Only three satellites are known for the Terrestrial planets. One is the Moon, which we discussed in Chapter 7. The other two are the little satellites of Mars, which were discovered in 1877. Phobos, the inner one, and Deimos, the outer one, are potato-shaped bodies with cratered surfaces. Phobos orbits eastward, just as our Moon does, and in the same direction that Mars rotates, in a period of 12.5 hours at a distance of about 6000 km from the surface of Mars. This gives it an angular size, as seen from the surface of Mars, of about half that of our Moon. Since it revolves around Mars much faster than the planet rotates, it rises on the western horizon and sets on the eastern horizon 5.5 hours later. As observed from its primary, Phobos is the only natural satellite in the Solar System whose motion is of this form.

Phobos (Figure 10.1) is about 27 km long, 21 km high, and 19 km wide. Both Phobos and Deimos have been shaped by high-velocity impacts, which appear to have sheared off large sections of each satellite. In addition, both have many craters, but no significant amount of ejected material or craters with central peaks, a reasonable result since their gravitational attraction is very small. Some of the material ejected during cratering simply escapes and does not fall back to the satellites' surfaces.

Phobos seems more heavily cratered than Deimos, the largest crater, Stickney, being about 10 km across. It also has mysterious long parallel grooves across a large part of its surface, as can be seen in Figure 10.1. They are a few hundred meters wide and a few tens of meters deep and may have been formed by the same impact that caused the crater Stickney.

Deimos is about half the size of Phobos--some 15 km by 12 km. It orbits Mars some 20,000 km from the planet's surface in a period of 30.3 hours. Its angular size, as seen from Mars, is quite small, roughly equivalent to a quarter viewed at a distance of about 40 m. Its orbital period is somewhat longer than the rotational period of Mars, so it rises on the eastern horizon and sets on the western horizon nearly 3 days later, while going through its phases twice.

The darkness of the surface of both satellites is probably due to carbon- and water-rich minerals, such as are found in black, crumbly meteorites known as carbonaceous chondrites. A number of asteroids appear to have similar surfaces. This has led to the speculation that Mars's satellites may be captured asteroids acquired early in the planet's life.

10.2.1. The Asteroid Belt

On January 1, 1801, a Sicilian astronomer, Giuseppe Piazzi (1746-1826), accidentally discovered a faint object whose orbital motion was that of a body 2.8 AU from the Sun. Although Piazzi thought it was a comet, others noted that it was located about where a major planet would be expected according to Bode's law (Section 3.6). The object was later named Ceres, after the Roman goddess of agriculture. Shortly afterward, three more objects were discovered with orbits near 2.8 AU: Pallas in 1802, Juno in 1804, and Vesta in 1807. Since photographic techniques were introduced into astronomical research in the 1890s, more than 3000 of these bodies have been discovered.

So instead of one planet in the slot at 2.8 AU, many small bodies orbit in the region between Mars and Jupiter. William Herschel called these objects asteroids because in a telescope they looked like stars. Better than 90 percent of them have orbits in an asteroid belt between 2.2 and 3.3 AU, with periods from 3 to 6 years. For several asteroids, their orbits are more elliptic than are those of the planets and more inclined to the ecliptic. However, they move in the same direction as the planets around the Sun.

Asteroids vary in size from Ceres (960 km) down to an estimated 1 million that are more than 1 km in diameter and countless numbers of even smaller ones. All the asteroids together add up to no more than six ten-thousandths of Earth's mass. Ceres alone constitutes about 20 percent of the mass of all the asteroids (Figure 10.2). Six of the larger asteroids are listed in Table 10.2.

Photometric studies of asteroids have for some time been interpreted as showing that they differ in size, shape, and rotation. All but the largest ones are too small to show a measurable disk. From the variations in their brightness, it has been assumed that most have somewhat irregular shapes with periods of rotation measured in hours.

Their colors put nearly all asteroids into two categories: Some are bright reddish or sandy-colored, a sign of iron and magnesium silicates, and they populate mostly the inner part of the asteroid belt. Most asteroids, however, have the darker neutral color of material containing various carbon- or water-rich compounds (carbonaceous) and occupy mainly the outer part of the belt.

Collisions between two asteroids may produce effects ranging from craters (if a small one collides with a large one) to fragmenting of both asteroids (if they are of comparable size). For example, if the body producing crater Stickney on Mars's satellite Phobos (Figure 10.1) had been a little larger, Phobos might have been broken into many small pieces. As it is, the grooves on Phobos may be large cracks produced by the impact.

