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SOL (ROMAN MYTHOLOGY) An ancient god of Mesopotamian origin, Sol was introduced into Roman religion in the 3d century AD as the Roman equivalent of the Greek sun god HELIOS. He was worshiped by the Roman emperors as their principal protector.
SUN Light...Plasmic Eye...Sun Master- Overseer of disorder- shaping entropy to order- energy to evolution. Controlling-driving- guiding this immense engine of Creation by His Word. Nuclear fusion spinning gas to dust- small particles to planets. Life: de-novo, from where There Was-nothing carlyle miller

Sun (pic 1)

Sun (pic 2)

Sun (pic 3)

Sun (pic 4)

Sun: 1977 eclipse

Sun: 1980 eclipse

Sun: 1991 eclipse

Sun: 1994 eclipse

Sun: Magnetic Fields

Sun: Prominences



The Sun, the central body of the SOLAR SYSTEM and the closest
STAR, is an immense sphere of glowing gas 1.39 million km
(860,000 mi) in diameter at an average distance from the Earth
of 149,591,000 km (92,960,000 mi).  It is composed mainly of
hydrogen, with about 5 percent by number of helium and heavier
elements.  Its mass of 1.99 X (10 to the power of 33) is
sufficient for the mutual gravitational attraction of the
molecules to prevent the hot solar gases from expanding rapidly 
into the relative vacuum of interstellar space.  The Sun
generates energy at the rate of 3.9 X (10 to the power of 33)
ergs/sec by burning hydrogen to helium through nuclear FUSION
reactions in its interior.  This energy is radiated into space
mostly in visible and infrared light and is largely responsible 
for the continuation of life on our planet.
Compared with the largest known stars, with diameters a
thousand times larger and masses several hundred times greater, 
the Sun merits its astronomical designation as a dwarf star.
Its mass and radius are close to the average mass and size of
all stars in the Galaxy, however, because many stars are even
smaller and less massive than ours.  The Sun's spectrum,
surface temperature, and color lead to its further
classification as a G2 dwarf in the scheme of spectral types
used by astronomers.  The spectral intensity of light radiated
by its surface gases is a maximum at wavelengths near 5000
angstroms, thus giving sunlight its characteristic yellow
Modern study of the Sun began in 1611 with Galileo's
observations of sunspots and his discovery of solar rotation
from their motions.  The first approximately correct
determination of the Sun's size and distance from the Earth was 
made in 1684, from data obtained by the French Academy from
triangulation observations of Mars during its close approach to 
the Earth in 1672.  The discovery of the solar absorption-line
spectrum by Joseph von Fraunhofer in 1814, and its physical
interpretation by Gustav Kirchhoff in 1859, opened the era of
solar astrophysics, during which the effective study of the
physical state and chemical composition of the solar material
became possible.  The strong magnetic fields of sunspots were
detected by George Ellery Hale in 1908, and the role of nuclear 
fusion in producing solar energy was elucidated by Hans Bethe
in 1939.  Modern developments continue to change scientists'
perception of the Sun.  The solar wind was not detected
directly until 1962, and the sources of its high-speed
recurrent streams awaited the observations of coronal holes in
From its innermost core to its corona, and to the solar wind
that extends even to the Earth, the Sun has a structure typical 
of most stars of its kind.
Inner Core
The weight of the Sun's outer layers compresses the gas of the
innermost region to a density about 100 times that of water and 
raises the central temperature to about 15 million K
(27,000,000 deg F).  Throughout the Sun's interior, atoms
collide frequently and with enough energy to ionize the gas,
which is then referred to as a plasma (see PLASMA PHYSICS).  In  
the inner third of the Sun the collisions among ions are
energetic enough to cause nuclear reactions at a rate
sufficient to liberate the energy required to give the Sun's
observed luminosity.  The specific set of reactions thought to
be most effective in generating energy in the Sun involves the
burning of hydrogen to helium, following the specific sequence
of reactions known as the PROTON-PROTON REACTION.  Present
evidence suggests that the plasma of the central nuclear
burning region of the Sun is not mixed with the material in the 
outer shells.  Thus the proton-proton reaction will continue
only until the hydrogen of the central region, some 10 percent
of the Sun's mass, becomes transformed into helium after about
10 billion years.  The Sun's age is estimated to be about 5
billion years (see STELLAR EVOLUTION).  The gamma rays and X
rays emitted by the nuclear reactions travel outward with
little absorption through the solar interior, because the
electrons that allow an atom to absorb light have mostly been
stripped from the nuclei by interatomic collisions.
The Convection Zone and the Photosphere
Nearer the Sun's visible surface, as the weight of overlying
gas diminishes, the gas pressure and thus the density and
temperature required to support this layer in hydrostatic
equilibrium decrease rapidly.  At a distance of about
two-thirds the solar radius from the center, where the
temperature has dropped to about 1 million K, the hydrogen and
helium are no longer completely ionized.  The neutral atoms
absorb radiation moving outward from the central nuclear
burning regions.  In this region the heating and consequent
expansion of parcels of the fluid cause them to rise because of 
their lower densities, and transport their heat upward.  The
net upward flux of heat carried by the resulting pattern of up-  
and down-flowing convection is the dominant mode of energy
transport in the outer third of the Sun.  Convection continues
to be efficient in transporting heat until layers are reached
where the density is so low that radiation from the hot
up-flowing gas can escape directly into space.  This layer is
the visible surface of the Sun, known as the photosphere.
Direct evidence for the size scales, velocities, and shapes of
solar convective scales can be deduced from observations of
convectionlike cellular motions at the photosphere.
Small-scale cells called granules are about 1,000 km (620 mi)
in diameter and are formed by hot up-flowing gases, surrounded
by cooler down-flowing gases, moving about 1 km/sec (22,000
mph).  Supergranules form a larger set of polygonal cells, of
diameter roughly 30,000 km (18,600 mi), detected by their
horizontal velocities of about 0.5 km/sec (1,100 mph).
