All Space Missions to the Outer Solar System

SPACE MISSIONS 
The space age dawned in the outer solar system when the Pioneer 10 and 77 spacecraft flew a first reconnais­sance by jupiter in December of 1973 and 1974. With gravitational assistance from jupiter Pioneer 77 went on to rendezvous with Saturn in September 1979, passing within about 21,400 kilometers of the planet. Pioneer 10 and 11 will cross the orbit of Pluto (but far from the planet) sometime between 1987 and 1990 on their way out of the solar system.
In the late summer of 1977 Voyager 1 and 2 were launched toward jupiter, Saturn, and their satellites. These spacecraft carried 11 different scien­tific instruments. For fear of radiation damage to measuring instruments, Voyager 1 was targeted to pass within about 5 Jupiter radii in its March 1979 en­counter with the planet; and Voyager 2, no closer than about 10 Jupiter radii in july 1979. Besides studying Jupiter, both spacecraft "looked at" its innermost sat­ellite Amalthea and the Galilean satellites (10, Europa, Ganymede, and Callisto). The Galilean satellites (shown in the photograph at the beginning of this chapter) were photographed with a resolution as good as that in Mariner 9's photographs of Mars.
After departing from jupiter, Voyager 1 and 2 headed for an encounter with Saturn, using the grav­itational assist of Jupiter for their acceleration toward Saturn. Voyager 1 made its closest approach to Saturn in November 1980, Voyager 2 following some 10 months later, in August 1981. Voyager 2 also received a gravitational assist from Satu rn for a rendezvous with Uranus in January 1986 and if all goes well, with Nep­tune in August 1989.
The next space mission to the outer solar system, Project Calileo, is currently scheduled for launch sometime in the next decade. This mission will send to Jupiter a 2-ton orbiter with a detachable probe that will enter the planet's atmosphere on the sunlit side. The orbiter will then go into orbit about the planet.

All about Terrestrial Planets - The secretry Overview

Terrestrial Planets
Surfaces of the terrestrial planets. For the terrestrial planets, the two major terrain-shaping processes have been impact cratering and thermal-tectonic activity. Crate red surfaces are presumably older terrain since evidence suggests that the intense period of cratering was some 4 billion years ago. The surface of Mercury is heavily cratered, resembling that of the moon but with some differences. This suggests that Mercury has had a history unlike that of the earth, which is domi­nated by thermal-tectonic activity. Mercury's surface features have remained relatively unchanged for 3 to 4 billion years.
Because of the obscuring cloud covers on Venus, direct observation of its surface is not possible. How­ever, from a variety of measurements we can infer that both impact cratering and thermal-tectonic activity have shaped the surface of Venus. The younger por­tions of the surface may be on the order of millions of years old. Mars is more like the moon than the earth. Cratering and to some extent thermal-tectonic activity are responsible for the appearance of the Martian su r­face. Almost half the surface has remained relatively stable and unchanged for 3 to 4 billion years. What appear to be channels carved by flowing liquid are present on Mars, suggesting that a warmer, denser atmosphere long ago allowed liquid water to exist on the surface.
Earth, unlike the other terrestrial planets, pos­sesses a relatively young surface that has been shaped by thermal-tectonic activity which has erased evi­dence of the earlier impact-cratering phase. The oldest rocks are at most 3.8 billion years old. Earth proba­bly never resembled the moon, Mercury, or Mars since thermal-tectonic activity has probably always been vigorous with that for Venus being less active and that for Mars, even less significant.
Internal structure of the terrestrial planets. Astron­omers can infer the internal structure of a planet from knowledge of such aspects as its mean density, mass, shape, rotation rate, and surface conditions. Extensive seismic data exist only for the earth and moon. Critical assumptions must be made in order to develop a model for a planet's interior. Such models for the terrestrial planets resemble a model for the earth with three layers - core, mantle, and crust. At the time of their formation the planets' chemical composition should have been determined by their distance from the sun. Closer to the sun, the lighter gaseous and icy materials vaporize at the higher temperatures, leaving as solids the iron-rich rocky materials. Mercury has an iron-rich core comprising 65 percent of the total mass of the planet while the percentage for Venus is 38 percent, earth 33 percent, and Mars 26 percent. Mars has the smallest core for its size.
Atmospheres of the terrestrial planets. Mercu ry and the moon have almost no atmosphere. Mars and Venus have atmospheres dominated by a carbon diox­ide composition with a small percentage of nitrogen. Earth's atmosphere has primarily a nitrogen com­position with about 21 percent oxygen, which comes from the biological development of the planet. The composition of a planetary atmosphere depends on the planet's distance from the sun, chemical com­position of the interior through outgassing and the greenhouse effect, evolution of the body, and inter­action with living organisms if they exist.
Satellites of the terrestrial planets. The two sat­ellites of Mars and the single satellite of the earth are the only satellites of the terrestrial planets. The sat­ellites of Mars are small irregular bodies of rocky ma­terial and may be captured asteroids.
Asteroids. About 3000 asteroids have been discov­ered that occupy the space between 1.6 and 3.3 AU from the sun in nearly the plane of the ecliptic. They are less than 1000 kilometers in size, most are irregular in shape, and they represent two broad classes of chemical composition: either a silicate-rich one or one of carbon-rich or water-rich compounds.
Meteoroids, meteors, and meteorites. More than a thousand tons of cosmic debris pepper the earth's atmosphere everyday. The smaller meteoroids, which are the majority, evaporate in the earth's atmosphere, leaving the long visible streak characteristic of the me­teor; larger ones fall to earth as meteorites. About 300 meteorite specimens have been recovered for study.
Interplanetary medium. Gas and interplanetary dust occupy the space between the planets. The dust particles, small solid grains located in the orbital planes of the planets, scatter sunlight, giving rise to the faint glow of the zodiacal light. Interplanetary gas, which is primarily protons and electrons, constitutes the solar wind rushing out from the sun through the solar system. The density of the interplanetary me­dium is quite low.

