We humans take the skies for granted. Like most life on Earth, we have evolved under the rhythms of the Sun, the Moon, and the stars, a situation that made us not only aware — but entranced — by the vast universe around us. 

But as we’ve journeyed deeper into the solar system, we’ve discovered an abundance of worlds that keep their oceans locked away deep in their bellies, hidden from view under crunchy shells of ice. The most famous examples are Jupiter’s moon Europa and Saturn’s moon Enceladus, but each passing year reveals more evidence of subterranean water seas on Jovian moons like Ganymede and Callisto, Saturnian moons like Titan and Mimas, and dwarf planets like Pluto and Ceres.

Several spacecraft are currently en route to the outer solar system to search for insights about that very mystery. If we ever do find life on these worlds—whether they be simple microbes or fantastical space whales—they might be literally in the dark about the great cosmic drama that is unfolding around us.

“Ocean worlds appear to be ubiquitous in our solar system, which means that the vast abundance of liquid water hidden beneath the ice shells of these moons could be the largest volume of habitable real estate in our solar system,” said Dr. Kevin Hand, an astrobiologist and director of Ocean Worlds Lab at NASA’s Jet Propulsion Laboratory, in an email to Supercluster.

“And if these worlds are so abundant in our solar system… just imagine what it means for alien oceans in the Milky Way and beyond,” he added.

While interior oceans appear to be extremely common, no two are alike. Europa’s ocean is estimated to contain at least twice as much water as all of Earth’s seas, with seafloor depths that extend down some 70 miles — ten times as deep as Earth’s seafloor. Enceladus, meanwhile, is a mere 300 miles in diameter, with depths similar to those on our own planet. Titan boasts hydrocarbon seas on its surface, and an ocean of water and ammonia underneath its crust. On Ganymede, the solar system’s largest moon, pressurized ice may surround the silicate rocky core, which could impact habitability.

“On the bigger moons, where you've potentially got this high pressure-formed ice area, you haven't got the interaction between the liquid water and the silicate rock, which is potentially important in generating the chemicals  for life,” said Dr. Grace Richards, an astrobiologist and postdoctoral researcher at Italy’s National Institute of Astrophysics. “You want to be in this sweet spot. You really need that water-rock interaction and to have an energy source, the elements that are essential to life, and the liquid water.”

The marine environments inside ice moons are also impacted by the tidal forces of their host planets, which stretch and squeeze their cores and may generate hydrothermal activity on many alien seafloors. On Earth, hydrothermal vents in the deep sea sustain vibrant microbial ecosystems that sometimes support more complex animals, like tubeworms or molluscs. 

“In the deep ocean on Earth, you get no light after a certain depth, and it’s very high pressure,” Dr. Richards said. “You think about how alien the fish and the octopuses look down there. It is so cool, and it's so different from what we know, looking around us.” 

she added. “Hopefully these extreme environments on Earth can inform what we know about how life might develop in these extreme environments in our solar system.”

Indeed, while these bizarre deep-sea ecosystems are the most common touchstone for envisioning subsurface marine biospheres, it may be that entirely different onramps to life exist in these waters.  

Fortunately, several missions are currently on their way to study some of these worlds up close to assess their potential habitability. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) and NASA’s Europa Clipper, launched respectively in 2023 and 2024, will arrive in the Jupiter system in the early 2030s. JUICE will conduct flybys of the Jovian ice moons Europa, Callisto, and Ganymede, and will eventually end up in orbit around the latter.

“JUICE will be the first time that we're going to have a ground-penetrating radar instrument for an icy moon,” Dr. Richards said. “That's going to give us loads of information about these icy moon interiors, because that's something that we really want to constrain.”

Europa Clipper also has ice-penetrating radar that can peer nearly 20 miles deep into its namesake moon’s icy shell, along with cameras and instruments designed to probe other properties. From its orbit around Jupiter, Clipper will perform a series of close flybys of Europa, eventually reaching altitude as low as 16 miles above its surface, providing unprecedented close-ups of this fan-favorite world.

