Friday, April 30, 2010

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.