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
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
“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.”
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
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
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.”
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
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.”
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
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 ”
“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) ”
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
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.
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.
Cite this Jupiter and its Moons
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