The cratered surface of Jupiter's moon, and Europa's neighbor, Callisto.
Click on image for full size
Scientists usually use craters to judge the age of a surface. The more craters the older the surface is. (That is because on an active planet, the surface turns over with time and wipes out the craters. The new surface without craters is younger than the surface with craters).
The lack of craters on Europa told scientists that they had misjudged the age of Europa's surface. Moreover, as they examined each new picture, the age of Europa's surface seemed to get younger and younger. First scientists thought it was 3 billions years old, then they guessed it was only 1 million years old, then 10,000 years old, and finally they guessed that it might be currently active!
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