Enceladus likely has a warm salty ocean, and liquid water lurks beneath the surface of Ganymede. The red planet Mars harbors evidence of being flooded by oceans and life forms, long ago. These findings of the past few years are continuing to chip away at the once-held belief that the rest of the solar system was dry and barren. As it turns out, there are quite a few neighboring moons and planets with water.
It seems there are few places in the solar systems without some amount of water, whether liquid or solid. There is even a small amount of water vapor on Venus, something like 20 parts-per-million. And every time a source of liquid water is found or suggested, it brings up the chances of life on that world because of the way water acts as a solvent—facilitating the metabolic processes at the most basic level of life. That’s why the hunt for extraterrestrial life (quite doubtfully of an intelligent sort, though we’ve found some quite remarkable octopuses on Earth) has turned from distant solar systems to our own cosmic backyard.
🪐 You want to know more about our universe. So do we. Let’s explore its mysteries together—join Pop Mech Pro.
Here’s the breakdown of all the planets with water (and other celestial bodies) that we know about in our solar system, and what form the water comes in.
Oceans All But Confirmed
Europa
Europa has been the biggest contender for life for years now, with a craggy, icy crust hinting in almost every way at an ocean below. Thanks to the tidal effects from Jupiter (friction inside the moon created by the pull of the planet’s gravity), the water would be kept liquid and possibly even warm below the icy crust, helped by possible hydrothermal vents.
There has been some evidence of ice geysers shooting from the surface of Europa, as well as evidence that the ocean could have Hadley cells—warm water radiating from the moon’s equator. Europa could provide the possibility not just for life, but, if the conditions were just right, even complex life.
NASA’s Europa Clipper will reach this ocean moon in 2030 to conduct a long awaited investigation.
Enceladus
We’ve known of tiny, sleepy Enceladus since 1789. Its diameter is just 310 miles, smaller even than Ceres and Vesta, the two largest objects in the asteroid belt. Compare that to Europa, slightly smaller than our moon at a 1,950-mile diameter. Never mind its small size: This is one of the most intriguing places in the solar system and best candidates for a warm, wet, and salty ocean. It has as high a probability for life, if not higher than Europa.
Enceladus orbits near the rings of Saturn. In fact, watery eruptions from Enceladus’ ice geysers comprise the F-Ring of the gas giant—the moon spews 1,000 tons of water into space every hour, according to Phil Plait at Bad Astronomy, along with organic molecules, salt, and other materials. Research suggests the ocean is also very warm thanks to the tidal effects from Saturn. The tug causes hydrothermal activity, which warms the oceans and causes the geysers, all in the kinds of hot beds that sprouted life on Earth.
The Probablies
Ganymede
Ganymede is the largest moon in our solar system, bigger even than the planet Mercury. Astronomers had long suspected that an ocean lies beneath the 100-mile-thick ice crust at the surface, and in a 2015 study, aurorae activity suggest that the ocean is somewhat warm and definitely salty. Unlike Europa, no ice geyser activity has yet been spotted. This could be because of a reduced tidal effect on the moon, which is further from Jupiter than Europa. Unlike Europa, it doesn’t have the cross-hatched icy surface showing consistent geologic surface activity. Still, the signs are good that Ganymede is harboring an ocean.
The Maybes
Callisto
Callisto is similar in composition to Ganymede and, as the furthest out of the four Galilean moon of Jupiter, is bombarded with the least amount of radiation. It also has a magnetic field, adding some additional protection.
We know there’s water here—what we don’t know is to what extent it’s liquid. Callisto’s relative lack of geologic activity suggests that the moon might not be able to sustain an ocean without the presence of some kind of anti-freeze compound within, meaning that there could be just a whole lot of ice in there. However, as the farthest out of the satellites, it could make an interesting destination for exploration, allowing you to avoid the harsher effects of Jupiter’s radiation while remotely exploring the other moons for signs of oceans and life.
