The so-called outer planets are Jupiter, Saturn, Uranus, Neptune, and Pluto (although some people debate whether or not Pluto qualifies as a planet). All these planets have moons, some of which are bigger than Earth’s moon. In this chapter, we’ll look at planetary moons larger than 1000 km (620 mi) in diameter.
Most of the moons of the outer planets always keep the same side facing their parent planets. This is the result of long-term tidal effects and is the same phenomenon as that which has happened to Earth’s moon. Most of the moons of the outer planets orbit near the equators of their parent planets and in the same direction as the planets’ rotations. In the cases of Jupiter, Saturn, and Uranus, the planet-moon systems are thought to have evolved like star systems in miniature.
Now that astronomers have had a chance to closely examine (by means of space probes) the major moons of the outer planets along with the planets themselves, how can anyone not be amazed at the variety of worlds our Solar System has produced? The more we learn about these worlds, the more mysterious they become.
Jupiter, the largest of the planets, has a system of moons that resembles a miniature “Solar System” with Jupiter as the “Sun.” Four of these moons can be seen through a good pair of binoculars or a small telescope. Galileo Galilei observed them and carefully recorded their behavior in the early 1600s. Their images were starlike points of light that appeared along a straight line passing through Jupiter’s disk. Galileo deduced that these points of light were natural satellites, or moons, because their relative positions changed from night to night in a way consistent with bodies traveling in more or less circular paths around Jupiter. These four little worlds, and only these four—Ganymede, Callisto, Io, and Europa—are called the Galilean moons in memory of Galileo.
This moon, Jupiter’s largest, is 5,270 km (3,270 mi) across, a little less than half the diameter of Earth but larger than Earth’s moon. Compared with Jupiter, Ganymede is tiny (Fig. 10-1). The satellite orbits Jupiter in a nearly perfect circle at a distance of about 1.1 million km (660,000 mi). Like virtually all planetary moons, Ganymede orbits near the equator of its parent planet and revolves in the same direction that the planet rotates. Also like most planetary moons, Ganymede keeps the same face toward Jupiter at all times. At the surface of Ganymede, the gravitational field is only about 15 percent as strong as it is on Earth’s surface. Thus, if you weigh 140 lb on Earth, you would weigh only 21 lb on Ganymede.
Ganymede is believed to consist of a metallic core surrounded by rocky material, in turn surrounded by a crust of rock mixed with ice. The moon’s density is approximately twice that of water. The surface was seen close up by the Voyager probe, and a number of features were observed, including many craters, evidence that Ganymede has been bombarded by meteorites. The shallow nature of the craters suggests that the surface of Ganymede is largely made of water ice. At the extreme cold temperatures at Jupiter’s distance from the Sun, ice behaves something like rock on Earth’s surface. However, over time the ice flows and settles so that the mountains and rims of the craters flatten out gradually. Nevertheless, this ice maintains crater imprints for millions of years.
Ganymede, like most of the other moons of the outer planets, has little or no atmosphere, although Ganymede has enough of a gravitational pull to hold down trace amounts of oxygen and other gases. The presence of water (in the form of ice) and oxygen in the environment of Ganymede does not imply that this moon bears life; temperatures are too cold and conditions far too tranquil for anything biological to evolve the way it has on our planet Earth. One of the ingredients for the development of life as we know it, interestingly enough, is an environment subject to change.
Figure 10-1. The relative sizes of Jupiter’s largest moon Ganymede, the planet Earth, and the planet Jupiter.
The second largest moon of Jupiter, Callisto, is 4,800 km (3,000 mi) in diameter, nearly as large as Ganymede. It is about the same size as the planet Mercury. Callisto orbits Jupiter almost twice as far away as Ganymede: 1.9 million km (1.2 million mi). Callisto is the outermost of Jupiter’s four large Galilean satellites. The density of Callisto is similar to that of Ganymede; it has about 1.8 times the specific gravity of water. The interior structure of Callisto appears to differ somewhat from that of Ganymede because Callisto reflects less light. Some regions of Callisto are almost black, whereas others are bright. The oldest geologic features appear in the darker areas.
