 |
 |
Planets | Sun | Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto |
|
||
The Planets .... brought to you by QuasArt Web Designs.
J
U P I T E R (5th planet)
Jupiter (a.k.a. Jove; Greek Zeus) was the King
of the Gods,
the ruler of Olympus and the patron of the Roman state.
Zeus was the son of Cronus (Saturn).
CHAPTERS
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIV
XX
XXI
XXII
XXIII
16 MOONS
INTRODUCTION
I
Jupiter (planet), fifth planet
from the sun and the largest planet in the earth's solar system. With
the exception of the sun, the moon, and Venus, Jupiter is the brightest
object in Earth's sky-more than three times brighter than Sirius,
the brightest star.
Jupiter orbits the sun at an average distance of 780 million km (484
million mi), which is about five times the distance from Earth to
the sun. Jupiter's year, or the time it takes to complete an orbit
about the sun, is 11.9 Earth years, and its day, or the time it takes
to rotate on its axis, is about 9.9 hours, less than half an Earth
day. Unlike the rocky inner planets of the solar system (Mercury,
Venus, Earth, and Mars), Jupiter is a dense ball of gas. It has a
relatively
small core of molten rock and iron, but Jupiter has no solid surfaces.
Jupiter's mass is about 318 times the mass of Earth and its diameter
is about 11.2 times the diameter of Earth. The force of gravity at
the level of the highest clouds in Jupiter's atmosphere is about 2.5
times the force of gravity on Earth.
Because Jupiter has such a large diameter and high rate of rotation,
material at the surface must travel quickly to circle the planet.
This speed gives the material a great deal of momentum, or a strong
tendency to fly away from the planet and continue traveling in a straight
line through space. Material at the equator has the highest speed
because, in a Jovian day, it must travel the greatest distance to
circle the planet. Hence, material at the equator has the greatest
momentum, and the strongest tendency to fly away from the planet.
Because of Jupiter's weak, gaseous structure, the planet can not hold
this material in as well as a more solid planet could, which results
in Jupiter having the distorted shape of a flattened ball. The diameter
of its equator is 143,000 km (89,000 mi), yet the diameter through
its axis of rotation is only 133,700 km (83,000 mi).
 |
Jupiter is the largest of the planets, with a volume 1400 times greater than that of the earth. Jupiter's colorful bands are caused by strong atmospheric currents and accentuated by a dense cloud cover. The massive planet, (left) is shown here with the four largest of its sixteen satellites: Europa, center, nearest Jupiter, Io upper left, Callisto lower left, and Ganymede lower right. Photo Researchers, Inc. |
OBSERVATIONS FROM EARTH
II
Jupiter was first viewed through a telescope in 1610 by Italian philosopher and scientist Galileo Galilei. Until that time, the dominant world view, which was developed by 2nd-century Alexandrian astronomer Ptolemy, held that all of the stars and planets move in orbits around the earth. Galileo, however, observed four satellites, or moons, in orbit around Jupiter. This simple observation of astronomical objects in orbit about another astronomical object other than the earth touched off what is known as the Copernican revolution, named after Polish astronomer Nicolaus Copernicus. Copernicus had earlier developed a cosmology in which the earth orbits the sun, which is now known as the Copernican System. The Copernican revolution was one of the key elements of the Renaissance and the Age of Enlightenment that continues to influence thinking to the present day. The moons that Galileo saw were collectively named the Galilean moons in honor of their discoverer. When viewed through a modern telescope, the oblate (flattened) disk of Jupiter has a pearly color with bands of pastel browns and blues. Earth-based observers can best observe Jupiter when it is near solar opposition-that is, when Jupiter is on the side of Earth opposite the sun, or when both planets are aligned with the sun on the same side of the sun. At opposition, the distance from Earth to Jupiter is at its annual minimum, and Jupiter appears as much as one and one-half times larger than it does at other times. Also at opposition, Jupiter rises at sunset and sets at sunrise, which means that it is visible all night long.
Because Jupiter orbits the sun in the same direction as Earth, Earth
has to travel a little more than a full year to catch up to Jupiter
from one opposition to the next. The time interval between exact oppositions
is about 399 days. In the mid-1950s radio astronomers discovered that
Jupiter emits strong radio waves at many frequencies (see Radio Astronomy).
The radio data indicates that Jupiter has a magnetic field similar to
Earth's, but much stronger: At its upper atmosphere Jupiter's magnetic
field is about ten times more intense than Earth's field at Earth's
surface. Also like Earth's field, Jupiter's field is tipped about 10°
relative to its axis of rotation. The interaction of Jupiter's magnetic
field with charged particles ejected from the sun creates radio noise
near the poles and auroras similar to Earth's aurora borealis, or northern
lights. As Jupiter rotates its north and south magnetic poles are obscured
to different extents, which makes the intensity of radio noise vary
in a regular pattern. The pattern repeats at intervals of 9 hours 55.5
minutes, indicating that this is the rate of rotation of Jupiter's interior.