In October of 1977, the asteroid Chiron was discovered traveling in a highly eccentric orbit (eccentricity = 0.38) at an angle of 6.9o to the plane of the ecliptic. It ranges between 8.5 and 18.9 AU, or roughly between the orbits of Saturn and Uranus, with a period of 50.7 years. Although a couple of other asteroids are known to have aphelion points near Saturn's orbit, Chiron is the most distant yet discovered. Because of its great distance from the asteroid belt, there is a question whether it might be the first discovery in an outer zone of asteroids. Or possibly it confirms the suspicion that asteroids and comets are objects related to each other, although not having the same composition. Together they are all that remains of the small bodies, called planetesimals, that once filled the Solar System and ultimately coalesced to form the planets 4.6 billion years ago (Section 11.4).

10.2.2. Asteroid Collisions with Earth

There are at least a thousand asteroids with diameters greater than 1 km that at some time during their orbit come close to Earth. Eventually, some of these will collide with Earth, as other asteroids have done in the past, for example, the 35-m-diameter one that caused the Barringer Meteor Crater (Figure 10.3). Current estimates are that, for those with diameters under 0.1 km, one will collide with Earth every 10,000 years, releasing as much energy as a 1000 atomic bombs (13 million tons of TNT). An asteroid with a diameter close to 10 km should strike Earth once every 100 million years with an energy equivalent to 1 billion atomic bombs (13 trillion tons of TNT). In fact, it has been proposed that the well-documented extinction, the so-called Cretaceous-Tertiary Extinction, of approximately 60 percent of all animal species that occurred about 65 million years ago was caused by climatic changes from the dust kicked up by the impact of a 10-km asteroid. So massive was the dust that was ejected into the atmosphere that a layer of clay 1 to 2 cm thick was deposited worldwide of which about 7 percent was carbonaceous matter from the asteroid. Work is currently in progress to determine whether or not other wholesale periods of extinction may have been caused by asteroid impacts.

10.2.3. Meteoroid Debris and Showers

As much as 1000 tons of cosmic debris--billions of microscopic particles--pepper the Earth daily. We are aware only of those weighing a significant fraction of a gram, which produce the so called shooting stars that flash across the sky. All but a few are too small to leave luminous trails. These solid particles are called meteoroids before they encounter Earth. Those large enough to survive flight through our atmosphere and land are called meteorites. And the luminous trails of the smaller particles that are completely vaporized in the atmosphere are called meteors. The largest and brightest meteors are referred to as fireballs or bolides.

Our atmosphere slows incoming meteoroids and transforms their kinetic energy into radiant and thermal energy. A meteoroid passing through the atmosphere leaves a wide, dense column of electrons stripped from atoms and molecules in its path. As ionized atoms regain their electrons, they deexcite, emitting photons that make the momentary luminous trail we see from the ground as a meteor or shooting star. Anything that remains of the vaporized meteoroid slowly filters down through the air as dust and solidified droplets of melted meteoroid.

When meteoritic particles strike the Earth, they are moving anywhere from about 10 up to 72 km/s, depending on the angle at which they encounter Earth. The velocities convince us that meteoroids belong to the Solar System, moving in independent orbits around the Sun.

The normal observed rate for meteors is about 10 per hour over the entire sky. Fewer meteors are seen before midnight than after midnight. This is so because during the evening hours we are on the back side of Earth, facing the direction opposite to Earth's orbital motion, and we see only the swift meteoroids overtaking us from the rear. During the morning hours, Earth's rotation has turned us so that we are facing in the same direction as its orbital motion. Hence we see those meteoroids that we overtake and those which meet us head on.

Several times during each year we see meteor showers, swarms of shooting stars darting from a small area in the sky. These showers can persist for hours or days. On such occasions, Earth is passing through a large group of particles moving in ribbonlike fashion along an orbit around the Sun. Perspective makes their tracks seem to diverge from a small spot in the sky called a radiant. Such showers are named after the constellation in which the radiant appears. Some of the better known showers are listed in Table 10.3.

About 100 years ago astronomers found that some meteoroids travel in orbits much like those of some comets. They had found a link between meteor showers and short-period comets (to be discussed later in this chapter). The particle swarms are the resulting debris left after the evaporation of a comet. For example, on the night of November 13, 1833, watchers in the southern part of the Atlantic seaboard were awestruck as over 100,000 shooting stars per hour plummeted from the constellation Leo for 3 hours. The great display was produced when Earth encountered a swarm of meteors orbiting the Sun in a period of 33 years and associated with comet Tempel (1866 I). The comet itself has long since vanished, leaving the meteor shower as a remainder of its existence. The subsequent meteoric displays of 1866, 1899, and 1932 were progressively weaker; then on November 17, 1966, a fairly spectacular meteor shower was observed in the southwestern part of the United States.

With the passage of time, a meteor stream--which is made up of conglomerates of fine dust and ice-covered particles--is strung out along the comet's orbit. This ribbon of particles typically averages about 50,000 km in cross section. Thus Earth must come fairly close to a meteor stream, therefore, in order for us to see a meteor shower.

