Photos: 10 extraordinary ocean worlds in our solar system

Europa is Jupiter's fourth largest moon, and the smoothest of all the celestial bodies. There are almost no craters, and despite a dense network of cracks and ridges covering this moon, none are higher or deeper than a few hundred meters. This suggests that Europa's surface is geologically young and possibly floating on a liquid mantle. The Hubble Space Telescope has also spotted plumes of water vapor spewing 124 miles (200 kilometers) into the air from the south pole. This lends weight to the idea that Europa has a subsurface saltwater ocean with a layer of ice that may be just a few kilometers thick in places, according to NASA. 

Tidal flexing and friction from the gravitational interaction with Jupiter generates enough heat to keep the interior ocean liquid, but because it is so far from the sun, the surface remains frozen. Europa also has a very thin oxygen atmosphere, generated when radiation splits water molecules in the surface ice. A tiny fraction of this could become trapped within the ice, and eventually would be carried down to the subsurface ocean by tectonic subduction. A 2007 study at Stanford University, California, calculated that it was possible for the oxygen levels in Europa's ocean to equal that of Earth's own deep seas, which further bolsters the moon's chances of harboring life. 

Ganymede, Jupiter's largest moon, is 8% larger than Mercury, but only half of its mass. Such a low density suggests that it should be made of equal parts rock and water. In the 1990s, the Galileo spacecraft found that Ganymede has its own magnetic field, which means that it must have a molten iron core. The heat from this core would be enough to melt the ice and create an enormous subterranean ocean. 

This ocean could be a 62-mile (100 kilometer) thick layer, sandwiched between an icy crust on the surface and another layer of ice below, which is held solid by the enormous pressures. Other models have suggested that there might be several different oceans, arranged in concentric rings like an onion, with different phases of solid ice separating them. Ganymede's ocean is trapped a long way underground, so we don't see any water plumes spewing at the surface like on other moons, but there are other observations that provide direct evidence of its ocean. 

As Ganymede completes its orbit around Jupiter, the parent planet's massive magnetic field creates polar aurorae in Ganymede's thin atmosphere. But the salt in Ganymede's seawater makes it electrically conductive, and this creates magnetic drag, which reduces the amount that the aurorae oscillate around Ganymede's poles. 

The Hubble Space Telescope has observed Ganymede's auroras, and discovered that the oscillations are damped in exactly the way that an underground ocean would predict.

Callisto is Jupiter's second largest moon. It is almost as large as Mercury, but one third as massive, which means that it is about 50% water. The strange thing about Callisto is that the surface is completely saturated with craters, with no breaks or smooth plains caused by geological processes below. Not only is Callisto geologically dead today, it probably always has been. Gravity measurements from the Galileo spacecraft show that the internal structure hasn't fully separated out into a rock core with a pure water/ice mantle. This means that the ice has never fully melted during Callisto's formation, according to a study from the NIH. 

Despite this, we know that Callisto does have a liquid ocean near the surface. Measurements of its interaction with Jupiter's magnetic field show that it must have an electrically conducting layer at least 6.2 miles (10 kilometers) thick just below the surface. Callisto orbits too far away from Jupiter to receive any significant tidal heating, so for this ocean to remain liquid, it must contain something besides water to act as antifreeze. A 5% mixture of ammonia would be enough, for example. Callisto lies outside Jupiter's main radiation belt, and has ample water ice on the surface, which makes it a good candidate for a future human base. But conditions within its underground ocean are much less hospitable. As well as being very cold, the liquid water is sandwiched between two layers of ice, so there is no influx of minerals, and only very slow heat transfer from the core.

Pluto is too small to have retained enough heat to keep its core molten. Radioactive heating under the surface only provides a fiftieth of the energy that radiates upwards on Earth. But that's still enough to melt the lighter elements and allow the heavier silicate minerals to sink. The result is a rocky core 1,056 miles (1,700 kilometers) across, surrounded by a layer of water and ice 62-112 miles (100-180 kilometers) thick. Pluto's surface is so cold that it is blanketed by snow made of solid nitrogen, methane, and carbon monoxide, but spectrometry data from New Horizons suggests the 'bedrock' is water ice, according to NASA. 

Deep in the mantle, the heat from the core could be keeping this as a mixture of slush and water. The heart-shaped Tombaugh Regio is in an area absent of craters, suggesting the surface is geologically active. The western half, Sputnik Planitia, lies close to Pluto's equator, keeping it at a stable temperature. For millions of years, the nitrogen ice on the surface has been slowly circulating on convection currents driven by the subterranean ocean. This provides a clue that the water inside Pluto behaves like the molten magma in Earth's mantle, according to a study at Purdue University. 

Ceres is the largest object in the Asteroid Belt, and the only dwarf planet in the inner solar system. It was originally formed as a mixture of porous rock with about 10% ice. 

Early in Ceres' formation, heating from the radioactive decay of the heavier elements melted the ice, which caused most of the rock to sink down towards the core. The heating wouldn't have been enough to melt all the way to the surface — the outer 6.2 miles (10 kilometers) or so has stayed frozen — but as the subterranean ocean warmed, it expanded and forced cracks in the surface. Over billions of years, convection currents have carried away the heat from the core, and allowed the interior to mostly freeze solid again, but Ceres still seems to have some liquid water beneath the surface. 

The Herschel Space Telescope has observed plumes that are ejecting water vapor into space at a rate of 13.2 lbs. (6 kilograms) per second. The total amount of water in Ceres' icy mantle is more than all the fresh water on Earth, but it's difficult to tell how much of this is liquid. Since Ceres doesn't have a large gas giant parent to generate significant tidal heating, all of its core energy comes from radioactive decay, and the proportion of radioactive isotopes in the core is currently unknown. 

