4 Why do some of the large moons of the Giant Planets have surfaces full of craters and others are smooth?

Answer 1

Large meteor impacts, although not common by human standards, do occur regularly by astronomical standards and most planets and moons show much cratering. So the question becomes, why don't all moons have lots of craters. It depends partly on what the moon is made of and how close it is to its mother planet. For example, Io, a large moon of Jupiter, is so close to Jupiter than the tidal friction melts the core of the moon and makes it the most active volcanic object in the solar system; the continual volcanic activity makes meteor craters disappear fairly quickly. Europa, similarly affected by tidal forces, is made largely of water and the surface melts and refreezes regularly enough to hide meteor impacts. The larger planets (Jupiter, Saturn, Uranus and Neptune) are gas giants - their surface is gaseous and therefore show no meteor impacts - although they do get hit. Earth does have meteor impact craters; but Earth's atmosphere is very corrosive - the oxygen and water will erode most metals, two thirds of the surface is covered in water and and the effects of life hide most of these.