With this rapid rotation, Jupiter's entire surface can be observed in
two days during the long observing periods of opposition.TOP OF PAGE
COMPOSITION AND STRUCTURE OF JUPITER
III
By measuring the velocity of Jupiter's satellites, astronomers have been able to calculate the gravitational force that Jupiter exerts on them. Because the gravitational force exerted by a planet is proportional to its mass, astronomers have thus been able to calculate Jupiter's mass. Spacecraft flying by Jupiter have made possible more detailed studies of Jupiter's gravitational field, giving clues about its inner structure. These spacecraft have also relayed close-up television images and the results of chemical studies of the composition of Jupiter's outer layers. Putting all of this information together, astronomers have assembled a detailed picture of Jupiter.
TOP OF PAGE
COMPOSITION OF JUPITER
IV
Jupiter's diameter is 11.2 times larger than Earth's, which means that its volume is more than 1300 times the volume of Earth. However, Jupiter's mass is only 318 times the mass of Earth. The density of Jupiter is therefore less than one-fourth of the density of Earth. Jupiter's low density (1.33 gm/cc vs. 5.52 gm/cc for Earth) indicates that the planet is composed primarily of the lightest elements-hydrogen and helium. The Galileo spacecraft, a probe launched by the National Aeronautics and Space Administration (NASA), began orbiting Jupiter in 1995. It measured high winds and a puzzling lack of water molecules deep in Jupiter's atmosphere. It also found that there are about 6.4 hydrogen molecules, or not quite 13 hydrogen atoms, for each helium atom. This is similar to the ratio of these elements scientists have measured in the outer envelope of the sun and supports the theory that Jupiter was formed from the same cloud of material as the sun (see Planetary Science). When spacecraft fly by a planet, the gravitational field of the planet causes them to accelerate, or change speed. Changes in the speed of the spacecraft are reflected by changes in the frequency of the radio signals that they send back to Earth (see Doppler Effect). Detailed analyses of radio signals sent back to Earth by several spacecraft that have flown near Jupiter indicate that Jupiter has a core made of material that is about as dense as the Earth's average density. Astronomers believe that this core material is rock and metal.
TOP OF PAGE
STRUCTURE OF JUPITER
V
Data from the spacecraft missions and from Earth-based observations indicate that Jupiter's outer layer is a gaseous mixture of hydrogen, helium, and traces of hydrogen-rich compounds such as ammonia, methane, and water. This outer layer is about 1000 km (600 mi) thick. Beneath this layer, the pressure is so great and the atmosphere is so hot and compressed that the hydrogen and helium atoms do not behave as a gas, but as what physicists call a supercritical fluid. The supercritical zone extends 20,000 to 30,000 km (12,000 to 18,000 mi) into Jupiter, which is about one-fourth to one-third of the radius of the planet. Beneath the supercritical fluid zone, the pressure reaches 3 million Earth
atmospheres. At this depth, the atoms collide so frequently and violently
that the hydrogen atoms are ionized-that is, the negatively-charged
electrons are stripped away from the positively-charged protons of the
hydrogen nuclei. This results in a sea of electrically-charged particles
that resembles in many important respects a liquid metal. This liquid
metallic hydrogen zone is 30,000 to 40,000 km
(18,000 to 24,000 mi) thick-about half the radius of the planet-and
extends to the molten rock core at Jupiter's center. The molten rock
core occupies a sphere with a radius of about 10,000 km-about one-fourth
of Jupiter's total radius-and has a mass of about 15 to 20 times the
mass of Earth.TOP OF PAGE
EVOLUTION OF JUPITER
VI
According to current theories, a disk of dust and gas encircled the sun in its early days. At Jupiter's distance from the sun, silicate and metal-rich grains in this disk combined with icy cometlike fragments to form seeds for larger bodies. The largest fragments swept up the most dust and surrounding gases and formed ever-larger bodies. Bodies that attained masses in excess of about 15 times the mass of the earth were able to gravitationally attract and keep individual hydrogen and helium atoms, which constituted most of the disk material. Eventually, nearly all of the matter of the disk was concentrated in a few bodies, with Jupiter being the largest one other than the sun. Despite the planet's large size, the pressure and temperature at Jupiter's core are too small to cause sustained fusion of hydrogen-the requirement for a body to become a star. Jupiter would need to have about 80 times its current mass for this to occur. Though Jupiter and the sun are made of the same material, the planet is too small to become a star in its own right.
TOP OF PAGE
CIRCULATION OF JUPITER'S ATMOSPHERE
VII
Jupiter is five times more distant from the sun than Earth, so the intensity of solar energy reaching Jupiter is only about 4 percent of the intensity of solar radiation reaching Earth. Studies of infrared radiation (heat energy radiation) from Jupiter reveal that the planet radiates 1.67 times as much energy as it receives from the sun. The source of the excess radiated energy is apparently stored heat that was created by gravitational compression of Jupiter's material when it formed 4.5 billion years ago. The difference in temperature between the top of Jupiter's atmosphere to its deepest layers drives circulating cells that transport heat from within outward. AAtmospheric Circulation In the upper reaches of Jupiter's atmosphere, the temperature drops below the freezing point of ammonia. In regions where gases tend to rise, the fresh ammonia freezes to form highly reflective ice crystals. The ice crystals are pushed horizontally by new material welling up from below, causing the formation of bright bands. Ultraviolet radiation interacts with molecules in the upper atmosphere and generates yellow-brown smog. This smog settles down on the clouds causing those that are deeper in the smoggy atmosphere to appear darker and more brown. Within the darker belts, the atmosphere tends to sink and the ammonia ice crystals melt, exposing more brown smog particles and causing further darkening.
