When we think of Earth, we picture a blue planet, overflowing with water from the highest mountain peaks to the deepest ocean trenches.
But in the grand scheme of the solar system, Earth’s water content is surprisingly modest.
In fact, some of our celestial neighbors have far more water—hidden beneath thick layers of ice.
Recent data from NASA suggests that Jupiter’s moon Europa harbors twice as much water as Earth, despite being smaller than our own Moon.
Even distant Pluto—once thought to be a frozen wasteland—may hold an underground ocean comparable in size to Earth’s seas.
With the increasing number of missions targeting these mysterious ocean worlds, we may soon discover alien life swimming beneath their icy crusts.
But before that happens, let’s take a deep dive into where the real water giants of our solar system exist.
Earth’s Place in the Solar System’s Water Hierarchy
To put things into perspective, Earth contains about 1.335 zettalitres (ZL) of water—that’s 1 billion cubic kilometers.
It’s an unfathomable amount, yet it pales in comparison to some of our neighboring moons.
Using estimates from NASA’s Jet Propulsion Laboratory (JPL), planetary scientist Steve Vance has compiled a ranking of celestial bodies with the most water.
Here’s how they stack up:
- Ganymede (Jupiter’s moon) – The undisputed king of water worlds. 69% of its entire volume is water, making it the wettest place in the solar system.
- Titan (Saturn’s moon) – This moon has vast subsurface oceans and methane lakes on its surface.
- Callisto (Jupiter’s moon) – Believed to have an ocean hidden beneath its thick ice shell.
- Europa (Jupiter’s moon) – A deep, global ocean locked beneath an icy crust—twice the water volume of Earth.
- Earth – The only known world where water freely exists on the surface.
- Pluto – Evidence suggests a massive underground ocean.
- Dione (Saturn’s moon) – Potentially harbors a deep internal ocean.
- Triton (Neptune’s moon) – Thought to have a subsurface sea.
- Enceladus (Saturn’s moon) – Famous for its water geysers shooting into space.
Beyond this list, scientists suspect that even Mimas (another moon of Saturn) and Ceres (the largest asteroid in the solar system) might have oceans, though their sizes remain unknown.
But Wait—Shouldn’t Earth Have the Most Water?
It’s a common assumption: since Earth is covered in oceans, it must be the wettest place in the solar system.
But when you compare Earth’s total water content to the size of other worlds, it’s actually quite dry.
Take Ganymede, for example.
Not only does it contain significantly more water, but nearly 70% of its entire volume is liquid.
Earth, on the other hand, is only 0.12% water by volume—meaning if our planet were the same size as Ganymede, its oceans wouldn’t even come close.
Another misconception?
That these ocean worlds are just frozen wastelands.
In reality, many of these moons likely have active oceans beneath their surfaces—oceans that are kept warm by tidal heating and geothermal activity.
This leads to a fascinating question: If Earth’s deep-sea hydrothermal vents can support thriving ecosystems without sunlight, could the hidden oceans of Europa or Enceladus harbor extraterrestrial life?
The Race to Explore Alien Oceans
NASA and other space agencies have already set their sights on investigating these watery worlds. Two major missions are in the works:
- Europa Clipper (NASA) – Scheduled to launch between 2022 and 2025, this mission will conduct multiple flybys of Europa, scanning its icy shell and searching for signs of habitability.
- Jupiter Icy Moons Explorer (ESA) – A European Space Agency mission set to reach Jupiter by 2030, conducting flybys of Europa and an extended orbit around Ganymede.
One of the most exciting objectives?
Sniffing out water plumes that erupt from beneath the ice.
If these geysers contain organic molecules or microbial life, it would be one of the most groundbreaking discoveries in human history.
What Happens If We Find Life?
The implications of discovering life beyond Earth—especially within our own solar system—would be staggering.
It would mean that life isn’t unique to our planet, and that it may be far more common than we ever imagined.
Some scientists argue that these subsurface oceans could be home to primitive microbes, while others even speculate about complex life forms akin to deep-sea creatures here on Earth.
If organisms can survive the crushing darkness of our own ocean trenches, why not beneath the ice of Europa?
Earth’s Water is Just the Beginning
While Earth may seem like the pinnacle of a water world, it’s only a small player in the grand scheme of the solar system.
Moons like Ganymede, Europa, and Titan hold vastly more water—some of which may be teeming with life.
As future missions venture into the unknown, we may soon uncover hidden ecosystems beneath the ice, proving once and for all that Earth is not alone in its ability to support life.
And if we do, it will change everything we know about our place in the cosmos.
