Saturn, imaged by the Cassini spacecraft in 2004

Saturn, imaged by the Cassini spacecraft in 2004 (full view here)     [Source]

Ten times more distant than Earth from the Sun, Saturn is the second largest of the planets. While all four gas giants are known to possess a ring system, none are remotely comparable to that of Saturn’s.

The rings of Saturn are primarily composed of water ice. They are extremely thin, with an average thickness of about 20 meters (~65 feet). Spacecraft have shown that the rings are marked by thousands of divisions and gaps, caused by gravitational resonances from Saturn’s moons. The most prominent of these is the Cassini Division, which separates the wide B ring (the brightest ring section) from the A ring further out. Even small telescopes have been able to reveal this 4800 km (3000 mi) wide lacuna.

Saturn's rings at equinox, seen by Cassini in 2009

Saturn viewed from orbit at equinox, with the rings barely casting a shadow on the planet’s surface     [Source]

Though the rings are never visible from Earth without optical aid, their orientation does appear to change as Saturn goes about its orbit around the Sun. Over a period of 29.5 Earth years, Saturn goes through its complete seasonal cycle, featuring an equinox every 14.7 years. During a Saturnian equinox (as last seen in 2009), the rings appear barely illuminated, oriented “edge-on” to any point on the ecliptic plane. However, in periods between equinoxes, the rings can comparatively achieve an inclination as great as 26.7°. This variation affects the display of the Cassini Division, as well as shadows cast by the rings on Saturn’s disk.

The surface of Titan

The surface of Titan     [Source]

Enceladus, illuminated by direct sunlight (on the right) and by reflected sunlight from Saturn (on the left)

Enceladus, illuminated by direct sunlight (on the right) and by reflected sunlight from Saturn (on the left)     [Source]

Modest-sized instruments should also be able to reveal as many as six of Saturn’s moons: Titan, Rhea, Dione, Tethys, Iapetus, and Enceladus. After Ganymede (of Jupiter), Titan is the second largest moon in the Solar System. It is the only Solar System moon that possesses a heavy atmosphere. Titan’s surface exhibits lakes and rivers of liquid methane (CH4). It is actually the only surface beyond the orbit of Mars that has been visited by a probe from Earth.

Though generally the toughest of these six moons to spot, Enceladus ironically reflects 99% of sunlight that strikes its surface. However, it is extremely small—just 500 km (310 mi) in diameter—and orbits close to Saturn, a sure justification for its visual difficulty.



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