9 min read

Ocean Worlds: Water in the Solar System and Beyond

Illustration of the surface of Europa - shown as icy blue - with Jupiter behind it, and the Europa Clipper spacecraft in front of Jupiter.
An illustration of NASA's Europa Clipper spacecraft above the surface of Europa and in front of Jupiter.
Credits: NASA/JPL-Caltech

The story of oceans is the story of life. Life as we know it requires three ingredients: energy, organic molecules, and liquid water. Our search for life beyond Earth is, in part, a search for planets and moons that harbor substantial liquid water. We call these places “ocean worlds,” and we’re learning that they could be ubiquitous in the galaxy.

Oceans define our home planet, covering the majority of Earth’s surface and driving the water cycle that dominates our land and atmosphere. But more profound still, the story of our oceans places our home in a far larger context that reaches deep into the universe and places us in a rich family of ocean worlds that span our solar system and beyond.

Origins of Oceans

Where do oceans come from?

What is the ultimate origin of water? A water molecule is made up of one atom of oxygen and two of hydrogen. Hydrogen was created in the Big Bang, and oxygen in the cores of stars more massive than the Sun. Enormous amounts of water, in gaseous form, exist in the vast stellar nurseries of our galaxy.

Swirls and knots of colorful gases and dark dust clouds fill the entire image, with bright clusters of stars sprinkled throughout.
This Hubble Space Telescope image shows the Carina Nebula, where a maelstrom of star birth – and death – is taking place.
NASA, ESA, N. Smith (University of California, Berkeley), the Hubble Heritage Team (STScI/AURA); N. Smith (University of California, Berkeley) and NOAO/AURA/NSF

The Hubble Space Telescope peered into the Helix Nebula and found water molecules. Hydrogen and oxygen, formed by different processes, combine to make water molecules in the ejected atmosphere of this dying star. The origins of our oceans are in the stars.

Black background. At image center is a colorful ring of gas and dust. The ring is a deep red along the outside. Moving inward toward the center of the ring, the colors change from red, to orange, to yellow, to pinkish-white, to a light blue sphere in the center.
The Helix Nebula is an example of a planetary nebula. In its center lies a white dwarf star. Though it represents the end of one planetary system, the nebula's gases will feed the formation of future stars and planets. Credit: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)
NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Water molecules exist in the Orion Nebula and are still forming today. The nebula is composed mostly of hydrogen gas; other molecules are comparatively rare. Even so, the nebula is so vast that it creates enough water every day to fill Earth’s oceans 60 times over. Water, along with every other molecule created in these stellar nurseries, becomes raw material for the formation of new planetary systems.

This region of the Orion Nebula is seen as a colorful, bright region of gas and dust, mostly in shades of green, yellow, orange and brown. It's brightest toward the center and darkens at the image edges.
This spectacular color view shows the center the Orion nebula as seen by the Hubble Space Telescope. The nebula is a region of intense star formation. Credit: NASA, ESA, C.R. O'Dell (Rice University), and S.K. Wong (Rice University)
NASA, ESA, C.R. O'Dell (Rice University), and S.K. Wong (Rice University)

Water molecules are abundant in planetary systems forming around other stars. For instance, they have been found around the 20-million-year-old star Beta Pictoris, where a huge disk of dust and gas hints at collisions between comets, asteroids, and young planets.

An illustration of a swirling disk of gas and dust in space, with a couple of planets forming, plus a couple of comets in the distance. Several comet-like fragments are seen crashing or about to crash into one of the planets.
Artist’s conception of the view toward the young star Beta Pictoris from the outer edge of its disk. This disk of dust and gas orbiting the star is produced by collisions between and evaporation of asteroids and comets. A giant planet may have already formed and terrestrial planets may be forming. Credit: NASA/FUSE/Lynette Cook
NASA/FUSE/Lynette Cook
A photo taken from the International Space Station shows the curving horizon of Earth below. In view is a brilliant blue ocean region with islands and small white clouds. Pieces of the station like the solar arrays are visible at the top of the photo.
Clouds trace out the islands of the Caribbean Sea in this photo taken by an astronaut aboard the International Space Station.

