belt’s largest denizen. Data from Voyager 2, which encountered Neptune in 1989, suggest Triton possesses a very thin atmosphere and more than a few geysers on its surface. “What is actually driving the cryovolcanism? It’s got to be pretty powerful because it’s shooting this material quite high above tiny Triton,” says Heidi Hammel, a senior research scientist at the Space Science Institute. “It’s not just that it’s leaking, it’s jetting these materials.” The answer, as far as researchers can tell, is the same combination of radioactive decay and tidal forces that sustains oceans on other frozen moons of the outer solar system. But geysers that shoot as high as Triton’s, she notes, would require a particularly potent heat source and a massive ocean. “There’s no doubt that there’s a case to be made to look at Triton in the light of a very habitable environment,” she says. “It’s an active world. It has cryovolcanoes and we’ve seen them. There’s a whole lot of material from these cryovolcanoes that is black, dark,” which indicates the presence of organic materials, according to Hammel.

Triton, it seems, is a hipster ocean world—it was seen spouting geysers decades before the plumes of Enceladus were astrobiology’s next big thing. That it has been overlooked for so long is due to its immense distance from Earth and the fact that it has only been visited once, for a handful of hours by Voyager 2. But if Triton is so promising, what then of Pluto, its Kuiper Belt cousin? On this world, planetary scientists may find the weirdest potential ocean world of all.

The Heart of the Matter

During its 2015 Pluto flyby NASA’s New Horizons probe only had about 12 hours to study the dwarf planet Pluto before it was a fading point of light in the rearview. The spacecraft had to make use of every available moment, and that intense scrutiny revealed signs of something spectacular but not yet entirely certain: Pluto appears to have an ocean, too.

Scientists had long suspected what New Horizons confirmed: Pluto is mostly made of ices—not only water, but also of more volatile substances like nitrogen and methane that freeze solid at extremely low temperatures. Yet the spacecraft measured Pluto to be more compact than it was expected to be, given its mass, because freezing water should have expanded and pushed the surface outward. There is also a surprising amount of geologic activity at Sputnik Planitia, the heart-shaped region sprawled across one of Pluto’s hemispheres. All that tumult on a world long thought to be locked in deep-freeze suggests some reservoir of heat must linger within—an ocean, perhaps. If so, though, that ocean would be decidedly atypical.

Bill McKinnon, a Washington University in Saint Louis professor and New Horizons team member, says ammonia is believed to be plentiful on Pluto, albeit hard to detect remotely from a spacecraft flying by at thousands of kilometers per hour. That ammonia, he says, should mix with any water below, which might still be in liquid form even billions of years after the dwarf planet’s formation. “If the ocean is able to cool and not freeze, then the ammonia helps to keep it from disappearing,” he says. This would make the ocean fairly viscous. “At those temperatures it ends up having the consistency of honey,” he says. But the scant bit of time New Horizons spent in the Plutonian system means one unfortunate thing: There is insufficient data to conclusively prove the model. “It’s a story, it’s not proof,” McKinnon notes. “At least it hangs together conceptually.”

The only way to further study these ocean worlds is via targeted missions. Europa, of course, will be lavished with attention by Europa Clipper and JUICE. But outside of that, Ganymede and Callisto are currently the sole also-ran ocean worlds with planned visits—from JUICE. Despite their revered status, even Enceladus and Titan lack committed follow-up investigations, but that could change by the end of the year if NASA chooses to pursue a mission to either as part of the agency’s New Frontiers program of midsize interplanetary missions. Uranus and Neptune missions have been discussed—and rejected—for decades, with no concepts for exploration ever advancing beyond the preplanning stages. Of all the lesser-known worlds, Turtle says, “the two that really cry out to me are Ariel and Dione. I think Dione has some surprises still.”

JUICE could make Ganymede and Callisto’s oceans seem less remote and more dynamic, and maybe even confirm some tentative evidence of cryovolcanism at Ganymede, once again revolutionizing our understanding of these mysterious moons. But without more missions to the outer solar system, inquisitive scientists—as well as the curious public they serve—will be left with more questions than answers for several decades to come. The oceans, of course, will wait. Will we?