Moons of the Solar System
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Unlike Callisto, Jupiter's moon Europa has very few craters, which adds to the evidence that the moon is geologically active and possibly has a liquid ocean not far beneath its icy surface. Imagine breaking ice cubes out of an ice tray. Any liquid water in the tray will try to fill in the cracks as it seeps out. The surface of Europa is very similar. If a crack forms in the ice, or a meteor punches a hole in it, the liquid underneath will quickly fill in the gap. This is why Europa has an almost uniform surface, marked by very few craters.
Fun Fact: Where there is water, there could be life. If life does exist on Europa, it is probably supported by undersea volcanic vents much like the ones that help life exist at extreme ocean depths of Earth.