Oceans of Earth

How did water arrive on Earth?

Asteroids and comets are debris left over from the formation of our solar system, and are rich in water. These small bodies are time capsules that contain tantalizing clues about what our solar system was like 4.5 billion years ago.

Asteroid Vesta as seen by NASA's Dawn spacecraft.

Most asteroids orbit the Sun between the planets Mars and Jupiter, but many swing nearer to Earth and even cross our orbit. Comets are found in the outer reaches of our solar system, either in the Kuiper Belt just beyond the orbit of Neptune, or in the vast, mysterious Oort Cloud that may extend halfway to the nearest star.

Over billions of years, countless comets and asteroids have collided with Earth, enriching our planet with water. Chemical markers in the water of our oceans suggest that most of the water came from asteroids. Other observations hint that ice, and possibly even liquid water, exists in the interiors of asteroids and comets.

Jet erupting from comet.
A jet of gas and fine ice and dust particles erupts from Comet 67P/Churyumov-Gerasimenko, imaged here by ESA's Rosetta spacecraft.

How are our oceans changing?

NASA’s research on oceans also includes our own planet and helps us to better understand the role of Earth’s ocean in our planet’s climate system. And as we learn more about our own oceans, we will better understand worlds beyond Earth.

Water on Earth is very abundant – about 71 percent of Earth’s surface is covered by water. There are more than 326 million trillion gallons of it on Earth, and the oceans contain about 96.5 percent of all the planet’s water. Less than 3 percent of all water on Earth is fresh water (usable for drinking), and more than two-thirds of Earth’s fresh water is locked up in ice caps and glaciers.

A view of Earth in the blackness of space, looking toward the Pacific Ocean, shows only blue ocean and clouds.
Earth is a water planet: three-quarters of the surface is covered by water, and water-rich clouds fill the sky.

As global temperatures increase, the ocean responds by expanding. Changing sea levels will affect everyone on our planet, and NASA has been monitoring these trends for decades.

A short animation clip shows a satellite orbiting above Earth, bouncing a radar beam off the ocean surface as it flies.
This animation shows the radar pulse from the Sentinel-6 Michael Freilich satellite's altimeter bouncing off the sea surface in order to measure the height of the ocean.

Currently, ocean levels are rising at the rate of 0.13 inches per year. The Greenland ice sheet is melting at the rate of 287 billion tons a year, and the Antarctic ice sheet is losing 134 billion tons a year. Both will be factors in sea level rise.

Oceans Lost

Do planets lose their oceans over time?

An illustration of Mars as it might have appears with water on its surface, shows the Red Planet with areas of blue water, with Sun glinting off the liquid's surface.
This artist’s impression shows how Mars may have looked about four billion years ago.
ESO/M. Kornmesser


Billions of years ago, Venus may have been our solar system’s first ocean world. Venus lacks a strong global magnetic field, which on Earth, helps to protect our atmosphere. A runaway greenhouse effect raised temperatures enough to boil off the water, which escaped into space due to the solar wind.

An illustrated view from the rocky surface of ancient Venus shows a smoking volcano in the distance, and a body of water in the foreground. The sky is bluish, with clouds like on Earth.
Long ago, Venus might have looked much more earthlike, with oceans and seas on its surface.
NASA's Goddard Space Flight Center Conceptual Image Lab

Ancient Mars

Mars was once much more Earth-like, with a thick atmosphere, abundant water, and global oceans (as in this artist’s conception). Billions of years ago, Mars lost its protective global magnetic field, leaving it vulnerable to the effects of our Sun: solar wind and space weather.

The MAVEN mission has measured Mars continuing to lose its atmosphere to the Sun at the rate of nearly 400 kilograms per hour. Scientists estimate that Mars has lost approximately 87 percent of the water it had billions of years ago.

The Mars rover Perseverence is seen at right, with the rust-colored, dusty and rocky surface of Mars in the background.
Present-day Mars is a cold, desert world, but NASA’s Perseverance Mars rover is studying what the planet's rocks tell us about its warmer, wetter past.

