Saturn’s Rings

Saturn, the sixth planet from the sun, is one of the most easily identified targets for astronomers, largely due to its large and distinct ring system. However, Saturn is not the only planet in the solar system to have rings — Jupiter, Uranus and Neptune also contain faint ring systems. The rings are wide — 170,000 miles across (273,600 km) — but only about 30 feet (10 meters) thick. They consist of countless small particles, ranging in size from micrometres to metres that orbit about Saturn. The rings are named alphabetically in the order of discovery.

In 1610, the year after Galileo Galilei first turned a telescope to the sky, he became the very first person to observe Saturn’s rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn.

The dense main rings extend from 7,000 km to 80,000 km above Saturn’s equator. With an estimated local thickness of as little as 10 metres and as much as 1 kilometre, they are composed of 99.9 percent pure water ice with a smattering of impurities that may include tholins or silicates. Thus, the main rings are, from farthest from the planet to closest, A, B and C. A gap 2,920 miles wide (4,700 kilometers), known as the Cassini Division, separates the A and B rings. Voyager 1 detected the innermost D ring in 1980. The F ring lies just outside of the A ring, while the G and E ring lay even farther out. Earth is visible as a “pale blue dot” at about the 4 o’clock position, between the G Ring and outer E Ring.

Radio signals of differing wavelengths were sent from Cassini through the rings to Earth. The attenuation of each signal reflects the abundance of particles of sizes similar to the wavelength and larger. Purple (much of the B Ring and the inner A ring) indicates few particles smaller than 5 cm are present. Green and blue (the C Ring and outer A Ring) indicate particles smaller than 5 cm and 1 cm, respectively, are common. White areas of the B Ring are densest and transmit too little signal for size estimation. Other evidence indicates all three rings have a broad range of particle sizes, up to meters across.

There is still no consensus as to the rings mechanism of formation; some features of the rings suggest a relatively recent origin, but theoretical models indicate they are likely to have formed early in the Solar System’s history. Several hypotheses exist as to the how Saturn’s rings were formed. One possibility is that the rings were once large moons that spiraled into the planet. Saturn has at least 62 moons. Only one (Titan) is large; the rest are small bodies. Only 13 of the moons are larger than 30 miles (50 km). The gravity of these moons affects the structure of Saturn’s rings, but also offer insight into possible formation methods. If the icy outer layers were stripped away, leaving the core to crash into Saturn, the result would be rings dominated by nearly pure water-ice.

Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O₂) produced when ultraviolet light from the Sun interacts with water ice in the rings. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.

Credits: NASA, Wikipedia