Callisto is covered by impact craters, as we would expect for a satellite of the planet with the most powerful gravitational field in the Solar System. There are also markings that look like concentric rings when photographs of this moon, taken by the passing Voyager, are examined. The most commonly held belief is that these “bull’s-eye” features are the remnants of craters produced by gigantic objects that smashed into Callisto in the early ages of the Solar System, just after the formation of Jupiter and its moons, when Callisto was still in a molten or partly molten state. Some of these features are thought to be in excess of 4 billion years old.
Despite the fact that Callisto is heavily cratered, the depths of the craters and the heights of the mountains associated with them are fairly shallow. This suggests a smoothing-out process that has taken place since most of the craters were formed. This is the same sort of thing that seems to have occurred on Ganymede. Callisto also has features that are believed to be eroding; these were first seen in a 2001 space-probe flyby and have been described as “spires” or “knobs” several hundred meters high. It is not clear how these structures were formed.
Some astronomers speculate that Callisto has a salty sea beneath its outer crust of rock and ice. This idea has come from the fact that there are numerous bright spots on the surface consisting of water ice that looks as if it flowed out of some of the craters and froze on the exterior. If this model is correct, then large meteorites striking Callisto occasionally have punctured the solid crust and created holes through which the subsurface ocean spilled out and froze solid when exposed to the cold. After such a crater formed, it would fill up with ice, creating the bright spot. Observations and analyses of the magnetic field, or magnetosphere, surrounding Callisto lend further support to the subsurface-ocean theory. The Galileo space probe, which began observing Callisto in the 1990s, has detected the presence of a magnetic field that behaves in a way consistent with the presence of a conductive liquid beneath the surface. Salty water is a fairly good electrical conductor and could carry currents sufficient to generate the magnetic field that has been observed.
Many astronomers think that Io, the third-largest moon of Jupiter, is the most interesting object in the Solar System other than our own Earth. When the first photographs of this moon were returned by space probes, scientists could hardly believe their eyes. Rather than a desolate, cratered scape typical of most other moons and asteroids in the Solar System, Io’s surface looked like a pizza.
Io is 3,630 km (2,260 mi) in diameter, somewhat smaller than either Ganymede or Callisto. Io is also the innermost major moon of Jupiter. Because of its proximity to Jupiter and its orbital position with respect to the other major moons, Io is constantly bearing the forces of a gravitational tug-of-war. This heats up the whole moon because of tidal flexing in much the same way as a piece of wire heats up if you bend it back and forth. The heat boils away water from the surface. As a result of this, and also because Io appears to have an iron core, the density of this moon is relatively high.
Io’s constant internal heating produces volcanic activity that has been photographed by space probes. If you could stand on the surface of Io, you would weigh only about 18 percent of your weight on Earth, but you would, if you didn’t know better, think it reasonable to suppose that you had gone to hell. Sulfur and other molten rock compounds are abundant on this little world. There are no impact craters visible from space; the constant flow of volcanic lava on the surface erases them before they can become old. There are plenty of volcanic calderas, however. Mountains several kilometers high also have been seen. Io has a thin atmosphere. This is the sort of thing we would expect from constant emission of gases from volcanoes.
Europa is the smallest of the Galilean moons, with a diameter of 3,140 km (1,950 mi). It orbits only 670,000 km (420,000 mi) from its parent planet.
Europa has a high albedo (proportion of light that it reflects), unlike almost all other planetary moons. When this moon was photographed close up by the space probes and the light from its surface was analyzed, scientists concluded that it is water ice. Europa has high density, however, suggesting that this water is only a thin veneer over a predominantly rocky planetoid. The surface is crisscrossed with fracture lines that resemble those in the Earth’s Arctic and Antarctic ice shelves. This suggests that the ice is the frozen surface of an ocean that covers the entire moon. There are very few impact craters visible on the surface. Astronomers think that liquid water from underneath the ice periodically floods the surface and freezes over, erasing old surface features and creating new ones. There is a thin atmosphere consisting largely of oxygen.
Europa has a weak magnetosphere. There are two theories concerning the origin of this magnetism: electric currents in a salty sea or electric currents in an iron core. The intensity of this field is affected by Jupiter’s vastly more powerful magnetic flux.
Whenever scientists find water on an extraterrestrial body, they are tempted to speculate concerning the evolution of life. There is only one way to find out if there are strange, fascinating marine organisms beneath the ice of Europa: Send a probe down to land on the surface and drill holes in the ice! Someday, if astronomers get their way, this will be done.
