The Solar System
Desktop Theme
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
-- T. S. Eliot
Contents
Part 1 - The theme and its components
Part 2 - The Solar System
01. Introduction
02. The Origin of the Solar System
03. The Sun
04. Mercury
05. Venus
06. Earth
07. The Moon
08. Mars
09. Jupiter
10. Saturn
11. Uranus
12.
13. Pluto
14. Asteroids
15. Comets
16. Meteoroids
17. Meteors
18. Meteorits
Part 1 - The theme and its components
This theme consists of 9 separate themes, one for each planet in the Solar System except for Pluto and Mercury, and also including the Sun and the Moon. The theme contains a total of 36 icons, 126 cursors, 12 MPEG Layer-3 format sound files, 9 wallpaper images and three logo screens. All components were derived from material found on different sites on the web.
Part 2 - The Solar System
01. Introduction
The solar system consists of the Sun--an average star in the Milky Way Galaxy--and those bodies orbiting around it: 9 major planets, at least 60 planetary satellites, countless asteroids and comets, and the vast interplanetary medium. Four of the major planets have ring systems, and seven have one or more satellites. The several thousand minor planets, or asteroids, are predominantly in orbits between Mars and Jupiter, while most of the several billion comets travel around the Sun in a spherical shell approximately 50,000 times farther out than the Earth. The interplanetary medium--an exceedingly tenuous plasma (ionized gas) laced with concentrations of dust--extends outward from the Sun to great distances.
Observations of the motions of the Sun, the Moon, and the visible planets by early investigators gave rise to the science of astronomy. These objects are still studied today in an attempt to understand their origin and evolution, which can aid in determining whether there may be other similar systems among the millions of stars in the galaxy.
Containing more than 99 percent of the mass of the solar system, the Sun lies at the centre of the system; all the planets (and the asteroids) move around it in elliptical orbits in the same direction as the Sun rotates. Looking down on the system from a vantage point above the North Pole of the Earth, an observer would find that all the orbital motions are in a counterclockwise direction. The shape of an ellipse is defined in terms of its eccentricity, e. For a circle, e = 0; for a parabola, e = 1.0. Venus and Neptune have the most circular orbits, with eccentricities of 0.007 and 0.009, respectively. Another attribute of a planet's orbit is inclination, which is the angle that it makes with the plane of the Earth's orbit. The closest and most distant planets have the greatest inclinations: Mercury's orbit is inclined at 7 and Pluto's at 17.
Many other physical characteristics of the planets, including their dimensions and distances from the Sun, have been determined. Knowing the diameter and the mass of a planet, astronomers can readily calculate its density, or mass per unit volume, which provides an important clue to its composition. The planets can be divided into two distinct categories on the basis of their densities. The inner, or terrestrial, planets--Mercury, Venus, Earth, and Mars--have rocky compositions and densities greater than 3 grams per cubic centimetre (g/cm3). In contrast, the outer, or Jovian, planets--Jupiter, Saturn, Uranus, and Neptune--are large objects with densities less than 2 g/cm3 and thus are composed primarily of gas. Pluto is unique in that it is an icy, low-density body smaller than the Earth's Moon, resembling a giant comet nucleus or an icy satellite of one of the outer planets.
The relatively small inner planets have solid surfaces, lack ring systems, and have few or no satellites. The atmospheres of Venus, Earth, and Mars are composed of a significant percentage of oxidized compounds such as carbon dioxide (CO2). Among the planets of the inner solar system, only the Earth has a strong magn 151u2011b etic field, which shields the planet from the interplanetary medium. The magnetic field traps some of the electrically charged particles of the interplanetary medium inside a region known as the magnetosphere around the Earth. Heavy concentrations of these high-energy particles occur in the so-called Van Allen belts in the inner part of the magnetosphere. (see also Index: geomagnetic field, Van Allen radiation belt)
The
outer planets are much more massive than the terrestrial planets and have
immense atmospheres composed mainly of hydrogen (H) and helium (He). However,
they have no solid surfaces, and their densities are so low that one of them,
Saturn, would actually float in water. Each of the outer planets has a magnetic
field, a ring system, and many satellites, ranging in number from 8 for
The more than 60 known satellites are extremely diverse, representing a wide range of environments. Jupiter is orbited by Io, a body wracked by intense volcanism, while Saturn's largest satellite, Titan, exhibits a primitive atmosphere denser than that of the Earth. Triton moves in a retrograde orbit around Neptune and features plumes of material rising through its tenuous atmosphere from a surface that is only 37 K above absolute zero (0 K, or -273.15 C).