What is Interplanetary Gas?

INTERPLANETARY GAS 
Most interplanetary matter is in the form of an ionized gas comprising 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 the earth the solar wind reaches a velocity of about 400 kilometers per second. Beyond the 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 the average, but it can rise on occasion to 100 particles per cubic centimeter.

What is Interplanetary Dust?

INTERPLANETARY DUST 
The space between the planets is a vacuum by terres­trial standards, but it is not devoid of some interplane­tary gas and small solid particles called interplanetary dust. The particulate matter consists of particles blown out from the sun's atmosphere by the solar wind, micrometeoric debris scattered by comets, anc~ perhaps less plentiful granular powder strewn about by asteroid and meteoroid collisions.
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 su rface. 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 grams, or about a hundred-millionth of the mass of the earth.

Meteoroids, Meteors, and Meteorites

METEOROID DEBRIS 

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 the 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. In order of increasing size and brightness, meteors are classified as (1) telescopic and radio meteors, (2) visual meteors, and (3) 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 the atoms and molecules in its path. As the 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 meteoroid slowly fil­ters down through the air as dust and solidified drop­lets of melted meteoroid.

When meteoritic particles encou nter the earth, they are moving anywhere from about 10 up to 72 ki­lometers per second, depending on their direction and the angle at which they stri ke the 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. Why do we see fewer meteors before midnight than after midnight? During the even ing hou rs we are on the back side of the 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 that meet us head on.

RECOVERED METEORITES 

Most meteorites are discovered accidentally years af­ter they fall. Of some three dozen 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 not likely to be observed. No known record tells of a community destroyed or an individual killed by a meteorite in spite of some close calls. Approximately 3000 meteorite specimens have been recovered and catalogued for study.

Meteorites striking the earth have probably formed thousands of craters, but only 200 or so have been found. One great collision in 10,000 years is a conser­vative estimate, and at that rate at least 50,000 giant meteorites must have fallen on the 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. Probably most of them have been obliterated by weathering, erosion, and geological processes.

One that we know about, near Winslow, Arizona, is the Barringer meteorite crater, created by a meteorite weighing at least 30,000 tons. It struck the earth about 24,000 years ago and must have devas­tated all plant and animal life within a large area. The crater is over 1 kilometer across. Thirty tons of shat­tered iron fragments have been picked up within about 6 kilometers of the crater.

At 7 A.M. on June 30, 1908, a tremendous fireball flashed across the sky in Siberia. A great fall 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 50 or so kilometers. 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 the ground, and completely vaporized.

Three classes of meteorites have been established based on their chemical and metallurgical properties:

also 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.

1. 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. 
2.  Stony-iron meteorites are 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. 
3. Iron meteorites are almost exclusively composed of iron, with some nickel. They 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 more fragile than the 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 that have been dated by thei r natural radioactivity average tens of millions of years for the stones and 600 million years for the 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 the 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 sure of the origin of meteorites. Are they the remains of comets? Perhaps, but the support­ing evidence for this idea is not strong. Another line of speculation is that most meteorites may be descended from a few chemically differentiated asteroids, whit­tled down by repeated collisions early in the planetary 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.

