- One of the four large satellites of Jupiter discovered by Galileo in 1610. Travelling in a 671 000km, 3 day orbit, Europa is 3100km in diameter and seems to have an icy crust and a rocky interior Its surface is blotchy and covered in a system of long fracture lines, but Europa’s density, slightly greater than that of the Moon, indicates that this icy surface must have rock beneath it, making up most of the satellite’s body. There may be liquid water below tens of kilometres of ice. The ice seems to be able to flow and generate new surface material, since the surface has few impact craters, indicating a young average age. In 1995 its extremely thin atmosphere, with a pressure of about one hundred thousandth that of the Earth’s atmosphere, was shown to consist of oxygen.