Figure 10-2. The relative sizes of Jupiter’s four Galilean satellites, along with Earth, as they would appear if placed size by side. Jupiter (gray curve) dwarfs them all.
Figure 10-2 shows the four major satellites of Jupiter, along with the Earth and the curvature of Jupiter itself for size comparison.
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.
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.
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.
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.
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 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 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 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.
Tethys has a diameter of 1,060 km (660 mi) and orbits Saturn at a distance of 290,000 km (180,000 mi). Like Dione and Rhea, this satellite is believed to consist mainly of water ice with some rocky material mixed in.
Tethys is noted for its long surface canyon and for a crater that is gigantic compared with the size of the moon itself. The size of this crater and the presence of the fracture suggest that a large asteroid smashed into Tethys and nearly broke the moon in two. Gravity, however, pulled the object back together again. According to one theory, Tethys was liquid at one time, and if this was the case when the violent impact took place, it might have saved the moon from being pulverized.
Like Dione, Tethys is involved in an orbital resonance with one of Saturn’s minor moons, Mimas. Mimas orbits the planet twice for every orbit of Tethys. There are also two tiny moons that orbit in exactly the same path around Saturn as Tethys but 60 degrees of arc (one-sixth of a circle) ahead and behind it. This is a common phenomenon for major satellites of planets and stars that have nearly circular orbits and is a result of gravitational interaction between the moon and its parent planet or between the planet and its parent star. The points 60 degrees ahead and behind an object in a nearly circular orbit are known as the Lagrange points.
Figure 10-4 shows the five major satellites of Saturn, along with the Earth and the curvature of Saturn itself for size comparison.
Uranus has numerous moons. Four of them can be considered major satellites, in the sense that they have diameters greater than 1,000 km (620 mi). These four moons are Titania, Oberon, Umbriel, and Ariel. As we have already learned, Uranus is tilted on its axis so much that its equator lies almost perpendicular to the plane of its orbit around the Sun. The moons of Uranus orbit in or near the plane of the planet’s equator, rather than the plane of the planet’s orbit around the Sun. This is to say that the entire planet-moon system is tipped almost perfectly on its side.
Figure 10-4. The five largest satellites of Saturn, along with Earth, as they would appear if placed in close proximity. They are all tiny compared with Saturn (gray curve).
The major moons of Uranus are believed to resemble “dirty snowballs,” a mixture of water ice and rock. The minor moons, with the exception of Miranda, are much smaller than the major ones and in some sense can be considered captive comets, containing a higher proportion of ice and less rocky material. None of the moons of Uranus has any appreciable atmosphere. Like most of the moons of major planets, they each keep the same side facing their parent at all times, and their orbits are nearly perfect circles.
Titania, the largest moon of Uranus, is only 1,580 km (980 mi) in diameter. It orbits its parent planet at a distance of 436,000 km (271,000 mi). Titania is much smaller than Uranus and between one-eighth and one-ninth the diameter of the Earth (Fig. 10-5). Observations of this moon and analysis of the light reflected from its surface indicate that it is made up of approximately half water ice and half rocky material.
In addition to the usual craters, the surface of Titania has long cracks or valleys. The reason for the existence of these fracture zones is unknown, but one popular theory holds that Titania was liquid at one time and then it froze from the outside in. As the water beneath the surface froze, the ice above cracked because water expands when it freezes. Another theory suggests that heat from the interior produces occasional eruptions of hot liquid or gas that penetrates the surface.
Oberon is just a little bit smaller than Titania, with a diameter of about 1,520 km (950 mi). It orbits Uranus at a distance of 583,000 km (362,000 mi). This moon has a composition similar to that of Titania, but there is some indication that the surface features are older. Fracture zones exist, and their origins suggest that Oberon was geologically active for a while after it formed, but it appears as if Oberon has been a “dead world” for much of its existence.
One of the most interesting features of Oberon is dark material inside many of its craters. The surface consists largely of water ice. At Uranus’ distance from the Sun, ice is as hard as granite unless heating occurs as a result of some other action such as tidal forces or internal activity. Neither of these factors seems to play a role on Oberon, and this makes the origin of the dark material somewhat mysterious. Some astronomers think that the dark material is volcanic lava, but there is little evidence to support this kind of activity on Oberon. Another theory holds that the floors of these craters are relatively smoother than the surrounding terrain and that this is why they appear darker. When a large meteorite strikes an icy body such as Oberon, the heat of impact melts the ice in and around the point of impact. The liquid water pools inside the crater and then refreezes, producing a smooth landscape that reflects relatively little light. You have seen this effect if you have ever looked at a smooth, well-kept outdoor skating rink surrounded by snow.