The asteroids and comets represent remnants of the planet-building process in the inner and outer solar system, respectively. Asteroids are rocky bodies, ranging in size from the largest known, Ceres, with a diameter of roughly 930 kilometres (578 miles) to the microscopic dust that is dispersed throughout the asteroid belt. The orbits of asteroids typically have both higher eccentricities and higher inclinations than those of the major planets. Some asteroids travel in paths that cross the orbit of the Earth, providing opportunities for collisions with the planet. The rare collisions with relatively large objects (those with a radius of more than 10 kilometres) can be devastating, as in the case of the asteroid impact that is thought to be responsible for the Cretaceous-Tertiary extinction (see DINOSAURS: Extinction: The asteroid theory). More commonly, the impacting objects are much smaller, reaching the Earth's surface as meteorites. Observations from Earth suggest that some asteroids are mainly metal (principally iron), others are stony, and still others are rich in organic compounds, presumably resembling the so-called carbonaceous chondrite meteorites. The October 1991 flyby of the asteroid Gaspra by the Galileo spacecraft (en route to Jupiter) revealed an irregularly shaped object pockmarked with craters, resembling one of the small satellites of Mars.
The physical characteristics of comets tend to be the precise opposites of asteroids. Ice is their main constituent, predominantly in the form of frozen water (H2O), but frozen carbon dioxide (CO2), carbon monoxide (CO), and other ices also are present. These cosmic ice balls are laced with rock dust and a rich variety of organic compounds, many of which are collected in tiny grains.
A typical comet is an irregularly shaped object whose nucleus has a diameter of a few kilometres. It spends most of its life at immense distances from the Sun, about one-third of the way to the nearest star. This is the realm of the Oort comet cloud, named after the Dutch astronomer Jan Oort. The Oort cloud is actually a spherical shell that surrounds the flat plane of the solar system that contains the planets and asteroids. The comets in this shell are not visible from the Earth; their existence there is presumed because of the highly elliptical orbits (e approaching 1.0) that may be observed on their perihelion passages (i.e., the part of their orbit nearest the Sun). As comet nuclei are warmed through solar heating, they begin to release the gases that form their familiar comas and tails. These comets have orbital periods of millions of years; their orbits can be inclined in any direction.
Based on information obtained during the 1986 spacecraft flybys, the most famous comet, Comet Halley, appears to exemplify what are termed short-period comets--i.e., those that have been captured into smaller orbits by a close encounter with Jupiter during their long journey from the Oort cloud in toward the Sun. Comet Halley has a period of only 76 years, as opposed to the several million years of many other cometary bodies, and has a retrograde orbit around the Sun.
As the comets trace out the arcs of their orbits that are closest to the Sun, they continuously shed mass. The subliming gas dissipates into space, but the grains of silicates and organic compounds remain to orbit the Sun along paths very similar to those of the parent comet. When the Earth's path around the Sun intersects one of these dust-populated orbits, a meteor shower occurs. During such an event tens to hundreds of so-called shooting stars are seen in the night sky each hour as the dust grains strike the upper atmosphere of the Earth. Although many random meteors can be observed nightly, they occur at a much higher rate during a meteor shower. Even on an average day, the Earth accumulates approximately 400 tons of asteroidal and cometary debris.
Besides the solid grains of such debris, the space through which the planets travel contains protons, electrons, and ions of the abundant elements, all streaming outward from the Sun in the form of the solar wind. Occasional giant flares (short-lived eruptions) on the Sun's surface expel matter, along with high-energy radiations, that contribute to this interplanetary medium.
Exactly where the boundary between the interplanetary medium and the interstellar medium lies has not yet been determined, but four spacecraft have recently passed the orbit of Pluto with velocities that will allow them to escape from the solar system. Thus, this boundary may well be crossed in the near future.
