http://www.flatoday.com/space/explore/probes/galileo/1999/012099e.htm
To quote:
If a 100 kilometer (60 mile) deep ocean existed below a
15 kilometer (10 mile) thick Europan ice crust, it would be
10 times deeper than any ocean on Earth and would contain
twice as much water as Earth's oceans and rivers combined.
Unlike Earth, magnesium sulfate might be a major salt component
of Europa's water or ice, while Earth's oceans are salty due
to sodium chloride (common salt).