Ceres
Ceres has been known since 1801, but its small size has made it difficult to study. Until recently, it was believed to be a relatively rocky body. But observations from Hubble and evidence from the Dawn spacecraft, which arrived there in 2015, have raised an intriguing probability: that Ceres is less a ball of rock and more a watery dwarf planet with an icy mantle and a slushy ocean below. Located in the asteroid belt between Mars and Jupiter, the world could be our nearest oceanus neighbor. If it’s true, it would be the nearest world to Earth with an ocean. Dawn revealed that as Ceres’ ocean froze, salts and other minerals left deposits scattered over the surface. Organic compounds also exist on the surface.
Mars
The red planet probably once had oceans, including one covering a good chunk of the northern hemisphere. There are traces of water left on the surface, including compounds from the evaporation of the ancient ocean, as well as seasonal water ices covering the surface of the planet. There is some evidence pointing to occasional melting on the surface as well.
That much is clear.
Missions like the NASA’s 2021 Perseverance rover and China’s first Mars rover, Zhurong, are investigating the intriguing possibility that Mars still has water underneath the surface, possibly in the form of aquifers. Theoretically, these underground waterways could still host microbial life under the surface of Mars. The question, then, is whether this water exists as ice or liquid, and how much of it lurks beneath the soil. An entire ocean’s worth is unlikely, but a significant amount of subsurface water isn’t. Both rovers continue looking specifically for signs of microbial life and water. Perseverance has already found rocks in the Jezero crater that appear to have been shaped by water.
Titan
With a surface slurry of water ice and ice made of other compounds, like hydrocarbons, Saturn’s largest and arguably most fascinating satellite has some of the most abundant pools of liquid found anywhere in the solar system. Titan’s methane is a hydrocarbon chain that’s good for life, but not necessarily as we know it.
Just beneath a hefty water ice crust is more liquid water, scientists suspect.
Titan also has dense clouds of hydrocarbons that burst with rain, plus a liquid-rich landscape of rivers, lakes, and ocean of liquid hydrocarbons including methane and ethane below the icy crust.
Dione
The team behind NASA’s Cassini probe, which has turned up many of the exciting findings about Saturn and its moon, jokingly called Dione a “weaker copycat of Enceladus.” This Saturnian moon is much less active now than in the past, but it shows signs of geologic activity, including giant mountain peaks and other evidence pointing to a warmer history. It’s possible that the moon retains enough of that heat for a small ocean to exist.
Pluto (and maybe Charon)
New Horizons flew by Pluto in 2015, becoming the first spacecraft to directly visit the world once known as the ninth planet. It discovered hints of something that once seemed unlikely: an ocean.
Pluto is still seen mostly as an icy world. However, the tidal forces from its orbit with its largest moon, Charon—along with scientists theorizing a large collision likely formed Pluto and its five moons out of the same materials—means Pluto could have hosted a ocean, and leaves open the outside possibility that it’s still around.
An Outside Chance of Water
Mimas
Mimas, the “Death Star moon,” is pretty much one big snowball. There doesn’t seem to be much more to it than water ice. Yet a few unusual features hint at something weird on Mimas. The moon wobbles as it orbits Saturn, which indicates something unusual going on beneath the surface. The Cassini team says that it could be an ocean. But only maybe. The other chief possibility is that Mimas has a football-shaped core giving it the unusual tilt.
At around the size of Enceladus, the moon is too small to retain the heat from its formation, so any ocean on Mimas would have to have an outside force acting on it—possibly radioactive decay.
Triton
Neptune’s largest moon, Triton, looks a lot like Pluto. There's a reason for that. Its retrograde (backwards) orbit in comparison to the rest of the system suggests that Triton could be a captured Kuiper Belt object, and not something that formed alongside the planet. The moon’s surface seems to be a mix of methane and water ices, much like Pluto, and there's the outside chance of an internal ocean, provided there is enough heating or radioactive decay.
The moon likely has geysers, but instead of water they probably shoot nitrogen, giving the moon a thin atmosphere. We just don’t know much else about Triton because the only close-up imaging came from Voyager 2’s flyby in 1989.
Titania, Oberon, and Umbriel
The same thing goes for Uranian moons: We need a better look at them. But preliminary indications show that Titania and Oberon are likely ice and rocky materials. Neither has, at the time, enough evidence to support liquid water hypotheses without an anti-freeze agent like ammonia.
Umbriel, too, is largely composed of ice, but is even less likely to have an ocean. It does, however, contain a bright spot of ice near one of its poles, likely the effect of a crater impact on the surface. There is also evidence of carbon dioxide gasses trapped under the surface.
