Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The Galileo spacecraft currently in orbit around Jupiter and send data from there over the next two years. Gaseous planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radius and diameter suitable for these planets refer to the level corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the top of the highest clouds in its atmosphere (slightly above the 1 atmosphere pressure). Jupiter has a 90% hydrogen and 10% helium (by number of atoms, or a 75/25% by mass) with traces of methane, water, ammonia and "rock." Which closely resembles the composition of the solar nebula from which the solar system formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.
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