In addition to transporting heat, convective motions of the
Sun's gases are also thought to have important consequences for 
solar rotation, solar magnetism, and for the structure of the
Sun's outer layers above the photosphere.  Convection may help
to explain the observation that the gases of the solar
photosphere do not rotate rigidly--the angular rate at the
equator is some 50 percent higher than that at latitudes of
plus or minus 75 degrees.  Although a satisfactory theory of
this basic solar property does not yet exist, models of the
fluid mechanics of rotating, convecting shells indicate that
such velocity differences might result from the forces exerted
upon rising and falling convecting gases as the Sun rotates
about its axis at the observed sidereal rate of about 25 days
at the solar equator.  The angular rotation rate also appears
to increase inward, at least immediately below the photosphere, 
at a rate of 5 percent in the first 15,000 km (9,300 mi).
The Sun's magnetic field, observed at the photosphere, does not 
have the basic north-south dipole symmetry observed in the
terrestrial magnetic field at the Earth's surface.  The solar
field lines seem to be wound around the Sun's rotation axis and 
roughly follow lines of constant latitude, rather than
longitude.  This property is inferred from the observed
alternation of magnetic polarity in bipolar sunspot groups.
The magnetic dipole axes of such groups tend to be oriented
east-west, and within a given hemisphere (above or below the
solar equator) the western half of all dipoles is generally of
the same magnetic polarity.  The polarity of dipoles in the
northern and southern hemispheres is opposite.  This law of
alternation of polarities is called the Hale-Nicholson law.
The plasma of the solar convection zone is about as good a
conductor as copper wire under room-temperature conditions.
When a large volume of this material moves through a magnetic
field, as in solar convection, it induces a large electric
current that deforms the original field so as to displace it
along with the motion.  The mutual influence of magnetic fields 
and moving plasmas is known as MAGNETOHYDRODYNAMICS (MHD).  MHD 
studies show that the Sun's differential rotation will tend to
stretch and pull out magnetic-field lines into the observed
toroidal geometry.
Near the photosphere the known temperature, the mean molecular
weight, and the acceleration of solar gravity indicate that the 
density decreases hydrostatically at the rapid rate of a factor 
of ten roughly every 1,000 km (620 mi) radially outward.  This
rapid decrease explains the sharp edge or limb of the Sun, even 
when seen with telescopes, because the shell in which the gas
passes from being opaque to transparent is less than 1,000 km
(620 mi) thick, and subtends less than 1 arc second as viewed
from the Earth.  When looking at the center of the Sun's disk,
it is possible to see deeper into the absorbing solar
atmosphere than when looking toward the limb, where the line of 
sight is more nearly tangent to the photosphere.  Because the
temperature increases inward below the photosphere, the line of 
sight toward the center of the disk sees hotter, and thus
brighter, layers.  This phenomenon explains the prominent limb
darkening seen in pictures of the photosphere.
A spectrogram of the solar light shows a bright background
continuum traversed by many dark absorption lines.  The
continuum radiation that is visible to the eye, roughly between 
4000 A and 7000 A, is emitted when electrons released from the
relatively easily ionized heavy elements are captured by
neutral hydrogen atoms.  The dark Fraunhofer lines, such as the H 
and K lines of ionized calcium, are formed when light of
certain discrete wavelengths is preferentially scattered by the 
particular species of neutral atoms or ions that are abundant
at the density and temperature of the photosphere.  The light
emerging through the photosphere at these wavelengths is
changed in frequency by multiple scattering of the photons from 
atoms and rapidly moving electrons, and is emitted instead in
the continuum.
The Chromosphere
Above the photosphere, the temperature drops to a minimum of
about 4,500 K, and then, remarkably enough, begins to rise.
During a few seconds around totality during a solar eclipse, a
thin ring (annulus) about 10,000 km (6,200 mi) thick around the 
limb is seen shining with a reddish glow, leading to its
designation as the chromosphere ("color sphere").  Upon
examination with a telescope and spectrograph at high
resolution, most of the chromospheric emission is seen to come
from very fine jets of outward-moving gas called spicules, at a 
temperature of about 15,000 K and a density of some (10 to the
power of 11) particles/cu cm.  A spicule lasts some 5 to 10
minutes and is typically 6,000 km (3,700 mi) high and perhaps
one-tenth as thick.  The gases are moving outward at speeds of
about 10 km/sec (22,000 mph).
The Corona
During a total solar eclipse, or with a CORONAGRAPH, the Sun's
atmosphere can be seen extending to several solar radii beyond
the photospheric limb as a faint glow, about one million times
less bright than the disk.  The height of the corona was for
some time puzzling to scientists, because it seemed that the
density should drop off so rapidly that no corona would be
visible at distances of even a small fraction of solar radius
above the limb.  The explanation for this discrepancy came in
1940 when certain unidentified lines seen in the spectrum of
the corona were demonstrated as arising from transitions in
iron ionized up to 13 times, implying temperatures of several
million K.  Because a hot gas is expected to be compressed
relatively less by the weight of overlying layers than a cool
one, the high coronal temperature explained why the corona
remains visible much farther above the limb than might be
The specific mechanism that heats the corona to such a high
temperature is still unclear, and this question is the focus of 
much of modern solar research from satellites.  The coronal gas 
close to the Sun is visible with the naked eye during eclipses
because it scatters photospheric light from electrons in the
plasma.  The hot coronal plasma also emits its own ultraviolet
and X-ray light when rapidly moving electrons collide with ions 
of the heavier elements.  For instance, the lines of
9-times-ionized magnesium and 11-times-ionized silicon are
prominent in the ultraviolet spectrum.  The heating of the
corona is not a matter of simple heat flow from the cooler
photosphere, by either conduction, convection, or radiation,
because such a heat flow would violate the second law of
thermodynamics.  Most likely, acoustic or other forms of waves
generated by gas motions at the photosphere may carry energy
into the coronal medium and dissipate it into heat, balancing
the corona's losses.  Another alternative is dissipation of
electric currents in the highly conducting coronal plasma, in
much the same way that joule heating raises the temperature of
a common resistor.
