95 Ring and Moon Systems Introduced
Learning Objectives
By the end of this section, you will be able to:
- Name the major moons of each of the jovian planets
- Describe the basic composition of each jovian planet’s ring system
The rings and moons (see the moons in Figure 1) of the outer solar system are not composed of the same materials as the mostly rocky objects in the inner solar system. We should expect this, since they formed in regions of lower temperature, cool enough so that large quantities of water ice were available as building materials. Most of these objects also contain dark, organic compounds mixed with their ice and rock. Don’t be surprised, therefore, to find that many objects in the ring and moon systems are both icy and dark.
Roughly a third of the moons in the outer solar system are in direct or regular orbits; that is, they revolve about their parent planet in a west-to-east direction and in the plane of the planet’s equator. The majority are irregular moons that orbit in a retrograde (east-to-west) direction or else have orbits of high eccentricity (more elliptical than circular) or high inclination (moving in and out of the planet’s equatorial plane). These irregular moons are mostly located relatively far from their planet; they were probably formed elsewhere and subsequently captured by the planet they now orbit. (Perhaps the fact that they were not born locally will excuse their ill-mannered behavior.)
The Jupiter System
Jupiter has 67 known moons (that’s the number as we write) and a faint ring. These include four large moons—Callisto, Ganymede, Europa, and Io (see Figure 1 in Introduction to Rings, Moons, and Pluto)—discovered in 1610 by Galileo and therefore often called the Galilean moons. The smaller of these, Europa and Io, are about the size of our Moon, while the larger, Ganymede and Callisto, are about the same size as the planet Mercury. Most of Jupiter’s moons are much smaller. The majority are in retrograde orbits more than 20 million kilometers from Jupiter; these are very likely small captured asteroids.
The Saturn System
Saturn has at least 62 known moons in addition to a magnificent set of rings. The largest of the moons, Titan, is almost as big as Ganymede in Jupiter’s system, and it is the only moon with a substantial atmosphere and lakes or seas of liquid hydrocarbons (such as methane and ethane) on the surface. Saturn has six other large regular moons with diameters between 400 and 1600 kilometers, a collection of small moons orbiting in or near the rings, and many captured strays similar to those of Jupiter. Mysteriously, one of Saturn’s smaller moons, Enceladus, has active geysers of water being expelled into space.
The rings of Saturn, one of the most impressive sights in the solar system, are broad and flat, with a few major and many minor gaps. They are not solid, but rather a huge collection of icy fragments, all orbiting the equator of Saturn in a traffic pattern that makes rush hour in a big