As it turns out, we are not the only ocean world in our solar system. You just have to look for them.
Europa, Enceladus and Ganymede are all moons of the outer Solar System. Europa and Ganymede orbit Jupiter, Enceladus is a satellite of Saturn. They all share one thing in common with each other, and with Earth. They contain vast quantities of liquid water.
This might sound weird to you. It should. Enceladus has a surface temperature of around -200°C, and the Jovian moons are only slightly higher. But there is water. Huge amounts of it, in fact – between these 3 moons, there exists more than 20 times as much water as on Earth.
The water is, of course, subsurface. All of these moons are covered in a crust of ice, similar to that of the Arctic Ocean. The difference is that the very thinnest crust, that on Enceladus, is 10km thick. Ganymede’s ice shell is hypothesied to be well over 800km thick in places.
But Europa. Ah, Europa.
Europa is a moon of Jupiter, about 3000km across. The surface is ice, mixed with a number of chemicals (including salts and clay-like compounds) which gives it the noticeable discolouring. But below the surface, about 20km down, is the ocean of Europa. It is about 100km deep and, if the evidence gathered by the Voyager and Galileo missions is correct, is in direct contact with the silicate crust below. Now, a water ocean in direct contact with a silicate crust is exactly the same as what we have here on Earth. And that is why Europa is the best candidate for being able to support life in the Solar System, other than Earth.
But I’m going to answer an interesting question, one that isn’t usually asked about science. I’m going to talk about how we know this information.
We, as a species, have known about the existence of Europa for a long time – it was discovered in 1610 by Galileo. But we only found out about the subterranean ocean in the last few decades. How?
There are three main methods that we used to discover the ocean under Europa. Here they are.
Method 1 – Spectroscopy
Spectroscopy is one of the main methods used in chemistry, as well as astronomy, to identify unknown chemicals. Here is, basically, how it works.
We know what spectrum of light is emitted by the Sun, because we measure it with satellites. We know what spectrum of light is reflected back from Europa because we can measure it with satellite telescopes. And by subtracting one from the other, we can work out what frequencies of light are being absorbed by Europa’s surface.
As you can see, the data gathered from Europa can be matched with existing data from different substances to work out what the surface might be made of. Ice, it turns out. We established in the 1950s that Europa’s surface is covered in a layer of ice.
But that isn’t enough. We still have to work out how thick the ice layer is – it could be just a layer a few metres thick, or it could extend for hundreds of kilometres into the planet.
Enter the Galileo probe.
Method 2 – Internal mass distribution
When the Galileo space probe did a flyby of Europa in 2001, it was being tracked with exceeding precision by the Deep Space Network – a series of radio telescopes designed to communicate with, and track the positions of, space probes in the outer Solar System. What it was doing was tracking the trajectory of the spacecraft as it flew low over Europa’s surface, in an attempt to calculate the internal mass distribution of the planet.
For a rocky planet like Earth, the mass of the planet is distributed fairly evenly. If you were to pull out a chunk of the Earth that went from the surface to the core, the centre of mass of that chunk would be fairly close to the middle of the section.
This is not true for Europa. Using a lot of extremely complicated maths that I’m not even going to try to understand, they managed to work out the internal mass distribution of Europa. In other words, they worked out the density and sizes of different components of the moon. So, here is the data that the mission got.
There exists, on Europa, a core made of (we presume, based on its density) iron and nickel similar to that on Earth. Above that, there is a mantle made of silicate-based rocks, similar to that of Earth. And for the last 150km below the surface, there is a substance with a density of around 1 gram per cubic centimetre.
But either liquid or solid. The results of the flyby were’t accurate enough to spot the difference between the density of ice (0.997 g/cm³) and liquid water (1 g/cm³). Darn. So we know that there is a layer of water around 150km thick, but we don’t know if it is liquid or solid. See Wikipedia’s helpful diagram.
We need a third method, a way to distinguish water from ice.
Method 3 – Magnetism
There is a fundamental property of all conductive materials in the universe. When you pass them through a varying magnetic field, they generate electrcity. This is the principle behind dynamos and ulitmately, modern power generation. This also generates a smaller magnetic field. This is called induced magnetism, and is the principle behind airport metal detectors. The ‘doorway’ generates a pulsing magnetic field. If you have any conductors on your person (like a phone, keys, or guns) then they generate an induced field, which sets the alarm off.
It turns out, because Europa has a slightly eccentric orbit (not perfectly circular), as it progresses it passes through a varying magnetic field – that of Jupiter. So when the Galileo spacecraft was performing a flyby, it measured the induced magnetic field coming from Europa. And from that data, the NASA team was able to calculate the strength and size of the conducting layers inside Europa. With that data, they checked it against the conductive properties of existing materials.
Solid ice is too weak a conductor. The same is true of solid ice with a high salt content. Pure water isn’t quite strong enough.
But water, with a salinity similar to that of Earth’s oceans, is exactly conductive enough. Bingo.
We, as a species, had now proven that there existed, under the surface of a moon orbiting Jupiter, a vast sea of liquid water.
And that, ladies and gentlemen, is how science works.