How do we know that there is a salty ocean underneath Europa’s icy surface?

Question

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?

In: 101

Answer 1

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.