TOP OF PAGE
WINDS
VIII
Rising air masses in Jupiter's atmosphere expand north and south. The air that moves toward the equator must travel around a longer path than it originally followed, while the air that moves toward the pole travels a shorter path. These deflections give rise to alternating winds that shear Jupiter's cloud layers into sharply defined bands. Similar mechanisms cause the trade winds of Earth, but Jupiter's winds are much stronger and form a more stable pattern than they form on Earth. At Jupiter's equator, individual cloud systems shift eastward 11° in 24 hours relative to rotation of the planet's interior, indicating that Jupiter's atmosphere circles the planet with extremely high winds. At Jupiter's equator, these winds reach up to 600 km/h (360 mph), but they decrease near the poles.
TOP OF PAGE
STORMS
IX
Jupiter's atmosphere is composed mostly of hydrogen and helium with lesser amounts of minor gases. White clouds of frozen ammonia crystals and other colored clouds, including the Great Red Spot, swirl around in atmospheric currents as the planet rotates. The Great Red Spot was photographed by Voyager 1 in 1979. Photo Researchers, Inc.
Major storms often appear suddenly on Jupiter's surface. Unlike storms on Earth, which are driven by solar heating of the atmosphere, Jupiter's storms appear to be caused by bubbles of gas that rise through the dense atmosphere from deep within the planet. These bubbles carry varying amounts of heat, creating storms that are often trapped between belts by strong winds blowing in opposite directions. Unable to move north or south, and with no solid land masses to create friction, the storms roll in these winds and feed off them for weeks or longer. Jupiter's most famous storm, the Great Red Spot, has persisted since the first telescope strong enough to see it was aimed at Jupiter centuries ago. The cause of the Great Red Spot is not yet known, but its motion is such that it must sustain itself on energy gained from the upper atmosphere. It cannot be linked to a heat source deep in the atmosphere. The red color of the spot appears to be caused by impurities that absorb ultraviolet and violet light, such as sulfur or phosphorus compounds.
TOP OF PAGE
COMET SHOEMAKER-LEVY
X
Comet Bombardment of Jupiter, 1994 (below). Fragments of Comet Shoemaker-Levy 9 collided with Jupiter between July 16 and July 22, 1994, stirring up the planet's atmosphere and enabling scientists on the earth to gain rare and revealing telescopic access to the planet. The comet had broken into 21 large fragments on July 8, 1992, when it ventured too close to Jupiter. Trapped by Jupiter's stong gravitational pull, these fragments bombarded the planet at speeds of about 210,000 km/hr (130,000 mph). This image, taken by the National Aeronautics and Space Administration's Hubble Space Telescope, reveals the impact sites (dark spots near the center of the image) created by two of the comet fragments. Space Telescope Science
In 1994 the comet Shoemaker-Levy 9 provided a unique opportunity to study the energy balance and circulation of Jupiter's atmosphere. The comet was torn apart as it entered Jupiter's gravitational field, and the resulting fragments collided with Jupiter's upper atmosphere at speeds of up to 216,000 km/h (134,000 mph). The collisions generated huge explosions in Jupiter's stratosphere that were observed on the earth by a worldwide net of ground-based telescopes and by the Hubble Space Telescope. Cameras aboard the Galileo spacecraft, which was near Jupiter at the time, also recorded this event. Jupiter's atmosphere responded to the impact of Shoemaker-Levy in four phases. As the fragments entered Jupiter's upper atmosphere, they left a bright meteor trail that lasted somewhat less than a minute, which was followed by an explosion that ejected a rapidly expanding cloud of material about 3000 km above Jupiter's cloud layer. When this material fell back into Jupiter's stratosphere, it generated shock waves and discharged enough energy to heat a local area several thousand kilometers in diameter from its normally frigid -100° C (about -148° F) to more than 700° C (about 1300° F). The resulting debris cooled and formed a dark layer in Jupiter's stratosphere that slowly settled into the deeper atmosphere. Winds then swept the debris around the planet and removed all trace of the event within months. The data collected during this event will be analyzed by planetary and atmospheric scientists for years to come.
TOP OF PAGE
JUPITER'S MAGNETOSPHERE
XI
The thick belt of liquid metallic hydrogen created by the high pressures and temperatures deep within Jupiter circulates in convection cells. The circulation of the metallic hydrogen generates electrical currents. These electrical currents create a large magnetic field that is quite strong even as far from the planet as the orbits of the Galilean moons, Jupiter's four largest satellites, a million kilometers away. Beyond the Galilean moons, charged particles emitted by the sun greatly distort the weak outer envelope of the field, pushing it in on the side facing the sun and dragging it out in a long tail on the opposite side. Closer to Jupiter, within the orbits of the Galilean moons, Jupiter's strong field traps the charged particles. The entire region of particle-field interactions is known as the magnetosphere. Particles that are trapped by the strong inner field of Jupiter's magnetosphere move in helical, or spiral, paths along the magnetic field lines toward the poles of Jupiter's field. Since the magnetic field is more concentrated near the poles, the particles frequently collide with each other and with molecules in Jupiter's upper atmosphere over the poles. These collisions create auroras that are similar to the earth's aurora borealis and aurora australis-the northern lights and southern lights.