The Solar Wind
Because the outward gradient of gas pressure in the hot corona
is too high to be balanced by solar gravity, this outermost
layer of the atmosphere expands into space at a steady rate.
At the Earth's orbit, the outward velocity of this SOLAR WIND
reaches between 300 and 700 km/sec (185 and 435 mi/sec).  The
density there, however, is only between 1 and 10 particles/cu
cm, so that the mass flux is only about (10 to the power of
-13) solar masses per year.  Nevertheless the solar wind has
observable effects on the upper atmosphere of the Earth;  for
instance, it is thought to be responsible for most of the
auroras seen at high terrestrial latitudes.
The intense magnetic fields produced in the solar interior
influence the physical structure of the photosphere,
chromosphere, and corona in a complex and time-varying way
described collectively as solar activity.
Sunspots, Faculae, and Flares
The magnetic fields emerge at the visible layers as toroidal
loops of magnetic flux up to 100,000 km (62,000 mi) in
diameter.  Their most obvious effect at the photosphere is to
produce the dark SUNSPOTS and bright faculae that constitute an 
active region.  If, as believed, the intense radially directed
fields inhibit convection, and thus reduce the efficiency of
the dominant heat-transport process to the photosphere, the low 
temperature and relative darkness of sunspots would be
explained.  How intense fields can also produce a net facular
brightening under similar circumstances is still unclear.
An active region grows in horizontal extent as the loop
emerges, from less than 5,000 km (3,100 mi) across, to more
than 100,000 km (62,100 mi) within 10 days.  During this period 
of rapid growth the probability of a spectacular eruption,
called a SOLAR FLARE, is highest.  A large flare is marked by a 
rapid brightening within a few minutes of a considerable area
of the active region by a factor of 5 to 10, as seen in
chromospheric radiations such as the H alpha line of hydrogen.
Only the very largest flares can be detected in integrated
white light against the bright photosphere.  The most violent
and spectacular effects of the eruption, however, take place in 
the corona above.  Here, a set of the magnetic loops above the
spots and faculae may increase their brightness in X-ray and
ultraviolet light by a factor of 100 or more, even more rapidly 
than the change seen in the chromosphere.  Charged particles
are accelerated to relativistic energies, and strong
centimeter-wave emission is generally detected.
Some flares also produce powerful meter-wave radio bursts, and
large volumes of hot plasma, called sprays, are often ejected
into space at speeds exceeding the escape velocity of 617
km/sec (380 mi/sec) from the solar gravitational field.  The
cataclysmic event decays more slowly, over a few hours, after
liberating up to (10 to the power of 32) ergs of energy, by a
mechanism that is not well understood and is at the center of
current research.
Sunspots generally last a few weeks, with the most persistent
large spots surviving for 2 to 3 months.  The faculae continue
to mark an active region for somewhat longer.  Eventually, it
appears that the random motions of convection near the
photosphere disassemble the magnetic flux loop and disperse it
into smaller magnetic elements distributed over the solar
Away from the active regions, less extended fields of
comparable intensity (1,000 to 2,000 gauss) are measured, but
they are confined to a polygonal network that coincides with
the edges of the supergranular convective cells mentioned
Loops, Prominences, and Coronal Holes
Above the photosphere the magnetic fields over an active region 
can be seen by their effect on the distribution of temperature
and density in the chromosphere and corona.  Here, prominent
loop-shaped structures seen in X rays and ultraviolet light
show how the field lines extend to 100,000 km (62,000 mi) or
more above a spot, and then connect back to the photosphere,
generally within the same active region.  In other regions of
the corona immense sheets of relatively cool (10,000 K as
opposed to the 1 to 3 million K of the corona) condensed
plasma, called prominences, are supported at heights up to
200,000 km (124,000 mi).
In certain large areas, called coronal holes, the coronal
emission is significantly depressed, indicating a low density
of the million-degree plasma.  Studies indicate that in these
regions the field lines continue radially outward and do not
form closed structures, as in loops or prominences.  Models
show that the hot corona can then flow out into interplanetary
space more easily, leaving a deficit of coronal material.  Such 
holes are particularly common at the north and south solar
poles, where no active regions with closed fields are observed. 
Solar Activity Cycle
Solar activity exhibits a cycle over a period of about 22
years.  The most easily observed feature of the cycle is the
approximately 11-year variation in the number of sunspots on
the disk.
At the beginning of a new cycle the first groups emerge at
relatively high latitudes, between 35 and 40 degrees.  Their
magnetic polarity (orientation of the dipole axis in solar
coordinates) is opposite to that of the last groups of the
preceding cycle in that hemisphere.  Thus two consecutive
11-year cycles of spot number are required to return to a given 
level of spot number and also of spot group polarity.
This 22-year solar magnetic cycle seems to have been quite
regular in the past 100 years and more.  Historical evidence,
however, indicates that between approximately 1640 and 1710
hardly any spots were visible at all, suggesting that the
present range of solar activity cannot be taken for granted.
Such long-term irregularities in solar activity are of
practical interest, because the solar fluxes of charged
particles and ultraviolet radiation are directly controlled by
the level of activity through active regions, flares, and
coronal holes.  The solar variation in these emissions is known 
to affect the upper atmosphere and may have important
influences on climate as well.  Solar-terrestrial effects are
under close study.

The Sun still holds many mysteries.  For example, the
proton-proton reaction thought to be the source of most solar
energy should also produce a certain number of the particles
called NEUTRINOS, yet studies thus far have detected
significantly fewer neutrinos than theory predicts.  One
radical suggestion is that the Sun produces fewer neutrinos
than expected because it has an iron-plasma core that amounts
to about 0.5 percent of its total mass.  Other physicists have
theorized that weakly interacting massive particles
(WIMPs)--predicted by GRAND UNIFICATION THEORIES and sometimes
invoked to account for the "missing matter" in the
universe--might exist deep within the Sun and lower its
temperature enough to explain the lack of neutrinos.  Yet
another proposal is that electron-type neutrinos in the Sun's
core change on the way out into muon-type neutrinos
unobservable by the detectors now in use.