10.2.4. Recovered Meteorites

Most meteorites are discovered accidentally years after they fall. Of some three dozen recovered meteorite falls weighing more than a ton, only a few were seen descending. Not many falls are ever recovered. Most meteorites land in the oceans or in unoccupied places, where their fall is less likely to be observed. No known record tells of a community destroyed or an individual killed by a meteorite despite some close calls. Approximately 3000 meteorite specimens have been recovered and catalogued for study.

Meteorites striking Earth have probably formed thousands of craters, but only 200 or so have been found. One major impact in 10,000 years is a reasonable estimate, and at that rate at least 50,000 giant meteorites must have struck Earth in the past 500 million years. But the fossil craters left by many of these may lie buried and unnoticed in the Earth's crust (Figure 8.9). Probably most of them have been obliterated by weathering, erosion, and geologic processes.

One that we know about, near Winslow, Arizona, is the Barringer Meteor Crater (Figure 10.3), created by a meteorite weighing at least 30,000 tons. It struck the Earth about 25,000 years ago and must have devastated all plant and animal life within a large area. The crater is over 1.2 km across and 167 m deep. Thirty tons of shattered iron fragments have been picked up within about 6 km of the crater.

At 7 A.M. on June 30, 1908, a tremendous fireball flashed across the sky in Siberia. A great ball of flame brighter than the Sun was seen leaping from a forested region near the Tunguska River. The sight of the fire was followed by the sound of an explosion powerful enough to level trees within approximately 50 km. Earth tremors were recorded on seismographs throughout Europe, yet no large crater was formed, only many small ones. The most plausible explanation for the event is that a small comet (possibly part of comet Encke) or a large, fragile, stony meteorite struck the Earth, dissipated its kinetic energy on the forest and ground, and completely vaporized.

Three classes of meteorites have been established based on their chemical and metallurgical properties. The first group, known as stony meteorites, is composed primarily of silicates of iron, magnesium, aluminum, and other metals. These generally have a relatively smooth, brown or grayish, fused crust indented with pits and cavities. Buried inside all but a small fraction of them are small pieces of glassy minerals, called chondrules, that apparently formed from molten droplets, presumably during the formation of the Solar System.

One subgroup of the stones is the carbonaceous chondrites, which contain large amounts of carbon, water, and other volatiles that would have been driven off with the slightest heating above about 500 K. Therefore, these are the most primeval samples of matter from the early Solar System that we have. They are doubly interesting because they contain organic compounds, such as hydrocarbons, amino acids, and lipids. These biologically important compounds evidently formed in the primordial solar nebula without the assistance of living organisms.

The second group, known as stony-iron meteorites, is a mix of stone and iron. Their brownish crust sometimes contains pockets of the yellow mineral olivine. Inside the meteorite the iron may have a veinlike or globular structure.

The third category, iron meteorites, is almost exclusively composed of iron, with some nickel. These meteorites are easily identified by their characteristic pitted, brownish exterior and high density. Cut, etched, and polished, they usually have a peculiar crystalline pattern unlike any in terrestrial iron. They show evidence of melting and signs of other heating and cooling processes.

Stones are the most brittle kind of meteorite, and they are far more fragile than irons. Even though most falls are stones, more of the recovered meteorites are irons because they are relatively easy to identify and they resist weathering.

Those meteorites which have been dated by their natural radioactivity average tens of millions of years for stones and 600 million years for irons. These are their ages only since the breakup of the larger mass of which they were probably a part. The most ancient specimens are about 4.6 billion years old, the same age as Earth. The chemical and mineralogical sequences in the different classes of meteorites indicate that they share the same heritage as that of the rest of the Solar System.

We are still not totally sure of the origin of meteorites. It is possible that meteoroids are primarily debris from asteroids and comets, in which case they formed by accretion in the early Solar System. However, some could be ejected bits and pieces of matter from the Moon and Mars originating during the intense cratering period early in the Solar System's history or later. Another possibility is that meteorites may be descended from a few chemically differentiated asteroids that were whittled down by repeated collisions early in the Solar System's history. In such a case, stony meteorites come from the original crusts, the stony-irons from the intermediate parts, and the irons from the core. Regardless of our ability to understand their origins, it is evident that asteroids and meteorites are representatives of the unused building material from which the Terrestrial planets formed at the birth of the Solar System.

10.3.1. Interplanetary Dust

The space between the planets is a vacuum by terrestrial standards, but it is not devoid of matter, since some gas and small, solid dust particles exist there. The particulate part of the interplanetary medium, called interplanetary dust, consists of particles blown out from the Sun's atmosphere by the solar wind, micrometeoric debris scattered by comets, and perhaps some granular powder strewn about by asteroid and meteoroid collisions. Interplanetary dust has even formed dust rings about the Sun analogous to Saturn's rings.

