How much of Jupiter's moon Europa is made of water? A lot, actually. Based on the Galileo probe data acquired during its exploration of the Jovian system from 1995 to 2003, Europa posses a deep, global ocean of liquid water beneath a layer of surface ice. The subsurface ocean plus ice layer could range from 80 to 170 kilometers in average depth. Adopting an estimate of 100 kilometers depth, if all the water on Europa were gathered into a ball it would have a radius of 877 kilometers. To scale, this intriguing illustration compares that hypothetical ball of all the water on Europa to the size of Europa itself (left) - and similarly to all the water on planet Earth.
With a volume 2-3 times the volume of water in Earth's oceans, the global ocean on Europa holds out a tantalizing destination in the search for extraterrestrial life in our Solar System
Image Credit & Copyright: Kevin Hand, Jack Cook and Howard Perlman
Explanation from: http://apod.nasa.gov/apod/ap120524.html