More water on the moons is the earth

We like to call Earth the "blue planet" because of the abundance of water here. But compared to Jupiter's largest moons, Earth might as well be called dry. The moons…

We like to call Earth the “blue planet” because of the abundance of water here. But compared to Jupiter’s largest moons, Earth might as well be called dry. The moons could have a total of 17 times more water than Earth.

Let’s follow the water! This is the mantra behind scientists’ search for alien life in the universe. And it makes sense, because our experience here on Earth is that where there is liquid water, there is also life.

In the solar system, Earth stands out from the other planets in that we have liquid water on the surface. On all the other planets, the temperature is either too high, i.e. water evaporates or is too low i.e. it freezes. Only at the right distance from the sun – the so-called habitable zone – is the temperature such that water is liquid.

Earth’s water is a drop

When we look at a globe or a world map, the Earth’s oceans cover 71 percent of the surface, so it is natural that we call this little friend of ours in the solar system the “blue planet”. But our world’s oceans are not very deep and water therefore not really a very large part of the earth.

If we imagine that we sucked up all the water on Earth and collected it in a sphere, it would only be 1400 km in diameter. Compared to Earth’s 12,746 km diameter, it would look like a small drop.

The Earth contains 1.4 billion cubic kilometers of water or about 0.1% of the globe.

Other planets in the solar system have much larger amounts of liquid water, just not on the surface. This is especially true for three of Jupiter’s large moons: Europa, Ganymede and Callisto. The astronomers believe that oceans over 100 km deep may be hidden under the thick ice sheets of these moons. In total, there could be up to 17 times more water on these moons than on Earth.

The three Jupiter moons thus form the largest reservoir of water in the solar system, and astronomers therefore want to look for alien life there.


Europe contains 2.9 billion cubic kilometers of water – or 18.2% of the globe.

The amount of Europe is best known

Possible volume of water: The ocean under the Europa ice sheet is probably about 100 km deep. On Earth, the oceans are on average about 4 km deep. There is probably twice as much liquid water on Europa as on Earth.

Evidence for water: It is on Europe that the most reliable traces of water have been found. The Hubble telescope detected water eruptions that reach 160 km into space. And in 2019, measurements from the ground showed that these eruptions contained water molecules. In addition, there are no meteor craters on Europa, and the most likely reason is that the surface is renewed by the movements of the subsurface water.


Ganymede contains 7.4 billion cubic kilometers of water – or 7.9% of the globe.

Ganymede has an ocean about 100 km deep

Potential water content: The largest moon in the solar system is covered in a 150 km thick layer of ice. Scientists believe that there may be a 100 km deep ocean and about five times more water than on Earth.

Evidence for water: Ganymede is the only moon in the solar system with its own magnetic field, and therefore northern and southern lights are formed there. In 2015, scientists used the Hubble Space Telescope to analyze how these light movements are affected by the interplay between the magnetic fields of Ganymede and Jupiter. The results showed the existence of some third element and the most reasonable explanation is considered to be electrically conductive salt water.


Possibly 14.3 cubic kilometers of water on Kallistó or 24.2% of the globe.

The most water could be on Kallistó

Possible amount of water: Jupiter’s second-largest moon could have a 250-km-deep ocean hidden under a 150-km-thick ice sheet. If so, there is ten times more liquid water on Callisto than on Earth.

Evidence for water: Callisto is the moon of Jupiter where liquid water is most uncertain. The moon is further from Jupiter than the other two and the effect of the gas giant’s gravity is therefore less and at the same time more uncertain whether water remains liquid there. Nevertheless, observations show that Callisto has a disruptive effect on Jupiter’s magnetic field, suggesting that an electric current forms there – most likely in salty water beneath the ice.

A spacecraft visits the largest reservoir of water in the solar system

In the search for life in space, astronomers now want to study Jupiter’s moons more closely. Loaded with instruments, the JUICE spacecraft is on its way to Europa, Ganymede and Callisto to explore their hidden oceans. Dive into this exciting project here.

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