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.

Titan: Saturn’s Highly Atmospheric Moon

If Io is the most geologically active moon in the solar system and Ganymede the largest, Saturn’s Titan enjoys the distinction of having the most substantial atmosphere of any moon. No wispy, trace covering, Titan’s atmosphere is mostly nitrogen (90 percent) and argon (nearly 10 percent) with traces of methane and other gases in an atmosphere thicker than the earth’s. The earth’s atmosphere consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent argon. Surface pressure on Titan is about 1.5 times that of the earth. But its surface is very cold, about 90 K. Remember 90 K is –183 C!
Titan’s atmosphere prevents any visible-light view of the surface, though astronomers speculate that the interior of Titan is probably a rocky core surrounded by ice, much like Ganymede and Callisto. Because Titan’s temperature is lower than that of Jupiter’s large moons, it has retained its atmosphere. The presence of an atmosphere thick with organic molecules (carbon monoxide, nitrogen compounds, and various hydrocarbons have been detected in the upper atmosphere) has led to speculation that Titan might support some form of life.

Jupiter’s Four Galilean Moons

The four large moons of Jupiter are very large, ranging in size from Europa, only a bit smaller than the earth’s moon, to Ganymede, which is larger than the planet Mercury. Certainly, they are large enough to have been discovered even through the crude telescope of Galileo Galilei, after whom they have been given their group name. In his notebooks, Galileo called the moons simply I, II, III, and IV. Fortunately, they were eventually given more poetic names, Io, Europa, Ganymede, and Callisto, drawn from Roman mythology. These four are, appropriately, the attendants serving the god Jupiter.
Io is closest to Jupiter, orbiting at an average distance of 261,640 miles (421,240 km);
Europa comes next (416,020 miles or 669,792 km); then Ganymede (663,400 miles or 1,068,074 km); and finally Callisto (1,165,600 miles or 1,876,616 km). Intriguingly, data from Galileo suggests that the core of Io is metallic, and its outer layers rocky—much like the planets closest to the sun. Europa has a rocky core, with a covering of ice and water. The two outer large moons, Ganymede and Callisto, also have more icy surfaces surrounding rocky cores.
This pattern of decreasing density with distance from the central body mimics that of the solar system at large, in which the densest planets, those with metallic cores, orbit nearest the sun, while those composed of less dense materials orbit farthest away. This similarity is no mere coincidence and can be used to discover more about how the Jupiter “system” formed and evolved.
Let’s look briefly at each of Jupiter’s large moons.
Because of our own moon, we are accustomed to thinking of moons generally as geologically dead places. Nothing could be further from the truth in the case of Io, which has the distinction of being the most geologically active object in the solar system. Io’s spectacularly active volcanoes continually spew lava, which keeps the surface of Io relatively smooth—any craters are quickly filled in—but also angry-looking, vivid orange and yellow, sulfurous. In truth, Io is much too small to generate the kind of heat energy that produces vulcanism (volcanic activity); however, orbiting as close as it does to Jupiter, it is subjected to the giant planet’s tremendous gravitational field, which produces tidal forces.These forces stretch the planet from its spherical shape and create the geologically unsettled conditions on Io. Think about what happens when you rapidly squeeze a small rubber ball. The action soon makes the ball quite warm. The forces exerted on Io by Jupiter are analogous to this, but on a titanic scale. Don’t invest in an Io globe for your desk. Its surface features change even faster than political boundaries on the earth! In contrast to Io, Europa is a cold world—but probably not an entirely frozen world, and perhaps, therefore, not a dead world. Images from Galileo suggest that Europa is covered by a crust of water ice, which is networked with cracks and ridges. It is possible that beneath this frozen crust is an ocean of liquid water (not frozen water or water vapor). Liquid water is certainly a requisite of life on Earth, though the presence of water does not dictate the existence of life. Still, the prospects are most exciting. Europa may be a literal lifeboat in the outer solar system, although before we get our hopes up, we need to realize just how cold Europa is at 130 K and how thin its atmosphere is—at a pressure approximately one billionth that on Earth. Ganymede is the largest moon in the solar system (bigger than the planet Mercury). Its surface shows evidence of subsurface ice that was liquefied by the impact of asteroids and then refrozen. Callisto is smaller but similar in composition. Both are ancient worlds of water ice, impacted by craters. There is little evidence of the current presence of liquid water on these moons.

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.