All about Structure, rings and atmosphere of Jovian Planets

Jovian atmosphere and clouds. The most funda­mental aspect in understanding a planet's atmosphere is the vertical temperature structure. Chemical con­stituents of Jupiter's and Saturn's atmospheres in­clude primarily hydrogen, presumably helium, some methane, and some ammonia. In composition these atmospheres more closely resemble that of the sun than those of the terrestrial planets. Traces of other molecular compounds have also been identified in Jupiter. Some of these compounds may be solids in the colder atmosphere of Satu rn, and that is why they have not been identified. Low-lying clouds of water­ice crystals, mid-level clouds of ammonium hydro­sulfide, and high-altitude clouds of ammonia are the most conspicuous physical feature of the atmo­spheres of Jupiter and Saturn. Large-scale motions in the atmospheres of the two planets are responsible for light- and dark-colored bands of clouds, vigorous eastward and westward winds, turbulent motions, and the formation of circulation cells. Especially notable is the Great Red Spot in the atmosphere of Jupiter.
Little is known about the atmospheres of Uranus and Neptune, which contain comparatively more oxy­gen, nitrogen, carbon, silicon, and iron than do Ju­piter and Saturn. Although similar to each other in some respects, there are differences with respect to each other and to Jupiter and Saturn. Neptune's atmosphere possesses a variable haze that is appar­ently not present in that of Uranus.
Internal structure of the Jovian planets. Because of the greater mass of the Jovian planets and a com­position dominated by hydrogen, helium, and other light elements, the internal structure of these planets differs from that of the terrestrial planets. Arou nd pos­sibly a small, dense core of rocky and icy materials, Jupiter and Saturn have metallic-liquid and molecular­liquid layers for an internal structure overlain by ex­tensive gaseous layers that are the atmosphere. The internal structures of Uranus and Neptune are different from those of Jupiter and Saturn because of their smaller masses and different compositions. Jupiter, Saturn, and Neptune radiate more energy than they receive from the sun. The source of energy is probably the slow contraction of these planets.
Magnetospheres about the Jovian planets. Jupiter is the strongest emitter of radio radiation in the solar system with the exception of the sun. This is the result of the magnetosphere, which is quite extensive. Saturn's magnetic field also defines a magnetospheric zone in which it can control the motion of subatomic particles. Whether or not magnetospheres exist about Uranus and Neptune will not be known with certainty until Voyager 2 reaches them in 1986 and 1989.
Ring systems of the Jovian planets. Ring systems are formed of countless individual particles in orbit about the planet and shaped by interactions of the particles with each other and by the gravitational influence of satellites. Saturn's ring system is more extensive than that of Jupiter or Uranus. Rings of Saturn are composed primarily of water-ice or ice­coated rocky particles. The three major rings are com­posed of thousands of narrow ringlets. Saturn's ring system is far more complex than had been anticipated prior to Voyager. The ring systems of Jupiter and Nep­tune are fainter than Saturn's and are probably com­posed of silicates or carbon-rich material. Their struc­ture appears to be quite different from that of Saturn.
Satellites of Jupiter and Saturn. Compared to the terrestrial planets, Jupiter and Saturn have many sat­ellites; Jupiter has at least 16 and Saturn, 17. Jupiter's Galilean satellites resemble in many respects the ter­restrial planets and have apparently been shaped b similar forces. Active volcanoes have been found on 10, although they do not much resemble those on earth. 10 and Europa, with size, density, and mas comparable to that of the moon, are probably a rock\ silicate-rich material similar in structure to that of the moon. While Ganymede and Callisto are as large as Mercury, they have a low mean density and are proba­bly composed of a mixtu re of rocky and icy materials. Saturn's satellite system contains the only satellite with an atmosphere: Titan is a rocky and icy mixture with an atmosphere dominated by nitrogen. Saturn also has four interesting, intermediate-sized satellites:
Rhea, Iapetus, Dione, and Tethys.
Comets. Comets have four principal parts: nu­cleus, coma (head), hydrogen cloud, and tail. The two types of tail point away from the sun. ComeK,ere an icy conglomerate of frozen water, methane, ammo­nia, carbon dioxide, and some small particulate matter and dust grains.

