Where Did Pluto Come From?

Except that it doesn’t orbit another planet—and, indeed, has a moon of its own—Pluto looks more like a jovian moon than a planet. It fits into neither the terrestrial nor jovian mold. Some astronomers believe that Pluto is really a renegade moon, escaped from Neptune’s gravitational influence due to a collision or interaction involving Triton, Pluto, Charon, and Nereid. Others regard it as a kind of spare part, something left over from the creation of the solar system, and perhaps only one of a number of such objects in the outer reaches of the solar system, the Kuiper Belt.

Charon, the Moon of Pluto

If, having been discovered in 1930, Pluto was a late addition to our known solar system, its moon, Charon, is almost brand new, having been found in 1978. Named, fittingly, for the mythological ferryman who rowed the dead across the River Styx to the underworld ruled by Pluto, Charon is a little more than half the size of its parent: 806 miles (1,300 km) in diameter versus Pluto’s 1,426 miles (2,300 km). Orbiting 12,214 miles (19,700 km) from Pluto, it takes 6.4 Earth days to make one circuit. Pluto and Charon are tidally locked—forever facing one another; the orbital period and rotation period for both are synchronized at 6.4 days. Like Venus and Uranus, Pluto’s rotation is retrograde spinning on its axis in the opposite direction of most of the planets.

A Dozen More Moons in the Outer Solar System

Thanks to Voyager, the six medium-sized moons of Saturn have also been explored. All of these bodies are tidally locked with Saturn, their orbits synchronous, so that they show but one face to their parent planet. They are frozen worlds, mostly rock and water ice. The most distant from Saturn, Iapetus, orbits some 2,207,200 miles (3,560,000 km) from its parent. Because these moons orbit synchronously, astronomers speak of their leading faces and trailing faces. That one face always looks in the direction of the orbit and the other in the opposite direction has created asymmetrical surface features on some of these moons. The leading face of Iapetus, for example, is very dark, while the trailing face is quite light. While some astronomers suggest that the dark material covering this moon’s leading face is generated internally, others believe that Iapetus sweeps up the dust it encounters.
The innermost moon of Saturn, Mimas is 115,320 miles (186,000 km) out. It is also the smallest of Saturn’s moons, with a radius of just 124 miles (200 km). Mimas is very close to Saturn’s rings and seems to have been battered by material associated with them. Heavily cratered overall, this small moon has one enormous crater named for the astronomer William Herschel, which makes it resemble the “Death Star” commanded by Darth Vader in Star Wars. Whatever caused this impact probably came close to shattering Mimas. Indeed, some astronomers believe that similar impacts may have created some of the debris that formed Saturn’s great rings. The Cassini mission will add greatly to our knowledge of these moons and rings. The Cassini-Huygens Mission to Saturn and its mysterious moon Titan was launched on October 15, 1997.
The spacecraft will separate into two parts as it approaches Saturn, sending the Huygens Titan probe on a mission to the surface of the atmosphereenshrouded moon. The mission will study the magnetosphere of Saturn, the planet Saturn itself and its atmosphere and rings, the moon Titan, and finally the other icy moons that orbit the planet. If there were any worries about the performance of the spacecraft, or what it will do when it arrives at Saturn on July 1, 2004, they were substantially allayed in early 2001 when the Cassini-Huygens Mission sped past Jupiter, snapping pictures of the gas giant. You can check on the progress of the mission and view its photos of Jupiter at www.jpl.nasa.gov/cassini/.
The medium-sized moons of Uranus are Miranda, orbiting 80,600 miles (130,000 km) above the planet; Ariel, 118,400 miles (191,000 km) out; Umbriel, 164,900 miles (266,000 km) out; Titania, 270,300 miles (436,000 km) out; and Oberon, 361,500 miles (583,000 km) out. Of these, the most remarkable is Miranda, which, in contrast to the other moons, is extremely varied geographically, with ridges, valleys, and ovalshaped faults. To the camera of Voyager 2, it presented a chaotic, violently fractured, cobbled-together surface unlike that of any other moon in the solar system. Clearly, this moon had a violent past, though it is unclear whether the disruptions it suffered came from within, without, or both. Some astronomers believe that Miranda was virtually shattered, its pieces coming back together in a near-jumble.

Triton, Neptune’s Large Moon

Triton’s distinction among the jovian moons is a retrograde (backward) orbit—in the reverse direction of the other moons. Moreover, Triton is inclined on its axis about 20 degrees and is the only large jovian moon that does not orbit in the equatorial plane of its planet. Many astronomers believe that these peculiarities are the result of some violent event, perhaps a collision. Others suggest that Triton did not form as part of the Neptunian system of moons, but was captured later by the planet’s gravitational field.
Triton’s atmosphere is so thin that Voyager 2 had no trouble imaging the moon’s surface, finding vast lakes of water ice or water-ammonia mixtures there. Nitrogen frost, found at the polar caps, appears to retreat and reforms seasonally.

Moons of Gas Giants

One of the key differences between the terrestrial and many jovian planets is that, while the terrestrials have few if any moons, the jovians each have several: 16 (at least) for Jupiter, over 25 for Saturn, 15 for Uranus, and 8 for Neptune. Of these known moons, only 6 are classified as large bodies, comparable in size to the earth’s moon. Our own moon is all the more remarkable when compared to the moons of the much larger jovian planets. It is larger than all of the known moons except for Ganymede, Titan, Callisto, and Io. The largest Jovian moons (in order of decreasing radius) are …

➤ Ganymede orbits Jupiter; approximate radius: 1,630 miles (2,630 km)
➤ Titan orbits Saturn; approximate radius: 1,600 miles (2,580 km)
➤ Callisto orbits Jupiter; approximate radius: 1,488 miles (2,400 km)
➤ Io orbits Jupiter; approximate radius: 1,130 miles (1,820 km)
➤ Europa orbits Jupiter; approximate radius: 973 miles (1,570 km)
➤ Triton orbits Neptune; approximate radius: 856 miles (1,380 km)

It is interesting to compare these to the earth’s moon, with a radius of about 1,079 miles (1,740 km), and the planet Pluto, smaller than them all, with a radius of 713 miles (1,150 km).
The rest of the moons are either medium-sized bodies—with radii from 124 miles (200 km) to 465 miles (750 km)—or small bodies, with radii of less than 93 miles (150 km). Many of the moons are either entirely or mostly composed of water ice, and some of the smallest bodies are no more than irregularly shaped rock and ice chunks. Thanks to the Voyager and Galileo space probes, we have some remarkable images and data about the moons at the far end of our solar system. Those that have received the most attention, since they are the largest, are the so-called Galilean moons of Jupiter; Saturn’s Titan; and Neptune’s Triton. They were first observed in 1610 by Galileo Galilei.

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.