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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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.
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