Europa Clipper is “not a mission designed to detect signs of life but the spectrometers will tell us about the chemistry of the ice,” explained Dr. Hand. “The ice shell of Europa serves as a window into the ocean below and thus we can use the chemistry of the ice shell to learn about the chemistry of the ocean. I’m particularly interested to see if we can find any organic compounds with our spectrometers because, of course, carbon is the key building block for life on Earth and I think life on Europa would also be carbon-based.”

Last, NASA’s Dragonfly mission will launch to Saturn’s moon Titan in 2028, where it will land in the mid-2030s and fly from location to location to study the moon’s rich surface chemistry, which is unlike anything else in the solar system. 

“Titan is an organic chemist’s dream world — there are liquid methane and ethane seas, lakes, and rivers; there are dunes piled high with grains made from complex organics; and there is a subsurface liquid water ocean beneath the organic-rich icy crust of Titan,” said Dr. Hand. “If we do someday find life on Titan, many of us speculate that its biochemistry would be unlike anything we see here on Earth.”

We don’t yet know if any of these oceans are actually inhabited, and we’ll probably have to wait at least a decade for any preliminary answers about their potential to host even the simplest forms of life. While the results will be exciting, it’s important not to get too ahead of our space skis with optimism about prospective aliens in the deep. 

“I don't think that we'd be finding any evidence of anything crawling around in the oceans, to be quite honest, which is maybe disappointing for people,” Dr. Richards said. “I think if we found evidence of life in our solar system, it would be single-celled organisms.”

“It's fun to talk about speculation of the deep oceans on Earth and Enceladus —the thermal vent systems where there's potentially huge biological communities, like there is on Earth,” she added. “But ultimately that is quite unlikely, and I think we need to try and be objective when we're thinking about it.”

Still, it’s tempting to wonder what kinds of life might be possible in these vast ice-covered abysses. Science fiction has already given us haunting visions, from the bioluminescent tentacled creature of Europa Report — a 2013 film that Dr. Hand consulted on — to spacefaring parasitic slug invaders from Titan in Robert Heinlein’s 1951 novel The Puppet Masters.

The discovery of even microbial life in one of these oceans would be among the biggest breakthroughs in scientific history, a finding that would suggest life is common in the wider universe. But the sheer scale and strangeness of these environments invites wilder speculation about the possibility that complex life — perhaps even intelligent civilizations — might emerge in the depths of these worlds.

Dr. Hand explores this concept in his 2020 book Alien Oceans: The Search for Life in the Depths and ponders the implications of these closed-off habitats for creatures that might emerge in them.  

“Despite the potential for a technologically advanced species on such a world, they would have no night sky to foster their sense of wonder,” Dr. Hand writes in the book. “They might never be able to sense beyond the ice to discover the stars above. Could they ever develop the impulse to explore beyond their own planet, not having the Sun or stars to compel them into the sky? Think about what that might mean for their philosophy, their art, their music and their concept of the meaning of life.”

Dr. Hand speculates that creatures in these worlds might base their mythologies on the acoustics of cracking ice, rather than the stars in the heavens above. What strange stories might be told about the vibrations rippling through the water, or the patterns of heat along the seafloor, or the bioluminescent displays that could illuminate these encased seas? 

It’s almost excruciating to imagine a universe teeming with such vibrant ecosystems, that are nonetheless sealed off from contact or even detection by miles of ice. But Dr. Hand holds out some optimism that somewhere, one of these civilizations might peel off the icy veil and grasp, even for a moment, a sense of the grand universe outside of these womblike worlds.

“I like to imagine that within some of these alien oceans beyond Earth, there are, in fact, biologically and perhaps technologically advanced civilizations,” Dr. Hand concludes in Alien Oceans. “Given the abundance of ocean worlds in our solar system, such worlds might be ubiquitous throughout the universe. I only hope that if they are out there, if there are intelligent creatures deep beneath the icy shells of these distant worlds, that some of them have found a way to peer through the ice and see the wonder of what lies beyond.”

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Becky Ferreira is a science reporter based in upstate New York. Her book First Contact, about the search for alien life, was published in September 2025.