Enceladus

Enceladus , 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, believed to be 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 volcanism, 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) geyserlike eruptions on the moon.

Enceladus (Saturn II) A major satellite of Saturn, discovered in 1789 by Sir William Herschel. It has the brightest surface of any body in the solar system (albedo more than 0.9), composed of clean, fresh ice (not necessarily water ice). The surface is cratered, but there are also smooth plains and long linear cracks and ridges. The surface appears to be young, probably less than 100 Ma old, indicating it has been geologically active until very recently, and possibly is still active with some kind of water volcanism. This activity may make Enceladus the source of the material comprising the tenuous E ring of Saturn. Enceladus is much too small to be heated by radioactive decay; the heat would have dissipated long ago. The orbit of Enceladus is locked in a 1:2 resonance with Dione, which may provide some tidal heating, but probably not enough to melt water ice. Enceladus is 238 020 km from Saturn; its radius is 249.1 km; mass 0.73 × 10 ²⁰ kg; mean density 1120kg/m³; visual albedo 1.0.