Jupiter Moons
Jupiter, the largest of the Jovian planets, reigns supreme throughout the
solar
system. Named after the Roman god Jove, the ruler of Olympus; "Jupiter
is
the fifth planet from the sun and is also the largest planet in the
Earth's
solar system. It is 318 times moremassive than Earth and is two
thirds of the
planetary mass in the solar system. Jupiter's surface, unlike
earth, is gaseous
and not a solid. It is about 90% hydrogen and 10% helium
with traces of methane,
ammonia, water and rock. Jupiter's interior is very
similar to the Sun's
interior but with a far lower temperature."(Columbia)
However, it is still
unknown for certain, but Jupiter is believed to have a
core of liquid metallic
hydrogen. This exotic element can only be achieved at
a pressure greater than 4
million bars. Jupiter radiates more energy in space
than it receives from the
sun. "Jupiter's orbit lies beyond the asteroid belt
at a mean distance of
c.483 million mi (773 million km) from the sun; its
period of revolution is
11.86 years." (Seeds) In order from the sun it is
the first of the Jovian
planets (Jupiter, Saturn, Uranus, and Neptune), very
large, massive planets of
relatively low density, having rapid rotation and a
thick, opaque atmosphere.
"Jupiter has a diameter of 88,679 mi (142,800 km),
more than 11 times that
of the earth. Its mass is 318 times that of the earth
and about 2 1/2 times the
mass of allother planets combined." (Columbia) A
measurement of the
diameter of Jupiter determined the planet's polar
flattening. The flattening of
Jupiter was revealed by Pioneer to be
slightly greater than that derived from
the best Earth-based measurements.
"The diameter of the planet was measured
at a pressure of 800 mbar near the
cloud tops (a bar is roughly equal to the
pressure of 1 atm of Earth). Its
polar diameter is 133,540 km (82,980 miles) and
its equatorial diameter
is142, 796 kilometers (88,732 miles)." (Seeds)
These values were
established by the timing of the occultation of the spacecraft
by Jupiter.
Thus, Jupiter is nearly 20 times more fattened than Earth,
principally
because of its non-solid state and its higher rate of rotation. The
average
density of Jupiter, calculated from its mass and volume, was confirmed
as
1.33 gm/cm^3 (the density of water is 1). The atmosphere of Jupiter
is
composed mainly of hydrogen, helium, methane, and ammonia. "It appears
the
atmosphere is divided into a number of light and dark bands parallel to
its
equator and shows a range of complex features, including an ongoing storm
called
the Great Red Spot, located in its southern hemisphere and measuring
16,150 mi
long by 8,700 mi wide (26,000 by 14,000 km)." (Columbia) This Great
Red
Spot is still present in Jupiter's atmosphere, more than 300 years
later. It is
now known that it is a vast storm, spinning like a cyclone.
Unlike a low-
pressure hurricane in the Caribbean Sea, however, the Red Spot
rotates in a
counterclockwise direction in the southern hemisphere, showing
that it is a
high-pressure system. "Winds inside this Jovian storm reach
speeds of about
270 mph. The Red Spot is the largest known storm in the
Solar System. With a
diameter of 15,400 miles, it is almost twice the size of
the entire Earth and
one-sixth the diameter of Jupiter itself." (Fimmel) The
Great Red Spot was
first detected by Robert Hooke in 1664. Jupiter has no
solid rock surface. One
theory pictures a gradual transition from the outer
ammonia clouds to a thick
layer of frozen gases and finally to a liquid or
solid hydrogen mantle.
"The Spot and other markings of the atmosphere also
provide evidence for
Jupiter's rapid rotation, which has a period of
about 9 hr 55 min. This rotation
causes a polar flattening of over 6%."
