Composition of Jupiter
Since Jupiter is a gas planet, there is no solid surface. This means that if one stepped onto the planet, he would sink into it and eventually get crushed by the massive increase in pressure (the gaseous compounds get denser with depth) or be vaporized by the hot temperatures near the center. Thus, what we see when we observe the surface of Jupiter is the atmosphere that extends deep into the planet.
Jupiter is comprised of 90% hydrogen and 10% helium (a 75/25% mass ratio) and contains small amounts of methane, water and ammonia. According to scientists, this chemical composition matches closely to the solar nebula that formed our solar system in the early stages of development. Saturn, Uranus and Neptune also have a similar chemical makeup, but Uranus and Neptune have less hydrogen and helium.
On the surface of Jupiter, the layer of gas consists mainly of gaseous hydrogen and helium. As we go deeper into the planet, the tremendous pressure causes the gasses to compress into liquid. In addition, scientists believe that there are three distinct layers of clouds in Jupiter. Consisting mainly of ammonia ice, ammonium hydrosulfide and combination of water and ice. This was unconfirmed, even after the Galileo spacecraft flew past the planet. The initial data from the probe did not show strong evidence of the three layers, but rather, only very faint signs of clouds. However, Galileo's unusual observation viewpoint may account for the unexpected results since it may have observed Jupiter in an area where there are fewest clouds.
In addition to the observation of clouds, Galileo also recorded data on Jupiter's chemical composition. It was surprising to many to find that the planet contains very little water since scientists had expected that Jupiter would be abundant in oxygen that would react to form massive quantities of water. Another finding was Jupiter's high temperature and higher than expected density in the outermost areas of the atmosphere.