We have learned about interplanetary dust from several sources. One is the zodiacal light, which is most easily observed in our Northern Hemisphere in spring after sundown in the west and in fall before dawn in the east. It appears as a faint pyramidal band of light tapering upward from the horizon along the line of the ecliptic. The spectrum of zodiacal light is a faint replica of the solar spectrum; it is produced by small particles lying in the plane of the planets' orbits that scatter solar photons in our direction.

Most direct evidence of interplanetary dust comes to us from spacecraft experiments. Electronic sensors on the skin of the spacecraft are arranged to count small dust particles as they strike the surface. From the numbers of impacts, it is estimated that the average spacing between interplanetary dust particles is many meters. The total mass of dust particles is estimated to be about 1020 g, or about a hundred-millionth of Earth's mass.

10.3.2. Interplanetary Gas

Most interplanetary matter is in the form of an ionized gas that comprises the solar wind. It consists of an almost continuous stream of particles, mostly protons and electrons, flowing out from the Sun's corona. As the solar wind moves forward, it forms an expanding spiral pattern due to the Sun's rotation, and its velocity increases until, several solar radii from the Sun, it equals the speed of sound in the plasma. Its velocity continues to increase as it flows outward, much as rocket gases are accelerated to supersonic velocities in a rocket nozzle. Near Earth the solar wind reaches a velocity of about 400 km/s. Beyond Earth its speed remains very nearly constant.

At Earth's distance the wind's density is down to about five protons and five electrons per cubic centimeter on average, but it can rise on occasion to 100 particles/cm3. Compare that with the number of molecules in your room, about 1019 particles/cm3. The temperature of the wind particles is about 200,000 K in the vicinity of the Earth. This is less than their approximately 1 million K temperature when they were in the inner parts of the Sun's corona. The density is so very low, however, that the wind transfers no appreciable quantity of heat to the Earth.

The solar wind rushes past Terrestrial planets and flows out to the outer Solar System, the realm dominated by four giant planets--the Jovian planets.

10.4.1. Satellites of Jupiter and Saturn

A total of 16 satellites are known to orbit Jupiter. The thirteenth was discovered in 1974 from Earth, and the last three were found by Voyager 1 and 2. Among the four largest, discovered by Galileo in 1610, Io and Europa are about the size of our Moon, whereas Ganymede and Callisto are about the size of Mercury. The four Galilean satellites, Amalthea, and the three discovered by Voyager orbit within Jupiter's magnetosphere. As they orbit Jupiter, the Galilean satellites and Amalthea keep the same face toward the planet as the Moon does with respect to Earth. The Galilean satellites, Amalthea, and the innermost small satellites form what can be called a regular satellite system in that they orbit in nearly circular orbits in Jupiter's equatorial plane and revolve in the same sense as Jupiter rotates. The outer eight are much smaller than the Galilean satellites and move in irregular orbits inclined at varying angles to Jupiter's equatorial plane. They form an irregular satellite system in many respects.

Before the Pioneer and Voyager spacecraft sailed past Jupiter and Saturn, astronomers were not generally aware that there were any similarities between the Galilean satellites and the Terrestrial planets. Even though there are compositional and internal structural differences, the Galilean satellites have and continue to evolve under processes similar to those operating in the Terrestrial planets. Astronomers now give the Galilean satellites (shown in the chapter opening photo) a great deal of attention. In order of distance from Jupiter, their mean densities in g/cm3 are as follows: Io, 3.55; Europa, 3.04; Ganymede, 1.93; and Callisto, 1.81. Hence Io and Europa, with size, density, and mass comparable to the Moon, probably have a rocky, silicate-rich composition and structure similar to the Moon, whereas Ganymede and Callisto are lighter and made of a mixture of rocky and icy matter.

The photo essay in Figure 10.4 shows what astronomers suppose to be the internal structure of the Galilean satellites along with photographs of their surfaces obtained by the Voyagers. Clearly there are fascinating differences in their surface appearances. Europa, Ganymede, and Callisto have icy crusts with surfaces that are covered by ice or a mixture of ice and rocks many kilometers thick, whereas Io is quite different and a most exotic body.

Like Jupiter's satellites, Saturn's 17 satellites include a regular satellite system of 15 bodies moving in near circular orbits in the planet's equatorial plane, their direction of motion the same as Saturn's direction of rotation. Iapetus, the second outermost satellite, and Phoebe, the outermost one, form an irregular system with orbits inclined by several tens of degrees to Saturn's equatorial plane. Phoebe's motion is retrograde, or opposite to Saturn's direction of rotation. Thus the 8 inner satellites of Jupiter, the 15 inner ones of Saturn, and all 5 of the known satellites of Uranus form regular satellite systems. Jupiter's 8 outermost satellites, the 2 outermost satellites of Saturn, and both of Neptune's satellites form irregular satellite systems. It is probably more than coincidental that the ring systems are attached to the three planets having regular inner satellite systems. If the rings and regular satellites form as natural consequences of the formation of the planet, then what causes the irregular satellite systems? They may be captured bodies, such as pristine comets or icy-composition asteroids. Possible in the next decade we shall have a better understanding of the differences after Voyager 2 visits Neptune.