(Columbia) The temperature of
Jupiter ranges from about -190° F (-124°C)
for the visible surface of the
atmosphere, to 9° F (-13° C) at lower cloud
levels; localized regions reach as
high as 40° F (4° C) at still lower cloud
levels near the equator. Jupiter
radiates about four times as much heat
energy as it receives from the sun,
suggesting an internal heat source. This
energy is thought to be due in part to
a slow contraction of the planet.
Jupiter is also characterized by intense
non-thermal radio emission; in the
15-m range it is the strongest radio source
in the sky. Jupiter has a simple
ring system that is composed of an inner halo,
a main ring and a Gossamer
ring. To the Voyager spacecraft, the Gossamer ring
appeared to be a single
ring, but Galileo imagery provided the unexpected
discovery that Gossamer is
really two rings. One ring is embedded within the
other. The rings are very
tenuous and are composed of dust particles kicked up
as interplanetary
meteoroids smash into Jupiter's four small inner moons Metis,
Adrastea,
Thebe, and Amalthea. Many of the particles are microscopic in size.
"The
innermost halo ring is toroidal in shape and extends radially from
about
92,000 kilometers (57,000 miles) to about 122,500 kilometers (76,000
miles)
from Jupiter's center. It is formed as fine particles of dust from the
main
ring's inner boundary 'bloom' outward as they fall toward the
planet."
(A.U.R.A.) The main and brightest ring extends from the halo
boundary out to
about 128,940 kilometers (80,000 miles) or just inside the
orbit of Adrastea.
Close to the orbit of Metis, the main ring's
brightness decreases. "The two
faint Gossamer rings are fairly uniform in
nature. The innermost Amalthea
Gossamer ring extends from the orbit of
Adrastea out to the orbit of Amalthea at
181,000 kilometers (112,000
miles) from Jupiter's center." (Hamilton) The
fainter Thebe Gossamer ring
extends from Amalthea's orbit out to about Thebe's
orbit at 221,000
kilometers (136,000 miles). Jupiter's rings and moons exist
within an intense
radiation belt of electrons and ions trapped in the planet's
magnetic field.
"These particles and fields comprise the Jovian
magnetosphere or magnetic
environment, which extends 3 to 7 million kilometers
(1.9 to 4.3 million
miles) toward the Sun, and stretches in a windsock shape at
least as far as
Saturn's orbit - a distance of 750 million kilometers (466
million miles)."
(Seeds) Jupiter has a huge magnetic field, much stronger
than Earth's called
the magnetosphere. The magnetosphere is not a true sense a
perfect sphere. It
is highly flattened due to the rapid rotation of Jupiter.
This magnetic
field causes phenomenon such as strong lightening and even an
aurora similar
to earth's aurora borealis. "Plasma flows in the dayside
outer magnetosphere;
the plasma rotates with the planet every 10 hours."
(Hamilton) Jupiter,
unlike earth, has three distinct weather-producing zones or
a troposphere.
They are believed to contain Ammonia ice, ammonium hydrosulfide,
and water
and ice. "In the apparent or uppermost atmosphere, ammonia ice
crystals
thrive in a temperature of about 150 degrees Kelvin." (A.U.R.A.)
Most
astronomers theorize that the next level of the atmosphere is primarily
made
up of Ammonium hydrosulfide crystals in a temperature of 200
degrees
Kelvin. It is also theorized that the third and final level
before the liquid
metallic hydrogen is a layer of liquid ammonia and water
droplets. Jupiter's
atmosphere is also plagued by high velocity winds that
move in wide bands. These
winds blow in opposite directions along the
latitude of the planet. Because of
chemical reactions and differences, they
can be seen wrapping around Jupiter in
colorful bands. The light colored
bands are called zones and the dark colored
bands are called belts. It is not
known whether the belts and zones are
permanent; they have not changed in
eighty years of observance. "One theory
is that the jet stream at the
belt-zone boundaries are linked to circulation
patterns deep in the liquid
interior" (Seeds). At least sixteen natural
satellites are known to orbit
Jupiter. Twelve of Jupiter's moons are relatively
small and seem to have been
more likely captured than to have been formed in
orbit around Jupiter. They
are conveniently divided into three groups. The four
largest- -Io, Europa,
Ganymede, and Callisto--were discovered by Galileo in
1610, shortly after
he invented the telescope, and is known as the Galilean
satellites. "These
large moons are believed to have accreted as part of the
process by which
Jupiter itself formed." (Fimmel) "Accretion is the
accumulation of dust and
gas into larger bodies." (Astronomy) Ganymede is
the largest satellite in the
solar system; with a diameter of 3,268 mi (5,262
km), it is larger than
the planet Mercury. In 1979 Io was observed by Voyager I
and II and was found
to have several active volcanoes actually in eruption
during the spacecraft
flyby. Io is the innermost of the Galilean satellites.