Jovian Planets and Their Ring System

Of all the aspects of the Jovian planets their ring sys­tems are among the most captivating. Galileo first observed what we know as Saturn's rings in July 1610, but it was not until 1655 that Christian Huygens pro­posed that they are a flattened disk of matter detached from the planet. Finally, in 1857 James Clerk Maxwell showed mathematically that they must consist of nu­merous tiny bodies in orbit about Saturn. This was experimentally demonstrated in 1895 from Doppler shifts showing that the ring particles move in Kepler­ian orbits, fastest close to the planet and slower far­ther away.
It can be shown with reasonable mathematical pre­cision that particles swarming about a planet even­tually form a thin ring system in the equatorial plane. This system is produced by the gravitational attraction of the planet and many gravitational interactions of the ring particles with each other. Satellites of the planet play an important role in sculpting the appear­ance of the ring system and in keeping it from spread­ing out in the equatorial plane. Also the ring system forms within several planetary radii of the surface of the planet and is not able to form at greater distances. Although the same basic principles underlie the three known ring systems, Saturn's rings are much more elaborate and complex than those of Uranus and Ju­piter.

Rings of Saturn

The circular rings lie in a plane coinciding with Sat­urn's equator. During the 29.5-year period of the plan­et's revolution around the sun the rings are observed obliquely at different angles from the earth.

Three concentric ri ngs have been known for some time and are labeled A, 8, and C in order of decreasing distance from Saturn. The bright ring 8 is sepa­rated from ring A by a space of about 5000 kilometers, called Cassini's division. Next is the semitransparent ring C, the so-called crepe ring, which lies inside the inner edge of the 8 ring. An exceptionally faint 0 ring, which lies inside the inner edge of the C ring, has been found by Voyager investigations. Outside the A ring other faint rings, known as E, F, and C, have been identified. The vertical extent of all the rings is less than a couple of kilometers. Given their immense di­ameters, they are proportionally thousands of times thinner than a razor blade.

The composition of the ri ng particles is suggested by the way they reflect sunlight. Their infrared reflec­tivity indicates that they are water ice or at least cov­ered with water ice. The particles are better reflectors of red light than they are of blue-which suggests that some other substance is mixed with the water ice. Ring particles vary in size from a few centimeters up to several meters. Each particle pursues its independent orbit around Saturn in accordance with Kepler's third law. The farther out from the planet, the lower are the particles' speeds where a solid ring would rotate fast­est at the farthest point from the planet. The entire ring system lies within the critical distance called the Roche limit, equal to about 2.4 Saturnian radii. This limit is named after the nineteenth-century French mathematician Edouard Roche, who found that inside this limit the gravitational attraction exerted by a planet on two adjacent orbiting particles is larger than the attraction of the two particles for each other. Whether the rings were formed inside the Roche limit by the breakup of a satellite, comet, or other body or whether Saturn's gravitational force prevented pri­mordial particles from coalescing to form a satellite is unknown.

Cassini's division and another known as Encke's division appear dark, suggesti ng an absence of parti­cles. However, high-resolution photographs made by the Voyager spacecraft, as in Figure 9.10, surprised astronomers when they revealed that the three major

rings, A, 8, and C, are made up of hundreds, if not thousands, of very narrow ringlets. Even Cassini's and Encke's divisions are crammed with ringlets, with something like 100 in Cassini's division alone (Figure 9.11). Apparently, particles in Cassini's division do not readily scatter photons in the backward direction so that they appear dark from the sunlit side.

The Voyagers provided evidence that some of the ringlets are not circular, while the F ring has knots, braids, and twists in it-which had not been predicted from gravitational theory. We are not sure what causes this strange behavior. Proba­bly the most unexpected aspect found was wedge­shaped spokes orientated radially out from the planet in the 8 ring. The spokes from the sunlit side, where they appear dark, and looking. back

toward the sun, where they appear bright. They are perplexing in that, if produced somehow by the ring particles, Keplerian motion should dissolve the spokes in a short time; but they are seen to last close to 10 hours. The spokes are a mystery for which we may not have a satisfactory solution for many years.

Jupiter Rings

The notion that jupiter possesses a ring system like that of Saturn was proposed some 20 years ago. Pio­neer 11 data were interpreted as consistent with the existence of a system of tiny satellites forming a ring about jupiter. This was at best speculation, and it was only Voyager 1's photograph of the Beehive star clus­ter that finally revealed the ring. The photograph showed a ring system extending some 0.7 to 0.8 ju­piter radii above the cloud tops of the planet. At most the ring is about 30 kilometers thick and 6,000 kilome­ters wide.

The particles composing the ring appear to be smaller on the average than Saturn's ring particles. Also unlike Saturn's ring particles, those of jupiter's and Uranus's systems are quite dark. Thus they are not water ice or coated with water ice. The evidence sug­gests that they are probably silicate particles whose origin is.not known.

There is also a diffuse disk, several times fainter than the bright ring, extending inside toward the planet. Surrounding the bright and diffuse rings is a faint halo some 20,000 kilometers thick. Saturn's rings are not embedded in such a halo.