Finally, whereas Jupiter has four planet-size satellites (the Galilean satellites), Saturn has only one, Titan, that is that large. However, Saturn has four intermediate-size satellites, and Jupiter has none.

10.4.2. Io: Most Volcanic Body of the Solar System

Io surprised Voyager scientists by its surface appearance, which is a collage of mottled yellows, reds, and blackish browns. Passing very close to Io, Voyager 1 was able to resolve features smaller than 1 km. The satellite has a thin, patchy atmosphere, with sulfur dioxide as its primary constituent. The greatest excitement about Io is the positive identification of active volcanos on its surface. Figure 10.4 shows an eruption occurring on the limb, with material being thrown up to altitudes of about 150 km at velocities of about 1 km/s. Such high speeds suggest that these are not Earthlike volcanic eruptions, since the latter seldom exceed 0.1 km/s. At least eight active volcanos have been identified in the Voyager 1 photographs, and seven were still erupting 4 months later when Voyager 2 arrived.

The bright array of colors of the surface is due to sulfur compounds from volcanic activity. As you may remember from a past chemistry course, sulfur is normally a bright yellow. If heated and cooled quite rapidly, however, sulfur can take on a range of colors from orange and red to black. Related to the discovery of volcanic activity is the fact that no impact craters, such as on the Moon, can been seen on Io's surface. There are also hot spots, up to 500 K, on the surface, whose typical temperature is 60 to 120 K. Finally, some 200 calderas, both with and without lava flows, dot the surface; Earth has only 15 or so. Thus the surface is active enough that impact craters are either eroded away or filled in by volcanic debris in time periods as short as 1 million years. Io must possess the youngest surface of any Solar System body we have examined, and it is the only body besides Earth to show significant volcanic activity, which is actually greater than that of Earth.

The probable reason for the extensive volcanic activity is heating by tidal friction. Moving in an elliptic orbit, Io slightly shifts in and out in Jupiter's powerful gravitational field, with the result that it is flexed rapidly by tidal forces that release an enormous amount of frictional heat in its interior. This thermal energy, as it works its way to the surface, powers the volcanism. Io's volcanic activity may be akin more to the eruption of a hot water geyser on Earth, such as Old Faithful in Yellowstone Park, than to a volcanic eruption, such as Mount St. Helens.

10.4.3. Europa: A Lowellian Maze of Canals

In stark contrast to Io is the next Galilean satellite out from Jupiter, Europa, shown in the Voyager 1 photograph in Figure 10.4. The satellite is Moonlike in size, density, and mass but has a much higher reflectivity than the Moon, which indicates an ice-rich surface. There is very little relief to the surface, which has been likened to a frozen ocean of water 100 km deep. The surface color is a lightly orange-hued off-white and has the least contrast of the four Galilean satellites. There appears to be a total lack of craters larger than about 50 to 100 km across and very few smaller ones. The low number of craters is indicative of a young surface, something less than 3 billion years of age.

What makes Europa the enigma that it is are the vast numbers of crisscrossing light and dark markings. The surface is a maze of these stripes such that at a distance Europa is reminiscent of Percival Lowell's imagined canals crisscrossing Mars and intersecting in oases. These features are tens of kilometers wide, and some are thousands of kilometers long. In fact, some appear to extend halfway around the satellite and may be up to 3500 km in length if they are truly continuous. These stripes are thought to be cracks in the thick icy surface layers, caused by tidal flexing by Jupiter, into which darker material has been forced.

10.4.4. Ganymede: The Ice Giant

Ganymede, which has a smaller mean density than Europa, as is also the case for Callisto, must possess a much thicker layer of water ice surrounding its rocky interior than the 100-km surface layer on Europa. Estimates suggest that both Ganymede and Callisto possess 1000-km thick mantles of water ice.

Voyager photographs of Ganymede and Callisto (Figure 10.4) reveal heavily cratered surfaces, like the ancient surface of the Moon, with the exception that these are craters made in an icy surface. Ganymede has two fairly distinct types of terrain. A dark terrain appears to be the oldest part of the surface, in that it is heavily cratered, whereas a lighter-colored, heavily grooved terrain possesses a smaller number of craters and is therefore younger. On Terrestrial planets, young craters are bright, often having a bright ray system of ejected material, while old craters are dark and lack ray systems. In addition to bright, rayed craters on Ganymede, there are also some dark-rayed craters that are relatively young; these craters suggest that ice-dominated soils can behave differently from the silicate-rich soils of Terrestrial planets.