Io's size and
density is very similar to our own Moon, and it the most dense of
the
Galilean satellites. "When the Galileo spacecraft flew by Io in
December
1995; it discovered that Io has an iron inner core. A
high-altitude
ionosphere was also revealed by the Galileo flyby. In 1979, the
Voyager
spacecraft flew through the Jovian system, and one of the most
exciting
discoveries made by the Voyager spacecraft was the presence of
active erupting
volcanoes on Io." (Helicon). It was discovered that Io was
the most
volcanically active planet in the solar system, even more active
than the Earth.
"The volcanism on Io is due to the internal heat generated by
the tidal
tug-of-war between Jupiter, Europa and Ganymede."(Helicon). The
largest
volcano on Io is named Pele. Pele was the first volcano discovered on
Io and it
was actively erupting with a plume an astonishing 300 km high at
the time of its
discovery. The Voyager spacecraft observed eleven active
volcanoes during their
flyby. "Hundreds of volcanic calderas have also been
observed." (Fimmel).
There are no impact craters on Io. Therefore, the
surface of Io is believed to
be younger than a millions years old, and is
continually being resurfaced by
volcanic activity. Also, the surface is very
colorful, covered with red, yellow,
white and orange black markings. The
surface composition on Io consists largely
of sulfur with deposits of frozen
sulfur dioxide. The surface on Io is mostly
flat plains rising no more than
1km. Mountain ranges up to 9 km high have also
been observed. "A torus of
sodium gas along with sulfur ions is spread out
over Io's orbit. This torus
is so large that it has been observed from
Earth." (Io;Helicon) Europa is
a strange looking moon of Jupiter with a
large number of intersecting
features. It is unlike Callisto and Ganymede with
their heavily cratered
crusts. "Europa has almost a complete absence of
craters as well as almost no
vertical relief." (Europa;Helicon). As one
scientist put it, the features
"might have been painted on with a felt
marker" (Seeds). There is a
possibility that Europa may be internally
active due to tidal heating at a
level one-tenth or less that of Io. Models of
Europa's interior show that
beneath a thin 5 km (3 miles) crust of water ice,
Europa may have oceans
as deep as 50 km (30 miles) or more. "The visible
markings on Europa could be
a result of global expansion where the crust could
have fractured, filled
with water and froze." (Europa;Helicon). Ganymede is
the largest moon of the
planet Jupiter, and the largest moon in the Solar
System, 5,260 km/3,270
mi in diameter which is larger than the planet Mercury.
It orbits Jupiter
every 7.2 days at a distance of 1.1 million km/700,000 mi.
" Its surface is a
mixture of cratered and grooved terrain. Molecular
oxygen was identified on
Ganymede's surface in 1994" (Ganymede;Helicon).
"The space probe Galileo
detected a magnetic field around Ganymede in 1996;
this suggests it may have
a molten core." (Hamilton). Galileo photographed
Ganymede at a distance
of 7,448 km/4,628 mi. The resulting images were 17 times
clearer than those
taken by Voyager 2 in 1979, and show the surface to be
extensively cratered
and ridged, probably as a result of forces similar to those
that create
mountains on Earth. "Galileo also detected molecules containing
both carbon
and nitrogen on the surface March 1997. Their presence may indicate
that
Ganymede harbored life at some time" (Hamilton). Callisto is the
eighth of
Jupiter's known satellites and the second largest. It is the outermost
of the
Galilean moons and was discovered by Galileo and Marius in 1610.