METEOR SHOWERS 

Several times a year we can see meteor showers, the swarms of shooting stars that dart from a small area in the sky. These showers can persist for hours or days. On such occasions the 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 the radiant. The shower is named after the con­stellation in which the radiant appears. Some of the better-known showers are listed in Table 8.3.

Long ago astronomers found that some meteoroids travel in orbits much like those of some comets. They had found a link between meteor showers and the short-period comets (to be discussed in Chapter 9). The particle swarms may be debris left by evaporation and tidal disruption of comets. 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 the 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 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 the meteor stream­which is made up of conglomerates of fine dust, ices, and ice-covered particles- is strung out along the comet's orbit. This ribbon of particles typically averages about 50,000 kilometers in cross section. Thus the earth must come fairly close to the meteor stream in order for us to see a meteor shower.

What are meteorites?

RECOVERED METEORITES 
Most meteorites are discovered accidentally years af­ter they fall. Of some three dozen 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 not likely to be observed. No known record tells of a community destroyed or an individual killed by a meteorite in spite of some close calls. Approximately 3000 meteorite specimens have been recovered and catalogued for study.
Meteorites striking the earth have probably formed thousands of craters, but only 200 or so have been found. One great collision in 10,000 years is a conser­vative estimate, and at that rate at least 50,000 giant meteorites must have fallen on the 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. Probably most of them have been obliterated by weathering, erosion, and geological processes.
One that we know about, near Winslow, Arizona, is the Barringer meteorite crater, created by a meteorite weighing at least 30,000 tons. It struck the earth about 24,000 years ago and must have devas­tated all plant and animal life within a large area. The crater is over 1 kilometer across. Thirty tons of shat­tered iron fragments have been picked up within about 6 kilometers of the crater.
At 7 A.M. on June 30, 1908, a tremendous fireball flashed across the sky in Siberia. A great fall 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 50 or so kilometers. 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 the ground, and completely vaporized.
Three classes of meteorites have been established based on their chemical and metallurgical properties:
also 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.

1. 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. 
2.  Stony-iron meteorites are 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. 
3. Iron meteorites are almost exclusively composed of iron, with some nickel. They 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 more fragile than the 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 that have been dated by thei r natural radioactivity average tens of millions of years for the stones and 600 million years for the 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 the 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 sure of the origin of meteorites. Are they the remains of comets? Perhaps, but the support­ing evidence for this idea is not strong. Another line of speculation is that most meteorites may be descended from a few chemically differentiated asteroids, whit­tled down by repeated collisions early in the planetary 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.

What are Meteor Showers?

METEOR SHOWERS 
Several times a year we can see meteor showers, the swarms of shooting stars that dart from a small area in the sky. These showers can persist for hours or days. On such occasions the 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 the radiant. The shower is named after the con­stellation in which the radiant appears. Some of the better-known showers are listed in Table 8.3.
Long ago astronomers found that some meteoroids travel in orbits much like those of some comets. They had found a link between meteor showers and the short-period comets (to be discussed in Chapter 9). The particle swarms may be debris left by evaporation and tidal disruption of comets. 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 the 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 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 the meteor stream­which is made up of conglomerates of fine dust, ices, and ice-covered particles- is strung out along the comet's orbit. This ribbon of particles typically averages about 50,000 kilometers in cross section. Thus the earth must come fairly close to the meteor stream in order for us to see a meteor shower.

What is Meoroid Debris?

METEOROID DEBRIS 
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 the 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. In order of increasing size and brightness, meteors are classified as (1) telescopic and radio meteors, (2) visual meteors, and (3) 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 the atoms and molecules in its path. As the 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 meteoroid slowly fil­ters down through the air as dust and solidified drop­lets of melted meteoroid.
When meteoritic particles encou nter the earth, they are moving anywhere from about 10 up to 72 ki­lometers per second, depending on their direction and the angle at which they stri ke the 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. Why do we see fewer meteors before midnight than after midnight? During the even ing hou rs we are on the back side of the 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 that meet us head on.

Discovery and Characteristics of Asteroids

Discovery of Asteroids

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. The object was later named Ceres, after the Roman goddess of agriculture. Shortly after­ward, three more objects were discovered with orbits near 2.8 A.U: Pallas in 1802, Juno in 1804, and Vesta in 1807. Since photographic techniques were introduced into astronomical research in the 1890s, nearly 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 asteraids because in a telescope they looked like stars. Almost 95 percent of them have orbits between 1.6 and 3.3 AU, with periods from 2 to 6 years. Their orbits are more elliptic than are those of the planets and more inclined to the ecliptic. They also move in the same direction as the planets around the sun.