02. The Origin of the Solar System
The problems faced
by any theory on the origin of the solar system have become increasingly
complex as astronomers' knowledge about the planets, satellites, comets, and
asteroids has expanded. The earliest of such theories were certainly much less
constrained. A scientific approach to the origin of the solar system became
possible only after the publication of Isaac Newton's laws of motion and
gravitation in 1687. Even after this breakthrough, many years elapsed while
scientists struggled with applications of
A
significant step forward was made by Pierre-Simon Laplace of
In the early decades of the 20th century, several scientists independently decided that these deficiencies of the nebular hypothesis were so great that it was no longer tenable. The Americans Thomas Chrowder Chamberlin and Forest Ray Moulton, along with Sir James Jeans and Sir Harold Jeffreys, both of Britain, independently developed variations on the idea that the planets were formed catastrophically--i.e., by the close encounter of the Sun with another star. The basis of this model was that, when the two bodies passed at close range, material would be drawn out from one or both stars, and this material would later coalesce to form planets. A somewhat discouraging aspect of this theory was the implication that the formation of solar systems must be extremely rare, because sufficiently close encounters between stars occur very seldom, and thus very few would have taken place during the lifetime of the galaxy. (see also Index: planetesimal)
The next significant development occurred during the middle of the 20th century, as scientists became more aware of the processes by which stars themselves must form and acquired a more mature understanding of the behaviour of gases under astrophysical conditions. This perspective led to the realization that hot gases stripped from a stellar atmosphere would simply dissipate in space; they would not condense to form planets. Hence the basic idea of solar system formation through stellar encounters was physically impossible. Furthermore, the growth in knowledge about the interstellar medium--the gas and dust distributed in the space separating the stars--indicated that large clouds of such matter exist and that stars form in these clouds. Planets must somehow be created in the process that forms the stars themselves. This awareness prompted scientists to reconsider certain basic processes that resembled some of the earlier notions of Kant and Laplace.
03. The Sun
The Sun is the
mother star around which the Earth revolves once a year. It is the source of
heat, light, and life itself on the Earth. The Sun is classified as a G2 V
star, with G2 standing for the second hottest stars of the yellow G class--of
surface temperature about 5,800 kelvins (K)--and the V representing a main
sequence, or dwarf, star, the typical star for this temperature class. (G stars
are so called because of the prominence of a band of atomic and molecular
spectral lines that the German physicist Joseph von Fraunhofer designated G.)
The Sun exists in the outer part of the Milky Way Galaxy and was formed from
material that had been processed inside a supernova. The Sun is not, as is
often said, a small star. Although it falls midway between the biggest and
smallest stars of its type, there are so many dwarf stars that the Sun falls in
the top 5 percent of stars in the neighbourhood that immediately surrounds it.
04. Mercury
Mercury,
designated in astronomy, is the planet closest to the Sun, revolving
around it at an average distance of 58 million kilometres. Mercury's orbit is
inside the orbit of the Earth, and this creates two important astronomical
effects. First, Mercury is never more than 2745' of angle away from the Sun and
is thus seen as a "morning" star just before sunrise or an
"evening" star just after sunset. Second, Mercury exhibits phases
much like the Moon: when it lies nearly between the Earth and the Sun (inferior
conjunction), it appears as a thin crescent; when it is at its greatest
separation (or elongation) from the Sun, the apparent disk is half-illuminated;
and when it is on the opposite side of the Sun from the Earth (superior
conjunction), its fully illuminated surface is visible. Since these changes in
phase occur because of the motion of Mercury in its orbit, its apparent size
also varies with the phase. It is largest at inferior conjunction (about
10" of arc, or 1/180 the apparent size of the Moon) and smallest at superior
conjunction (about 4 1/2" of arc, or 1/380 the apparent size of the Moon).
