Europa is one of Jupiter’s moons and apparently has a “water-ice crust”, “salty ocean of liquid water”, and a “rocky sea floor”. How could we know this much information in great detail from a telescope?
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There are mutliple indicators, but a big one is the surface. There are no craters. Every other celestial body we know of that doesn’t have an atmosphere (aside from the other ones we also think have liquid oceans) are covered in craters. This implies that something is renewing the surface regularly, with liquid flowing from the warmer interior to the surface and freezing there. On a rocky body, this liquid would be molten rock of some kind. Since Europa is an icy body, that liquid has to be molten ice: water.
Spacecraft have measured its gravity, from which we know about how much rock vs metal core vs water/ice it has.
But the most important evidence we have comes from measurements of its magnetic field. Unlike Earth, Europa does not generate its own intrinsic magnetic field, but it does have a detectable magnetic response to Jupiter’s magnetic field. Jupiter’s field at Europa changes over time, and this changing magnetic field causes an induced field in Europa. This means that Europa is basically a big electrical conductor, and the most reasonable way this could be is that it has a layer of liquid water. It has to be somewhat salty, because that increases the conductivity of the water, and it makes sense with our understanding of how Europa formed and evolved over time.
Add that to the fact that we see its surface covered in ice, and we see salts like sodium chloride at some recently resurfaced areas, and we think this picture of the interior is pretty well supported.
I know some icy moons like Enceladus shoot water out of their surface, and we use spectroscopy to find out what’s in the water. I looked it up and there’s a theory that Europa has some too. But it makes sense they’d be harder to spot, Europa has over 10x stronger surface gravity than Enceladus.
Spectroscopy is an excellent answer, but to distill the answer down even further for my kids –
It’s got the right colours for an ocean!
It doesn’t look like lava, it doesn’t look like dirt, and it’s white and shiny a bit like ice in winter. But we won’t *really* know for sure until a robot gets there to check for us; we’re only guessing.
When spacecraft (not telescopes) travelled near to Europa, they were able to measure its magnetic field. The only plausible explanation for Europa’s magnetic field is a conductive layer. The surface is ice (an insulator), so it can’t be the conductive layer. Europa’s core is too small to account for the magnetic field. So the only reasonable explanation is an ocean of water underneath the ice, with enough dissolved salt (or other minerals) to make the water conductive.
This is my research field, but I’m about to teach a class so I’ll edit this post with citations and more details later. — Update: added links.
Early suspicions of liquid water ocean came from [Voyager](https://en.wikipedia.org/wiki/Voyager_program) data which showed almost no craters and very little elevation change, suggesting that the ice crust had no solid support underneath. Supporting evidence came from Europa’s size, mass, shape and gravitational field, which showed that it has an overall density less than rock, but is much denser in the center than near the surface, so it’s probably rock on the inside and either ice or water near the surface.
But that’s not enough on its own to prove liquid water, it could just be ice. (Ice flows like a very thick liquid on geological scales: see glaciers and ice caps on Earth.)
“Proof” of a salty *liquid* ocean came from [Galileo](https://en.wikipedia.org/wiki/Galileo_(spacecraft)) [magnetic field data](https://www.nature.com/articles/27394), which showed that Europa responds to Jupiter’s changing magnetic field the same way an electrically conductive sphere would. So either Europa has a layer of copper metal under its surface (no way), or there’s a continuous layer of salt water down there. And no, a metal core wouldn’t explain the observations, nor would little pockets of water in an ice shell.
[Additional studies](https://www.sciencedirect.com/science/article/pii/S0019103507000656) have used more precise magnetic field modeling to roughly constrain *how* salty it is (saltier than soup, maybe as salty as the Dead Sea) and [chemical modeling](https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2000je001413) shows that if you get the rocks we guess Europa is made of wet, they will release lots of salt.
Two related points that may help you work through the question:
1- We don’t know, but we strongly *suspect* the ocean is there. We can’t see the ocean, but we can see plenty of other things! And one of the best explanations for what we see is that there is either an ocean down there or a lot of aquifer-type pools of (probably) salty water that seeps up to the surface from time to time.
2- We’re not limited to telescopes on Earth or in Earth orbit, we’ve sent several probes to/past Jupiter, some only went by on their way to further places, others dropped into orbit. We have some pretty close-up data from onboard camera-telescopes as well as a variety of other sensors that can do spectroscopy, sniff magnetic fields, bounce radar or similar, and so on.
There is a *lot* of work/data that’s been collected around Jupiter, including many of its moons, and there are potential future missions that would focus on the moons specifically, but it’s far more than can fit in a single comment so I’ll drop you the wikipedia page and you can check out the sources (at the bottom!) and the various reference links in the article: [https://en.wikipedia.org/wiki/Exploration_of_Jupiter](https://en.wikipedia.org/wiki/Exploration_of_Jupiter)
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