Uranus' Rings

Occasionally a planet will pass between the earth and a star. Such an event is called an occultation (from the Latin word meaning "hiding"). In recent years astron­omers have carefully monitored these occultations since the time and place on the earth at which the occultation will be visible can be calculated. It re­quires a precise knowledge of the planet's orbit to make such a calculation, and the precision with which the prediction is confirmed by the observation in turn tells us how well we really know the orbit.

As the planet begins to occult the star, its atmos­phere, which is partially transparent, covers the star first so that there is a gradual dimming of the star. If there were no atmosphere, the star's brightness would remain constant until the opaque body of the planet cut off all light; the change would be sudden, not gradual.

In this manner astronomers aboard the Kuiper Air­borne Observatory, an airplane fitted with an infrared telescope, flying high over the Indian Ocean discov­ered a ring system around Uranus on March 10, 1977. About a half hour before the occultation was to take place, the star's light dimmed unexpectedly for a few seconds, followed by four other dips in brightness minutes later. The sequence was repeated in reverse as the star passed beyond the disk of Uranus on the other side. Since the original discovery of five rings four less prominent rings have been identified; mak­ing a total of nine rings.

The rings appear to be very narrow, not more than 10 to 100 kilometers in width, and they lie close to the planet's equatorial plane. Hence the origin of the rings is closely related to that of Uranus since the planet's equatorial plane is almost perpendicular to its orbital plane. Six of the rings appear to be slightly elliptical, with the radii for all the rings lying between 1.6 and 1.95 planetary radii. All the rings are dark and have sharp edges. Since the ring particles are poor reflectors, it is hard to believe that they are coated with water (or ammonia or methane) ice. More likely they are a silicate- or carbon-bearing material.

Ring System of Uranus

Uranus' Rings
Occasionally a planet will pass between the earth and a star. Such an event is called an occultation (from the Latin word meaning "hiding"). In recent years astron­omers have carefully monitored these occultations since the time and place on the earth at which the occultation will be visible can be calculated. It re­quires a precise knowledge of the planet's orbit to make such a calculation, and the precision with which the prediction is confirmed by the observation in turn tells us how well we really know the orbit.
As the planet begins to occult the star, its atmos­phere, which is partially transparent, covers the star first so that there is a gradual dimming of the star. If there were no atmosphere, the star's brightness would remain constant until the opaque body of the planet cut off all light; the change would be sudden, not gradual.
In this manner astronomers aboard the Kuiper Air­borne Observatory, an airplane fitted with an infrared telescope, flying high over the Indian Ocean discov­ered a ring system around Uranus on March 10, 1977. About a half hour before the occultation was to take place, the star's light dimmed unexpectedly for a few seconds, followed by four other dips in brightness minutes later. The sequence was repeated in reverse as the star passed beyond the disk of Uranus on the other side. Since the original discovery of five rings four less prominent rings have been identified; mak­ing a total of nine rings.
The rings appear to be very narrow, not more than 10 to 100 kilometers in width, and they lie close to the planet's equatorial plane. Hence the origin of the rings is closely related to that of Uranus since the planet's equatorial plane is almost perpendicular to its orbital plane. Six of the rings appear to be slightly elliptical, with the radii for all the rings lying between 1.6 and 1.95 planetary radii. All the rings are dark and have sharp edges. Since the ring particles are poor reflectors, it is hard to believe that they are coated with water (or ammonia or methane) ice. More likely they are a silicate- or carbon-bearing material.

How were rings of Jupiter discovered?

Jupiter Rings
The notion that jupiter possesses a ring system like that of Saturn was proposed some 20 years ago. Pio­neer 11 data were interpreted as consistent with the existence of a system of tiny satellites forming a ring about jupiter. This was at best speculation, and it was only Voyager 1's photograph of the Beehive star clus­ter that finally revealed the ring. The photograph showed a ring system extending some 0.7 to 0.8 ju­piter radii above the cloud tops of the planet. At most the ring is about 30 kilometers thick and 6,000 kilome­ters wide.
The particles composing the ring appear to be smaller on the average than Saturn's ring particles. Also unlike Saturn's ring particles, those of jupiter's and Uranus's systems are quite dark. Thus they are not water ice or coated with water ice. The evidence sug­gests that they are probably silicate particles whose origin is.not known.
There is also a diffuse disk, several times fainter than the bright ring, extending inside toward the planet. Surrounding the bright and diffuse rings is a faint halo some 20,000 kilometers thick. Saturn's rings are not embedded in such a halo.