Mercury
was known to be a planet in Sumerian times, some 5,000 years ago. In classical
05. Venus
Venus, symbol
in astronomy, is the second planet from the Sun and the planet whose
orbit is closest to that of the Earth. When visible, Venus is the brightest
planet in the sky. Viewed through a telescope, it presents a brilliant,
yellow-white, essentially featureless face to the observer. The obscured
appearance results because the surface of the planet is hidden from sight by a
continuous and permanent cover of clouds. Features in the clouds are difficult
to discern and become evident only when the planet is viewed in ultraviolet
light. When observed at ultraviolet wavelengths, the clouds of Venus exhibit
distinctive dark markings, with complex swirling patterns near the equator and
global-scale bright and dark bands that are V-shaped and open to the west.
Venus' mean distance from the Sun is 108 million kilometres. Its distance from
the Earth varies from a minimum of about 42 million kilometres to a maximum of
about 257 million kilometres.
06. Earth
The Earth is the
third planet outward from the Sun. Its single most outstanding
feature is that its near-surface environments are the only places in the
universe so far known to harbour life. Scientists have applied the full battery
of modern instrumentation to studying the Earth in ways that have not yet been
possible for the other planets; thus, much more is known about its structure
and composition. It is convenient to consider separate parts of the planet in
terms of roughly spherical regions extending from the interior outward: the
core and mantle, the lithosphere (the rocky, near-surface crust of land), the
hydrosphere (dominantly the oceans, which fill in low places in the crust), the
atmosphere (itself divided into spherical zones such as the troposphere, where
weather occurs), and the magnetosphere (which includes the interface with the
upper atmospheric ionosphere, the radiation belts, and the bow shock). These
parts of the planet are treated briefly, in turn, in this section, while they
are treated in detail elsewhere.
Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe, enlightened thinkers have regarded the Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that the Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of the Earth's horizon that the conception of the Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation.
Humans for the first time saw the Earth as a complete globe in December 1968 when Apollo 8 carried astronauts around the Moon. In December 1990 the Galileo spacecraft, outfitted with an array of remote-sensing instruments, studied the Earth during the first of its two gravity-assisted flybys en route to the planet Jupiter. The information about the Earth gathered from Galileo was meagre compared with that obtained by the swarm of artificial satellites that have orbited the globe throughout the space age, but it provided some unique portraits of the Earth as a planet.
Viewed from another planet, the Earth would appear bright and bluish in colour. Most readily apparent would be its atmospheric features, chiefly the swirling white cloud patterns of mid-latitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white owing to the clouds above and the snow and ice below. Beneath the changing patterns of clouds are the much darker, blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space; not only do they constitute a modest fraction of the land area, which itself is a small fraction of the Earth's surface, but they are often obscured by clouds. Over the course of the seasons, some seasonal changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere.
07. The Moon
The Moon, symbol
in astronomy, is the natural satellite of the Earth. For centuries,
speculation and scientific investigation have been centred on the Moon; it was
the first new world to be visited by humans. Many questions remain about lunar
history, yet it is clear that the Moon holds keys to understanding the origin
of the solar system.
The Moon is a spherical, rocky body, possibly with a small metal core, orbiting the Earth in a slightly eccentric orbit at a mean distance of near 400,000 kilometres. Its radius is about 1,738 kilometres, and its shape is slightly flattened, with its longest axis along the radial direction from the Earth. Its mass distribution is not uniform; the centre of mass is displaced about two kilometres toward the Earth relative to the centre of the lunar sphere, and there also are surface mass concentrations, called mascons, that cause the Moon's gravitational field to increase over local areas. The Moon has no global magnetic field like that of the Earth, but some of its surface rocks have remanent magnetism, indicating one or more magnetic episodes in the past. The Moon presently has very slight seismic activity and little heat flow, indications that most internal activity ceased long ago. It is now known that billions of years ago the Moon was subject to violent heating--resulting in a differentiated crust--followed later by volcanic outpourings of lava. The Moon's mean density is 3.34 grams per cubic centimetre, close to that of the Earth's mantle. Because of the Moon's small size and mass, its surface gravity is only about one-sixth of the planet's and it retains little atmosphere. The molecules of any gases present on the surface move without collision, and so there is no shield to protect the surface from meteoritic and particulate bombardment. As a consequence, countless bodies have struck and cratered the Moon, forming a regolith consisting of rock fragments of all sizes down to the finest dust. In the ancient past giant impacts made great basins, some of which were later partly filled by enormous lava floods. These great dark plains, called maria, are clearly visible to the naked eye from the Earth. The dark maria and the lighter highlands constitute the two main kinds of lunar territory. The mascons are regions where dense lavas rose up from the mantle and flooded into basins. Lunar mountains, mostly along the rims of ancient basins, are tall but not steep or sharp-peaked, because all lunar landforms have been eroded by the unending rain of impacts.