Figure 10-5. The relative size of Uranus’ largest moon Titania, the planet Earth, and the planet Uranus.
This satellite has a diameter barely large enough to satisfy our arbitrary minimum size limit to qualify it as a major moon: 1,170 km (730 mi). It orbits 266,000 km (165,000 mi) above its parent planet and takes a little more than 4 Earth days to revolve once around.
Umbriel is notable for its low albedo. The surface is almost entirely charcoal black. The only reason we can see it at all is that it isn’t a perfect light absorber; it reflects approximately one-fifth of the light it receives from the Sun. The orb resembles a gigantic dirty ice ball. Most of the solid material on the surface is water ice mixed with rocky material, but there also appears to be some frozen methane and trace amounts of other frozen elements and compounds that are gases in the familiar environs of our planet Earth.
The entire surface of Umbriel is pitted with craters. One feature, a bright ringlike mark, is thought to be the outline of a crater in which the rim mountains have more exposed ice than either the interior or exterior lowlands. Umbriel shows no signs of geologic activity in the recent past (meaning within the last several million years). Some scientists believe that it has not undergone much change since it was formed as part of the Uranian system.
Ariel is, in terms of size, practically a twin of Umbriel. It measures 1,160 km (720 mi) in diameter. It is much closer to Uranus, orbiting at a mean altitude of 190,000 km (120,000 mi). It takes only 2½ Earth days to orbit once around the planet. Like all the other moons of Uranus, Ariel orbits in a nearly perfect circle and keeps the same face toward Uranus all the time.
Ariel reflects about twice as much light as Umbriel, leading astronomers to surmise that its surface consists of relatively more icy material and less rock. The whole surface is cratered, but there are huge rift valleys and canyons too. There is evidence that a mixture of liquid ammonia and methane once flowed across the surface of this moon.
The long cracks in the surface of Ariel suggest that the moon has expanded or contracted since it was formed, resulting in fault lines. Some of the canyons have ridges inside them, as if liquids once flowed out from the interior and then froze solid when exposed to the chill of space in the outer Solar System.
Figure 10-6 is a size comparison of Titania, Oberon, Umbriel, and Ariel, along with the Earth and the curvature of Uranus.
Figure 10-6. The four largest satellites of Uranus, along with Earth, as they would appear if placed in close proximity. The relative size of Uranus is represented by the gray curve.
The two outermost known planets in our Solar System, Neptune and Pluto, each have only one moon that is more than 1,000 km across. Neptune’s lone major satellite is called Triton, and Pluto’s is called Charon.
Neptune’s dominant moon has a diameter of 2,700 km (1,680 mi). It orbits at 385,000 km (240,000 mi) from Neptune, just about the same distance as Earth’s moon is from Earth. Figure 10-7 compares Triton in terms of size with Earth and Neptune.
This little world has the distinction of possessing the chilliest surface of any known planet or moon, approximately –235°C (–390°F). Triton is also unique in another way, for it is the only known moon in our Solar System that orbits its parent planet in the opposite direction from that of the planet’s rotation. As if this does not make Triton peculiar enough, its orbit is greatly tilted with respect to the plane of Neptune’s equator.
Figure 10-7. Triton as it would appear if placed next to our own planet Earth. The relative size of Neptune is shown by the gray curve.
Measurements of Triton’s density indicate that it is made up of relatively less ice and more rock than the other major moons of outer planets. This fact, along with the retrograde orbit, has given rise to the theory that Triton was not originally a moon at all but instead was a planet in its own right when the Solar System was formed. It ventured too close to Neptune, and the large planet captured it. Strangely, however, Triton’s orbit is essentially a perfect circle, and this fact can be used to argue against the “once it was a planet” hypothesis. Triton keeps the same side facing Neptune all the time, but this generally takes place with planetary moons as the parent planets’ gravitational fields create tidal bulging over millions of years, pulling the rotation and revolution rates into synchronicity.
