A1.  After the Challenger accident, it was decided to launch Galileo with a small conventional rocket instead of from the shuttle. As a result, it had to circle the sun three times to make close passes by Venus and Earth in order to gain enough speed to reach the outer solar system. Voyager, on the other hand, was launched with the much larger Saturn V rocket on a direct path to Jupiter.

A2.  The Galileo spacecraft consists of both an atmospheric probe and an orbiter. The probe plunged straight into the atmosphere of Jupiter to observe the properties of the atmosphere (temperature, composition, cloud particles, etc.). The orbiter continues to orbit Jupiter, taking observations of the planet and its moons.

A1.  From the outside, the layers are composed primarily of liquid molecular hydrogen, liquid atomic metallic hydrogen, and liquid rock and ice. Metallic hydrogen occurs under very high pressure, when the hydrogen atoms are able to conduct electricity by sharing their electrons.

A2.  The type of solid material available to form a planet depends upon the temperature at that location in the cloud. The temperature of the cloud diminishes with increasing distance from the sun. At Jupiter's distance from the sun, the temperature was low enough that the ices could solidify. Earth is too close to the sun for that to have happened, so Earth is all rocky material while Jupiter's core contains both rock and ice. When ice solidifies, there is a great deal more solid material available to form a planet, since the ices are much more common than rocky material. Hence, Jupiter's core is much larger than Earth.

A3.  Two mechanism of heat generation are present. The gas giants (except Uranus) are still radiating some heat as they continue to slowly collapse. In addition, helium condenses into a liquid and falls toward the interior somewhere in the molecular hydrogen layers. This falling rain gains enough energy as it falls to significantly heat up the gas it falls through. This heat eventually escapes from the surface of the planet, making it warmer than it would otherwise be.

A4.  Jupiter has a core of rock surrounded by ice. Outside the core is a large region of atomic hydrogen which has the unusual property (for hydrogen) of being a good conductor of electricity. For this region it is referred to as metallic hydrogen. Outside this region is a region of more normal molecular hydrogen.

A5.  We can learn about the interior of Jupiter both from the average density of the planet and from measurements of the flattening of the planet, which measures the interior response to Jupiter’s rotation.

A6.  Jupiter’s core is about the same size as Earth’s core, but contains between 3 and 30 times as much material. Obviously, the density of Jupiter’s core is quite a bit higher than ours, because of the greater compression of a more massive planet. Even at the high end of the range of possible masses, the core of Jupiter is still only about 10% of the mass of the whole planet. The additional material was attracted to Jupiter during its formation, because its core had grown large enough for its gravity to be strong enough to attract gas from the surrounding cloud. Earth never made it that far.

A7.  A metal is defined as a material which conducts electricity effectively. Hydrogen becomes electrically conductive under very high pressures. The layer of metallic hydrogen inside Jupiter creates the very strong magnetic field observed around Jupiter

A1.  The bands on Jupiter result from a combination of the effects of rapid rotation (which creates the straight bands of winds from the cooler night side to the hotter day side) and convection (rising elements of hot gas bring material to the surface which makes the clouds visible, while sinking cooler material takes them away again).

A2.  Jupiter’s surface is covered by alternating bands of light and dark clouds in rather uniform, straight bands. Sprinkled throughout these bands are many oval storms, actually giant hurricanes. The largest of these ovals is called the Great Red Spot.

A3.  The three cloud layers in Jupiter’s atmosphere are (from the top) ammonia droplets, ammonium hydrosulfide, and water. The Galileo probe must have gone through an especially clear portion of the atmosphere because the first layer was much more scattered than expected, the second layer was much thinner than expected, and the water clouds were completely missing.

A4.  The bands in Jupiter’s atmosphere are the result of the interaction of two processes: convection, which brings warmer, light colored material to the surface, and causes cooler, darker material to sink; and rapid rotation, which stretches the clouds into linear bands parallel to the equator.

A5.  The three cloud layers of Jupiter are believed to be composed of ammonia (NH₃), ammonium hydrosulfide (NH₄SH), and water ice (H₂O). The bottom two layers are probably composed of ice particles, not liquid droplets. However, the Galileo atmospheric probe did not measure any water at any depth in Jupiter’s atmosphere.

A6.  The Galileo probe penetrated well below the bottom of the visible cloud layers in Jupiter’s atmosphere, but never did detect any layer with a significant concentration of water. Did it penetrate an especially dry window in the atmosphere, or is the whole atmosphere much drier than expected?

A7.  The visible bands on Jupiter are the result of the combined effects of rapid rotation, which creates strong winds around the planet, and strong convection, which brings warmer gas to the surface. Adjacent hot and cool regions have different colors from the different molecules which form at different temperatures.

A8.  The red spot is a giant hurricane, three times bigger than Earth and lasting for several hundred years so far. It is the result the turbulence created by opposing winds in the atmosphere of Jupiter. Unlike hurricanes on Earth, it does not dissipate because there is a constant source of heat from the interior and no friction from a solid surface.

A9.  Winds observed at the top of Jupiter’s atmosphere are fairly strong, but alternate in direction with each of the bands visible in the atmosphere. The winds at Saturn are much stronger (up to about 900 mph compared to 250 mph at Jupiter) and do not alternate as much as those at Jupiter.