08. Mars
Mars, symbol in astronomy, is the fourth planet in order of distance from the Sun and seventh in order of diminishing size and mass. It orbits the Sun once in 687 Earth days and spins on its axis once every 24 hours and 37 minutes.
Mars moves around
the Sun at a mean distance approximately 1.52 times that of the Earth from the
Sun. Owing to the relatively large eccentricity (0.0934) of its orbital
ellipse, the distance between Mars and the Sun varies from 206.6 million to
249.2 million kilometres. Mars completes a single orbit in roughly the time in
which the Earth completes two. At its closest approach, Mars is less than 56
million kilometres from the Earth, but it recedes to almost 400 million
kilometres.
Owing to its blood-red colour, Mars has often been associated with gods of war. It is named for the Roman god of war; as far back as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The Greeks called it Ares for their god of battle; the planet's two satellites, Phobos (Fear) and Deimos (Terror), were named for two of the sons of Ares and Aphrodite.
09. Jupiter
Jupiter, symbol
in astronomy, is the most massive of the planets and the fifth in
distance from the Sun. When ancient astronomers named the planet Jupiter for
the ruler of the gods in the Greco-Roman pantheon, they had no idea of the
planet's true dimensions, but the name is appropriate, for Jupiter is larger
than all the other planets combined. It has a narrow system of rings and 16
known satellites, one larger than the planet Mercury and 3 larger than the
Earth's Moon. Jupiter also has an internal heat source--i.e., it emits more
energy than it receives from the Sun. This giant has the strongest magnetic
field of any planet, with a magnetosphere so large that it would exceed the
apparent diameter of the Moon if it could be seen from the Earth. Jupiter's
system is the source of intense bursts of radio noise, at some frequencies
occasionally radiating more energy than the Sun.
Knowledge about the Jovian system grew dramatically during the 1970s. The new information came in part from Earth-based observations but especially from two sets of spacecraft-- Pioneers 10 and 11 in 1974-75 and Voyagers 1 and 2 in 1979 The Pioneer spacecraft served as scouts for the Voyagers, showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment. The more sophisticated instrumentation on the later spacecraft then filled in the details, providing so much new information that it was still being analyzed in the early 1990s.
10. Saturn
Saturn, symbol
in astronomy, is the second largest of the planets in mass and size.
Its dimensions are almost equal to those of Jupiter, while its mass is about
three times smaller; it has the lowest mean density of any object in the solar
system. Both Saturn and Jupiter resemble stellar bodies in that their bulk
chemical composition is dominated by the light gas hydrogen. However, Saturn's
structure and evolutionary history differ significantly from its larger
counterpart. Like the other giant planets Jupiter, Uranus, and Neptune, Saturn
has an extensive satellite and ring system, which may provide clues to its
origin and evolution. Saturn's dense and extended rings, which lie in its
equatorial plane, are currently the most impressive in the solar system.
Saturn is the sixth planet in order of distance from the Sun, with an orbital semimajor axis of 1.427 billion kilometres. Its closest approach distance from the Earth is never less than about 1.2 billion kilometres, and thus Earth-based observations of Saturn always show a nearly fully illuminated disk.
11. Uranus
Uranus, symbol
in astronomy, is the seventh planet in order of distance from the Sun.
Its low density (1.285 grams per cubic centimetre) and large size (radius four
times that of the Earth) place it among the four giant planets, which have no
solid surfaces and are composed primarily of hydrogen, helium, water, and other
volatile compounds. Absorption of red light by methane gas gives the planet a
blue-green colour. Uranus spins on its side; its rotation axis is tipped at an
angle of 98 relative to its orbit axis. In addition, the magnetic field is
tipped at an angle of 59 relative to the rotation axis. Uranus has 15 known
satellites, ranging up to 789 kilometres in radius, and 10 narrow rings. Its
mean distance from the Sun is 2,870.99 million kilometres, and its mean
distance from the Earth at closest approach is 2,721.39 million kilometres.
