Saturn

When Italian astronomer Galileo Galilei (1564–1642) first pointed his telescope towards Saturn in 1610, he saw two features protruding from the planet's disk. These puzzling side-lobes were in reality Saturn's ring feature, though Galileo's telescope was too small to resolve their shape and extent. When these side-lobes started vanishing, as the rings began gradually assuming a position edgewise to the Earth, Galileo was not able to explain the nature of his observations. Dutch astronomer Christiaan Huygens (1629–1695) was the first scientist to suggest, in 1659, that Saturn was surrounded by a flattened ring.

Soon after Huygens had suggested that a ring existed around Saturn, the Italian-born French astronomer Figure 1. Main ring features visible from Earth. Illustration by Hans & Cassidy. Courtesy of Gale Group. Jean-Dominique Cassini (1625–1712) discovered, in 1675, that there were in fact several distinct rings about the planet. Several divisions in Saturn's rings are now recognized, the dark band between the A and B rings being known as the Cassini Division. The A ring is further subdivided by a dark band termed the Encke Division after German astronomer Johann F. Encke (1791–1865), who first observed this feature in 1838.

Saturn's rings are best seen when the planet is near opposition, that is, at its closest approach to the Earth. At this time the rings are seen at the greatest angle. The rings are aligned with Saturn's equator, thus tilted at 26.7 degrees to the ecliptic (i.e., the plane of the planets' orbits about the Sun). During the course of one Saturnian year the rings, as seen from Earth, are alternately viewed from above and below. Twice each Saturnian year (i.e., once every 15 years) the rings are seen edge-on, an event termed a ring-plane crossing. That the rings nearly disappear from when seen edge-on indicates that they must be thin. Recent measurements suggest that the rings are no more than 1.24 mi (2 km) thick.

That the rings of Saturn cannot be solid was first proved mathematically by Scottish physicist James Clerk Maxwell (1831–1879) in 1857. Maxwell showed that a solid planetary disk would literally tear itself apart, and concluded that the rings must be composed of orbiting particles. Subsequent observations have confirmed Maxwell's deductions, and it is now known that the rings are made of pieces of ice and ice-coated rock varying in size from dust particles to chunks on the order of 10 yards across.

Images obtained by the Voyager and Pioneer space probes have shown that Saturn's rings are really composed of numerous ringlets. The apparently empty regions between ringlets are thought to be caused both by gravitational resonance with Saturnian moons (which boost particles out of those regions by delivering periodic pushes or pulls) and by a mechanism called shepherding. Just as gaps (Kirkwood gaps) have been produced in the asteroid belt through gravitational resonance with Jupiter, so gaps have been formed in Saturn's rings due to resonance with its major satellites. The Cassini Division, for example, is the result of a 2-to-1 resonance with the moon Mimas. Some of the narrower rings, on the other hand, are believed to be maintained as distinct objects by shepherding satellites. Orbiting nearby on either side of a ring, the shepherding satellites prevent the ring particles from dispersing into higher or lower orbits. The faint F ring, 62 mi (100 km) wide, for example, is maintained by two small satellites, Prometheus (62 mi [100 km] in diameter) and Pandora (56 mi [90 km] in diameter). Indeed, the F ring, which was discovered by the Pioneer 11 space probe in 1979, shows some remarkably complex structures, with the ring being made of several interlaced and (apparently) braided particle strands.

In 1995 and 1996, ring-plane crossings occurred three times with respect to Earth, as well as once with respect to the Sun. This provided a unique opportunity for observing the rings and satellites of Saturn, since glare from the rings is greatly reduced during ring plane crossing, enabling astronomers to observe faint objects. On the ring-plane crossing of May 22, 1995, the Hubble Space Telescope discovered four new Saturn moons, including a third shepherd satellite which may account for the braiding of the F ring. The Hubble also discovered that the orbit of Prometheus had shifted, perhaps due to a collision with the F ring. During the August 10, 1995, ring plane crossing, the Hubble detected clouds of debris near the outer edge of the ring system. These may be the remains of small satellites shattered in collisions. These clouds may be the source of material for Saturn's rings.