In the early 1960s, radial oscillations of the photosphere were 
detected that have since been explained as the resonant
trapping of acoustic waves between certain layers of the
convection zone of the Sun; the WIMP theory helps to explain
some details of the oscillations, as well.  Close studies of
these oscillations are enabling scientists to probe the
density, temperature, and velocity patterns of the invisible
subphotospheric layers.  Scientists have also observed that the 
Sun's diameter fluctuates by about 0.01 percent of the average
diameter over a nearly 80-year cycle, and longer-period
pulsations may be possible.

Bibliography:  Bartusiak, Marcia, "Seeing into the Sun,"
MOSAIC, Spring 1990;  Foukal, Peter, Solar Astrophysics (1990); 
Stix, Michael, The Sun (1989);  Time-Life Books Editors, The
Sun (1990);  Wentzel, D.L., The Restless Sun (1989);  Whitmire, 
Daniel, and Reynolds, Ray, "The Fiery Fate of the Solar
System," Astronomy, April 1990.
SOLAR SYSTEM The solar system is the group of celestial bodies, including the Earth, orbiting around and gravitationally bound by the star known as the SUN, one of at least a hundred billion stars in our galaxy. The Sun's retinue includes nine planets, at least 54 satellites (see SATELLITE), more than 1,000 observed comets (see COMET), and thousands of lesser bodies known as minor planets (asteroids) and meteoroids (see ASTEROID; METEOR AND METEORITE). All of these bodies are immersed in a tenuous sea of fragile and rocky interplanetary dust particles, perhaps ejected from comets at the time of their passage through the inner solar system or resulting from minor planet collisions. The Sun is the only star known to be accompanied by such an extensive planetary system. A few nearby stars are now known to be encircled by swarms of particles of undetermined size, however (see PLANETS AND PLANETARY SYSTEMS), and evidence indicates that a number of stars are accompanied by giant planetlike objects (see BROWN DWARF). Thus the possibility of a universe filled with many solar systems remains strong, though as yet unproved. HISTORY OF SOLAR SYSTEM STUDIES Since primitive times humanity has been aware that certain of the stars in the sky are not fixed, but wander slowly across the heavens. The Greeks gave these moving stars the name planets, or "wanderers." They were the first to predict with accuracy the positions of the planets in the sky, and they devised elaborate theoretical models in which the planets moved around combinations of circles that in turn circled the Earth. The Greek mathematician Claudius Ptolemy systematized an elaborate geocentric scheme of this kind in the 2d century AD, which passed with minor changes through the Middle Ages and on to the Polish astronomer Nicolaus Copernicus (see ASTRONOMY, HISTORY OF). In his work of 1543, Copernicus proposed that planetary motions centered on the Sun rather than on the Earth, but he retained the description of planetary motions as being a series of superimposed circular motions, mathematically equivalent to the Ptolemaic theory. In the same year Copernicus died. During the 17th century a German mathematician by the name of Johannes Kepler abandoned the concept of circular motion in favor of an elliptical scheme, in which the motions of the planets describe a simple series of ellipses in which the Sun is at one of the foci. Basing his work on the observations of Tycho Brahe, his former employer and a renowned astronomer, Kepler found (1609, 1619) three important empirical relationships, concerning the motion of the planetary bodies, now known as KEPLER'S LAWS. Kepler's labors laid the groundwork for Sir Isaac Newton's law of GRAVITATION (1687), from which it became possible for astronomers to predict with great accuracy the movements and positions of the planets. Only the planets Mercury, Venus, Mars, Jupiter, and Saturn were known to the ancients. The English astronomer William Herschel accidentally discovered Uranus in 1781 as the result of telescopic observations. Discrepancies between the observed positions of Uranus and those predicted led John Couch Adams and Urbain Jean Joseph Leverrier to propose (1846) that another large planet was exerting a gravitational force on Uranus. In the same year the planet Neptune was found close to its predicted position. In the 20th century smaller residual discrepancies in the apparent positions of Uranus and Neptune led to predictions of the presence of still another planet, and in 1930, Clyde Tombaugh discovered the planet Pluto close to one of the areas of prediction. Pluto's mass, however, is so small that the discovery is now considered to have been an accident resulting from intense scrutiny of that part of the sky to which the predictions had called attention. Yet another planet may remain to be discovered. Galileo was in 1609 the first to use the telescope for astronomical purposes, and it has since become an essential tool in planetary studies. In the 19th century planetary astronomy flourished, thanks to the construction of large telescopes and their systematic use for planetary observations. Two new tools, the spectroscope and the photographic plate, were also developed in the 19th century and gave rise to the new science of astrophysics. For the first time it became possible to determine not only the orbits and masses of objects in the solar system, but also their temperatures, compositions, and structures (see ASTRONOMY AND ASTROPHYSICS). During the early years of the 20th century great advancements took place in the understanding of the physics and chemistry of the planets in the solar system, and during the middle years of the century important further advances were derived from RADIO ASTRONOMY and RADAR ASTRONOMY. Although most astronomers gradually turned their attention away from the solar system to the study of stars and galaxies, the launch (1957) of the first artificial satellite ushered in an age that transformed solar-system studies. During the 1960s, 1970s, and 1980s spacecraft accomplished flyby, orbiting, or landing missions to many of the planets. At the present time the reconnaissance of the planets in the solar system has been accomplished for Mercury through Neptune. The U.S. MARINER spacecraft have provided a good model of the atmosphere of Venus, and the Soviet VENERA spacecraft have returned pictures from the surface of that planet. Mariner and VIKING (U.S.) spacecraft have extensively photographed Mars from orbit, and the Viking landers have carried out important initial measurements of surface properties. U.S. PIONEER and VOYAGER probes have returned data and images from the outer planets Jupiter, Saturn, Uranus, and Neptune. The investigation of the Moon has progressed through the stages of flybys, orbiters, and landers both of the manned variety (U.S. Apollo) and the unmanned variety (U.S. RANGER, SURVEYOR, and LUNAR ORBITER, and Soviet LUNA). The success achieved in bringing to the Earth samples from several different lunar landing sites has made possible a continuing series of laboratory investigations and further intensive study of Earth's satellite (see SPACE EXPLORATION). THE