Ocean Worlds

Oceans help make life on Earth possible. So if there are oceans beyond Earth, do living things exist on those worlds, too? This is a question that NASA scientists are trying to answer. Right now, we know there are moons and dwarf planets in our solar system where oceans do exist. And there are more places where they could exist. To find out more, we will need to send spacecraft to study these places up close. But we already have some good clues to get us started.

Flip through the slideshow to see what we know about ocean worlds in our solar system. Which of these places do you think is most likely to have living things?

Our home planet, Earth, is the only place we know of where life exists. Earth is called the “ocean planet" because it is covered mostly by water (71 percent water and 29 percent land).

Size: Earth has an equatorial circumference (the distance around its middle) of nearly 25,000 miles (40,000 km). Earth is not the biggest planet, but it is the biggest ocean world that we know of in our solar system.

Distance from the sun: On average, Earth is 1 astronomical unit, or AU, from the sun. That's nearly 93 million miles (150 million kilometers)! When scientists search for worlds that may have living things, they look at the distance between those worlds and our sun. That's because one of the things that makes our lives on Earth possible is our distance from the sun. We’re not too close to the sun, where it's burning up, and we’re not too far away, where it's really cold. That doesn't mean that life doesn't exist farther away from the sun. It just may be very different than what we have here on Earth.

Type of ocean world: Earth has what's called an active ocean. That means that our oceans have systems and patterns that keep our planet working. Life on Earth would not exist without this active ocean.

› More about Earth

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Ceres isn't a moon and it isn't a planet. It is a dwarf planet. (Watch this video to find out what makes a dwarf planet different from a planet.) Scientists estimate that some of Ceres (about 25 percent by mass) is made of water. And some of that water might be in a liquid form. Based on computer models, scientists predict that Ceres has a significant liquid water reservoir, or even an ocean, far beneath the surface. In 2015, a NASA spacecraft called Dawn arrived at Ceres to find out more about it.

Size: The distance around the Ceres' equator is 1,859 miles (2,992 km). It would take nearly 31 hours to drive all the way around Ceres’ equator if you were in a car going 60 miles per hour. That may sound like a long time, but driving around Earth would take more than 17 Earth days – and that's without stopping!

Distance from the sun: On average, Ceres is about 2.8 AU from the sun. That's almost three times farther than Earth is from the sun.

Type of ocean world: There are clues that Ceres may have salty liquid below the surface and that it could have had an ocean deep underground, but scientists can't say for sure. So we'll have to do more research. That makes Ceres a "possible ocean world.”

> More about Ceres

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Scientists have strong evidence that there's a salty ocean on Jupiter's moon Europa beneath a layer of ice. Europa is stretched and squeezed by Jupiter’s gravity as it orbits the planet. This process heats Europa’s ocean and keeps it from freezing. The heat may also melt parts of the moon's outer shell, creating lakes within the ice. NASA is launching the Europa Clipper spacecraft in 2024. Clipper will perform nearly 50 flybys of the moon to learn more about this mysterious world.

Size: It would take more than four days to drive all the way around Europa at 60 miles per hour without stopping. The distance around its equator is 6,093 miles (9,807 km).

Distance from the sun: Europa orbits Jupiter, which is about 5.2 AU from the sun. That's more than five times farther than Earth is from the sun.

Type of ocean world: There are lot of clues that Europa has an active ocean. If it does, that means the ocean may be moving chemical nutrients that are useful to microbes (tiny living things) from its rocky interior and icy crust into the ocean. If scientists can confirm that Europa's ocean is active, it will be a major discovery. This is because similar cycles of activity affect Earth's ocean and help make life here possible. For now, though, Europa’s ocean is thought of as "possibly active."

› More about Europa

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Jupiter's moon Ganymede is the largest moon in our solar system. It is also the only moon with its own magnetic field. Scientists have found clues that Ganymede may have a large, underground, saltwater ocean. The moon may even have several layers of ice and water sandwiched between its crust and core.