Triton has an atmosphere, but it is so thin that it would make a good laboratory vacuum for most Earthly purposes. The surface is believed to contain frozen methane along with nitrogen ice because of the pinkish cast to much of its surface. Clouds form occasionally, and these apparently consist of tiny particles of frozen nitrogen. There is evidence of wind erosion on the surface.
Volcanic activity apparently occurs on Triton, but the eruptions are entirely different from the volcanoes we know on Earth or from those that dominate the surface of Jupiter’s restless satellite, Io. Instead of hot lava from the interior, the ejected material is believed to be liquid nitrogen or methane that freezes as soon as it comes into contact with the bitter-cold surface environment.
Charon, with a diameter of 1,190 km (740 mi), is the only known satellite of Pluto. Charon is small in absolute terms, but it is significant compared with Pluto. Figure 7-11 (in Chap. 7) compares Charon for size with Earth and Pluto. Charon orbits Pluto at an average distance of approximately 20,000 km (12,500 mi). Charon is unique not only in that it is the largest moon in size relative to its parent planet, but it is also extremely low in its orbit. In fact, the two bodies tidally affect each other to the extent that they always keep the same sides facing each other.
Charon is believed to be composed of essentially the same stuff as Pluto, a combination of rocks and ices. Charon’s surface, however, is mainly frozen water, unlike that of Pluto, which is largely frozen nitrogen with traces of methane ice. While Pluto has a pinkish or reddish tinge when observed in visible light, Charon appears gray.
Some astronomers think that Charon and Pluto are surviving members of an originally much larger group of icy, comet-like bodies in orbits outside that of Neptune. According to one theory, most of these objects congealed to form Pluto and Charon. During this process, there were countless collisions. Some objects got hurled in toward the Sun and became comets in the classic sense as they got close enough to the Sun to develop tails. The collisions also gave the Pluto-Charon system its eccentric orbit around the Sun. While major collisions of these outer denizens of the Solar System are a thing of the past, smaller collisions and gravitational interactions still take place, and every few years a new comet happens across the watchful eye of some comet-seeking astronomer.
The Pluto-Charon system seems distant and insignificant as you read about it in a book, but Pluto and Charon are relatives of objects that have played crucial roles in the evolution of life on our planet. Some scientists think that a comet brought the first primitive life forms to Earth or produced the energy necessary for amino acids to form. A comet is believed to have struck the Earth in the present-day Gulf of Mexico about 65 million years ago, leading to the extinction of dinosaurs and an upsetting of the terrestrial equilibrium that, had it not been perturbed, would still give sanctuary to the giant ruling lizards today.
QuizRefer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. Most planetary moons rotate on their axes
(a) once for every orbit they complete around their parent planets.
(b) in a retrograde manner.
(c) perpendicular to the axes of their parent planets.
(d) in synchronicity with the parent planet’s rotation on its axis.
2. The greatest danger that will face astronauts who plan to visit any of the inner moons of Jupiter or Saturn is
(a) the parent planet’s radiation belts.
(b) the moon’s gravitation.
(c) the moon’s lack of an atmosphere.
(d) the lack of sufficient visible light.
3. Tethys and Mimas are notable because they
(a) orbit each other.
(b) are in orbital resonance with each other.
(c) have retrograde orbits.
(d) both have exceptionally high albedo.
4. Continuous, moonwide volcanic activity is observed on
(a) Ganymede.
(b) Io.
(c) Charon.
(d) no planetary satellite.
5. Umbriel is a moon of
(a) Jupiter.
(b) Saturn.
(c) Uranus.
(d) Neptune.
6. Which of the following is a Galilean moon of Saturn?
(a) Ganymede
(b) Titan
(c) Triton
(d) There are no Galilean moons of Saturn.
7. The Lagrange points in a satellite’s orbit are the result of
(a) radiation from the Sun.
(b) volcanic activity on the parent planet.
(c) thermal heating from inside the satellite.
(d) gravitational interaction.
8. Which of the following moons is noted for its difference in albedo between the leading side and the trailing side?
(a) Charon
(b) Titan
(c) Iapetus
(d) Callisto
9. The planetary moon with the most extensive atmosphere is
(a) Titan.
(b) Charon.
(c) Deimos.
(d) Rhea.
10. On the surfaces of moons of the outer planets, water ice has a hardness and consistency similar to that of
(a) molasses at room temperature.
(b) putty at room temperature.
(c) rock at room temperature.
(d) steel at room temperature.