12.
13. Pluto
The never-before-seen surface of the planet Pluto, symbol in astronomy, is normally the planet farthest from the Sun. Named for the god of the underworld in Roman mythology (Greek: Hades), Pluto is so distant that sunlight traveling at 299,792 kilometres per second takes more than five hours to reach the planet. An observer standing on the planet's surface would see the Sun as an extremely bright star in the dark sky, providing Pluto with only 1/1600 the amount of sunlight as that reaching the Earth. Pluto's surface temperature is therefore expected to be only 35 -50 above absolute zero, suggesting that its surface may consist largely of ices of nitrogen, methane, and carbon monoxide. Pluto has a single natural satellite, Charon. Their dimensions are sufficiently similar that it has become common to speak of the Pluto-Charon system as a double planet.
14. Asteroids
Asteroids are
rocky bodies about 1,000 kilometres or less in diameter that orbit the Sun
primarily between the orbits of Mars and Jupiter. Because of their small size
and large numbers relative to the nine major planets, asteroids are also called
minor planets. The two designations are frequently used interchangeably, though
dynamicists, astronomers who study individual objects with dynamically
interesting orbits or groups of objects with similar orbital characteristics,
generally use the term minor planet, whereas those who study the physical
properties of such objects usually refer to them as asteroids. The term
planetoid is sometimes used as well.
15. Comets
The traditional definition of a comet is a nebulous body with a "hairy" tail that makes a transient appearance in the sky. The word comet comes from the Greek kometes, meaning "hairy one," a description that fits the bright comets noticed by the ancients. Many comets, however, do not develop tails. Moreover, comets are not surrounded by any nebulosity during most of their lifetime. The only permanent feature of a comet is its nucleus, which is a small body that may be seen as a stellar image in large telescopes when tail and nebulosity do not exist, particularly when the comet is still far away from the Sun. Two characteristics differentiate the cometary nucleus from a very small asteroid--namely, its orbit and its chemical nature. A comet's orbit is more eccentric; therefore, its distance to the Sun varies considerably. Its material is more volatile. When far from the Sun, however, a comet remains in its pristine state for eons without losing any volatile components because of the deep cold of space. For this reason, astronomers believe that pristine cometary nuclei may represent the oldest and best-preserved material in the solar system.
During a close
passage near the Sun, the nucleus of a comet loses water vapour and other more
volatile compounds, as well as dust dragged away by the sublimating gases. It
is then surrounded by a transient dusty "atmosphere" that is steadily
lost to space. This feature is the coma, which gives a comet its nebulous
appearance. The nucleus surrounded by the coma makes up the head of the comet.
When it is even closer to the Sun, solar radiation usually blows the dust of
the coma away from the head and produces a dust tail, which is often rather
wide, featureless, and yellowish. The solar wind, on the other hand, drags
ionized gas away in a slightly different direction and produces a plasma tail,
which is usually narrow with nods and twists and has a bluish appearance.
16. Meteoroids
The solar system contains many small bodies that move in orbits sufficiently eccentric to cross over and intersect the orbit of the Earth. When their orbits do intersect that of the Earth, the probability of the objects colliding with the planet becomes quite high. A body of this kind entering the Earth's atmosphere is called a meteoroid. Such bodies range from small particles less than one micrometre in size (about 10-12 gram in mass) up through objects several centimetres or metres in diameter and grade into kilometre-size bodies large enough to be observed as astronomical objects through telescopes. They enter the upper atmosphere with velocities of 11 to 72 kilometres per second. Interaction with the atmosphere heats incoming objects that are larger than about 0.01 centimetre to temperatures high enough to cause them to become incandescent, vaporize, and heat the surrounding air. As a result of this sequence, such objects are observable from the ground as meteors or, in more popular language, "shooting stars" or "falling stars." Strictly speaking, the term meteor refers only to the phenomena associated with the collision of a meteoroid with the Earth's atmosphere. Scientific usage is not all that strict, however, and the body itself is often called a meteor. Unusually luminous meteors are termed fireballs or bolides.