Triton is the largest moon of Neptune. It is slightly larger than Pluto, and has almost the same composition. It's likely that they were both formed in the Kuiper Belt, and later fell deeper into the solar system as a result of the gravitational pull of Neptune and Uranus. Neptune gravitationally captured Triton, but unusually, the moon has a retrograde orbit — it orbits in the opposite direction to Neptune's own spin. When it was first captured, its initial orbit was very eccentric, and this generated a lot of tidal heating as Triton flexed and relaxed with each orbit. This heat was enough to melt the interior and cause it to separate into a dense core with a liquid water mantle and a solid crust of water and nitrogen ice. Once the crust was isolated from the core by this liquid layer, it was free to flex, which increased the effect of tidal heating, and helped to stop the ocean refreezing as Triton's orbit decayed. 

Eventually, after a billion years, Triton's orbit became circular enough to lose most of its tidal heating, but it still receives energy from the core's radioactive elements. Computer models show it would only take a small amount of dissolved impurities in the water, such as ammonia, to lower the freezing point and keep Triton's ocean liquid.

Saturn's moon, Mimas, may mostly be composed of water ice with a smattering of rock — like a gritty snowball. It is only just large enough to be pulled into a rounded shape by its own gravity (it's actually slightly ovoid). Unlike its slightly larger cousin, Enceladus, there are no visible plumes or geysers, and its surface is very heavily cratered, which suggests that the crust has remained frozen for billions of years, according to NASA, and doesn't get recycled into the moon's interior. This is odd, because Mimas orbits closer to Saturn and in a more eccentric orbit, so it should receive much more tidal heating. 

However, recent analysis of images from Cassini found that Mimas does wobble slightly in its orbit, according to a report from Cornell University, and there are only two theoretical models that explain this. Either Mimas has a dense, elongated core that throws it off balance, or it has a liquid ocean under the crust that lets the core move around inside. If Mimas does have a liquid ocean, it must be capped with a very thick, strong crust to prevent any cracking or geysers. That doesn't fit in with our observations of other moons and dwarf planets around the solar system. But then, current models of moon formation also can't explain why Enceladus has a liquid mantle and Mimas doesn't. 

In 2005, NASA's Cassini probe observed plumes of water vapor erupting near the south pole of Saturn's moon, Enceladus. Because the gravity on Enceladus is only 1% of Earth's, the ice crystals are easily flung into orbit, and we now know they are responsible for most of the material in Saturn's E Ring, according to NASA. Enceladus has a rocky core around 230 miles (370 kilometers) across, surrounded by a 28-mile (10 kilometer) deep ocean under an icy crust. Initially, scientists thought the ocean was only present as an underground lake at the south pole, as that's where the plumes have all been seen. 

But measurements of Enceladus' slight wobble, or libration, show that the rocky core is likely completely detached from the crust, according to NASA. This means that the ocean envelopes the moon and probably accounts for 40% of its volume. The reason that the plumes only occur at the south pole is that the surface ice is believed to be much thinner — just 3.1 miles (5 kilometers) thick, compared with 12-28 miles (20-45 kilometers) thick surface across the rest of Enceladus. If this view of the moon were correct, Saturn's tidal heating wouldn't be enough to explain its liquid ocean. Instead, there may be more geothermal heat coming from the core than was previously thought. This might help to generate hydrothermal upwellings of nutrients and organic molecules, offering hope that life evolved there.

Saturn's moon Dione could be 50% water with a heavier rocky core. Dione is twice as large as Enceladus, but it has a much less eccentric orbit, so it receives less heat from tidal stresses. This gives it a much thicker frozen crust — some 62 miles (100 kilometers) thick. By analyzing the variations in the trajectory of Cassini, as it made several flybys of Dione between 2011 and 2015, one group of scientists at NASA have concluded that this crust could be floating on a liquid ocean 22-59 miles (35-95 kilometers) deep. 

Dione is heavily cratered, and doesn't have any of the geysers found on Enceladus, but one hemisphere is covered with huge cliffs of ice that are hundreds of meters high and hundreds of kilometers long. These are probably scars left over from early in Dione's life when the surface was still geologically active. An important feature of Dione is that its ocean may be liquid all the way down to the bedrock, rather than sandwiched between two layers of ice, according to a study at the Royal Observatory of Belgium. 

Titan is unusual because it is the only body in the Solar System, besides Earth, that has a substantial atmosphere and bodies of surface liquids. Titan's surface temperature is -292 degrees Fahrenheit (-180 degrees Celsius), so it's far too cold for liquid water on the surface, but it's just about right for liquid methane and ethane. These organic compounds evaporate into the atmosphere and rain down to form rivers, lakes, and seas. The lakes and rivers only cover about 3% of the surface, so Titan is still much drier than Earth. Titan's thick orange haze comes from sooty tholin particles formed when the sun ultraviolet light breaks up the methane in the atmosphere. This ought to have used up all the methane on the surface billions of years ago, so Titan must have some underground reservoir that is replenishing it, according to a study by Juan Lora at Yale University. So far, scientists haven't found any strong evidence of cryovolcanoes that could be supplying this methane. 

Like Callisto, Titan may have an ocean that is kept liquid by the antifreeze effects of dissolved ammonia. It would be equally hard for life to evolve there, as the liquid ocean is probably sandwiched between solid, impermeable ice layers. Life might have evolved in the hydrocarbon seas on the surface, according to NASA, but without access to liquid water, it would have a very different chemistry to life on Earth.