Saturn and its Ring System

Rings of Saturn
The circular rings lie in a plane coinciding with Sat­urn's equator. During the 29.5-year period of the plan­et's revolution around the sun the rings are observed obliquely at different angles from the earth.
Three concentric ri ngs have been known for some time and are labeled A, 8, and C in order of decreasing distance from Saturn. The bright ring 8 is sepa­rated from ring A by a space of about 5000 kilometers, called Cassini's division. Next is the semitransparent ring C, the so-called crepe ring, which lies inside the inner edge of the 8 ring. An exceptionally faint 0 ring, which lies inside the inner edge of the C ring, has been found by Voyager investigations. Outside the A ring other faint rings, known as E, F, and C, have been identified. The vertical extent of all the rings is less than a couple of kilometers. Given their immense di­ameters, they are proportionally thousands of times thinner than a razor blade.
The composition of the ri ng particles is suggested by the way they reflect sunlight. Their infrared reflec­tivity indicates that they are water ice or at least cov­ered with water ice. The particles are better reflectors of red light than they are of blue-which suggests that some other substance is mixed with the water ice. Ring particles vary in size from a few centimeters up to several meters. Each particle pursues its independent orbit around Saturn in accordance with Kepler's third law. The farther out from the planet, the lower are the particles' speeds where a solid ring would rotate fast­est at the farthest point from the planet. The entire ring system lies within the critical distance called the Roche limit, equal to about 2.4 Saturnian radii. This limit is named after the nineteenth-century French mathematician Edouard Roche, who found that inside this limit the gravitational attraction exerted by a planet on two adjacent orbiting particles is larger than the attraction of the two particles for each other. Whether the rings were formed inside the Roche limit by the breakup of a satellite, comet, or other body or whether Saturn's gravitational force prevented pri­mordial particles from coalescing to form a satellite is unknown.
Cassini's division and another known as Encke's division appear dark, suggesti ng an absence of parti­cles. However, high-resolution photographs made by the Voyager spacecraft, as in Figure 9.10, surprised astronomers when they revealed that the three major
rings, A, 8, and C, are made up of hundreds, if not thousands, of very narrow ringlets. Even Cassini's and Encke's divisions are crammed with ringlets, with something like 100 in Cassini's division alone (Figure 9.11). Apparently, particles in Cassini's division do not readily scatter photons in the backward direction so that they appear dark from the sunlit side.
The Voyagers provided evidence that some of the ringlets are not circular, while the F ring has knots, braids, and twists in it-which had not been predicted from gravitational theory. We are not sure what causes this strange behavior. Proba­bly the most unexpected aspect found was wedge­shaped spokes orientated radially out from the planet in the 8 ring. The spokes from the sunlit side, where they appear dark, and looking. back
toward the sun, where they appear bright. They are perplexing in that, if produced somehow by the ring particles, Keplerian motion should dissolve the spokes in a short time; but they are seen to last close to 10 hours. The spokes are a mystery for which we may not have a satisfactory solution for many years.

Magnetospheres of Jovian Panets

JUPITER'S MAGNETOSPHERE 
jupiter is the strongest radio emitter in the solar sys­tem after the sun. It emits both thermal and nonther­mal radiation. At times its radio emission exceeds even the sun's in intensity. The non­thermal radiation is a type of synchrotron radiation, and it results from jupiter's having a magnetic field and energetic, free electrons in radiation belts anal­ogous to the earth's Van Allen radiation belts.
There are also occasional bursts having energies up to 10 million kilowatts. The bursts are more intense when the nearest Galilean satellite, 10, appears on one side of jupiter as viewed from the earth. Why should the position of 10 make a difference? We suspect that it is due to the motion of 10 through jupiter's magnetic field, disturbing the field and the electrons trapped in it.
Pioneer space probes ran into the bow shock wave formed by the solar wind's interacting with jupiter's magnetic field as far out as 108 jupiter radii. Data from the two Pioneer craft and the two Voyagers indicate that the boundary of the magnetosphere in the direc­tion of the su n varies between about 50 and 100 jupiter radii. The planet's inner radiation region is like earth's Van Mien belts but from 5000 to 10,000 times more intense.
Far out the magnetic field flattens into a disk ex­tending several million kilometers from the planet, and its long tail, flowing out opposite to the direction of the sun, extends an unknown distance beyond the orbit of Saturn. The shape is influenced by the large centrifugal force that results from the planet's rapid rotation.
SATURN'S MAGNETOSPHERE 
Saturn's magnetic field also defines a zone about it, or a magnetosphere in which it can control the motions of subatomic particles. The Saturnian magnetosphere is intermediate, in size and in the intensity of the mag­netic field, between that of jupiter and the earth. AI three are based on a common framework of physica principles; yet each possesses its own distinctive char­acter.
Prior to the late summer of 1979 astronomers could only speculate on the magnetic field and radiation belts around Saturn. During that summer Pioneer 11 detected the boundary of the magnetosphere lying some 24 Saturnian radii from the planet (its rings ex­tend about six radii from the center of the planet). Saturn's magnetosphere is apparently more disklike than that of the earth, which is more spherical but less so than Jupiter's larger magnetosphere.
Beyond Saturn we expect to find that Uranus and Neptune possess magnetic fields to create magneto­spheres about themselves as do Jupiter and Saturn. However, until Voyager 2 makes its pass by Uranus in 1986 and Neptune in 1989, we shall not have confirm­ing evidence for their existence.

