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 (-124C) 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.


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