SUN The Sun is the only star whose surface can be studied in detail from the Earth. This surface presents a scene of churning, turbulent activity, largely dominated by strong magnetic fields. Magnetic lines of force emerging from the solar surface appear as sunspots. Arches of the magnetic lines of force extending across the surface give rise to bright, shining solar prominences. Wave motions generated below the surface of the Sun flicker across the surface and mount into the atmosphere. Brilliant flares appear in the vicinity of sunspots, generating bursts of ultraviolet and X-ray emissions from the Sun and accelerating ions and electrons to create the high-energy particles known as cosmic rays. The upper levels of the Sun's atmosphere are of very low density, but the solar activity heats the gases there to very high temperatures. Here the electrons are stripped from atoms to form ions, and the two types of particles together form a plasma. The gravitational field of the Sun is unable to retain this superhot plasma, and it streams outward into space as the solar wind. Measurements of the properties of the solar wind are routinely carried out by U.S. spacecraft at many different locations within the solar system. SOLAR SYSTEM Most of the mass (99.86 percent) of the solar system is concentrated in the Sun, which thus exerts the gravitational force that holds the scattered members of the system together. There is a remarkable degree of orderliness in the motions of the members of the solar system under the influence of the Sun's gravity. With the exception of the comets, some of the asteroids, and Pluto, the motions of the bodies in the solar system are confined to approximately the same plane, called the plane of the ecliptic. There is a striking similarity in the way in which these bodies revolve and rotate. The planets all revolve around the Sun in the same direction, and the Sun rotates in this direction as well. With only two exceptions, Venus and Uranus, the planets also rotate in this common direction. Many of the planets, particularly in the outer solar system, are accompanied by swarms of satellites, and again, with a few exceptions, these also tend to revolve in a plane close to the plane of the ecliptic and with the same sense of motion. All of these tendencies can be summarized by saying that the angular momentum vectors of the bodies in the solar system are for the most part aligned. THE PLANETS The nine planets of the solar system may be divided into two groups: the inner, or terrestrial, planets, and the outer, or Jovian, planets. This division is based not only on distance from the Sun, but also on the physical properties of the planets. The Inner Planets The inner planets are all comparable in size, density, and other characteristics to the Earth and so are generally referred to as the terrestrial, or Earth-like, planets. Included are Mercury, Venus, Earth, and Mars. The Earth is the largest of the terrestrial planets. By far the most massive constituents of the Earth are the iron core and the rocky mantle and crust. The water in the oceans and the gases in the air form only a thin veneer of volatile materials surrounding the rock of the planet proper. The Sun provides the heat and light that make the Earth habitable for life as we know it. The oceans and atmosphere of the Earth absorb and redistribute the heat in a complex fashion. Various types of geological evidence indicate that the Earth has passed through ice ages in the past, but it is not known whether some unknown variability in the Sun, the great complexity of the Earth's atmospheric weather system, or some other factor has been responsible for these (see also MILANKOVITCH THEORY). The early years of the Earth were apparently rather violent, as no geological record is preserved of the first half-billion years of its existence. The Earth-Moon system is often referred to as a "double planet" system, because the Moon is more nearly comparable in size to the Earth than the other satellites are to their primaries (except for Pluto and its moon). The Earth's MOON is 81 times less in mass than the Earth but only 4 times less in mass than the planet Mercury. It is one of a group of the six largest satellites in the solar system that have approximately comparable mass, and the only such large one in the inner solar system. Compared to the mass of its primary, the Earth, the Moon is abnormally massive. The return of samples from several lunar sites during the Apollo program, and the establishment of stations to measure seismic activity and other physical quantities at these sites, has provided more knowledge about the Moon than currently exists for any other body in the solar system except the Earth. If the Moon has a central iron core, it is unexpectedly small, compared to that of the Earth, and of surprisingly little mass; the bulk of the Moon is mantle and crust that has had an extensive history of melting and chemical differentiation. The Moon contains no atmosphere, and its surface is heavily cratered. Its topmost soil is a very fine-grained substance with little chips of rock sprinkled throughout. This is called the lunar regolith. The Moon is heavily depleted in the more volatile elements and compounds as compared to the Earth. The next inner planet toward the Sun is VENUS, long considered a mystery planet because it is shrouded in clouds that hide the details of its underlying surface. Venus is nearly as large and as massive as the Earth, contains relatively little water, and has nothing resembling the oceans of the Earth. Instead, carbon dioxide in an amount comparable to that in the carbonate rocks of the Earth fills the Venusian atmosphere, producing a pressure at the surface about 100 times higher than that at the surface of the Earth and a temperature far too high to support life of any kind as we know it. Venus has a slow retrograde rotation, so that it rotates in a direction opposite to that of most of the other objects in the solar system. The next planet outward from the Earth away from the Sun is MARS, which is only about one-tenth of the mass of the Earth. Its tenuous atmosphere is composed principally of carbon dioxide, with a pressure at the surface more than 100 times smaller (0.7 percent) than that at the surface of the Earth. The surface of Mars can be considered to be roughly divided into two hemispheres, one a surface of ancient, heavily cratered terrain and the other a geologically younger terrain having a much lower density of cratering. Mars has long been suspected to be a possible abode for other forms of life within the solar system, and apparent seasonal differences in its appearance were attributed to the presence of life. Experiments performed by the Viking spacecraft landers, however, found no evidence for the presence of Martian life forms, however, and it has been found that the Martian surface apparently contains oxidizing agents highly incompatible with any form of organic life. The planet closest to the Sun is MERCURY, a planet whose mass is half as great as that of Mars. Mercury has only a trace atmosphere, consisting of such elements