Mars today

Most of the remaining water on Mars is frozen in the ice caps or trapped beneath the soil, but Mars once rippled with rivers and ponds. Billions of years ago, the Red Planet provided a potential habitat for microbial life. As the planet’s atmosphere thinned over time, that water evaporated, leaving the frozen desert world that NASA’s Mars missions study today.

An illustration shows a spacecraft in silhouette above an icy moon's surface with reddish fractures. Beyond the moon's horizon, the planet Jupiter sits in the distance.
Illustration of NASA's Europa Clipper spacecraft above Jupiter's ocean moon Europa. The spacecraft will use its powerful suite of science instruments to determine if the moon has the ingredients to support life as we know it. Could simple life have developed in its ocean?

Oceans of Our Solar System

Which planets and moons in our solar system are considered ocean worlds?

Earth isn’t the only ocean world in our solar system. Water on other worlds exists in diverse forms on moons, dwarf planets, and even comets. Ice, water vapor in the atmosphere, and oceans on other worlds offer clues in the quest to discover life beyond our home planet.

Jupiter’s Moon Europa

Scientists strongly suspect that a salty, subsurface ocean lies beneath Europa's icy crust. Tidal heating from its parent planet, Jupiter, maintains this ocean's liquid state and could also create partially melted pockets, or lakes, throughout the moon's outer shell. The Hubble Space Telescope has spotted possible water plumes erupting from the moon's surface, which may be excellent targets for NASA’s upcoming Europa Clipper mission.

The pale gray of Europa is seen against the blackness of space.
This view of Jupiter’s icy moon Europa was captured by the JunoCam imager aboard NASA’s Juno spacecraft during the mission’s close flyby on Sept. 29, 2022. The agency’s upcoming Europa Clipper spacecraft will explore the moon.
Image data: NASA/JPL-Caltech/SwRI/MSSS Image processing: Kevin M. Gill CC BY 3.0

Jupiter’s Moon Ganymede

Ganymede is the largest moon in our solar system, and the only moon with its own magnetic field. Studies indicate a global, underground saltwater ocean is present there. Ganymede could in fact have several layers of ice and water sandwiched between its crust and core.

Ganymede Full Disk
Ganymede as seen in 1979 by NASA's Voyager 1 spacecraft.

Jupiter’s Moon Callisto

Callisto’s cratered surface lies at the top of an ice layer estimated to be about 124 miles (200 km) thick. An ocean, which is thought to be at least 6 miles (10 km) deep, could be directly beneath the ice.

Jupiter's moon Callisto appears in space, pockmarked by many bright craters in its dark, brown surface.
Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto in this image of Callisto from NASA's Galileo spacecraft.

Saturn’s Moon Enceladus

Scientists on NASA’s Cassini mission discovered that Enceladus has a global ocean of liquid water beneath its icy shell. This underground ocean is venting into space, feeding the moon's impressive plume, which sprays from deep fissures called "tiger stripes" near the moon's south pole. Cassini flew through the plume, and over the course of the mission, its instruments detected organic and nitrogen-bearing molecules such as acetylene, ammonia, carbon dioxide, and methane. And silica grains in the plume are of a size that likely formed in the kind of hydrothermal vents that, on Earth, are the foundation of rich ecosystems.

The icy white surface of a moon in space is completely covered in wrinkles, folds, and fractures, some appearing bluish in color.
NASA's Cassini spacecraft found that Enceladus, an icy and geologically active moon of Saturn, also contains a global ocean with evidence of hydrothermal activity.
NASA/JPL/Space Science Institute

Saturn’s Moon Titan

Titan is believed to have a salty subsurface ocean – as salty as the Dead Sea on Earth – beginning about 30 miles (50 km) below its ice shell. It is also possible that Titan’s ocean is thin and sandwiched between layers of ice, or is thick and extends all the way down to the moon’s rocky interior.

Seas, lakes, and rivers are found on Titan’s surface, but they are not filled with water. Instead, they are liquid hydrocarbons – primarily methane and ethane.

Ligeia Mare, shown here in a false-color image from NASA's Cassini mission, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region.

