Planets - The Gas Giants

[Ian King]

I believe Im correct in saying that Jupiter, Saturn and Neptune are what we gall the Gas Giants. I take this to mean that they are made from nothing but gases.

If this is so, when Comet Shoemaker-Levy 9 hit Jupiter, why did it not pass strait through?

[QmunkE]

Just because they are made of gas doesn't mean they don't have an atmosphere...

The comet fragments probably burned up and exploded as they smashed into the dense atmosphere of Jupiter, causing atmospheric disturbances viewable as dark splodges on the surface of the gas clouds.

[QmunkE]

Check out this Wikipedia entry for some more detailed explanation:

http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9

[Clanad]

To add to QmunkE's information: At its center, Jupiter is thought to have solid metal-rock core, similar in composition to Earth, with a diameter of about 24,000 km and a mass of 10 to 15 Earth-masses. Surrounding this, out to a diameter of about 100,000 km, is a metallic mixture of hydrogen and helium. On Earth we know these two as gases; in Jupiter's interior the pressure is so high that the hydrogen takes up a state in which it behaves like a metal. Outside this metallic hydrogen zone is a shell of liquid molecules, mainly hydrogen and helium, with the cloudy atmosphere, richer in ammonia and methane, about 1,000 km deep, above. (Source: The Encyclopedia of Astrobilogy)

[Loosehead]

Ditto the above. The density of the gas is ever greater from the outside in. It's like when you dive into a pool. From 10 feet ? no problem from 100 feet very similar to concrete! so for height read speed. At the comment speed the atmosphere of Jupiter would have been the same as hitting a solid.

[KebabMeister]

Also have a look at Jupiter on wikipedia.  Incidently, in astronomy terms, the gas giants are called the Jovian planets.

[BigDogsWang]

And you missed out Uranus in your list.