Magnetosphere of Saturn

Saturn's Magnetosphere
Saturn's magnetic field also defines a zone about it, or a magnetosphere in which it can control the motions of subatomic particles. The Saturnian magnetosphere is intermediate, in size and in the intensity of the mag­netic field, between that of jupiter and the earth. AI three are based on a common framework of physica principles; yet each possesses its own distinctive char­acter.
Prior to the late summer of 1979 astronomers could only speculate on the magnetic field and radiation belts around Saturn. During that summer Pioneer 11 detected the boundary of the magnetosphere lying some 24 Saturnian radii from the planet (its rings ex­tend about six radii from the center of the planet). Saturn's magnetosphere is apparently more disklike than that of the earth, which is more spherical but less so than Jupiter's larger magnetosphere.
Beyond Saturn we expect to find that Uranus and Neptune possess magnetic fields to create magneto­spheres about themselves as do Jupiter and Saturn. However, until Voyager 2 makes its pass by Uranus in 1986 and Neptune in 1989, we shall not have confirm­ing evidence for their existence.

Magnetosphere of Jupiter

Jupiter is the strongest radio emitter in the solar sys­tem after the sun. It emits both thermal and nonther­mal radiation. At times its radio emission exceeds even the sun's in intensity. The non­thermal radiation is a type of synchrotron radiation, and it results from jupiter's having a magnetic field and energetic, free electrons in radiation belts anal­ogous to the earth's Van Allen radiation belts.
There are also occasional bursts having energies up to 10 million kilowatts. The bursts are more intense when the nearest Galilean satellite, 10, appears on one side of jupiter as viewed from the earth. Why should the position of 10 make a difference? We suspect that it is due to the motion of 10 through jupiter's magnetic field, disturbing the field and the electrons trapped in it.
Pioneer space probes ran into the bow shock wave formed by the solar wind's interacting with jupiter's magnetic field as far out as 108 jupiter radii. Data from the two Pioneer craft and the two Voyagers indicate that the boundary of the magnetosphere in the direc­tion of the su n varies between about 50 and 100 jupiter radii. The planet's inner radiation region is like earth's Van Mien belts but from 5000 to 10,000 times more intense.
Far out the magnetic field flattens into a disk ex­tending several million kilometers from the planet, and its long tail, flowing out opposite to the direction of the sun, extends an unknown distance beyond the orbit of Saturn. The shape is influenced by the large centrifugal force that results from the planet's rapid rotation.

What is the internal Structure of Uranus and Neptune?

Uranus and Neptune

Simillar But Different

Like Jupiter and Saturn, Uranus and Neptune have a three-layered structure, but unlike the solar·system giants, each layer is of quite different chemical com· position. The core of each planet is probably a rocky (iron and silicates primarily) and icy (methane, ammo­nia, and water principally) material. For Uranus the pressure of overlying layers may not be sufficient to make the core solid, but it remains a thick viscous liquid with convective motions in it. On the other hand, Neptune's greater mean density suggests that its core is solid,

Surrounding the core of each planet is a liquid mantle of water, methane, and ammonia, in which t,here may be some convective motions (for Neptune but not for Uranus). Finally, each planet has a thick crust of hydrogen and helium that is compressed by gravity into a very dense gas. The crusts gradually give way to low·density atmospheres. Thus like Jupiter and Saturn, these planets have no solid surface sur· rounded by a thin atmosphere as the terrestrial plan­ets have.

Calculated models for the interiors of both planets suggest that their central temperatures are on the or­der of 7000 K. Since Jupiter and Saturn emit more radiant energy than they receive from the sun, is it not likely that the same is true for Uranus and Neptune? Yes, one might well expect that to be the situation for both; yet it is not true for Uranus and is true only for Neptune, which radiates about twice as much heat as it receives from the sun. Why this difference in what should be reasonably similar bodies? We really do not know.

What are Sun-Like Planets and what is their structure?

