Wednesday, March 31, 2010

More Rings on the Far Planets

During a 1977 Earth-based observation of Uranus in the course of a stellar occultation (the passage of Uranus in front of the star), the star’s light dimmed several times before disappearing behind the planet. That dimming of the star’s light revealed the presence of nine thin, faint rings around the planet. Voyager 2 revealed another pair. Uranus’s rings are very narrow—most of them less than 6 miles (10 km) wide—and are kept together by the kind of shepherd satellites that are found outside of Saturn’s F ring. Neptune has rings similar to those of Uranus.

Looking at Saturn with Voyager

The Voyager probes told us much more about the rings than we could have discovered from our earthly perspective.
First, data from Voyager confirmed that the rings are indeed made up of particles, primarily of water ice. Voyager also revealed additional rings, invisible from an earthly perspective. The F ring is more than twice the size of the A ring, stretching out to 186,000 miles.
The D ring is the innermost ring—closer to the planet than the innermost ring visible from the earth, the C ring. F and E are outside of the A ring.
But these additional rings are only part of what
Voyager told us. Voyager 2 revealed that the six major rings are composed of many thousands of individual ringlets, which astronomers liken to ripples or waves in the rings.
Voyager 2 also revealed many gaps within the rings, which are believed to be caused by small moonlets, which may be considered very large ring particles—a few miles in diameter—in orbit around Saturn. The gaps are, in effect, the wake of these bodies. Perhaps the most remarkable Voyager discovery concerning Saturn’s rings concerns the outermost F ring. Its structure is highly complex, sometimes appearing braided. Apparently, the structure of the F ring is influenced by two small outlying moons that bracket the ring called shepherd satellites, which seem to keep the F ring particles from moving in or out.

Looking at the Saturn from Earth

Galileo’s telescope, a wondrous device in 1610, would be no match even for a decent amateur instrument today. When he first observed the planet, all Galileo could tell about Saturn was that it seemed to have “ears.” He speculated that this feature might be topographical, great mountain ranges of some sort. Or perhaps that Saturn was a triple planet system. It wasn’t until a half-century later, in 1656, that Christian Huygens, of the Netherlands, was able to make out this feature for what it was: a thin ring encircling the planet. A few years later, in the 1670s, the Italian-born French astronomer Gian Domenico Cassini (1625–1712) discovered the dark gap between what are now called rings A and B. This feature is now called the Cassini division.
Six major rings, all lying in the equatorial plane of Saturn, have been identified, of
which three, in addition to the Cassini division and a subtler demarcation called the
Encke division, can be seen from the earth with a good telescope. With a typical amateur
instrument you should be able to see ring A (the outermost ring), the Cassini division, and inside the Cassini division, ring B.
If you become a serious Saturn observer, you will notice that the rings of Saturn are seen at different angles at different times. Sometimes we look down on the top of the ring system, and at other times we see
it “edge-on.” When the angle is right, it is possible to see the dramatic image of Saturn’s shadow cast onto its rings. Consult any of the guides in Appendix E for information on where to look for Saturn and when to view it.
The rings readily visible from the earth are vast, the outer radius of the A ring stretching more than 84,800 miles.
Big as the rings are, they are also very thin—in places only about 65 feet (20 m) thick. If you wanted to make an accurate scale model of the rings and fashioned them to the thickness of this sheet of paper, they would have to be a mile wide to remain in proper scale.
Speculation as to the composition of the rings began with their discovery in the mid-seventeenth century. In 1857, James Clerk Maxwell, the British physicist who had been critical of the nebular hypothesis of the formation of the solar system, concluded that the rings must consist of many small particles in orbit around Saturn. By the end of the century, the instrumentation existed to measure reflectivity, the differences in the way sunlight was reflected from the rings. These observations showed that the rings behaved as was to be expected if they were made up of particles; that is, orbital speeds closer to the planet were faster than those farther out—they were in differential rotation, not rotating as a solid disk might.
Where do the rings come from? There are two ways to think about the question, and both involve the gravitational field of the host planet. First, the rings may be the result of a shattered moon. According to this theory, a satellite could have been orbiting too close to the planet and have been torn apart by tidal forces (the same sort of forces that pulled comet Shoemaker-Levy 9 into pieces), or it might have been shattered by a collision. In either case, the fragments of the former moon continued to orbit the planet, but now as fragmentary material. The other possibility is that the rings are material left over from the formation of the planet itself, material that was never able to coalesce into planets due to the strong gravitational field of the host planet.