Owing to its fairly low entry velocity, large mass, and physical strength, a meteoroid sometimes survives its passage through the atmosphere, falls to the ground, and may be recovered as a meteorite. Recovered meteorites, ranging in mass from a few grams to several tons, are often exhibited in museums. Most meteorites either consist of rocky--chiefly silicate--material (stony meteorites) or are composed primarily of nickel-iron alloy. In stony-iron meteorites, massive nickel-iron alloy is intermixed with silicate material.
In addition to these relatively large meteorites, it is possible to recover much smaller objects about 0.001 centimetre in diameter called micrometeorites on filters attached to aircraft flying in the stratosphere. These micrometeorites (often referred to as interplanetary or cosmic dust) also accumulate on the bottom of the deep ocean. The larger ones can be identified and separated from cores drilled from the muddy deposits on the seafloor.
The largest meteoroidal bodies are observable through telescopes as astronomical objects. These include the Apollo objects, bodies of asteroidal appearance with diameters ranging from a few hundred metres to several kilometres that come closer to the Sun than to the Earth's orbit. There are about 700 of these larger than one kilometre in diameter. Because comets can strike the Earth, they, too, can be thought of as large meteoroidal bodies.
When meteoroids are sufficiently large (i.e., 100 metres to several kilometres in diameter), they can pass through the atmosphere without slowing down appreciably. When they strike the Earth's surface at velocities of many kilometres per second, the kinetic energy released is sufficient to produce an impact crater. In many ways such craters resemble those produced by nuclear explosions. They are often called meteorite craters, in spite of the fact that the impacting meteoroids themselves are almost entirely vaporized during the explosion. High-velocity impact by objects of this kind on the Moon, Mercury, and Mars are in large part responsible for the heavily cratered appearance of the surface of these bodies. The cratering record on the Earth and Moon also shows that there are many meteoroids in the intermediate mass range between the larger recovered meteorites (a few metres in diameter) and the Apollo objects.
17. Meteors
On any clear night in the countryside beyond the bright lights of cities, one can observe with the naked eye several meteors (or shooting stars) per hour as they streak through the sky, with durations ranging from a small fraction of a second up to several seconds. Quite often they vary in brightness along the path of their flight, appear to emit "sparks" or flares, and sometimes leave a luminous train that lingers after their flight has ended. These meteors are the result of the high-velocity collision of meteoroids with the Earth's atmosphere. Nearly all such interplanetary bodies are small fragments derived from comets or asteroids.
The observed apparent brightness of these easily observable meteors covers the same range of brightness as the stars visible to the unaided eye (i.e., from about zero to fifth astronomical magnitude). They constitute a portion of an Earth-impacting interplanetary flux of similar bodies ranging in mass from less than one nanogram up to millions of tons. The smaller bodies are too faint to be seen with the naked eye but are observable with the aid of binoculars and telescopes or by radar reflection. Brighter meteors--of magnitudes ranging in brightness from that of Venus (-4 magnitude) to greater than that of the full Moon--are less common but are not really unusual. These are produced by meteoroids with masses ranging from several grams up to about one ton.
The
brightest meteor (possibly of cometary origin) for which historical
documentation exists struck on June 30, 1908, in the Tunguska region of central
The visibility of meteors is a consequence of the high velocity of meteoroids in interplanetary space. Before entering the region of the Earth's gravitational influence, their velocities range from a few kilometres per second up to as high as 72 kilometres per second. As they approach the Earth, within a few Earth radii, they are accelerated to even higher velocities by the planet's gravitational field. As a consequence, the minimum velocity with which a meteoroid can enter the atmosphere is equal to the Earth's escape velocity of 11 kilometres per second. Even at this minimum velocity, the kinetic energy of a meteoroid would be 6 104 joules per gram of its mass. This can be compared with the energy of about 4 103 joules per gram produced by chemical explosives, such as TNT. As the meteoroid is slowed down by friction with atmospheric gas molecules, this kinetic energy is converted into heat. Even at the low atmospheric density at altitudes of 100 kilometres (6 10-10 gram per cubic centimetre compared with 10-3 gram per cubic centimetre at sea level), this heat is sufficient to vaporize and ionize the surface material of the meteoroid and dissociate and ionize the surrounding atmospheric gas as well. Electronic transitions effected by this excitation of atmospheric and meteoroidal atoms produce a luminous region, which travels with the meteoroid and greatly exceeds its dimensions. At deeper levels in the atmosphere, a shock wave may be produced in the air ahead of the meteoroid. This shock wave interacts with the solid meteoroid and its vapour in a complex way. About 0.1 to 1 percent of the original kinetic energy of the meteoroid is transformed into visible light.
