The cratered surface of Jupiter's moon, and Europa's neighbor, Callisto.
Click on image for full size
The significance of the lack of craters is this. There was a period of heavy bombardment of planetary surfaces by craters at the conclusion of the formation of the
solar system. The surfaces of most solar system bodies also stopped forming at or
near this time, preserving the record of cratering. However, the surfaces of some planets/moons continued to change long after this period stopped due
to interior activity, wiping out the record of cratering which appears of the planet's surface. Thus the number of craters on a surface help scientists pinpoint the age of a surface
With each new picture, the lack of craters suggested that the age of Europa's surface was 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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