TOP OF PAGE
SATELLITES AND RINGS OF JUPITER
XII
Scientists announced on September 15, 1998, that data from the Galileo spacecraft had helped confirm
the
composition of Jupiter's rings. They are composed of dust thrown off
Jupiter's four innermost moons as a result of collisions with asteroids,
comets, and other material. The two-part outer ring, known as the gossamer
ring, is made up of dust from Amalthea and Thebe, while the smaller
main ring is made up of dust from Adrastea and Metis. The cloudlike
halo is comprised of dust pulled close to Jupiter by the planet's powerful
electromagnetic field. NASA/JPL/Caltech Jupiter Rings View from IO's Moon Video- MOV 2.5 MB
Jupiter is encircled by at least 16 satellites and a series of thin rings that appear to be the remnants of a disk similar to the planetary disk that encircled the sun in the early stages of the solar system, eventually becoming the planets. For this reason, Jupiter is of much interest to planetary scientists and others who are concerned with the formation of planetary systems. The United States spacecraft Voyager 1 obtained images of a ring of debris concentrated in the equatorial plane of Jupiter. The ring has no well-defined inner boundary, and material appears to be slowly spiraling into the planet. The sharply defined outer edge is located 128,500 km (8000 mi), or about twice the radius of Jupiter, from the center of the planet. In 1998 astronomers at the University of Colorado at Boulder discovered a much less dense and much larger ring of dust around Jupiter. This diffuse ring begins where the inner ring ends and extends to about 350,000 km (about 220,000 mi) from the center of the planet. The particles in the outer ring orbit Jupiter in a clockwise direction as seen from Jupiter's north pole-the opposite direction as the planet's other ring and most of its moons.
 |
EA major part of the Galileo mission to Jupiter was a detailed study of the four Galilean moons, from bottom left, Ganymede, Callisto, Io, and Europa. Some of the mission's most intriguing findings regarded the surface of Europa, top left, which some astronomers believe is a fractured ice crust overlying a liquid-water ocean. Enlarged black-and-white and color-enhanced images of one fracture, top right, reveal what is probably pure water ice at the ridge crests (in red) with darker material (blue) in between. The cracks appear to have been created when the ice sheets pressed against each other, piling broken ice in ridges on either side, like ice ridges found on Earth. Courtesy of German Aerospace Center, Berlin |
The Voyager investigators found four small moons revolving about Jupiter at average distances of 127,960, 128,980, 181,300, and 221,900 km (79,510, 80,140, 112,700 and 137,900 mi). These moons are Metis, Adrastea, Amalthea, and Thebe, respectively. They are small, dark, irregularly-shaped moons. Amalthea is 135 km (84 mi) across its largest dimension, and the other moons are 10 to 50 km in diameter. Astronomers conclude that the two innermost satellites, Metis and Adrastea, which orbit Jupiter at the outer edge of the inner rings, sweep up material in their paths and thereby act as "shepherds" to keep the outer edge of the ring sharp.
TOP OF PAGE
THE GALILEAN MOONS
XIII
Jupiter is encircled by at least 16 satellites. As is the case of the planets and the sun, the closer a moon is to Jupiter, the more dense it is. Planetary scientists believe that these parallel trends reveal much about how the planets and the solar system formed and evolved over the intervening ages. The innermost satellites, Io and Europa, which orbit Jupiter at 421,000 and 671,000 km (262,000 and 417,000 mi), are dense and rocky like Mercury, Venus, Earth, and Mars, the innermost planets of the solar system. Ganymede and Callisto, at greater distances from Jupiter-1,070,000 and 1,883,000 km (660,000 and 1,117,000 mi)-are composed of lower density materials and are similar to the outer planets, especially Neptune and Uranus.
Two additional families of small satellites are located in inclined elliptical orbits at large distances from Jupiter. Leda, Himalia, Lysithea, and Elara orbit at average distances of about 11 million km (about 6,600,000 mi), and Ananke, Carme, Pasiphae, and Sinope orbit at average distances of about 21 to 23 million km (about 13 to 14 million mi). The 12 inner satellites and the rings revolve about Jupiter in the same direction that the planet rotates on its axis, but the four outermost satellites revolve in the opposite direction. This suggests that the outermost group could be fragments of two larger bodies that collided and destroyed each other.