This great release of energy destroys meteoroids of small mass--particularly those with relatively high velocities--very quickly. This destruction is the result both of ablation (the loss of mass from the surface of the meteoroid by vaporization or as molten droplets) and of fragmentation caused by aerodynamic pressure that exceeds the crushing strength of the meteoroid. For these reasons, numerous meteors end their observed flight at altitudes above 80 kilometres, and penetration to as low as 50 kilometres is unusual. Nevertheless, some meteoroids survive to much lower altitudes owing to a combination of relatively low entry velocity (< 25 kilometres per second), large mass (>100 grams), and fairly high crushing strength (>107 dynes per square centimetre). Those that are recoverable as meteorites lose their kinetic energy before the meteoroid is completely destroyed. They are effectively stopped by the atmosphere at altitudes of 5 to 25 kilometres. Following this atmospheric braking, they begin to cool, their luminosity fades, and they fall to the Earth at low terminal velocities of 100 to 200 metres per second. This "dark flight" of the meteoroid may be several minutes in duration, in contrast to the few seconds of visible flight.
The passage of meteoroids through the atmosphere produces atmospheric shock waves that penetrate to the ground. The penetration of a meteoroid in the kilogram range to altitudes of about 40 kilometres can thereby produce sounds on the ground similar to sonic booms or thunder. These sound waves can be intense enough to become coupled to the ground and recorded by seismometers.
The effect of the final impact with the ground of meteorites in the kilogram mass range could be considered an anticlimax. The fall can go unnoticed by those near the impact site, the impact being signaled only by a whistling sound and a thud. For this reason, many meteorites are recovered only because at least one of the meteoroid strikes a house, drawing the attention of the residents to an unusual event.
18. Meteorits
Meteorites are meteoroids that survive passage through the Earth's atmosphere. Any source that can eject such material into interplanetary space should therefore, at least in principle, be thought of as a candidate source of meteorites. There is no fundamental reason why all meteorites must come from similar sources.
It turns out, however, that in practice there are some regions in the solar system that are much more effective in introducing material of substantial strength into Earth-crossing orbits than others. Recent laboratory and theoretical studies fully confirm the older belief that most meteorites are fragments of asteroids. These same studies show that a small fraction, less than 1 percent of the meteorites, come from nonasteroidal sources. The lunar origin of several meteorites is well-established, and it is probable that at least eight others come from Mars. There is evidence from fireball data that a small part of the material in cometary orbits (i.e., with aphelia beyond Jupiter) may possess sufficient strength to successfully penetrate the atmosphere. It is not known if any of this material is present in existing meteorite collections. If it is, the best candidate material would be carbonaceous stony meteorites, probably those of type CI, of which five separate falls have been recovered.
With these few
exceptions, it is safe to regard all meteorites as samples broken from outcrops
of rock or metal, which until fairly recently in solar-system history were part
of asteroidal bodies, mostly in the inner region of the asteroid belt (between
about 2.2 and 2.6 AU). Like rocks from the Moon, the Earth, or any other
similar planetary body, their present state is determined by the total effect
of events that occurred on the body throughout the entire history of the solar
system. There is no a priori reason why such samples must be pristine samples
of a primordial solar nebula from which the present solar system evolved. On
the other hand, the principal driving force behind asteroid studies has been
the plausible belief that small "primitive" bodies such as asteroids
and comets are those most likely to preserve evidence of events that took place
in the early solar system. Insofar as this belief is correct, meteorites,
samples of these bodies, share this property. Evidence derived from the study
of meteorites themselves supports this conclusion.
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