Jupiter and Saturn

The Sun-Like Planets

The rapid rotation of Jupiter and Saturn, coupled with their composition of low-density materials, argues that their internal structures are more fluid than solid. Another significant factor is that Jupiter and Saturn give off more heat than they receive from the sun. In the case of Jupiter it is about 1.5 to 2 times the amount from the sun, and for Saturn it is between 2 and 3 times the amount. Hence Jupiter and Saturn have internal sources of heat. It is ex· tremely unlikely that the heat source is anything as exotic as that in the sun and the stars; Jupiter and Saturn are not small stars. But it is fair to say that they are more like the sun than like the earth, and they are clearly an intermediate type of body. The internal heat source is more likely the conversion of gravitational potential energy into thermal energy as the two plan­ets contracted during their formation and after. In fact it is likely that they are still contracting but very slowly.

Both Jupiter and Saturn have dense cores of rocky and icy materials- rather than compressed hydrogen and helium. The core is about 4 percent of the mass of Jupiter and 25 percent for Saturn, with temperatures in the range of 20,000 to 30,000 K and densities ranging from 2 to 20 grams per cubic centimeter. Surrounding the core is a layer ex· isting under a pressure in excess of 3 million times the earth's atmospheric pressure. In it hydrogen and he­lium behave more like liquid metals than solids. The upper boundary of the metallic-liquid zone is rather abrupt, giving way to a molecular·liquid mantle of hy­drogen and helium. Through both the metallic- and molecular-liquid zones, which are 96 and 75 percent, respectively, of the mass of Jupiter and Saturn, the temperature and density decrease. The molecular· liquid mantles gradually change to molecular gases, which are then the atmospheres of the two planets.

What is the internal structure of Jovian Planets?

Internal Structure of Jovian Planets
Larger masses and the fact that the Jovian planets contain far more easily vaporized materials than the terrestrial planets do mean that the resulting internal structures for the Jovian planets do not resemble those of the terrestrial. Jupiter and Saturn are the only planets in the solar system composed primarily of hydrogen and helium (as is the sun): Only hydrogen and helium could give Jupiter and Saturn their mean densities of 1.31 and 0.69 grams per cubic centimeter, respectively, for the temperatures and pressures that characterize each planet. On the other hand, the mean densities of Ura­nus and Neptune are about twice that of Saturn even though their masses are much smaller. Thus their compositions are probably dominated by oxygen, car­bon, and nitrogen, the third, fourth, and fifth most abundant elements in the solar system, even though their atmospheres contain significant amounts of hy­drogen (and possibly helium).
Jupiter and Saturn
The Sun-Like Planets
The rapid rotation of Jupiter and Saturn, coupled with their composition of low-density materials, argues that their internal structures are more fluid than solid. Another significant factor is that Jupiter and Saturn give off more heat than they receive from the sun. In the case of Jupiter it is about 1.5 to 2 times the amount from the sun, and for Saturn it is between 2 and 3 times the amount. Hence Jupiter and Saturn have internal sources of heat. It is ex· tremely unlikely that the heat source is anything as exotic as that in the sun and the stars; Jupiter and Saturn are not small stars. But it is fair to say that they are more like the sun than like the earth, and they are clearly an intermediate type of body. The internal heat source is more likely the conversion of gravitational potential energy into thermal energy as the two plan­ets contracted during their formation and after. In fact it is likely that they are still contracting but very slowly.
Both Jupiter and Saturn have dense cores of rocky and icy materials- rather than compressed hydrogen and helium. The core is about 4 percent of the mass of Jupiter and 25 percent for Saturn, with temperatures in the range of 20,000 to 30,000 K and densities ranging from 2 to 20 grams per cubic centimeter. Surrounding the core is a layer ex· isting under a pressure in excess of 3 million times the earth's atmospheric pressure. In it hydrogen and he­lium behave more like liquid metals than solids. The upper boundary of the metallic-liquid zone is rather abrupt, giving way to a molecular·liquid mantle of hy­drogen and helium. Through both the metallic- and molecular-liquid zones, which are 96 and 75 percent, respectively, of the mass of Jupiter and Saturn, the temperature and density decrease. The molecular· liquid mantles gradually change to molecular gases, which are then the atmospheres of the two planets.
Uranus and Neptune
Simillar But Different
Like Jupiter and Saturn, Uranus and Neptune have a three-layered structure, but unlike the solar·system giants, each layer is of quite different chemical com· position. The core of each planet is probably a rocky (iron and silicates primarily) and icy (methane, ammo­nia, and water principally) material. For Uranus the pressure of overlying layers may not be sufficient to make the core solid, but it remains a thick viscous liquid with convective motions in it. On the other hand, Neptune's greater mean density suggests that its core is solid,
Surrounding the core of each planet is a liquid mantle of water, methane, and ammonia, in which t,here may be some convective motions (for Neptune but not for Uranus). Finally, each planet has a thick crust of hydrogen and helium that is compressed by gravity into a very dense gas. The crusts gradually give way to low·density atmospheres. Thus like Jupiter and Saturn, these planets have no solid surface sur· rounded by a thin atmosphere as the terrestrial plan­ets have.
Calculated models for the interiors of both planets suggest that their central temperatures are on the or­der of 7000 K. Since Jupiter and Saturn emit more radiant energy than they receive from the sun, is it not likely that the same is true for Uranus and Neptune? Yes, one might well expect that to be the situation for both; yet it is not true for Uranus and is true only for Neptune, which radiates about twice as much heat as it receives from the sun. Why this difference in what should be reasonably similar bodies? We really do not know.