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 " (Callisto;Helicon).
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"
(Callisto;Helicon). "The
largest craters are surrounded by a series of
concentric rings that 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 " (Callisto;Helicon). "In
terms of the mass of Earth's Moon,
the masses of the Galilean satellites in
order of distance from Jupiter were
found to be: Io, 1.21; Europa, 0.65;
Ganymede, 2.02; and Callisto, 1.46. The
mass of Io was 23% greater than that
estimated before the Pioneer odyssey. The
density of the satellites decreases
with increasing distance from Jupiter and
was refined as a result of
Pioneer's observations. Io's density is 3.52;
Europa's, 3.28; Ganymede's,
1.95; and Callisto's, 1.63 gm/cm^3. The outer
satellites, because of their
low density, could consist largely of water and
ice. All four satellites were
found to have average daylight surface
temperatures of about-140 C (-220 F) "
(Columbia). A second group is
comprised of the four innermost
satellites--Metis, Adrastea, Amalthea, and Thebe.
Discovered by E. E.
Barnard in 1892, Amalthea has an oblong shape and is 168 mi
(270 km) long.
Metis and Adrastea orbit close to Jupiter's thin ring system;
material
ejected from these moons helps maintain the ring. The final group
consists of
the eight remaining satellites, none larger than c.110 mi (180 km)
in
diameter. "Four of the outer eight satellites located from 14 million
to
16 million mi from Jupiter (22 million-26 million km), have retrograde
motion,
i.e., motion opposite to that of the planet's rotation. The other
four have
direct orbits. It is speculated that all eight might be captured
asteroids"
(Seeds). When it is in the nighttime sky, Jupiter is often the
brightest
"star" in the sky (it is second only to Venus, which is seldom
visible
in a dark sky). The four Galilean moons are easily visible with
binoculars; a
few bands and the Great Red Spot can be seen with a small
astronomical
telescope. Jupiter is very gradually slowing down due to the
tidal drag produced
by the Galilean satellites. How will this effect it and
its moons? We currently
know that the same tidal forces that are slowing
Jupiter down are changing the
orbits of the moons, very slowly forcing them
farther from Jupiter. As
additional data is gathered and technology enables a
new fronitier, only then
will we know the fate of Jupiter. Until then we can
merely speculate it's final
life as a Jovian
planet.
Bibliography
Bibliography The Columbia Encyclopedia, Fifth
Edition. Copyright ©1993,
Columbia University Press. Licensed from
Lernout & Hauspie Speech Products
USA, Inc. Pioneer: First to
Jupiter, Saturn, and Beyond: Chapter 6A Results At
The New Frontier;
Fimmel, Richard O.; Van Allen, James; Burgess, Eric;
09-01-1990 Ganymede;
( The Hutchinson Dictionary of Science ) ; 01-01-1998,
Helicon Publishing
Ltd. 1998. Io ; ( The Hutchinson Dictionary of Science ) ;
01-01-1998,
Helicon Publishing Ltd. 1998. Callisto; ( The Hutchinson Dictionary
of
Science ) ; 01-01-1998, Helicon Publishing Ltd. 1998. Europa; (
The
Hutchinson Dictionary of Science ) ; 01-01-1998, Helicon Publishing
Ltd. 1998.
Seeds, Michael A., Foundations of Astronomy; copyright 1994,
Wadsworth Inc.
Copyright © 1997-1999 by Calvin J. Hamilton. Copyright ©
1998 The Association
of Universities for Research in Astronomy, Inc. Author
not available, Astronomy:
Common Terms in Astronomy. , The New York
Public Library Science Desk Reference,
01-01-1995.