For some time the asteroid Hidalgo was thought to have the largest orbit. Its orbital period is 14 years, with an aphelion just outside the orbit of Saturn. How­ever, in October, 1977, a new asteroid, named Chi ron, was discovered; it travels in a highly eccentric orbit (eccentricity = 0.38) at an angle of 6.9° 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. 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 is not even an asteroid like those between Mars and Jupiter but is something related to a comet.

Asteroids vary in size from Ceres (1025 kilometers) down to an estimated 100,000 that are no more than 1 kilometer in diameter and countless numbers of even smaller ones. All the asteroids together may add up to no more than a few ten-thousandths of the mass of the earth. Ceres constitutes about 20 percent of the mass of all the asteroids.

Characteristics of Asteroids

Photometric studies of the asteroids have for some time been interpreted as showing that they differ in size, shape, and rotation. All but the largest of the asteroids are too small to show a measurable disk. From the variation in their brightness, it has been as­sumed that most have somewhat irregular shapes with periods of rotation measured in hours. Recent evi­dence suggests, however, that the brightness varia­tions for some are the resu It of two or more asteroids in mutual orbit about each other; that is, some aster­oids are binary systems.

Their colors put nearly all asteroids into two cate­gories: Some are bright reddish, a sign of silicates and iron-nickel or other metallic grains, and they populate mostly the inner part of the asteroid belt; but most have the darker neutral color of material containing various carbon-rich or water-rich compounds (car­bonaceous) and occupy 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 fragmentation of the two asteroids (if they are of comparable size). For example, if the body producing crater Stickney on Mars's satellite Phobos 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.

Asteroids Characteristics

Characteristics of Asteroids
Photometric studies of the asteroids have for some time been interpreted as showing that they differ in size, shape, and rotation. All but the largest of the asteroids are too small to show a measurable disk. From the variation in their brightness, it has been as­sumed that most have somewhat irregular shapes with periods of rotation measured in hours. Recent evi­dence suggests, however, that the brightness varia­tions for some are the resu It of two or more asteroids in mutual orbit about each other; that is, some aster­oids are binary systems.
Their colors put nearly all asteroids into two cate­gories: Some are bright reddish, a sign of silicates and iron-nickel or other metallic grains, and they populate mostly the inner part of the asteroid belt; but most have the darker neutral color of material containing various carbon-rich or water-rich compounds (car­bonaceous) and occupy 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 fragmentation of the two asteroids (if they are of comparable size). For example, if the body producing crater Stickney on Mars's satellite Phobos 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.

How were Asteroids Discovered

Discovery of Asteroids
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. The object was later named Ceres, after the Roman goddess of agriculture. Shortly after­ward, three more objects were discovered with orbits near 2.8 A.U: Pallas in 1802, Juno in 1804, and Vesta in 1807. Since photographic techniques were introduced into astronomical research in the 1890s, nearly 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 asteraids because in a telescope they looked like stars. Almost 95 percent of them have orbits between 1.6 and 3.3 AU, with periods from 2 to 6 years. Their orbits are more elliptic than are those of the planets and more inclined to the ecliptic. They also move in the same direction as the planets around the sun.
For some time the asteroid Hidalgo was thought to have the largest orbit. Its orbital period is 14 years, with an aphelion just outside the orbit of Saturn. How­ever, in October, 1977, a new asteroid, named Chi ron, was discovered; it travels in a highly eccentric orbit (eccentricity = 0.38) at an angle of 6.9° 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. 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 is not even an asteroid like those between Mars and Jupiter but is something related to a comet.
Asteroids vary in size from Ceres (1025 kilometers) down to an estimated 100,000 that are no more than 1 kilometer in diameter and countless numbers of even smaller ones. All the asteroids together may add up to no more than a few ten-thousandths of the mass of the earth. Ceres constitutes about 20 percent of the mass of all the asteroids.

Does Mars has Satellites?

MARTIAN SATELLITES 
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 7.5 hours at a distance of about 6000 kilometers from the surface of Mars. This gives it an angular size, as seen from the surface of Mars, of about half that of ou r moon. Since it revolves about Mars much faster than the planet rotates, it rises on the western horizon and sets on the eastern horizon 5.5 hours later. This is counter to any other natural satellite in the solar system, as observed from its pri­mary.
Phobos is about 27 kilometers long, 21 kilometers high, and 19 kilometers 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 ejecta or craters with central peaks, a reason­able feature since their gravitational attraction is very small. Material ejected during cratering impact simply escapes and does not fall back to the satellites' sur­face.
Phobos seems more heavily cratered than does De­imos, the largest crater, Stickney, being about 10 kilo­meters across. It also has mysterious, long, parallel grooves across a large part of its surface. 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-about 15 kilometers by 12 kilometers. It orbits Mars some 20,000 kilometers 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 meters. 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-rich and water-rich minerals, such as are found in black, crumbly meteorites known as carbonaceous chondrites. A number of the 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.

