"During several flybys, Galileo took hundreds of snapshots of the moon Europa, showing a surface covered with dark spots and crisscrossing lines. In his new book, Greenberg walks readers through the Europa photo gallery like a curator in an art museum. He interprets the meaning of these wonderful images and recounts how he and his colleagues came to see Europa's strange features as evidence that the outer crust is a thin layer of ice riding over a deep ocean.

This is not the mainstream opinion, however. Most scientists who study Europa believe the ice is much thicker: tens of kilometers as opposed to only a few kilometers. In the course of defending his minority position, Greenberg blames the hierarchical structure of big science projects for creating a politically-motivated "thick ice" cabal that refused to go back on its initial interpretations even when later data seemed to contradict them."
Unmasking Europa

Linked from:
http://www.spacedaily.com/reports/Unmasking_Europa_999.html

It seems to me the maximum height of highest mountains on Europa would tend to indicate whether the crust was thin or thick.
The highest mountain on Io:
"Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest"
http://www.absoluteastronomy.com/topics/Io_(moon)#encyclopedia

Other than statement about Europa being the smoothest object I didn't see anything about ridges or crater rims being hundreds or thousands of feet high.
If the crust was 10 km or thicker it seems that would allow features 5000' or higher, whereas if the highest feature was 100 or 200 feet this indicates to me a very thin crust.