Also known as lunar craters, they are a type of concave terrain that covers the surface of the moon and are traces left by ancient volcanic activity and meteorite impacts. The size of volcanic craters varies, ranging from tiny millimeter sized craters caused by micro meteorite impacts to large structures several miles wide, forming a unique topographic map of the lunar surface. The formation process can be traced back to the early geological period of the moon, when magma activity was frequent inside the moon. As magma erupted onto the surface of the moon, it cooled and formed many circular depressions, which later became known as volcanic craters. It may also have been formed by meteorite impacts, which left permanent marks on the surface of the moon

Callisto, also known as Jupiter's moon Callisto (pronounced/k əˈ l ɪ sto ʊ, Greek: Καλλιστώ), is a satellite that orbits Jupiter and was first discovered by Galileo Galilei in 1610. Europa is the third largest moon in the solar system and the second largest moon of Jupiter, second only to Ganymede.