Secret Mars and Its Atmosphere

MARTIAN ATMOSPHERE 
As on Venus, carbon dioxide is the most abundant constituent of the thin Martian atmosphere, amount­ing to about 95 percent. We know that the atmosphere also contains about 3 percent nitrogen, about 2 percent argon, lesser amounts of atomic and molecular oxygen, and traces of other constituents.
A small, daily, and seasonally variable amount of water vapor has been detected on Mars. The abun­dance of water in the atmosphere, however, is far too low for rain. Because the atmospheric pressure on Mars's surface is low (less than 1 percent of the earth's sea-level atmospheric pressure), water vapor cannot exist as a liquid on the open, flat ground, and rain could not fall even if water were more abundant.
Early-morning fog lying in craters and other low places is probably evidence of an exchange of water vapor between subsu rface or su rface ice and the at­mosphere. The Martian atmosphere also possesses clouds, which are most probably condensations of water ice and carbon dioxide ice.
The warmest daytime temperature is around 30° C at the Martian equator, while the nighttime tempera­ture drops to -130°C. Over the polar regions it is even colder. During summer the north polar ice cap gets up to only about -70°C-though very cold, not cold enough for the residual cap to be made of carbon dioxide ice. Thus it appears that it is water ice, which is consistent with finding more water vapor in the atmosphere at high latitudes near the poles.
One of the most exciting things to happen since the landing of the Vikings was the photographing of frost on the surface at the Viking 2 site. The frost occurred during the n~rthern winter, be­tween May and November, 1977. The composition of the frost is not known; the air temperature was too warm for it to have been pure carbon dioxide ice, and the air was too dry for it to have been pure water ice. The best speculation is that it is some kind of mixture of carbon dioxide and water.
There is speculation that water ice may be a rem­nant of a denser atmosphere that Mars may have had in the first billion years or so of its existence. If that early atmosphere was a denser carbon dioxide (say 1 00 to 200 times more than at present) and water-vapor one, then it could have acted to trap infrared radiation in the greenhouse effect, and it would have been warm enough to contain substantial amounts of water vapor. This increased amount of carbon dioxide could easily have been provided by outgassing from the body of the planet early in its history. However, over time the formation of carbonate rocks removed car­bon dioxide from the atmosphere and lowered both the temperature and pressure. In such conditions the
atmosphere could no longer retain much water, and water could not exist as a liquid on the surface. Thus the era of water erosion ended for Mars some time ago.
The skies at the locations of Viking 1 and 2 are yellowish brown in color and seem to remain that way over the course of the Martian year. This color is prob­ably due to dust particles suspended in the atmos­phere below about 50 kilometers. Sur­face winds can stir the atmosphere sufficiently to hold dust particles. From the Viking data we know that the prevailing winds are westerly, as on earth, with veloc­ities up to 70 kilometers per hou r at the su rface and over 360 kilometers per hour at altitudes above 10 kilometers. The winds are strong enough to create major dust storms that can engulf almost the whole planet and last for months. Undoubtedly the winds come from unequal solar heating among different portions of the Martian surface, driving air from high­pressure areas to low-pressure areas (as on the earth).

