Enceladus (ĕnsĕlˈədəs) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn II (or S2), Enceladus is 310 mi (500 km) in diameter, orbits Saturn at a mean distance of 147,900 mi (238,020 km), and has equal orbital and rotational periods of 1.37 earth days. It was discovered in 1789 by the English astronomer Sir William Herschel. Enceladus has the highest reflectivity (almost 100%) of any body in the solar system. Its surface, apparently dominated by fresh, clean ice, is marked by few craters, smooth plains, and extensive fissures and ridges. Observations indicate that Enceladus has had five distinct geologic periods. The fresh surface suggests relatively recent cryovolcanism, caused perhaps by tidal forces exerted by Saturn and the moon Dione, with which Enceladus forms a satellite pair (that is, they interact gravitationally). In 2005 the space probe Cassini discovered Enceladus has an atmosphere, albeit one that must be replenished by a source on the moon, because its gravity is too weak to permanently retain an atmosphere. The probe also discovered (2006 and subsequent flybys) geyserlike eruptions on the moon. These eruptions contribute material to the replenishment of Saturn's E ring, and may be fed by a saltwater ocean beneath the surface ice.