as helium, sodium, and hydrogen. Its surface is heavily cratered. The planet possesses an interesting resonance with its orbital motion, presenting first one face and then the other during its closest approaches to the Sun. The Outer Planets The terrestrial planets just described have in common a rocky composition whose major constituents have high boiling points and are therefore described as refractory. It is believed that the entire solar system, including the Sun, was formed from the gravitational contraction of a large cloud of gas and dust composed mainly of hydrogen and helium and only a small percentage of heavier atoms such as oxygen, silicon, and iron. The Sun's composition, which is about three-quarters hydrogen and nearly one-quarter helium, with less than two percent heavy elements, is believed to be essentially the same as that of the original nebula. The inner planets lost most of their lighter, volatile elements early as a result of their proximity to the hot Sun, whereas the more distant, cold, outer planets were able to retain their light gases. The result is that the outer planets became far more massive than the terrestrial planets and were able to hold very extensive atmospheres of light gases such as hydrogen, as well as light, icy substances such as water, ammonia, and methane. The most massive planet in the solar system, with about one-thousandth the mass of the Sun and more than 300 times the mass of the Earth, is JUPITER. Composed primarily of hydrogen and helium, Jupiter may have an interior composed of ice (and other frozen volatiles) and rocks, or both, exceeding several times one Earth mass of rocky material and three Earth masses of the ices. The total amount of material heavier than hydrogen and helium is unknown but is probably in the range of 10-20 Earth masses. Jupiter rotates rapidly on its axis, so that its figure is significantly flattened toward its equatorial plane, and the gases in its surface show a banded structure along lines of latitude. Infrared measurements from high-flying aircraft on the Earth and from flyby spacecraft have determined that Jupiter radiates into space about twice as much energy as it absorbs from the Sun; the additional heat emerges from the interior of the planet. Spacecraft also revealed that Jupiter is ringed. The next planet outward from Jupiter is the strikingly ringed SATURN, another gas giant also thought to be composed predominantly of hydrogen and helium. Its mass is slightly less than a third that of Jupiter, but it also appears to have something approaching 20 Earth masses of heavier materials in the form, presumably, of icy or rocky materials. Saturn also rotates rapidly, is highly flattened toward its equatorial plane, and exhibits a banded structure along latitude lines. Beyond Saturn are URANUS and NEPTUNE, two planets of similar size. Uranus has a mass about 15 times and Neptune a mass about 17 times that of the Earth. Hydrogen and helium predominate in the atmospheres of both planets. The planetary interiors lie hidden beneath thick atmospheres, but data from Voyager 2 suggest that Uranus has a superheated water ocean, up to 10,000 km (6,000 mi) deep, surrounding an Earth-size core of molten rock materials. Although Neptune receives comparatively little energy from the Sun, it has an active atmosphere and apparently has some form of internal energy source. The rotation period of Uranus is a little more than 17 hour; that of Neptune a little longer than 16 hours. Uranus is unique among the planets in being tilted on its rotation axis by about 98 degrees with respect to the plane of the ecliptic, so that its rotation is retrograde. Uranus and Neptune both have ring systems. PLUTO is a planet whose characteristics were largely unknown until the discovery of its moon, Charon, in 1978. Astronomers report that Pluto's diameter is 2,302 km (1,430 mi) and that Charon's is 1,186 km (737 mi). The density of the planet is about the same as that of water, so that it may be composed of an ice-rock mixture. Pluto has a rather elliptical orbit that at times takes the planet closer to the Sun than Neptune. From 1979 until 1999, for example, Pluto will be within Neptune's orbit. This would ordinarily be a rather unstable state of affairs, but perturbations of the Pluto orbit caused by Neptune occur in such a way that a collision between the two planets cannot occur. Astronomers have also observed perturbations in the orbits of Uranus and Neptune. Pluto is too small to cause these irregularities, and the Pioneer spacecraft have detected no other sources of gravity. Some scientists hypothesize that a tenth planet, called "Planet X," is responsible for these perturbations. THE SATELLITES Of the more than 50 known satellites in the solar system, only three circle the inner planets. Earth has its abnormally massive Moon, and Mars has two tiny satellites, DEIMOS and PHOBOS. Very dark and heavily cratered, the Martian satellites may resemble the chondritic meteorites (fragile, low-density, stony-type meteorites that contain large amounts of carbon, water, and other volatile substances). Most of the outer planets have large swarms of satellites attending them. In many cases the satellites are arranged in regular orbits that are suggestive of miniature solar systems. Jupiter has four giant satellites, each comparable in mass to Earth's Moon, called the Galilean satellites for their discoverer. The internal densities of these satellites are now reasonably well known as the result of measurements made by the flyby Pioneer spacecraft. The innermost two Galilean satellites, IO and EUROPA, are largely rocky in composition. On the other hand, the outer two giant satellites, GANYMEDE and CALLISTO, are of a lower density, suggestive of a much higher ice content. Closer to Jupiter than these Galilean satellites is a much smaller one, Amalthea. These five satellites lie in the plane of Jupiter's equator and have very nearly circular orbits. Because of this ordered arrangement, they are called the regular satellites. Three further, very small satellites were discovered by Voyager spacecraft. Orbiting far from these regular satellites are the irregular satellites, in two swarms of much smaller bodies, each only a few kilometers in radius. Eight of these bodies are so far known to exist, and there are indications of additional members. The orbits of these satellites are inclined at substantial angles with respect to the plane of Jupiter's equator, and the orbits themselves are quite elliptical. Four of these small satellites rotate in a direct (west to east) sense, but the others rotate in a retrograde (east to west) sense. Saturn also has a system of regular satellites. One of these, TITAN, is larger than the planet Mercury and is unique among the satellites in the solar system in that it has a substantial atmosphere. Four other satellites of Saturn have diameters of more than 1,000 km (600 mi), but the rest are much smaller. One of them, Phoebe, has a retrograde orbit. Studies of Voyager data have brought the total number above 20. The five satellites of Uranus visible to Earth-based telescopes are