Lighter-colored terrain on Ganymede displays a number of features reminiscent of tectonic activity on Terrestrial planets, which further supports the contention that it is the youngest part of the surface. Dominating the light regions is a system of grooves that is very unlike the dark stripes on Europa. The grooves are parallel sets of ridges and troughs whose widths are up to tens of kilometers, and they may be a few hundred meters deep. Forming bands or sets of grooves, they wander for thousands of kilometers across Ganymede's surface to intersect in intricate patterns. As mentioned, the grooves have craters superimposed on them, so they are not very recent additions to the surface. The offset of grooves across what appear to be fault lines suggests that the breakup of dark crustal blocks is responsible for the grooves. Thus a global scale of tectonic activity may have existed on Ganymede some time during the first one and a half billion years of its existence.

10.4.5. Callisto: The Cratered Satellite

The outermost of the Galilean satellites is Callisto, which is shown in a Voyager 1 photograph in Figure 10.4. It is a bit smaller and a little less dense than Ganymede, and it too probably has an ice-rock composition. Callisto has about 10 times as many craters as Ganymede on its bitterly cold surface (daytime temperatures reaching only -118oC and nighttime temperatures down to -193oC). In fact, craters dominate the entire surface and stand nearly shoulder to shoulder. Callisto is unique in having no plains or regions where later processes have obliterated the craters. It also appears to lack fractures in its crust, which Ganymede has.

Callisto has several large, circular impact basins that are surrounded by an almost concentric sequence of rings. The rings are raised, separated by 50 to 200 km, and possessing diameters up to 3000 km. For many features on Callisto there does not appear to be a significant difference in elevation, which is somewhat puzzling. This characteristic may be an indication of a relatively weak surface material that, because of icy composition, is unable to support much vertical relief.

10.4.6. Saturn's Major Satellite, Titan

Titan, shown in Figure 10.5, is another of the planet-size icy bodies of the outer Solar System like the Galilean satellites of Jupiter. Estimates are that it must be 52 percent rocky and 48 percent icy materials in order to have a mean density of almost 2 g/cm3. It is unique--and thus highly intriguing--in that it is the only known satellite to possess a substantial atmosphere; Io's atmosphere is rarefied and very spotty in its density. Above a unit of surface, Titan's atmosphere contains more gas by a factor of 500 than does the atmosphere of Mars and 10 times that of Earth.

From data provided by the close approach of the Voyagers (several thousand kilometers for Voyager 1), it seems that Titan and Earth are the only bodies in the Solar System with atmospheres dominated by molecular nitrogen. Estimates for Titan lie between 82 and 94 percent for nitrogen, about 12 percent for argon, and a few percent for methane, which was found in 1944. In addition to these primary constituents, there is a smattering of molecular hydrogen that is escaping from the satellite and hydrocarbon molecules.

In the very complicated atmospheric chemistry of Titan, methane clouds form some 3 km above the surface and smog particles form from about 200 km down to the surface. The smog is so thick that nowhere are there holes through which the surface could be seen by the Voyager cameras. Thus the satellite has a rather bland reddish-brown color, with the only markings a polar hood and a change in reflectivity at the equator.

In addition, the low surface temperature has prompted speculation that methane, or natural gas, might exist in gaseous, liquid, and solid forms at the same time near the surface of Titan. Thus there could be oceans of liquid methane in warmer regions and methane polar ice caps. Whether or not continents of icy soil and rocklike ice divide the methane oceans is pure speculation. However, methane ice clouds could exist in the low atmosphere, from which methane rain falls on occasion. Methane could apparently play the same role on Titan that water plays on Earth, and thus they may be unique in the Solar System in having substantial amounts of liquid covering their surfaces.

10.4.7. Saturn's Intermediate-Size Satellites

The intermediate-size satellites in order of distance from Saturn are Tethys, Dione, Rhea, and Iapetus. Rhea and Iapetus are the second and third largest of Saturn's satellites, with nearly identical radii of about 750 km. The fourth and fifth largest are Dione and Tethys, with radii of about 550 km. All four, shown in Figure 10.5, have about the same mean density of between 1 and 1.5 g/cm3, and thus they are all presumably icy conglomerates. Iapetus is part of the irregular satellite system, whereas the others are part of the regular system. Iapetus is a strange body in that half its surface is bright and the other half nearly black. The dark hemisphere is the one that appears to always face forward as the satellite orbits Saturn for reasons of which we are not sure.

Rhea and Dione possess heavily cratered leading hemispheres, whereas their trailing hemispheres are covered by a network of strange wispy markings. These wisps may be troughs and valleys in the icy surface, or they may be fresh deposits of frozen water formed by outgassings from the satellites' interiors. Tethys, the closest to Saturn of the intermediate size satellites, is completely different: Its surface is heavily cratered and shows only small variations in brightness globally. Thus it does not have the hemispheric pattern of Rhea and Dione.

