Saturn's Major Moons Help (page 3)

Saturn’s Major Moons—Titan

Saturn has more known natural satellites than any other planet. Most of Saturn’s moons are ice-covered orbs; the smaller ones are irregular chunks, some of which are doubtless asteroids that were captured by Saturn long after the planet and its main moon system were formed. Only five of Saturn’s moons exceed 1,000 km (620 mi) in diameter. These are Titan, Rhea, Iapetus, Dione , and Tethys .

Titan

The largest and most interesting satellite of Saturn is Titan, measuring 5,150 km (3,200 miles) in diameter. It is almost as large as Ganymede, Jupiter’s largest moon. Nevertheless, the gigantic, gaseous planet Saturn dwarfs it (Fig. 10-3). Titan is the only planetary moon that has a significant atmosphere. As viewed through the most powerful telescopes, and even from space probes flying by, Titan looks something like an orange little sister of Venus. The cloud layer is so thick that it hides the surface features from visual view.

The atmosphere of Titan is comprised mainly of nitrogen and methane and is cold by Earthly standards, far below 0°C at the surface. The atmospheric pressure at the surface is about half again as great as the normal atmospheric pressure at the surface of the Earth. Thus, although we would not be able to breathe Titan’s “air,” we would at least not have to worry about being crushed to death by its pressure, as would be the case on Venus.

Figure 10-3. The relative size of Saturn’s largest moon Titan, the planet Earth, and the planet Saturn. The rings are shown edgewise, in proportion.

The main reason scientists find Titan so interesting is that it contains an abundance of organic chemicals. The term organic does not mean that these chemicals were produced by or are necessarily indicative of living things in the environment. Methane and ethane, hydrocarbons similar to natural gas, are considered organic because they have the potential to give rise to amino acids under the right conditions. The impact of a large meteorite or comet or an electrical discharge caused by a thunderstorm creates the high temperatures necessary for the formation of amino acids, which are the building blocks of life.

Titan is a candidate for exploration by humans. The main problem to be overcome in such a visit, besides the enormous distance that separates the Saturnian system from Earth, is the powerful magnetic field surrounding Saturn, which accelerates subatomic particles from the Sun, producing intense belts of ionizing radiation. Although this radiation is less intense than that in the vicinity of Jupiter, it is still much greater than the intensity of the Van Allen radiation belts surrounding the Earth. Anyone who lands on Titan also would have to be prepared for the possibility of hitting a turbulent, liquid hydrocarbon surface, perhaps with floating icebergs of frozen methane and heavy methane rain or snow blowing down out of the red sky.

Rhea

Rhea is 1,530 km (950 mi) in diameter. It orbits in an almost perfect circle 530,000 km (330,000 mi) from its parent planet. This moon is only a little more dense than water, and this fact has led astronomers to conclude that it must be comprised mainly of ice and very little rocky material.

Rhea, like most moons, keeps the same face toward Saturn at all times. As a result, one side of the moon “leads the way” through space, whereas the opposite side “trails behind.” There is a considerable difference in the appearance of the leading side of Rhea compared with the trailing side. The leading side is as densely packed with craters as any part of Earth’s moon, even though the surface of Rhea is mostly water ice. The trailing side has far fewer craters. This would be expected because the leading side would be more exposed to bombardment by meteorites.

Most of the rock in Rhea is believed to be contained in a small core. The moon’s small size and its relatively large distance from Saturn prevent heating from tidal effects, keeping the surface far below 0°C and hardening the ice so that it resembles granite and can maintain crater and mountain formations for a long time. Rhea has essentially no atmosphere.

Iapetus

Iapetus orbits at a great distance from Saturn: 3.6 million km (2.2 million mi). Its diameter is about 1,450 km (900 mi). Like Rhea, Iapetus is only a little more dense than water, and analysis of light reflected from its surface indicates that this moon is made up mostly of water ice.

The leading side of Iapetus is much darker than the trailing side. This is the opposite of the situation with Rhea. The contrast is great; the leading side is nearly as white as snow, whereas the trailing side is nearly as dark as tar. Also in contrast with Rhea, most of the craters on Iapetus are on the trailing side. This has caused some befuddlement among astronomers. Did something stain the leading side of Iapetus and cover up the craters there? Did this “dye” come from inside Iapetus, or did it come from space? Or is it the result of some reaction of material on the surface with ultraviolet light or high-speed particles from the Sun?

Iapetus is the only major moon of Saturn that does not orbit almost exactly in the plane of Saturn’s equator. Instead, Iapetus is inclined by 15 degrees. One theory concerning this inclination is that Iapetus did not form along with the Saturnian system but instead was once a huge wandering protocomet or planetoid that was captured by Saturn’s gravitation. Another theory holds that Iapetus originally orbited in the plane of Saturn’s equator but was knocked out of kilter by a large asteroid.

Dione

Dione has a diameter of 1,120 km (690 mi) and orbits Saturn in an almost perfect circle at a distance of 377,000 km (234,000 mi). Dione’s density is about 1.4 times that of water. This fact and the analysis of the light reflected from its surface indicate that Dione, like most of the other moons of Saturn, is made up largely of water ice. It is thought that the proportion of ice to rock is higher near the surface and lower near the core.

There are variations in the reflectivity of the surface of this moon, but the demarcation is not as great as is the case with Iapetus. The leading side is generally brighter than the darker side. The trailing side has wispy markings that suggest that volatile material, perhaps water vapor, has escaped from the interior and fallen back on the surface to freeze. Some areas of Dione are heavily cratered, whereas other regions contain virtually no craters.

Dione exhibits a property that is sometimes found in the satellite systems of large planets: orbital resonance with one of the other moons. In this case the other moon is Enceladus , one of the minor satellites of Saturn. While Dione takes 66 Earth hours to orbit once around Saturn, Enceladus takes 33 hours, exactly half that time. Orbital resonance effects are caused by mutual gravitation between celestial objects, such as moons, when they both orbit around a common, larger object, such as a planet. This resonance effect is believed to be responsible for tidal forces on Enceladus that cause it to generate heat from inside.