About Clouds and Atmosphere of Venus

Clouds and Atmosphere of Venus
The atmospheric pressure at the surface of Venus is about 90 times greater than that of the air in your room. Analysis of the lower atmosphere suggests that it is about 96 percent carbon dioxide and about 3.5 percent nitrogen, with the remainder water vapor and some others. Because of the high surface temperature (about 730 K), the carbon dioxide was apparently not depleted, as it was on the earth, by reacting with the primitive rocks to form carbonates and limestones and by absorption by water. Above 150 kilometers atomic oxygen is the most abundant spe­cies. And finally a huge cloud of hydrogen surrounds the planet far above the atmosphere.
More than almost any other aspect of Venus, the mysterious clouds that perpetually obscure the sur­face have been the subject of extensive speculation. In 1973 it was suggested that the clouds are composed of sulfuric acid droplets. The clouds begin around 46 kilometers above the planet's surface and seem to be confined to a fairly distinct layer, rising up to about 70 kilometers. Thin-haze regions lie above and below the cloud layer; the lower one has a surprisingly abrupt cutoff some 32 kilometers above the surface.
From the Pioneer Venus results we think that the clouds are indeed composed of sulfuric acid droplets and other particles, possibly free sulfur, so thick that during the probes' descent they appeared to be pass­ing through a blizzard. Early analysis of the data also suggests that possibly several sulfur compounds also exist in the atmosphere. From the bottom of the haze down to the su rface the atmosphere appears to be surprisingly clear.
If one assumes that Venus formed with about the same relative amount of water as the earth did, then the challenging question is what happened to it since it is not on the surface in pools nor in the atmosphere. Most likely it stayed in the vapor form because of the high temperature. Incoming ultraviolet photons from the sun could dissociate the molecule, with free hydrogen then able to escape over the planet's lifetime. Some water is also consumed in making the sulfuric acid droplets in the clouds, and a tiny amount of water vapor is still present in the atmosphere.
Why did this same escape of water not occu r on the earth since its early atmosphere was probably similar in composition to that of Venus? It is probable that the advent of life and photosynthesis on the earth began to replace carbon dioxide with oxygen and prevent a substantial greenhouse effect; thus the water on the earth stayed primarily in pools on the surface. More­over Venus receives about twice as much radiant en­ergy from the sun as does the earth.
The atmospheric circulation is the same in both hemispheres. A vigorous, equatorial, east-west jet stream is quite evident in the upper atmosphere, mov­ing around the planet in only 4 days, opposite to the direction of the planet's slow spin. The wind velocity decreases at lower altitudes until at the surface it slows to a gentle breeze. The lower atmosphere ap­parently circulates because of differences in solar heating between the equatorial and polar regions. Clouds rise near the equator, spiral toward the poles, and descend in what appears to be an almost con­tinuous flow. But why such high winds reverse direc­tion in the upper atmosphere we do not know.

A Little Hydrogen and Helium

MERCURY: A LITTLE HELIUM AND HYDROGEN 
The atmosphere on Mercury is very tenuous and is approximately a million billion times less dense than ours. Mercury's atmosphere seems to be supplied and constantly replenished by the solar wind. Helium and a couple of percent of atomic hydrogen have been identified as its principal constituents. The abundance of other atomic or molecular species, if they are present, is insignificant. No evidence has been found for atmospheric modification of any landform.
Just after Mercury formed some 4.6 billion years ago, gases like carbon dioxide escaping from the inte­rior of the planet may have temporari Iy created an atmosphere of some extent. But soon after forming it would have escaped into space and vanished. This is because as mentioned above, the planet is not mas­sive enough to hold much of an atmosphere at such a small distance from the sun, and probably was en­dowed with less gaseous material during its formation than the other terrestrial planets.

4 Secrets and Realities on Which Atmosphere Depends

WHY IS AN ATMOSPHERE THE WAY IT IS?
We are all aware of the characteristics of the earth's atmosphere and its consequent importance in main­taining life; Mercury and the moon, in contrast, have almost no atmosphere. How did the physical diversity of planetary atmospheres arise?
The nature of an atmosphere results from several factors:

• The planet's distance from the sun along with its size and mass, which influence its ability to retain an atmosphere 
• Its chemical composition, which determines what processes go on in the atmosphere 
• Geological and chemical evolution of the plan­et's surface layers 
• finally the atmosphere's interaction with bio­logical life, if living organisms are in fact present 