TOP OF PAGE
MOONS AND SATELLITES
XIV
Two additional families of small satellites are located in inclined elliptical orbits at large distances from Jupiter. Leda, Himalia, Lysithea, and Elara orbit at average distances of about 11 million km (about 6,600,000 mi), and Ananke, Carme, Pasiphae, and Sinope orbit at average distances of about 21 to 23 million km (about 13 to 14 million mi). The 12 inner satellites and the rings revolve about Jupiter in the same direction that the planet rotates on its axis, but the four outermost satellites revolve in the opposite direction. This suggests that the outermost group could be fragments of two larger bodies that collided and destroyed each other.
The following four moons are closest to jupiter.
METIS
Metis was a Titaness who was the first wife of Zeus (Jupiter).
Metis ( "MEE tis" ) is the innermost of Jupiter's known satellites:
orbit: 128,000 km from Jupiter
diameter: 40 km
mass: 9.56e16 kg
Discovered by Synnott in 1979 (Voyager 1). Metis and Adrastea
lie within Jupiter's main ring. They may be the source of the material
comprising the ring. Small satellites within a planet's rings are
sometimes called "mooms".
TOP OF PAGE
ADRASTEA
Adrastea, the distributor of rewards and punishments,
was the daughter of Jupiter and Ananke.
Adrastea ("a DRAS tee uh") is the second of Jupiter's known satellites:
orbit: 129,000 km from Jupiter
diameter: 20 km (23 x 20 x 15)
mass: 1.91e16 kg
Discovered by graduate student David Jewitt (working under Danielson)
in 1979 (Voyager 1). Metis and Adrastea orbit inside the synchronous
orbit radius and inside the Roche limit. They may be small enough to
avoid tidal disruption but their orbits will eventually decay. Adrastea
is one of the smallest moons in the solar system.
TOP OF PAGE
AMALTHEA
Amalthea was the nymph who nursed the
infant Jupiter with goat's milk.
Amalthea ("am al THEE uh") is the third of Jupiter's known satellites:
orbit: 181,300 km from Jupiter
diameter: 189 km (270 x 166 x 150)
mass: 7.17e18 kg
Discovered by Barnard September 9, 1892 using the 36 inch (91 cm)
refractor at Lick Observatory. Amalthea was the last moon to be
discovered by direct visual observation (as opposed to photography).
Amalthea and Himalia are Jupiter's fifth and sixth largest moons; they
are about the same size but only 1/15 the size of next larger one, Europa.
Like most of Jupiter's moons, Amalthea rotates synchronously; its long
axis is pointed toward Jupiter. Amalthea is the reddest object in the solar
system. The reddish color is apparently due to sulfur originating from Io.
Its size and irregular shape imply that Amalthea is a fairly strong, rigid
body. Its composition is probably more like an asteroid's than like the
Galilean moons. Like Io, Amalthea radiates more heat than it receives
from the Sun (probably due to the electrical currents induced by
Jupiter's magnetic field).
TOP OF PAGE
THEBE
Thebe was a nymph, daughter of the river god Asopus.
Thebe ("THEE bee") is the fourth of Jupiter's known satellites:
orbit: 222,000 km from Jupiter
diameter: 100 km (100 x 90)
mass: 7.77e17 kg
Discovered by Synnott in 1979 (Voyager 1). The image above shows
Thebe's leading side which has three or four large (compared to
Thebe's size) craters. The image at left shows the trailing side.
TOP OF PAGE
MOONS AND SATELLITES
XV
The following four moons are called Galilean Moons.
This is because of their large size compared to the
other smaller Moons of Jupiter.
IO
Io was a maiden who was loved by Zeus (Jupiter) and transformed
into a heifer in a vain attempt to hide her from the jealous Hera.
Io ( "EYE oh" ) is the fifth of Jupiter's known satellites and the third largest;
it is the innermost of the Galilean (Large) moons. Io is slightly larger than Earth's Moon.
orbit: 422,000 km from Jupiter
diameter: 3630 km
mass: 8.93e22 kg
The pronunciation "EE oh" is also acceptable.
Discovered by Galileo and Marius in 1610.
The Voyager 1 spacecraft launched by the United States National Aeronautics and Space Administration (NASA) photographed both hemispheres of Io, the innermost moon of Jupiter, in 1979. The hemisphere shown at below always faces Jupiter because Io's period of revolution around the planet is equal to its rotation around its own axis. The moon's colors depict its many volcanoes and the large lava flows and sulphur-dioxide snow resulting from Io's tremendous volcanic activity. During the three months between the photos of Io taken by Voyager 1 and Voyager 2, the surface of the moon changed dramatically-some volcanos stopped erupting while previously dormant volcanos became active.
Photo Researchers, Inc.
Beyond the rings and small inner satellites are Jupiter's famous Galilean moons-the bodies first observed by Galileo. These four satellites are much larger than Jupiter's other satellites and range in size from the earth's moon to the planet Mercury. The Galilean moons orbit Jupiter at distances ranging from 421,000 to 1,883,000 km (262,000 to 1,170,000 mi).
 |
Io's
Pele volcano stands out sharply in photographs taken by the Voyager
1 spacecraft in April 1979, inset, top left, the Voyager 2 spacecraft
in July 1979, inset, bottom left, and the Galileo spacecraft in
June 1996, center. In July 1998 scientists announced that some
of Io's volcanoes spew lava onto the surface at temperatures possibly
as high as 1700° C (3100° F). These are the hottest surface temperatures
ever recorded in the solar system outside those recorded on the
surface of the sun. NASA/JPL/Caltech |
 
|
Io
has an amazing variety of terrains: calderas up to several kilometers
deep, lakes of molten sulfur (Bottom left), mountains which are
apparently NOT volcanoes (Top left) extensive flows hundreds of
kilometers long of some low viscosity fluid (some form of sulfur?),
and volcanic vents. Sulfur and its compounds take on a wide range
of colors which are responsible for Io's variegated appearance.