Size: Ganymede is the biggest moon in our solar system. It’s even bigger than Mercury! But it is still much smaller than Earth. It has a circumferences of about 10,300 miles (16,500 km). That is less than half of Earth’s circumference. It would take about seven days to drive all the way around the moon at 60 miles per hour without stopping.

Distance from the sun: Ganymede orbits Jupiter at 483 million miles (5.2 AU) from the sun. Given that the average school day is about 7 hours, it would take more than 1 million school days to drive to Ganymede from the sun at 60 miles per hour! Luckily, space travel is much faster than that.

Type of ocean world: While there is a good chance that Ganymede has an ocean, it’s less likely to have things living in it. That's because the ocean is probably trapped between layers of ice. If it is trapped, there might not be a way for nutrients – key ingredients for life on Earth – to reach the ocean from above or below. So Ganymede is what we call a "locked" ocean world.

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The deepest known place on Earth is the Mariana Trench in the Pacific Ocean. It is more than 6 miles deep. Jupiter's moon Callisto may also have an ocean about that deep. But on Callisto, this ocean is trapped below an ice layer that is covered in craters and estimated to be 60 miles thick! So if Callisto were a layered cake, it would take a big fork to get down to the ocean layer.

Size: Callisto is just a little smaller than Ganymede. The circumference at its equator is about 9,400 miles (15,150 km).

Distance from the sun: Callisto orbits Jupiter with Ganymede and Europa at 5.2 AU from the sun.

Type of ocean world: Because of that thick ice layer covering Callisto's ocean, it probably doesn't have any creatures or other kinds of life inside it. Callisto, like Ganymede, is considered a "locked" ocean world.

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Scientists have discovered there's a global ocean inside Saturn's moon Enceladus. This body of water is under the moon’s icy crust. But there are deep cracks (nicknamed "tiger stripes") in the shell, near the south pole, that are letting some of the ocean’s water escape. While flying by Enceladus, NASA’s Cassini spacecraft took pictures of these jets of water escaping from the moon and even grabbed samples to determine what the jets are made of. (Check out our slideshow all about Enceladus to find out more.)

Size: Enceladus is pretty small, especially for one of Saturn’s moons. The circumference at its equator is 984 miles (1,584 km). That's much smaller than Earth's moon. It would only take 16 hours to drive around Enceladus at 60 miles per hour.

Distance from the sun: Enceladus orbits Saturn way out at 9.5 AU. Remember how long it would take us to drive to one of Jupiter's moons from the sun? It would take almost twice that long to get to Saturn and its moons.

Type of ocean world: Thanks to those deep cracks in the ice near Enceladus’ south pole, its ocean isn't trapped. Water from the ocean is shooting up to the surface through the cracks, providing “free samples” for spacecraft that pass by. By “tasting” these samples, the Cassini spacecraft found evidence that hot water full of minerals is pouring into the ocean from vents on the seafloor. There are even chemicals some Earth microbes could use for making food. This makes Enceladus an "active" ocean world. And it makes scientists think that Enceladus might be able to host living things.

› More about Enceladus

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Saturn's moon Titan is covered in a thick haze that makes it hard to know what's going on beneath it. But NASA's Cassini mission has helped us make new discoveries. The Cassini spacecraft started exploring Saturn and its moons in 2004 and flew by Titan many times. The spacecraft also dropped a probe onto Titan that collected images and information as it floated down to the surface. Thanks to the mission, scientists now believe that Titan has a very salty ocean under its outer, icy crust. Since the ocean might be in contact with a rocky core, some scientists wonder if the ocean could be a place that has the chemical ingredients for life. Titan is a double ocean world since it also has lakes and seas on its surface. However, these features are made of chemicals that are not ideal for life as we know it on Earth.