closely clustered in the plane of the Uranian equator, so that the plane of their orbits is also rotated 98 degrees to the plane of the ecliptic. These satellites are relatively small, comparable in size to the lesser regular satellites of Saturn. Several much smaller satellites were discovered by Voyager 2. The unusual system of Neptune contains one major satellite, TRITON--whose mass is not exactly known but may be comparable to that of the Moon--which moves in a circular but inclined retrograde orbit. Neptune also has a smaller, direct-rotating satellite. A single moon of Pluto was discovered on June 22, 1978, and named Charon. It appears to have about 5-10 percent of the mass of Pluto, meaning that it is the solar system's largest moon compared to its planet. ASTEROIDS AND METEOROIDS The major planets in the solar system are greatly outnumbered by the swarms of smaller bodies called minor planets, or asteroids, and by the even more numerous and smaller bodies known as meteoroids. Most of the asteroids exist within the relatively large gap lying between the orbits of Mars and Jupiter, whereas meteoroids are randomly distributed. A few large asteroids have radii of a few hundred kilometers, but most are much smaller. The smaller meteoroids produce meteor trails when they enter the Earth's atmosphere, and the larger ones form meteorite craters. A large number of the asteroids appear similar to the carbonaceous chondritic meteorites and are probably of relatively lower density than ordinary rocks. Nearly 2,000 of the asteroids have accurately determined orbits and have been given names. It is generally believed that the smaller asteroidal bodies have been created in collisions involving larger ones, so that there probably exist many small bodies that have not been detected by photographic surveys. Many asteroids have orbits that cross the orbit of Mars; some cross the orbit of the Earth or go even further into the inner solar system. These are called the Apollo asteroids. It has been suggested that many of the meteorites that strike the Earth are chips of the Apollo asteroids caused by collisions. These asteroids can collide with the Earth or one of the other terrestrial planets, and some of the major craters that exist on these planets have probably been caused by such collisions. Other asteroidal bodies, called Trojan asteroids, have been observed both 60 degrees ahead of Jupiter in its orbit and 60 degrees behind. These positions of special orbital stability are called Lagrangian points. It is possible that swarms of dust particles are concentrated in the Moon's orbit, both 60 degrees ahead of the motion of the Moon and 60 degrees behind it. These are sometimes called the L4 and L5 Lagrangian points. Although there has not been clear confirmation of the presence of these dust swarms, they may exist in a manner similar to that of the Trojan asteroids with respect to Jupiter. There have been suggestions that future human colonies in space might be established at one of these Lagrangian points. Until recently it was believed that minor planets were confined to the inner solar system. In 1977, however, an object was discovered called CHIRON, a body some hundreds of kilometers in radius that orbits between Saturn and Uranus. This object has since been classified as a huge comet. COMETS Comets are sometimes spectacular objects from the outer regions of the solar system, as far away as a substantial fraction of the distance to the nearest star. They appear to be typically a few kilometers in radius and are composed largely of icy substances. Their chemistry is, however, clearly complex. As a comet enters the inner solar system, it emits large amounts of volatile materials that are transformed by the energy of sunlight and of the solar wind into a variety of individual atoms, molecules, and ions, mostly of the common materials carbon, nitrogen, oxygen, and hydrogen, and combinations that include these. Many complex molecules have been detected by spectroscopic analysis of comet tails. Comets also emit a large number of tiny dust particles. The Dutch astronomer Jan H. OORT recognized (about 1950) that most of the apparently fresh comets coming into the inner solar system started from initial distances beyond 50,000 astronomical units (the distance from the Earth to the Sun is defined as one astronomical unit). Furthermore, he recognized that the ease with which planetary perturbations can change the orbits of the comets meant that typical comets were unlikely to endure many orbital passages through the inner solar system. Because several comets are observed each year, this means that there must be a very large reservoir of them in the outer solar system. Oort suggested that a thick shell of cometary material surrounds the Sun about 1,000 times farther out than the orbits of Neptune and Pluto. The Dutch-American astronomer Gerard Kuiper further suggested a nearer ring of cometary material in the plane of the solar system. Any disturbance of these clouds can send some material plunging into the solar system to be observed as a comet. DUST RINGS The sun is also encircled by rings, or disks, of interplanetary dust. One, lying in the zone between the orbital paths of Jupiter and Mars, has long been known and is the cause of ZODIACAL LIGHT. Another ring was found in the region of the asteroids, between Mars and Jupiter, by the Infrared Astronomy Satellite (IRAS) launched in 1983. Also detected in 1983, by a team of Japanese and Indonesian astronomers, is a third ring only two solar diameters away from the Sun. The dust in this ring is theorized to spiral slowly inward from the outer solar system, due to differential absorption and reradiation of solar energy, until it is vaporized by the Sun and the resulting gases are driven back by the pressure of solar radiation. ORIGIN OF THE SOLAR SYSTEM For more than 300 years there has been serious scientific discussion of the processes and events that led to the formation of the solar system. For most of this time lack of knowledge about the physical conditions in the solar system prevented a rigorous approach to the problem. Explanations were especially sought for the regularity in the directions of rotation and orbit of objects in the solar system, the slow rotation of the Sun, and the Titius-Bode law, which states that the radii of the planetary orbits increase in a regular fashion throughout the solar system. In a similar fashion, the radii of the orbits of the regular satellites of Jupiter, Saturn, and Uranus increase in a regular manner. In modern times the slow rotation of the Sun has been explained as resulting from the deceleration of its angular motion through its magnetic interaction with the outflowing solar wind, so that this feature should not have been considered a constraint on theories of the origin of the solar system. The many theories concerning the origin of the solar system that have been advanced during the last three centuries can be classified as either dualistic or monistic. A common feature of dualistic theories is that another star once passed close to the Sun, and tidal perturbations