The planet's distance and mass are important in the following way: From basic physics we know that, the higher the temperature or the smaller the mass of a molecule (or both), the greater will be the average velocity of gas particles in the atmosphere. Since a particle moving outward might gain enough kinetic energy after colliding with another atmospheric parti­cle to escape into space, can we determine the condi­tions necessary for the escape of a particular atom or molecule from a planet's gravitational field? Using the planet's temperature, mass, and radius and the masses and thermal velocities of its atmospheric con­stituents, astronomers have reached the following conclusion about the escape of different constituents:
If a molecule's velocity is near a third of the velocity needed for escape, about half of that chemical species will escape from the atmosphere within weeks. For a planet to preserve various molecular components of its atmosphere indefinitely, the mean velocity of the gases must be less than a tenth of the velocity of es­cape.
The massive Jovian planets, with their large escape velocities (several tens of kilometers per second), have held their primeval atmospheres of hydrogen and helium, while the less massive terrestrial planets, with smaller escape velocities (several kilometers per second), have lost these light gases. Venus, the earth, and Mars have managed to retain atmospheric water molecules as well as some heavy gases. Mercury and the moon lack any appreciable atmosphere since these two bodies have small masses; moreover Mer­cury is close to the sun.
In spite of the noble-gas abundance in the sun, the surprising scarcity of neon, argon, krypton, and xe­non on the terrestrial planets suggests that they may not have retained their original atmospheres. A sec­ondary atmosphere for Venus, the earth, and Mars may have formed out of gases escaping from their interiors during volcanic eruptions.
The second factor affecting a planet's atmosphere is the atmosphere's chemistry, which for the terrestrial planets is very different from that of the Jovian planets. Because the terrestrial planets formed from rocky materials-such as iron and iron silicates and metal oxides-their early atmospheres should have been largely composed of such gases as carbon diox­ide, nitrogen, and some water. Venus and Mars still have that kind of atmosphere. Planets like Jupiter and Saturn are more nearly like the sun in chemical com­position than like the terrestrial planets. Depending upon the chemical composition and chemical activity of the planet's atmosphere, the greenhouse effect will be more or less effective in trapping incoming solar radiant energy. This causes a warming of the low atmosphere and the surface, which will influence much of the atmospheric chem­istry. Venus is a good example of the long-term conse­quences of the greenhouse effect. Estimates are that the mean surface temperature of Mars, the earth, and Venus are about 5,35, and 500 K warmer, respectively, than they would be without the greenhouse effect.
The third factor, the ways geological and chemical evolution affect a planet's atmosphere, is important in the case of the earth, Venus, and Mars. Outgassing from these planets' interiors in their early history con­sisted mainly of water vapor, carbon dioxide, and ni­trogen, in approximately the same proportions that we observe in terrestrial volcanic gases today. On earth water vapor condensed to form the oceans, but nitrogen remained in a gaseous state. Most of the carbon dioxide combined with silicate rocks of the crust to forr:n carbonate rocks, such as limestone, a reaction that occurs most efficiently in the presence of liquid water. If it could be released from crustal rocks along with the small amount dissolved in the oceans, carbon dioxide in the earth's atmosphere would equal about one-half of the amount in the dense atmos­phere of Venus.
If at some earlier time water vapor condensed on Venus, the high su rface temperature due to the green­house effect prevented water from remaining in a liq­uid form and kept carbon dioxide in a gaseous form. Most of the water vapor apparently dissociated into hydrogen and oxygen by absorbing ultraviolet sun­light. Hydrogen escaped, and the heavier oxygen may have combined with crustal rocks to form oxides. On Mars the outgassing of water vapor, carbon dioxide, and nitrogen was probably less complete than it was on the earth; yet carbon dioxide forms the largest part of the atmosphere of Mars. Mars appears at present to be in a cold phase, and a large amount of water is apparently stored in the polar caps and under the surface of the planet as permafrost.
Finally we need to consider the effect of biological life, if any, on an atmosphere. Living organisms on a planet are .bound to affect its atmosphere if the inter­action between its biosphere (the zone in which life exists) and atmosphere is anything like that on earth:
Large expanses of liqu id water will moderate a planet's climate and can provide an environment conducive to the development of life if there is adequate protection from solar ultraviolet radiation; conversely most of the earth's free oxygen, so necessary to animal life, comes from photosynthesis. The oxygen is constantly replenished by green plants, plankton, and some bacteria. When living organisms became able to extract carbon dioxide from the atmosphere, they helped save the earth from the heat death that Venus has apparently experienced.

Magnetospheres and Terrestrial Planets

MAGNETOSPHERES OF THE TERRESTRIAL PLANETS 
It has been known for many years that the earth has a magnetic field that extends far beyond the body of the earth to form a magnetic envelope called a magneto­sphere. The earth's magnetic field is attributed to elec­trical currents flowing in the outer core; so the earth is somewhat like a giant electromagnet. If the other terrestrial planets also have electrical currents flowing in their deep interiors, then they too will have mag­netospheres. Or we can turn the argument around:
The spacecraft detection of a magnetic field tells us something about the interior of a planet; that is, it is sufficiently fluid for matter to flow in the deep interior to generate electrical currents.
Mariner 10 found evidence for Mercury's having a magnetic field, but it is much too weak to hold radiation belts such as the earth's Van Allen belts.
Before the advent of planetary exploration with spacecraft, astronomers had suspected that the earth's sister planet, Venus, might have an internally generated magnetic field comparable to that of the earth. However, with the first few Mariner and Venera spacecraft it became evident that the magnetic field for Venus is much smaller than that of the earth. Ve­nus has a well-developed bow shock formed by solar wind particles impinging on the planet's magnetic field; but that weak field, like that of Mercury, is too feeble to trap solar wind particles in a radiation belt. The weakness of Venus's magnetic field is probably due to the very slow rotation of the planet.
In 1965 Mariner 4 accomplished the first measure­ments of Mars's magnetic field. The intensity of the magnetic field is much less than that of the earth's field. There is a feeble bow shock formed between the onrushing solar wind particles and the field, but no radiation belts appear to exist.
In summary the earth is the only one of the terres­trial planets with a field intense enough to maintain radiation belts. This suggests that the earth is the only one possessing in its core a circulation pattern capable of generating a strong magnetic field.