Atmosphere Structures of Neptune and Uranus

Atmopheres of Uranus and Neptune
The masses of Uranus and Neptune are 5 and 6 per­cent, respectively, that of Jupiter, while their mean densities are about equal to or larger than Jupiter's. This strongly suggests that, whereas Jupiter and Sat­urn are made primarily of hydrogen and helium, Ura­nus and Neptune contain greater percentages of oxy­gen, nitrogen, carbon, silicon, and iron. That is, these two planets are not likely to have solar compositions.
In the 1930s spectroscopic studies revealed meth­ane in the atmospheres of Uranus and Neptune as in those of Jupiter and Saturn. Since then, hydrogen has been identified, helium has been inferred indirectly, and some other hydrogen-containing molecules are also known. Greatly distant from the sun, Uranus and Neptune are very cold, and thus a number of molecu­lar combinations are probably frozen into a crystal or liquid-drop form.
Although alike in many respects, evidence shows that the atmospheres of Uranus and Neptu ne are not highly similar to each other or to those of Jupiter and Saturn. The atmosphere of Uranus appears to be cold and clear to great depths: In the upper atmosphere there rarely appear to be any haze and no observed clouds; but in the lower atmosphere, it probably be­comes misty or hazy. Finally, the greenish cast to Ura­nus is thought to be due to the methane molecules in its atmosphere.
By contrast, the atmosphere of Neptune possesses a variable haze of unknown chemical composition. At times nearly half the planet's atmosphere is hazed over. This haze can dissipate and reform in a matter of weeks or even a few days. The haze is partly respon­sible for trapping solar radiation in the atmosphere so that Neptune's upper atmosphere is warmer than that of Uranus.
Important for Uranus is the fact that its axis of rota­tion lies almost in its orbital plane, causing regions near the poles to remain alternately in sunlight or darkness for periods approaching 42 years. What ef­fect such a phenomenon has on the overall structure of the atmosphere and how much of a difference it produces between Uranus and Neptune is not known.

Dynamics of Jupiter and Saturn Atmospheres

Jupiter and Saturn Atmospheres
The alternating light and dark cloud bands that parallel the equator of Jupiter and Saturn are constantly un­dergoing changes in color and intensity on a small scale within the bands. Apparently this is because of the formation or dissolution of clouds of differing chemical compositions at different altitudes and latitudes. There are large-scale patterns, such as the bands themselves and the Great Red Spot on Jupiter, that last for years and sometimes centuries. This com­plex behavior involves the dynamics of the atmos­phere for both planets.
The dominant observable motions in the atmos­pheres are alternating eastward (direction of rotation) and westward winds that correlate with the colored bands. As shown in Figure 9.4, Jupiter has five or six eastward- and westward-moving wind streams in each hemisphere, while Saturn has fewer but stronger wind streams. Those winds are measured relative to each planet's rotation. In the case of the earth there is only one low-latitude westward wind stream, known as the "trade winds," and one mid latitude eastward-moving
jet stream. Jupiter and Saturn also have some vertical streaming.
Evidence suggests that these eastward- and westward-moving winds have been constant in lati­tude and velocity for the last 80 or 90 years. However, the cloud bands with which they correlate are chang­ing, with small eddies between the wind streams sheared apart in 1 or 2 days. eddy currents are deviations in what are otherwise alternating streams flowing east or west in the atmosphere. Where the steady winds have velocities up to 100 meters per second or so, the eddy velocities are a few tens of meters per second.
The cloud motions on a small scale are by no means orderly. Voyager scientists were unprepared for the diversity and sometimes high state of turbulence in the cloud motions as photo­graphed by the spacecraft. Surprisingly, photographs failed to reveal cloud features smaller than about 100 kilometers across. Narrow bands appear to coalesce and widen, while wide bands break apart. Material seems to be transferring between bands.
Conspicuous in the southern hemisphere of Jupi­ter is the oval Great Red Spot, which has varied both in size and intensity since its telescopic discovery three centuries ago. It measures about 14,000 by 40,000 kilometers. The sense of circulation of gas in the spot and other ovals is almost always counter­clockwise in the southern hemisphere and clockwise in the northern. This indicates that they are high­pressure cells.
Great Red Spot and circulate around it in a week or so i the interior is relatively calm by comparison. In addition to the Great Red Spot, smaller white and dark­colored ovals are also apparently circulation cells.
Saturn also has circulation cells in its atmosphere.
It is not known whether the eddies and ovals on both Jupiter and Saturn extend as deep into their respective planets as do the wind streams. The long-term persis­tance of the winds and the short life for eddies and ovals are possibly related to the mass of material in­volved in the phenomena. Thus the winds probably extend deep into the planet, while the shorter-lived eddies are relatively shallow structures. However, this is still quite speculative.
The winds on the earth draw their energy from unequal heating by the sun between the equator and the poles, and in general the temperature decreases poleward at almost all levels of the atmosphere. On Jupiter the temperature difference between equator and pole is no more than 3 K, while for the much shorter distance of the earth it is more like 30 K. Even though the sun heats the equatorial region of Jupiter and Saturn more than it does the polar regions, just as it does for the earth, some mechanism must transport heat from the interior of the planet into the polar regions, reducing the temperature difference.

