This is a close-up image of the Pwyll crater taken by the Galileo spacecraft.
Click on image for full size
Image courtesy of NASA
Now let's discuss some of the geologic evidence for an ocean. We will not cover all the fine details which geologists are studying, just some of the major points. The first piece of evidence comes from a look at some of the craters. Some craters in particular exhibit a multi-ringed structure which suggests that whatever hit the surface of Europa relaxed into a warm layer of slushy ice. An example of such a crater is shown in this image. This is the crater called Pwyll. This crater is reminiscent of a footprint left in a partially frozen puddle in the sidewalk on a snowy day.
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