The Planet Like Moon

MARS: MORE LIKE THE MOON 
Besides the earth and moon, Mars is the only planet for which scientists have seismic data. Both Viking landers carried instruments to record quakes on Mars. Unfortunately, only the one on Viking 2 worked, and on November 24,1976, it appears to have detected its first Martian quake. If real, it suggests that the Martian crust has an average thickness of about 40 kilometers, with a maximum thickness of about 75 kilometers un­der the Tharsis ridge and a minimum thickness in the Hellas basin of about 10 kilometers. By comparison the earth's average crust thickness is 30 kilometers or so, and it covers a planet with nearly twice the radius of Mars. Hence the crust of the earth is about 0.5 percent of its radius, while that of Mars is about 1.2 percent. The moon's crust is about 4 percent of its radius.
The seismic activity on Mars, although somewhat more extensive than that of the moon, is much less than that of the earth. This, as noted, suggests a litho­sphere (crust and outer portion of the mantle) a cou­ple of hundred kilometers thick on a chemically differ­entiated body, with silicate mantle and iron-rich core of about 1500 kilometers radius. Thus of the four ter­restrial planets Mars has the smallest percentage of iron and the smallest core for its size.

The Planet Which is Similar to Earth?

VENUS: SIMILAR TO THE EARTH? 
Since Venus is the terrestrial planet closest to the earth in size and mass, it is reasonable to expect that it will generally be the planet most like the earth. Of critical importance in developing a model for its inte­rior is Venus's chemical composition: As we have seen, from estimates of what chemical elements were likely to have been present at the time of formation of the terrestrial planets and from observations of the planets' influences on the motion of spacecraft, as­tronomers have made estimates for the iron and nickel content in the terrestrial planets; as percen­tages of total mass, the iron and nickel content is as high as 65 percent for Mercury, up to 38 percent for Venus, up to 33 percent for the earth and the moon, and only up to 26 percent for Mars.
Such an iron content for Venus suggests that it ought to have a molten core (like that of the earth), occupying about the same fraction of Venus's interior as does the earth's (or significantly smaller than the core of Mercury). Overlying the molten iron-rich core is a silicate mantle and a crust on top of the mantle. It is possible that Venus (also like the earth) has an inner core of solid iron-rich material. Speculation for and against an inner core depends on estimates of the iron content of the planet.

Iron-Rich Core - Astronomical Objects

MERCURY: A LARGE IRON-RICH CORE 
The values of its mass and radius imply that Mercury must contain a large fraction of iron, the only heavy element sufficiently abundant to account for the plan­et's high mean density. The iron and nickel content may be as much as 65 percent of the total mass of Mercury. By analogy with terrestrial, lu­nar, and meteoritic chemical abundances, we pre­sume that silicates and oxides of iron are also preva­lent.
An unexpected discovery by the Mariner 10 mission was that Mercury has a shock front like the wave sur­rounding the bow of a ship plowing through water. It is caused by the onrushing solar wind particles colliding with the planet's magnetic field. Thus it is apparent that Mer­cury has a magnetic field, which is about 1 percent as strong as that of the earth. The magnetic axis of its field almost coincides with Mercury's axis of rotation.
The magnetic field is intrinsic to the planet and is most likely the result of an internal mechanism that continuously generates the field in much the same way as the earth's does. This is addi­tional evidence for a large iron core. Chemical differ­entiation appears to have occurred very early in the planet's history, probably the first half billion years. Since then, the surface has been largely undisturbed by thermal and tectonic processes.
The model that has been derived for Mercury is one with a crust overlying a silicate mantle, which in turn surrounds a molten (or partially molten) iron-rich core. The core radius may be as much as 75 percent of the planetary radius, a percentage that is considerably greater than that for any of the other terrestrial plan­ets, including the earth. Such a core should be ade­quate to generate the magnetic field observed by Mar­iner 10.