Tethys, Rhea, and Iapetus
These moons of Saturn appear similarly frozen, though there’s an outside chance of liquid water on Rhea. These worlds are relatively inert, though Iapetus shows evidence of water sublimation (moving directly from solid to gas) on the surface. While these moons may not be good candidates for liquid water, they demonstrate the sheer abundance of water in the outer solar system.
The Kuiper Belt
There are hundreds of known objects out in the Kuiper Belt, where Pluto resides, many of them believed to be icy. The dwarf planets Eris and Haumea are believed to be similar to Pluto in composition, with water ice on the surface. But these small worlds were discovered almost two decades ago. There are also a few dwarf planet candidates that are known to be icy in nature, including Varuna, Quaoar, and Orcus. The latter has some indication of cryovolcanism and could potentially have a liquid ocean.
There are also a number of comets in the Kuiper Belt and beyond that are believed to be composed of water. This includes the first identified member of the Oort Cloud, Sedna.
A Little Bit of Water
Mercury
Perhaps the most surprising place water has been detected in the solar system is Mercury, the closest planet to the sun. While the surface is scorching, the poles are often untouched by the sun’s heat, leading to an area where ice can accumulate. The Mercury-observing MESSENGER spacecraft snapped some polar photos of the frozen ice caps. Liquid water is unlikely because Mercury is so hot, but MESSENGER found signs that some of the accumulations were recent.
Our Moon
The moon and Mercury, they’re really not so different in appearance. Both are airless, rocky worlds, and both, it seems, have accumulations of water ice at the poles. Scientists had long suspected ice could be on the moon. India proved it in 2009… by crashing the Chandrayaan-1 probe headlong into the ice and seeing the plumes it formed.
While it’s far from abundant, the water ice on the moon could help out moon colonization some day.
Neptune and Uranus
Here’s another way that your grade school science textbook has been upended: Some researchers have begun calling Neptune and Uranus “ice giants” rather than “gas giants.” In part, this is due to the abundance of ices in the lower layers of the planets, in weird states made possible by the intense pressure.
Water vapors have been spotted in the upper atmospheres of these worlds, while ices in the lower parts of the atmospheres are suspected, especially in the “mantle,” the area of hot ices in the lower atmosphere. Some astronomers have gone as far as to propose “oceans” in both planets, though they would be nothing like the big blue bodies of water we know. The boiling temperatures would keep what would otherwise evaporate into a sort of solidified state under incredible pressure.
Just Vapors
Venus, Jupiter, Saturn... and more
There are hundreds more places in the solar system where water can be found, whether tiny, ice-packed moonlets never given official mythological names or just areas with a moderate accumulation of ice. Trace amounts of water vapor have been detected on Venus’, Jupiter’s, and Saturn’s atmospheres. And water is also embedded in Saturn’s dazzling rings. Still, it speaks to the abundance in our solar system, and the ways our views have changed from a dry solar system with a pale blue dot in its midst to one of abundant water and rife with possibilities for life.
Right now, Earth is the only true pale blue dot, the only place where life as we know it can exist, where temperature variables create a wide array of ecosystems and vegetation, where a thick, luscious atmosphere enables life by air, by sea, and by land. But it may not be the only genesis in our solar system. Life could thrive in dark depths of distant oceans, in forms large and small, life we can’t fathom the shape of because it has no point of reference to any we’ve experienced. We also don’t need to travel light years to find it.
There’s an exobiology truism that where there is water, there is life. And where once we believed that we were the only place to find water, we have instead proven that it’s abundant. While it may not mean life in the depths of Neptune or on the frigid, dark poles of Mercury, it could open the door for new explorations not just at Europa and Enceladus, but below the crust of Ganymede or in the depths of Dione. Not just fossil hunting on Mars, but looking for real, tangible, living organisms on Ceres.
It could provide way stations as we move out into the farther reaches of space. And if there’s abundant water in our own backyard, it shows that it may not be so rare to find, and that we may not be alone in the universe—whether that’s on a mini-Neptune 2000 light years away, or on a cold moon orbiting Saturn inside its rings.
Before joining Popular Mechanics, Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.