Analysis of the Voyager images led scientists to believe that
the lava flows on Io's surface were composed mostly of various
compounds of molten sulfur. However, subsequent ground-based infra-red
studies indicate that they are too hot for liquid sulfur. One
current idea is that Io's lavas are molten silicate rock. Recent
HST observations indicate that the material may be rich in sodium.
Or there may be a variety of different materials in different
locations. This is a close up view of Io's northern hemisphere.
The central feature has been named Loki Patera. The large dark area might be a lake of liquid sulphur with a raft of solid sulphur inside. (left) |
 |
Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede. Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases. Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out. However new evidence states that is subjected to tidal flexing. Whereby gravitational forces are so great from Jupiter and it other moons that they pull and tug on the moon and it flattens becoming eliptical at the equator. Acting like a machine it produces a molten rock surrounding the core generating heat and volcanic activity. |
MOONS AND SATELLITES
XVI
EUROPA
Europa was a Phoenician princess abducted to Crete by Zeus,
who had assumed the form of a white bull, and by him the
mother of Minos.
Europa ("yoo ROH puh") is the sixth of Jupiter's known satellites and
the fourth largest; it is the second of the Galilean moons. Europa
is slightly smaller than the Earth's Moon.
orbit: 670,900 km from Jupiter
diameter: 3138 km
mass: 4.80e22 kg
 |
Europa
is the fourth largest satellite of Jupiter. It is by far the far
the most interesting amoug Jupiters moons. Europa's streaked surface
resembles frozen seas at the poles of the earth. Scientists studying
data sent from the Galileo spacecraft believe liquid water may
exist underneath Europa's icy crust. Discovered by Galileo and Marius in 1610. Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock. Unlike Io, however, Europa has thin outer layer of ice. Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core. But Europa's surface is not at all like anything in the inner solar system. It is exceedingly smooth: few features more than a few hundred meters high have been seen. The prominent markings seem to be only albedo features or very low relief. |
There are very few craters on Europa; only three craters larger than 5 km in diameter have been found. This would seem to indicate a young and active surface. However, the Voyagers mapped only a fraction of the surface at high resolution. The precise age of Europa's surface is an open question.
The images of Europa's surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.
Europa's most striking aspect is a series of dark streaks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter material. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.
Recent observations with HST reveal that Europa has a very tenuous atmosphere (1e-11 bar) composed of oxygen. Of the 61 moons in the solar system only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.
The Voyagers didn't get a very good look at Europa. But it is a principal focus of the Galileo mission. images from Galileo's first two close encounters with Europa seem to confirm earlier theories that Europa's surface is very young: very few craters are seen, some sort of activity is obviously occurring. There are regions that look very much like pack-ice on polar seas during spring thaws on Earth. The exact nature of Europa's surface and interior is not yet clear but the evidence is now strong for a subsurface 'ocean'.
Galileo has found that Europa has a weak magnetic field (perhaps 1/4 of the strength of Ganymede's). And most interestingly, it varies periodically as it passes thru Jupiter's massive magnetic field.
This is very strong evidence that there is a conducting material beneath Europa's surface,
most likely a salty ocean. The most interesting question is whether life lives in that ocean.
TOP OF PAGE
MOONS AND SATELLITES
XVII
GANYMEDE
Ganymede was a Trojan boy of great beauty whom
Zeus carried away to be cup bearer to the gods.
Ganymede ("GAN uh meed") is the seventh and largest of Jupiter's
known satellites. Ganymede is the third of the Galilean (Large) moons.
Ganymede is largest moon of all in our solar system.
orbit: 1,070,000 km from Jupiter
diameter: 5262 km
mass: 1.48e23 kg
Image (above) shows an entire hemisphere of Ganymede. The prominent dark region, called Galileo Regio, is about 3,200 km in diameter. The bright spots are relative recent impact craters. Part of the Galileo Regio may be covered with a bright frost.
Discovered by Galileo and Marius in 1610.
Ganymede is the largest satellite in the solar system. It is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto.
Before the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice surface (and that Titan and Triton were similar). Preliminary indications from the Galileo data now suggest that Callisto has a uniform composition while Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfur core surrounded by a rocky silicate mantle with a icy shell on top. In fact, Ganymede may be similar to Io with an additional outer layer of ice.
Ganymede's surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).
Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has been found recently by HST (note that this is definitely NOT evidence of life).
Similar ridge and groove terrain is seen on Enceladus, Miranda and Ariel. The dark regions are similar to the surface of Callisto.
Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters both overlay and are cross cut by the groove systems indicating the the grooves are quite ancient, too. Relatively young craters with rays of ejecta are also visible (left).
Unlike the Moon, however, the craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow over geologic time and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests (right). Galileo's first flyby of Ganymede discovered that Ganymede has its own magnetosphere field embedded inside Jupiter's huge one. This is probably generated in a similar fashion to the Earth's: as a result of motion of conducting material in the interior.
TOP OF PAGE
MOONS AND SATELLITES
XVIII
CALLISTO
Callisto was a nymph, beloved of Zeus and hated by Hera. Hera
changed her into a bear and Zeus then placed her in the sky as
the constellation Ursa Major.
Callisto ("ka LIS toh") is the eighth of Jupiter's known satellites and
the second largest. It is the outermost of the Galilean moons.
orbit: 1,883,000 km from Jupiter
diameter: 4800 km
mass: 1.08e23 kg
Discovered by Galileo and Marius in 1610.
Callisto is only slightly smaller than Mercury but only a third of its mass.
Callisto has a very tenuous atmosphere composed of carbon dioxide.
Galileo has detected no evidence of a magnetic field on Callisto.
 |
This image (left) of an chain of craters on Callisto is 620 kilometers long. The largest crater is 40 kilometers across. This is the longest of 12 or so such chains on Callisto. The chain probably formed from the collision of a comet that was tidally disrupted during close passage of Jupiter, such as the comet Shoemaker-Levy 9. |
Unlike Ganymede, Callisto seems to have little internal structure; However there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.
Callisto's surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.
The largest craters are surrounded by a series of concentric rings which look like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla (right). 4000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples are Callisto's Asgard (left), Mare Orientale on the Moon and Caloris Basin on Mercury.
Like Ganymede, Callisto's ancient craters have collapsed. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury. Detailed images from Galileo (left) show that, in some areas at least, small craters have mostly been obliterated. This suggests that some processes have been at work more recently, even if its just slumping.
Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists; (it may be related to the orbital and tidal evolution of Ganymede). "Simple" Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.
TOP OF PAGE
JUPITER'S OUTER 8 MOONS
XIX
LEDA
Leda was queen of Sparta and the mother, by Zeus in
the form of a swan, of Helen and Pollux.
Leda ("LEE duh") is the ninth of Jupiter's known satellites and the smallest:
orbit: 11,094,000 km from Jupiter
diameter: 16 km
mass: 5.68e15 kg
Discovered by Kowal in 1974. Leda, Ananke, and Sinope are
among the smallest moons in the solar system.
TOP OF PAGE
HIMALIA
Himalia was a nymph who bore three sons of Zeus (Jupiter).
Himalia ("hih MAL yuh") is the tenth of Jupiter's known satellites:
orbit: 11,480,000 km from Jupiter
diameter: 186 km
mass: 9.56e18 kg
Discovered by Perrine in 1904.
Unlike the inner satellites, the orbits of Leda, Himalia, Lysithea and
Elara are significantly inclined to Jupiter's equator (about 28 degrees).
TOP OF PAGE
LYSITHEA
Lysithea was a daughter of Oceanus and one of Zeus' lovers.
Lysithea Jupiter X Lysithea ("ly SITH ee uh") is the eleventh of
Jupiter's known satellites:
orbit: 11,720,000 km from Jupiter
diameter: 36 km
mass: 7.77e16 kg
Discovered by Nicholson in 1938.
TOP OF PAGE
ELARA
Elara was the mother by Zeus of the giant Tityus.
Elara ("EE lar uh") is the twelfth of Jupiter's known satellites:
orbit: 11,737,000 km from Jupiter
diameter: 76 km
mass: 7.77e17 kg
Discovered by Perrine in 1905.
Leda, Himalia, Lysithea and Elara may be remnants of a
single asteroid that was captured by Jupiter and broken
TOP OF PAGE
ANANKE
Ananke was the mother of Adrastea, by Jupiter.
Ananke ("a NANG kee") is the thirteenth of Jupiter's known satellites:
orbit: 21,200,000 km from Jupiter
diameter: 30 km
mass: 3.82e16 kg
Discovered by Nicholson in 1951.
Ananke, Carme, Pasiphae and Sinope have unusual but similar orbits.
TOP OF PAGE
CARME
Carme was the mother, by Zeus of Britomartis, a Cretan goddess.
Carme ("KAR mee") is the fourteenth of Jupiter's known satellites:
orbit: 22,600,000 km from Jupiter
diameter: 40 km
mass: 9.56e16 kg
Discovered by Nicholson in 1938.
Ananke, Carme, Pasiphae and Sinope are especially unusual in
that their orbits are retrograde.
TOP OF PAGE
PASIPHAE
Pasiphae was the wife of Minos and mother, by a white bull, of the Minotaur.
Pasiphae Jupiter VIII Pasiphae ("pah SIF ah ee") is the fifteenth
of Jupiter's known satellites:
orbit: 23,500,000 km from Jupiter
diameter: 50 km
mass: 1.91e17 kg
Discovered by P. Melotte in 1908.
Ananke, Carme, Pasiphae and Sinope have orbits highly inclined
to Jupiter's equator (about 150 degrees).
