Europa is Jupiter's fourth largest moon, slightly smaller than Earth's moon. It has a thin icy outer layer and a possible liquid ocean underneath. Some scientists believe life could exist in Europa's subsurface ocean.
Jupiter rotates very quickly, completing an orbit every 10 hours. Its rapid rotation causes it to bulge at the equator and generates strong magnetic fields.
Mars is the fourth planet from the sun and about the same age as Earth. It appears reddish due to iron in its soil.
The moon is Earth's only natural satellite. It reflects sunlight and was the only astronomical body visited by humans until recently. At about 4.6 billion years old, it is much smaller than Earth