between the two stars drew out filaments of gas from which the planets condensed. Theories of this type encounter enormous difficulties in trying to account for modern information about the solar system, and they have generally been discarded. By contrast, monistic theories envisage a disk of gas and dust, called the primitive solar nebula, that formed around the Sun. Many of these theories speculate that the Sun and the planets formed together from the primeval solar nebula. A photograph taken in 1984 of a nearby star, Beta Pictoris, appears to show a solar system forming in this way from a disk of surrounding material. The large amount of activity that has taken place in the last 20 years in the renewed exploration of the solar system has also provided a great impetus for renewed studies of the origin of the system. One important component of this research has been the detailed studies of the properties of meteorites that has been made possible by modern laboratory instrumentation. The distribution and abundance of the elements within different meteoritic mineral phases has provided much information on the physical conditions present at the time the solar system began to form. Recent discoveries of anomalies in the isotopic compositions of the elements in certain mineral phases in meteorites promise to give information about the local galactic interstellar environment that led to the formation of the solar system. Investigations of the properties of other planets has led to the new science of comparative planetology, in which the differences observed among the planets not only lead to a better understanding of the planets, but also pose precise new questions concerning the mechanisms by which the planets may have been formed. Studies of the stars within our galaxy have shown that the age of our galaxy is much greater than the age of the solar system. Therefore, processes observed in the formation of stars within our galaxy today are likely to be found relevant to the formation of our solar system. Stars appear to form in groups or associations, as a result of the gravitational collapse of clouds of gas and dust in the interstellar medium. Modern monistic theories envisage the gas and dust in the primitive solar nebula to be the collapsed remnant of a fragment of an interstellar cloud. There has been much discussion of how the planets might have formed from the primeval solar nebula. In recent years attention has focused on the possibility that two types of gravitational instabilities might have played an important role in this process. One type is a gravitational instability in the gas of the primitive solar nebula, from which there would be formed a giant gaseous protoplanet. From the evolution of such protoplanets there could arise, in the outer solar system, the giant planets that are observed today. In the inner solar system, the possibility exists that giant gaseous protoplanets formed rocky cores at their centers, which survived the stripping away of the gaseous envelopes caused by gravitational and thermal forces from the growing Sun. The other form of gravitational instability involves the condensed materials in the solar nebula. Small dust particles that may have been present in the gas of the solar nebula could be expected to settle toward the midplane of the nebula if the gas were not subject to extensive turbulent churning. Gravitational instabilities acting on a thin dust layer might have formed bodies ranging from tens to hundreds of kilometers in radius. Collisions among these bodies may have played a major role in accumulations of material to form the planets. It must be stressed that all theories of the origin of the solar system currently being formulated respond to and are limited by the rapid accumulation of facts about planetary bodies within the solar system. Because of the rapid rate of progress in such studies, it is generally recognized that such theories are preliminary and simplified, so that ideas and theories in this area of research can be expected to continue to evolve rapidly. SOLAR APEX Finally, the movement of the solar system as a whole through space is defined in terms of the CELESTIAL SPHERE, the imaginary sphere of the heavens that has Earth at its center. The solar system appears to be moving toward a point on the sphere at the velocity, relative to nearby stars, of about 20 km/sec (12 mi/sec). This point, called the solar apex, lies in the constellation Hercules near the star Vega, at a right ascension of about 18 hours and a declination of about 30 degrees north. A. G. W. Cameron Bibliography: Beatty, J. Kelly, et al., eds., The New Solar System, 2d ed. (1982); Dermott, S. F., The Origin of the Solar System (1978); Frazier, Kendrick, Solar System (1985); Hardy, D. A., Atlas of the Solar System (1982); Hartmann, W. K., Moons and Planets, 2d ed. (1983); Jones, B. W., and Keynes, Milton, The Solar System (1984); Moore, Patrick, et al., The Atlas of the Solar System (1983); Morrison, David, and Owen, Tobias, The Planetary System (1988); O'Leary, Brian, and Beatty, J. Kelly, eds., The New Solar System, 2d ed. (1982); Smoluchowski, Roman, et al., eds., The Galaxy and the Solar System (1987); Time-Life Book Editors, The Far Planets (1989) and The Near Planets (1989). CHARACTERISTICS OF THE PLANETS --------------------------------------------------------------- Planet Mean Distance Length of ------ ---------------------------------- year Astronomical Millions Millions (Earth days Units of km of mi and years) --------------------------------------------------------------- SOLAR SYSTEM Mercury 0.387 57.9 36.0 88d Venus 0.723 108.2 67.0 224.7d Earth 1 149.6 93.0 365.26d Mars 1.524 227.9 141.6 687 d Jupiter 5.203 778.3 483.3 11.86 yr Saturn 9.539 1,427.0 886.4 29.46 yr Uranus 19.218 2,875.0 1,786.0 84.01 yr Neptune 30.06 4,496.6 2,794.0 164.8 yr Pluto 39.44 5,900.0 3,660.0 248.4 yr -------------------------------------------------------------- Length of day Inclination (Earth days, of Orbit to Planet hours, minutes, Inclination Ecliptic, Eccentricity and seconds) of Axis Degrees of Orbit --------------------------------------------------------------- Mercury 58.6 d 7deg 7.00 0.2056 Venus 243 d (retrograde) 3deg24' 3.39 0.0068 Earth 23 hr 56 min 4 sec 23deg27' ---- 0.0167 Mars 24 hr 37 min 23 sec 23deg59' 1.85 0.0934 Jupiter 9 hr 50 min 30 sec 3deg05' 1.30 0.0485 Saturn 10 hr 14 min 26deg44' 2.49 0.0556 Uranus 23 hr 15 min (retr) 97deg54' 0.77 0.0472 Neptune 22 hr 28deg48' 1.77 0.0086 Pluto 6 d 9 hr >50deg 16.00 0.0249 --------------------------------------------------------------- Equatorial Diameter Mass Planet ------------------- (compared Density km mi to Earth) (g)/cm(3) --------------------------------------------------------------- Mercury 4,880 3,030 0.054 5.4 Venus 12,104 7,517 0.815 5.2 Earth 12,756 7,921 1 5.51 Mars 6,787 4,210 0.107 3.9 Jupiter 143,000 88,800 317.9 1.32 Saturn 120,000 74,500 95.2 0.7 Uranus 51,100 31,750 14.58 1.2 Neptune 49,500 30,750 17.2 1.67 Pluto 2,302 1,186 0.0026 -1 ---------------------------------------------------------------
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