The Voyager spacecraft have given us a glimpse of new worlds in the satellites of Jupiter and Saturn that could not have been imagined from our Earth-based studies. As is so often the case, the glimpse has raised more questions than it answers. Scientists who eagerly look to the future are hopeful that several new ventures to the outer Solar System will be approved. However, the pace of scientific exploration can often be depressingly slow.

10.4.8. Satellites of Uranus and Neptune

Uranus has five satellites visible in ground-based telescopes--Ariel, Umbriel, Titania, Oberon, and Miranda (Figure 10.6). After Voyager 2's visit, ten new ones have been added to the list of known Uranian satellites. There was good reason to suspect that additional tiny satellites would be found even before the arrival of Voyager 2, since both Jupiter and Saturn have in excess of 10 satellites, most of which are very small bodies. Two of the new satellites orbit close to the planet's largest and outermost ring, while the other eight follow nearly circular orbits that lie inside the orbit of Miranda, the innermost of the five previously known satellites. All but one of the new satellites possess diameters between 40 and 80 km. The exception, designated 1985U1 pending the assignment of a name, is 160 km across.

Ariel, Umbriel, Titania, and Oberon are quite similar in size and approximately the size of the Saturnian satellites Tethys, Dione, and Rhea. Miranda is considerably smaller than the other four Uranian satellites. Spectra of reflected sunlight from the surfaces of the larger Uranian satellites suggest the presence of water ice on their surfaces. And since their densities are between 1.3 and 1.6 g/cm3, it is likely that the Uranian satellites are similar to the Saturnian satellites being composed of about half silicate rock and half icy materials, primarily water ice. Judging from the Voyager 2 pictures of the five largest satellites, all, and especially Ariel and Miranda, seem to have been geologically active early in their history. Although all show evidence of having undergone an early and intense impact cratering period as do other bodies in the Solar System, several of their surfaces bear the dramatic evidence of global tectonics in the form of rift valleys and suggestions of lava flooding.

Triton, Neptune's major satellite, is one of the larger satellites in the Solar System. The satellite was discovered only a month after the planet itself was discovered in 1846. Mass and radius determinations for Triton are quite difficult to make and are, consequently, very uncertain. If the mass is as large and the radius as small as estimated, then the mean density of the planet is large enough to suggest that Triton is not primarily and icy body, but is a rocky, silicate body. Until Voyager 2 arrives in the vicinity of Neptune in August of 1989, we can not be sure exactly what the probable composition of Triton is.

10.5.1. Discoveries and General Appearance

Comets are among of the most spectacular objects in the Solar System and appear unexpectedly in all parts of the sky. They are often discovered accidentally in photographs taken by professional astronomers for other purposes or by amateur astronomers methodically searching for them. As an honor, comets are named after their discoverers. Only once every other year or so does a comet become bright enough to be seen with the naked eye. A spectacularly bright comet appears about once or twice each decade. The usual telescopic comet appears as a small hazy object with a roundish nebulosity, called a coma (head), and occasionally a short tail.

Bright naked-eye comets, however, possess a more interesting structure, in that they have a large, teardrop-shaped coma surrounding a small bright nucleus and a well-formed tail pointing away from the Sun. While observing bright comets from night-to-night, it is apparent that their coma and tail undergo changes in appearance, sometimes making observable changes in matters of hours. The changing appearance results from the flow of matter from the nucleus into the coma and from there out into the tail.

These observations, along with satellite studies, show that comets have four principal parts: a nucleus or the principal mass of the comet, a nebulous coma surrounding the nucleus, an invisible hydrogen cloud enclosing the coma, and a tail (two types of tail may both be present), as shown in Figure 10.7. The size of the coma may vary from tens of thousands of kilometers to well over a million kilometers. Surrounding the coma, an immense hydrogen cloud spans millions of kilometers. The comet's mass is concentrated in the nucleus, which is the actual comet and which appears to be on the order of many kilometers in diameter. Well-developed tails usually form when the comet is within Earth's orbit and can be millions to hundreds of millions of kilometers long. Very rarely will the tail be longer than Earth's distance from the Sun. There can be two distinct types of tail, with both often present: One is a yellowish sweeping arc of dust, and the other is a long, straight, bluish tail of plasma.

10.5.2. Orbits of Comets

The first positive evidence that comets are extraterrestrial objects was found by the sixteenth-century astronomer Tycho Brahe. He tried to find the parallactic displacement of the comet of 1577 relative to the background stars by comparing measurements from his observatory and with those from other European centers. He decided that the object was more distant than the Moon. A century later, Isaac Newton and Edmund Halley demonstrated that comets are members of the Solar System and move in elliptic orbits under the gravitational attraction of the Sun. For example, old records reveal that the most famous comet, Halley's comet, has been observed at every return since 239 B.C. Its appearance in A.D 1066 is recorded in the historic Bayeux tapestry.

