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THE COMPLETE COSMOS
Chapter 10: Jupiter
Bigger than the other planets combined, a turbulent gas giant with 16 moons - a voyage through
this mini Solar System.
Outline
Larger than all the other planets combined, Jupiter lies 780 million kilometers from the Sun, has
an almost 12-year orbit, and the shortest planetary day. Eleven Earths are needed to span its
diameter and 1,300 to fill its volume. Jupiter captures Comet Shoemaker-Levy 9. Its fragments
impact the planet.
The twin Voyager spacecraft pass the giant planet in 1979. Detailed images of Jupiter acquired by
the Voyagers, including the Great Red Spot, a tempest three times larger than Earth. Ten years
later, the Galileo probe parachutes into Jupiter's clouds and makes further discoveries before
being crushed by atmospheric pressure. Jupiter's 30,000-degree Celsius core generates more heat
than the planet receives from the Sun - and drives 500 kilometer-per-hour winds. Internal
structure and its magnetic field. Galileo's orbital tour of Jupiter's 16 moons, focusing on the four
majors - Io, the most volcanically-active body known; frozen Ganymede, largest moon in the
Solar System with a surface temperature of minus 150 degrees; Callisto, Ganymede's craterscarred twin; and Europa, icy world with a possible sub-surface ocean that may contain extraterrestrial life. Jupiter and its moons are a mini solar system.
Sub-chapters
Largest Planet
• Jupiter, a giant ball of gas, bigger than all the other planets combined.
• Spinning like a top, Jupiter's short day, orbit and distance from the Sun.
• Jupiter/Earth comparisons: 11 Earths are needed to span Jupiter's diameter, and 1,300 to
fill its volume.
• Jupiter's role as a "cosmic vacuum cleaner", Jovian gravity capturing comets and drawing
them into the planet.
• Brief overview of the Comet Shoemaker-Levy 9 story.
Weather Bands and Storms
• The two Voyager probes, launched in 1977, reach Jupiter two years later.
• Whirling belts of weather, storms racing around the planet, and its most famous blemish,
the Great Red Spot - an anti-cyclone much larger than Earth.
Descent into Hell
• Galileo's circuitous route to Jupiter. The probe's descent through the Jovian clouds buffeted by 500-kilometre-per-hour winds (driven by heat from Jupiter's core), and
eventually crushed by atmospheric pressure.
• Jupiter's composition: 90 per cent hydrogen, the rest mainly helium. The planet's
atmosphere - layer by layer - and its internal structure, down to the rocky core.
• Generation of Jupiter's magnetic field in a layer of liquid metallic hydrogen.
• Jupiter's magnetosphere - after the Sun, the biggest entity in the Solar System.
• Auroral halos around the Jovian poles, created by the solar wind.
Volcanic Io
• Galileo's tour of Jupiter's four largest satellites. First stop, Io - flexed by the pull of
Jupiter and neighboring moons. An extraordinary colored surface and erupting volcanoes.
Ganymede and Callisto
• Ganymede, the Solar System's largest moon with a thick icy crust. The magnetic field
detected by Galileo, suggests an internal structure of slush, rock and ice.
• Callisto, Ganymede's twin, but heavily cratered with a multi-ringed impact basin.
Icy Europa
* Europa, a possible water-world. Is a salty ocean hidden beneath its cracked, icy surface?
* Tidal flexing may warm the waters, making Europa an incubator for life. With the surface ice
so thick, any search for life must await a landing.
Background
The Great Red Spot
A large, reddish oval feature called the Great Red Spot is usually visible in Jupiter's southern
hemisphere - lying within the planet's south tropical zone. Since the spot was first observed in the
mid-17th century, observers have reported many variations in its size and color. At its largest, the
spot was so huge that three Earth's could have fitted side-by-side along its length. In the mid1970s, however, the spot faded from view. At the time of the Voyager flybys in 1979, the Great
Red Spot was only slightly larger than Earth.
The Great Red Spot is a very long-lived storm in the planet's ever changing atmosphere.
Observations of cloud motions in and around the spot show that it spins anti-clockwise,
completing a full revolution in about six days. Furthermore, winds to the north of the spot blow
from the east, those south of the spot blow from the west. Consequently, the Great Red Spot is
rather like a wheel spinning between two surfaces moving in opposite directions, producing a
surprising stable wind pattern. The spot is a high pressure weather system that protrudes above
the surrounding visible cloud tops.
Jupiter's Volcanic Moon
In 1979, the Voyager 1 spacecraft flew past Io, the innermost of Jupiter's four large moons,
uncovering an extraordinary world. The surface displayed a broad range of colors from white,
yellow and orange, to black, which scientists jokingly compared to a cheese and tomato pizza, or
a rotten orange. A major clue to this puzzling appearance was found several days after Voyager's
Jupiter flyby when a scientist noticed a large umbrella-like plume protruding from Io in one
photograph - an erupting volcano.
Careful examination of Io images obtained by both Voyagers 1 and 2 - and the more recent
Galileo spacecraft - have shown that Io's surface is pocked with numerous volcanic vents from
which eruptions occur. These vents appear as black spots typically ten to 50 kilometers in
diameter, many with radiating lava flows. The plumes and fountains of material spewing from
Io's volcanoes rise to heights of 70 to 300 kilometers above the surface. To reach these altitudes,
material must be shot out of the vents at speeds of between 300 and 1000 meters per second.