TOP OF PAGE
SINOPE
Sinope was a woman said to have been unsuccessfully (!) courted by Zeus.
Sinope Jupiter IX Sinope ("sah NOH pee") is the outermost of Jupiter's known confirmed satellites:
orbit: 23,700,000 km from Jupiter
diameter: 36 km
mass: 7.77e16 kg
Discovered by Nicholson in 1914.
Ananke, Carme, Pasiphae and Sinope may be remnants of a single
asteroid that was captured by Jupiter and broken up.
TOP OF PAGE
SPACECRAFT MISSIONS
XX
An era of close observation of Jupiter began with spacecraft explorations by NASA. The first such spacecraft launched toward Jupiter was Pioneer 10, launched in March 1972. Pioneer 10 was followed in April 1973 by Pioneer 11. These simple spinning spacecraft carried instruments that provided excellent information on Jupiter's gravitational field, magnetosphere, and upper stratosphere.
The next spacecraft explorations of Jupiter were the Voyager 1 and Voyager 2 missions of 1979, also launched by NASA. The Voyager craft were designed to maintain a stable orientation in space, so that onboard cameras and other imaging instruments could be used to map Jupiter in ultraviolet (UV), visible, and infrared (IR) light. The visual images provided detailed maps of Jupiter's cloud deck, the IR data provided information about how heat escaped and the relative abundances of elements and molecules in Jupiter's upper atmosphere, and the UV data provided information about the interaction of Jupiter's magnetic field with the solar wind and the creation of auroras.
In 1990 NASA launched the spacecraft Ulysses from an orbiting space shuttle to study the sun over its poles. To do this, astronomers used Jupiter as a gravitational slingshot to put Ulysses in the proper flight path. While flying by Jupiter in 1992 Ulysses took measurements of Jupiter's magnetosphere and gravitational field. The Ulysses craft will again measure Jupiter's magnetic field during another pass in 1998. In 1989, prior to the launch of Ulysses, NASA launched the spacecraft Galileo.
The Galileo craft took a slower route to Jupiter, reaching the planet after six years in 1995. Upon achieving a stable orbit about Jupiter, the spacecraft launched a remote probe toward the planet. The probe plunged through Jupiter's opaque cloud deck, and the orbiting Galileo spacecraft relayed information gathered by the probe to Earth. The probe transmitted its readings until it reached a depth in Jupiter's atmosphere where the pressure was twenty Earth atmospheres, at which point high temperatures caused its transmitter to fail when the probe was crush by the intense gravity. Galileo's remote probe provided direct measurement of the abundances of the elements in Jupiter's outer atmosphere, the strength of its winds, and other information about Jupiter's atmosphere within the opaque cloud deck. The Galileo spacecraft remains in orbit about Jupiter, gathering and transmitting information about its magnetic field, its atmosphere, and its moons. The Galileo spacecraft is scheduled to continue relaying information about Jupiter through late 1997.
A Europa Orbitor due to be launched in 2008. Delayed until 2010.
The probe launched at the moon Europa will not just fly by the moon. It will actually go into orbit around the moon and map it's surface. It will also be equipped to measure whether an ocean lies beneath it's surface and how deep they might be. Over a period of 17 months it will arrive and begin to operate orbiting the Europa. The radiation from Jupiter will be so intense and the heat so great that an astronaut would be killed from radiation poisoning within miinutes even with sheilding. The orbitor will only last for 30 days in this environment.
TOP OF PAGE
DIRECTIONS FOR FUTURE STUDIES OF JUPITER
XXI
Astronomers, planetary scientists, and exobiologists are interested in studying Jupiter further to learn about its origins, the origins of the solar system, and perhaps even the origins of life. In order to do this, it will be necessary to closely observe Jupiter from many directions over a period of several years. One proposal for such a project is to place a long-lived general-purpose transmitter in high orbit about Jupiter and use small, relatively inexpensive single-purpose probes to gather specific pieces of information. The transmitter would relay the information gathered by the smaller, short-lived craft. The small craft could be used to map Jupiter's magnetic field, sample its atmosphere, or perform other tasks, including tasks with a high probability that the probes will not survive, for example sampling the inner atmosphere. This scheme allows more flexibility, timeliness, and cost-effectiveness than previous spacecraft explorations: as data comes in, new probes can be constructed and launched in less time and at less expense than required to send larger spacecraft such as Galileo.
TOP OF PAGE
RECOMMENDED READING
XXII
Destination: Jupiter (Paperback)
Jupiter : The Planet, Satellites and Magnetosphere
(Cambridge Planetary Science) (Hardcover)
The Planets (Hardcover)
Jupiter : A Novel (The Grand Tour) (Paperback)
Fantasy Adventure by Ben Bova
WEB LINKS
XXIII
COMING SOON
TOP OF PAGE
|
T
H E
P L A N E T S
2 0 0 1
|
||
|
|
Got
a Question? Send email to
Planets@quasart.50megs.com
© 2001 QuasArt Web Designs. All Rights Reserved. Privacy Statement |
|
Planets | Sun | Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto