We need to measure the distance of the orbiting satellite from the planet and it's period of revolution, and from that we get the mass. We'll actually be doing an exercise in lab where we make such a measurement. You told us that the Jovian planets are icy. Does that mean they have a surface or are they just balls of gas with ice in them. Think of them as gritty snowballs for the most part.
Comets are also icy balls.
The Mantle effectively traps it in. What was the highest mountain range in all of our planet's history. But there seems to be no reason not to regard the present planetary surface as typical, with the highest mountains about 5 miles above sea level. There are other higher mountains in the solar system, though. Mons Olympus on Mars is 15 km high.