Saturn’s Moon Mimas

Research suggests that Mimas has either a subsurface ocean or that its core is shaped like a football. If Mimas is hiding a liquid water ocean, it lies 15 to 20 miles (25 to 30 km) beneath the moon’s impact-battered surface.

crescent moon with large crater
This view of Mimas from NASA's Cassini spacecraft features the moon's most prominent feature – the giant Herschel Crater.
NASA/JPL/Space Science Institute

Neptune’s Moon Triton

Active geysers on Triton spew nitrogen gas, making this moon one of the known active worlds in the outer solar system. Volcanic features and fractures mark its cold, icy surface, likely results of past tidal heating. A subsurface ocean at Triton is considered possible, but is unconfirmed.

A closeup view of one hemisphere of Triton shows frosty, rippling terrain on a light brown surface. There is a difference in the appearance of the terrain from top to bottom, with the lower part appearing brighter.
A view of Triton from NASA’s Voyager 2 spacecraft during its flyby of the Neptune system in 1989.

Dwarf Planet Pluto

With towering mountains of water ice and flowing glaciers of nitrogen and methane ice, Pluto is a surprisingly active world. Mysterious fault lines, some hundreds of miles long, suggest that Pluto has a hidden subsurface ocean.

View of Pluto in space, featuring the heart-shaped bright area amidst the dark reds, pinks, and purples of its icy landscape.
NASA’s New Horizons spacecraft captured this high-resolution, enhanced color view of Pluto in 2015.
NASA/JHUAPL/SwRI | Full caption and image
exoplanet K2-18b illustration
This Illustration shows the super-earth exoplanet K2-18b, its host star and an accompanying planet in this system. K2-18b is known to host both water and temperatures that could support life.
ESA/Hubble, M. Kornmesser

Oceans Beyond

Are there oceans on planets around other stars?


Water vapor has been discovered on a planet roughly the size of Neptune; the smallest exoplanet known to have water. HAT-P-11b is 120 light years away in the constellation Cygnus and sits close to its star in a five-day orbit. This world is likely too warm for oceans, but has water vapor and clear, cloudless skies.

The silhouette of a planet is seen agains the surface of its star. A hazy envelope of atmosphere can be seen surrounding the planet.
A Neptune-sized planet with a clear atmosphere, HAT-P-11b, is shown crossing in front of its star in this illustration.
NASA, ESA, and R. Hurt (JPL-Caltech)


Kepler-22b is the first planet in a confirmed orbit in a star's habitable zone – the region around a star where liquid water could persist on its surface. Kepler-22b is a “super-Earth,” about 2.4 times Earth’s size. Scientists do not yet know if the planet has a rocky, gaseous, or liquid composition. It's possible that the world would have clouds in its atmosphere.

An illustration of a greenish-blue, ocean-covered planet in space, with swirls of white cloud in places.
Illustration of Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star.


Kepler-452b is a near-Earth-size world in the habitable zone of a star similar to our Sun – about 10 percent larger and 20 percent brighter. This world is about 60 percent larger than Earth, with an orbit of 385 days, which is slightly longer than Earth’s year. Kepler-452b is about 6 billion years old, marginally older than our solar system.

An exoplanet in space is shown, with its host star in the distance. The planet displays a rocky surface, with ice, clouds, water, and an atmosphere.
Illustration depicting one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of star that is similar to our sun. The habitable zone is a region around a star where temperatures are right for water -- an essential ingredient for life as we know it -- to pool on the surface.
NASA Ames/JPL-Caltech/T. Pyle


The five planets of the Kepler-62 system orbit a star two-thirds the size of the Sun and only one-fifth as bright. At 7 billion years old, the system is older than our Sun.

Kepler-62 is home to two habitable zone worlds, Kepler-62f and Kepler-62e. Kepler-62f orbits every 267 days and is only 40 percent larger than Earth, making it one of the smallest exoplanets known in the habitable zone of another star.

An earthlike planet with oceans and clouds is seen in space, with its host star in the background.
This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. The discovery of Kepler-186f confirmed that Earth-size planets exist in the habitable zones of other stars – a significant step closer to finding a world similar to Earth.
NASA/Ames/SETI Institute/JPL-Caltech