Size: Titan is another large moon that is similar in size to Ganymede and Callisto. The circumference at its equator is about 10,000 miles (16,200 km). It would take about 7 days to drive around it at 60 miles per hour.

Distance from the sun: Titan also orbits Saturn at 9.5 AU from the sun. To appreciate how far that is, consider that the Cassini spacecraft traveled at an average speed of 10 miles per second to get to Saturn. Even at that speed, it took seven years to get to Saturn (with some twists and turns).

Type of ocean world: If the ocean inside Titan doesn’t reach the surface, then it is a “locked” ocean world. We probably wouldn’t find life in Titan’s ocean if it is locked because living things would have a hard time surviving in that kind of environment. But it is possible that life as we don’t know it could exist in the seas of liquid methane and ethane on Titan’s surface. The seas have tides and waves and are fed by rivers and rain, which makes them active places.

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Scientists can study how a moon or planet moves to learn about what might be inside. Scientists who studied the movements of Saturn’s moon Mimas found that it wobbled much more than they expected. One reason might be that the moon could have an ocean 15 to 20 miles (25 to 30 km) below its surface. Or the wobble could be caused by a football-shaped core at the center of the moon. It will take more scientific study before anyone knows for sure what is below the surface of this crater-covered moon that looks strikingly similar to the Death Star from “Star Wars.”

Size: Mimas is tiny. The distance around its equator is about 800 miles (1,250 km). It would take about half a day to drive all the way around it at 60 miles per hour.

Distance from the sun: Mimas can be found near Saturn’s other moons at 9.5 AU from the sun. If you were to do a tour of Saturn’s possible ocean worlds from the ringed planet itself, you would reach Mimas first, then Enceladus and finally Titan.

Type of ocean world: Scientists are still trying to find out whether Mimas has an ocean at all. Until they do, they won’t know whether living things are likely to exist there. For now, Mimas is known as a “possible” ocean world.

› More about Mimas

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Neptune’s moon Triton was named after a Greek god of the sea, but scientists aren’t sure whether there are any oceans on Triton. Geysers that spew nitrogen gas can be found all across the icy moon. There is also evidence of volcanic activity. All of this activity makes scientists wonder about the possibility of an ocean below Triton’s surface. Voyager 2 is the only spacecraft to have ever visited Triton and that was in 1989. So there is still more science that needs to be done before anyone can say for sure whether Triton is an ocean world.

Size: Triton is close in size to Jupiter’s moon Europa, which may be an active ocean world. The circumference at its equator is about 5,300 miles (8,500 km). It would take about the same amount of time to drive around Triton’s equator as it would to drive from Los Angeles to New York City and back.

Distance from the sun: Triton orbits Neptune, which is the most distant planet in our solar system. Neptune and its moons are 2.8 billion miles (4.5 billion km) or 30 AU from our sun!

Type of ocean world: Like Mimas, Triton is a “possible” ocean world. There’s evidence of an ocean, but it will take more studying to know for sure.

› More about Triton

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In 2015, NASA’s New Horizons spacecraft made the first up-close flyby of Pluto. The spacecraft collected new information and images that are helping scientists learn more about the faraway world. Even before the flyby, there was some evidence that Pluto has an ocean beneath its surface. Scientists are still studying all the new information they have collected. But we may get more clues about Pluto’s potential as an ocean world soon.

Size: Pluto is a little smaller than Triton. The circumference at its equator is about 4,500 miles (7,200 km). Pluto is called a “dwarf planet” partly because it is not big or massive enough to have cleared the debris around it. (Watch this video to find out what makes a dwarf planet different from a planet.)

Distance from the sun: Pluto is about 40 AU from the sun. That’s 3.7 billion miles (5.9 billion km)! It took the New Horizons spacecraft more than nine years to get from Earth to Pluto.

Type of ocean world: Scientists are still studying Pluto for signs of an ocean. So for now, it is in the same category as Ceres, Triton and Mimas – a “possible” ocean world.

› More about Pluto

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