Average densities are low, similar to water:   Jupiter is 1.3 grams/cm³ and Saturn is 0.7 grams/cm³

Composition similar to Sun -- (especially Jupiter and Saturn)

• 80% hydrogen
• 19% helium
• 1% other elements

Liquid or icy surfaces

Dense atmospheres with violent and long lasting storms

Strong magnetic fields (Jupiter and Saturn)

Here is a comparison of their interiors (by G. Rieke). We have put in the earth for comparison. It is not very different in mass than the rocky/metallic cores of the giant planets, indicating that all the planets started with similar reservoirs of raw material. The differing properties have to arise from how the planets formed from these reservoirs.

	The densities and compositions of the planets are correlated with how close they are to the sun, and hence the temperature at which they had to form. (From The Essential Cosmic Perspective, by Bennett et al.)
	Low density ices could only condense in the zone of giant planets. (by G. Rieke, some data from Univ. of Michigan Global Change Program, http://www.sprl.umich.edu/GCL/)

The average speed of an atom or molecule goes as the square root of the temperature divided by the mass of the particle. If the speeds are close to the escape velocity, gases will leak away into space. At the high temperatures at the orbit of Mercury (and its relatively small gravity), all gaseous materials escaped. For Venus and Earth, the lower temperature and larger gravity allowed heavier gases to be retained (oxygen, nitrogen) but hydrogen and helium escaped. Mars lost most of its gases because its surface gravity is small. The giant planets were out where temperatures were low enough to retain virtually all the gases.