• 
  What are jovian planets made of?
  Jupiter and Saturn are made almost entirely of hydrogen and helium, while Uranus and Neptune are made mostly of hydrogen compounds mixed with metal and rock. These differences arose because all four planets started from ice-rich planetesimals of about the same size, but captured different amounts of hydrogen and helium gas from the solar nebula.
• 
  What are jovian planets like on the inside?
  The jovian planets have layered interiors with very high internal temperatures and pressures. All have a core about 10 times as massive as Earth, consisting of hydrogen compounds, metals, and rock. They differ mainly in their surrounding layers of hydrogen and helium.
• What is the weather like on jovian planets? The jovian planets all have multiple cloud layers that help determine the colors of the planets, fast winds, and large storms. Some storms, such as the Great Red Spot, can apparently rage for centuries or longer.

• 
  What kinds of moons orbit the jovian planets?
  We can categorize the more than 100 known moons as small, medium-size, or large. Most of the medium-size and large moons probably formed with their planet in the disks of gas that surrounded the jovian planets when they were young. Smaller moons are often captured asteroids or comets.
• 
  What makes Jupiter's Galilean moons unusual?
  Io is the most volcanically active object in the solar system, thanks to an interior kept hot by tidal heating-which occurs because Io's close orbit is made elliptical by orbital resonances with other moons. Europa may have a deep, liquid water ocean under its icy crust, also thanks to tidal heating. Ganymede and Callisto may also have subsurface oceans.
• What makes Titan different from other moons?Titan is the only moon in our solar system with a thick atmosphere. It may even have lakes or oceans of liquid ethane and methane on its surface.
• Why are small icy moons more geologically active than small rocky planets?Ices deform and melt at much lower temperatures than rock, allowing icy volcanism and tectonics at surprisingly low temperatures.

• 
  What are Saturn's rings like?
  Saturn's rings are made up of countless individual particles, each orbiting Saturn independently like a tiny moon. The rings lie in Saturn's equatorial plane, and they are extremely thin.
• 
  Why do the jovian planets have rings?
  Ring particles probably come from the dismantling of small moons formed in the disks of gas that surrounded the jovian planets billions of years ago. Small ring particles come from countless tiny impacts on the surfaces of these moons, while larger ones come from impacts that shatter the moons.

Copyright © 1995-2005 by Addison Wesley A division of Pearson Education Legal Disclaimer