The plumes from Io's volcanoes contain particles of sulphur and the acrid gas sulphur dioxide.
When this gas is erupted into the vacuum of space it crystallizes into white crystals of sulphur
dioxide frost or snow. It is the sulphur ejected from Io's volcanoes that causes the brilliant colors.
Although normally bright yellow, when heated and suddenly cooled, sulphur can assume a range
of colors from orange to red and black. Io's volcanic vents are not located at the tops of volcanic
peaks, as on Earth, but are more akin to terrestrial geysers, similar to those at Yellowstone Park in
the USA. Both sulphur and sulphur dioxide will be molten at depths of a few kilometers, but as
they are forced up to Io's surface, they are explosively converted from a liquid to a high pressure
gas. The heating of Io's interior is a consequence of the continual squeezing
and flexing of the moon by tidal stresses caused by the gravitational tug-of-war between
Jupiter, on one side, and Europa and Ganymede on the other.
Jupiter's Water-Worlds
At least two of Jupiter's four largest moons are probably water-worlds, with hidden salty oceans
beneath their icy surfaces. One of the moons, Europa, has long been speculated to have a subsurface ocean of slushy or liquid water. Europa's water is kept warm by powerful gravitational
tidal forces exerted by both Jupiter and the moons Io and Ganymede. Although nothing could
survive on Europa's frozen crust, which looks like Arctic pack ice, scientists have postulated that
water-based organisms, probably of a primitive microbial kind, could exist in Europa's hidden
ocean. Measurements have shown that the liquid layer might be up to 100 kilometers deep,
beneath an icy crust several kilometers thick.
More surprising was the discovery in October, 1998, that another Jovian moon, Callisto, may also
have a hidden salty sea. Callisto was previously thought to be a solid sphere of rock and ice. A
liquid ocean on Callisto, however, seems to be the only explanation for data sent back by the
Galileo spacecraft while in orbit around Jupiter. Observations by Galileo have confirmed that
neither Europa nor Callisto have an appreciable internal magnetic field. But evidence from the
spacecraft shows that both moons disturb Jupiter's magnetic field as they pass through it. The
only way this could happen is if electrical currents flowing within the moons induced a magnetic
field. This would require sub-surface layers on both moons that are very good conductors of
electricity. Deep salty liquid oceans on both moons seem to be the only adequate explanation.
Callisto is not subject to the same tidal forces as Europa. So flexing cannot be the reason that
Callisto's sub-surface water - if it exists - doesn't freeze. Explanations could be warming from
radiation, high pressures, eddy currents or the presence of some kind of "anti-freeze" in the form
of salts or ammonia.
There may also be a sub-surface ocean on Ganymede. But Gannymede has an appreciable
internal magnetic field which would obscure any induction effect. So, for the time being, there is
no way of proving that Ganymede has a layer of water beneath the surface.
Links for Further Information
Comprehensive Jupiter page, information on its atmosphere and internal structure, Jupiter's
moons, facts, missions to the planet, recent news and images.
http:// www.windows.umich.edu/cgi-bin/tour.cgi?link=/jupiter/jupiter.html
Good Jupiter page, images of the planet and moons with text links.
http://pds.jpl.nasa.gov/planets/welcome/jupiter.htm
Voyager 1 mission page, including information on the mission, extensive images and FAQs.
http://nssdc.gsfc.nasa.gov/
imgcat/html/mission_page/JP_Voyager_1_page1.html
Voyager 2 mission page, as above.
http://nssdc.gsfc.nasa.gov/imgcat/html/mission_page/JP_Voyager_2_page1.html
JPL's Galileo homepage, including up-to-date information on Galileo's orbital tour, details of the
mission and the craft, background and images of Jupiter and its moons, plus frequently asked
questions.
http://www.jpl.nasa.gov/galileo/
Questions and Activities for the Curious
1. If the Earth were the size of a tennis ball, how big would Jupiter be on the same scale? And
using this scale, how big would the Sun be and how far would be Jupiter from the Sun?
2. Describe the appearance of Jupiter's atmosphere. What kinds of features would you expect to
see through a telescope?
3. The Galileo spacecraft, launched to Jupiter from the Space Shuttle in 1989, took more than six
years to reach its destination. Describe its circuitous route from Earth to Jupiter, and research why
it was necessary to use this route?
4. Discuss the various layers that make up Jupiter's atmosphere and its internal structure. Where is
its magnetic field produced?
5. Imagine there had been a camera aboard Jupiter's Galileo probe as it parachuted through the
outer layers of the planet's atmosphere. Describe what this camera might have seen during the
probe's hour long descent, before it was destroyed.
6. Why do you think astronomers believe that Jupiter does not have a large, iron-rich core, even
though the planet possesses a strong magnetic field?
7. Describe the colorful surface of Jupiter's moon Io, and explain how volcanic eruptions are
responsible for its unusual appearance.
8. Describe the similarities and differences between the four largest moons of Jupiter.