Comets fall into two groups, long-period and short-period comets, depending on the period of orbital revolution around the Sun. Of the 600 or so for which information exists, about 500 are long-period comets with periods varying from thousands to millions of years, and approximately 100 are short-period comets with periods of less than 200 years.

The long-period comets, which astronomers believe are the vast majority of all comets, travel in highly elongated ellipses inclined at all angles to the ecliptic plane. The brighter members are usually among the most magnificent comets. Some of these comets are approaching the Sun for perhaps the first time. Several comets in the long-period category have grazed the outer parts of the Sun. Frequently, forces produced by the Sun break them apart during this close encounter, and the fragments travel on as independent comets along orbits nearly identical with the parent's orbit.

By contrast, the short-period comets, orbits the Sun at small or moderate angles of inclination to the ecliptic plane, nearly all in the same sense as the planets. Roughly, every third or fourth new comet discovered has a short period, ranging from 3.3 to 200 years. Approximately half have their aphelion (the point in the orbit most distant from the Sun) somewhere near Jupiter. Following up this suggestive face, one finds that, when their orbital history is traced back mathematically, the short-period comets initially moved in long-period eccentric orbits, bringing them on one critical occasion into a chance encounter with Jupiter. Thus all comets are initially long-period ones. Apparently, the great planet's gravitational attraction has so modified their orbits that they now form a Jovian family of short-period comets.

10.5.3. Physical and Chemical Properties

Astronomers know that comets overall are rather flimsy structures of low density from the following evidence: First, they cannot be observed on the solar disk when they pass in front of the Sun; secondly, we can see stars through the tail and the outer portions of the coma (Figure 10.8); third, there are the changes from night to night in brightness and size in the coma that are even more pronounced in the tail; and fourth, they are perturbed by the solar wind and by tidal, gravitational, or other disruptive forces.

Today most astronomers agree that comets are just large "dirty icebergs," that is, icy conglomerates composed mostly of frozen water with some carbon dioxide and other ices impregnated with small pieces of particulate matter and fine dust. This conglomerate is the nucleus of the comet, where the bulk of the mass is located.

Being a flimsy structure as a comet approaches the Sun, the surface of the comet is vaporized by solar radiation releasing matter to form the coma. The solar ultraviolet radiation breaks complex molecules down into simpler molecules of hydrogen, carbon, oxygen, nitrogen and sulfur. These molecules are identified by the bright bands they produce in the emission spectrum of the comet. Emission lines of gaseous sodium are also present in the spectrum, along with some lines of iron, magnesium, and silicon when the comet comes very near the Sun. The emission lines and bands are superimposed on the weak background spectrum of sunlight reflected from dusty material.

The nuclei of short-period comets have lifetimes of a few millennia, since they lose about one percent of their mass on each perihelion passage. They eventually evaporate away their basic constituents of gas and dust, leaving a rocky remnant that should be indistinguishable from an asteroid or meteoric debris in appearance.

A typical bright comet's structure is illustrated in Figure 10.7. The comet's head plowing through the onrushing solar wind creates a bow shock wave (Figure 10.9). High-energy electrons in the solar wind ionize the molecular gases in the coma. Ultraviolet photons from the Sun probably dissociate the hydroxyl radical, releasing the hydrogen to form a huge hydrogen cloud around the coma. Chaotic magnetic fields in the solar wind sweep the charged molecules away from the coma at high speeds, forming the narrow, bluish ion tail (item 4 in Figure 10.7). What causes the wide, yellowish, curved tail (item 3 in Figure 10.7)? The Sun's electromagnetic radiation can push the dusty material flowing from the coma at different velocities away from it at comparatively low speeds.

The Dutch astronomer, Jan Oort, studying how cometary orbits are distributed, suggested in 1950 that a cloud of comets, the Oort Cloud, of not more than a few Earth masses surrounds the Sun at an average distance of 50,000 AU. For comparison, the nearest stars are over 300,000 AU from the Sun. Detached from this great reservoir (the number may be in the tens of millions) by perturbations from nearby stars, a few begin to orbit the Sun as long-period comets. Around 100,000 comets might have come close enough to the Sun to be observable.

From their apparent structure and composition, it seems probable that comets are primeval material, basically unchanged since the origin of the Solar System some 4.6 billion years ago. Comets appear to be a link between the Solar System and the interstellar medium. We shall defer a complete discussion of the interstellar medium until Chapter 16, but we may note here that many complex molecules are being discovered in dark clouds in the interstellar medium that, when frozen, could form icy structures, which are or may be similar to comets.