Terrestrial atmosphere - Atmospheres of Jupiter and Saturn

Atmospheres of Jupiter and Saturn
There are many aspects of a planet's atmosphere that astronomers want to know about, such as chemical composition, temperature, density, cloud composi­tion, winds, and how these change with height, posi­tion over the surface, and time. Such detail is not now available for the earth's atmosphere much less for the atmospheres of the other planets. Probably the most fundamental aspect when trying to understand a planet's atmosphere is the vertical temperature struc­ture for jupiter and Saturn. On the way up through jupiter's and Saturn's tropospheres the meas­ured temperature profile first declines and then in­creases into the stratosphere, where photons from the sun can be directly absorbed.
The first constituents of Jupiter's atmosphere to be identified were methane and ammonia in the 1930s. Some 30 years later the most abundant element, hy­drogen, was identified and estimated to be 1000 times more prevalent than methane and ammonia. From these identifications estimates for the hydrogen, car­bon, nitrogen, and oxygen abundances indicate that jupiter's chemical composition (and similarly for Sat­urn) is more like that of the sun rather than like that of the terrestrial planets. In the 1970s and 80s, primarily through infrared observations, several additional mol­ecules were found to be minute constituents of Jupiter's atmosphere. Many of these molecules are probably also present in Saturn's atmosphere, but Saturn is colder than Jupiter, so that some of these compounds probably freeze, forming solid crystals; thus they are not in a gaseous state capable of being observed spectroscopically.
Helium, the second most abundant element in the composition of the sun and presumably in Jupiter and Saturn, is not directly observable by spectroscopic means. From data from the Pioneer and Voyager missions indirect determinations of the helium abun­dance have been made. The values derived, 10 per­cent for Jupiter and 6 percent for Saturn, are consistent with the solar-composition hypothesis.
The most conspicuous aspect of Jupiter's and Saturn's atmospheres in visible light is their clouds. Knowing something about the vertical temperature profile and the chemical composition of the atmos­phere has given astronomers clues to the basic con­stituents forming the clouds. For Jupiter and Saturn there appear to be three distinct cloud layers, as we have tried to show in Figure 9.3. The lowest is formed from water-ice crystals or possibly liquid drops, the next from ammonium hydrosulfide crystals (NH4SH), and the highest from ammonia crystals (NH3). The middle one can also be thought of as a compound of the more elementary molecules ammonia and hydro­gen sulfide. All the molecules forming the basic cloud particles should lead to white particles; so other mol­ecules are responsible for coloring the clouds, which are red, yellow, brown, blue, and white. The most likely coioring agent is the element sulfur, which forms a variety of colored particles depending upon its molecular structure. This has not been confirmed.
Infrared images of jupiter and Saturn show that the
cloud color correlates with altitude. Seen from the outside the blue clouds lie at the deepest levels in the atmosphere and are visible only through holes in the upper clouds. Brown clouds are the next highest, above which lie the white clouds; and finally the red clouds are the top layer. Compared to jupiter the greater spread in altitude for the clouds i Saturn's atmosphere results from the smaller mass of ­Saturn, whose gravity is not so effective in compres­sing the atmosphere as is the more massive jupiter.

What medium is Interplanetary medium?

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.
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.

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.