The most massive individual objects in the universe are stars, and stars are entirely made of gases. Gravity does not require solid material, it only requires matter, in any form, gaseous, liquid, or solid. Next to the Sun (our local star), Jupiter is the most massive object in the solar system. Hence, it also has the greatest gravitational force of any object (other than the Sun) in the solar system.

Without getting too technical, oxygen is extremely rare in the atmosphere of Jupiter. We know there is a large amount of water vapor well below the Jupiter clouds, and chemical breakup of water molecules must yield some percentage of oxygen, but very little (if any) reaches the atmosphere above the clouds.

The abundance relative to hydrogen in the atmosphere above the Jupiter clouds for water vapor is about 0.000 001 (i.e., one water vapor molecule for every million hydrogen molecules); the abundance of carbon monoxide is 0.000 000 002 (two parts in a billion). Free oxygen is lower in abundance than either of these and, to my knowledge, has not been detected in Jupiter's atmosphere.

The situation is slightly different for Saturn. While similar conditions exist there, Saturn has an extensive ring composed mostly of water ice particles. Sunlight acting on that water ice frees some of the oxygen, which has been detected as a constituent of the "atmosphere" of the rings (although not in the atmosphere of Saturn itself).

Jupiter is a giant gas planet and because of that, there really isn't a place to land. There is just a whole lot of gas. A spacecraft could have exploded once it entered Jupiter's atmosphere, but NASA was successful because they designed the spacecraft and its components such that it could take Jupiter's intense pressure and heat. Galileo had a probe that penetrated Jupiters atmosphere on December 7, 1995. The probe was able to record information for 59 minutes, almost an hour, before it melted and vaporized because of the heat. You can see how it was designed here: http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_spacecraft.html.

Jupiter, together with its four largest moons, which range from the size of the Earth's moon to the size of the planet Mercury, are in some ways analogous to a mini-solar system, Jupiter being analogous to the sun as the central body, and Jupiter's moons being analogous to the planets. By studying this mini-solar system, it should be possible to better understand the actual solar system, how it originated, and what processes affect its evolution.

Pictures of Jupiter, which always view the ammonia clouds, indicate that the clouds tend to be organized into a series of cloud bands which are oriented east-west. The bright bands, called zones, are believed to represent regions where jovian atmosphere is rising. The darker cloud bands, called belts, are regions where the atmosphere which has risen in the zones is descending back into the deeper part of the atmosphere. By tracking cloud features in photographs taken of Jupiter over a period of time, we know that at the level of the ammonia clouds there exists a system of winds blowing basically in the east-west direction.

At the equator, and extending to about 15 degrees latitude north and south, the wind blows in the same direction as Jupiter rotates (eastward), and wind speeds are about 100 meters per second, or about 200 miles per hour. Beyond 15 degrees latitude either north or south, the winds change direction and blow westward reaching speeds of about 50 meters per second. The winds continue to oscillate in direction and diminish in speed as latitude increases, until near 45 degrees latitude the winds appear to die out and the banded structure disappears.

It is almost certain that the winds occur at atmospheric levels other than just near the ammonia clouds, but we do not know what the vertical extent of the wind system is either above or below the ammonia clouds. It is known that Saturn, Uranus, and Neptune also have wind systems, and their winds exceed in magnitude those so far observed on Jupiter. Since the energy available to drive winds is far less on these other planets than for Jupiter, we are presented with a major puzzle as to what causes any of these winds.

Determining the vertical variation of the winds on Jupiter is a major scientific objective of the Galileo Probe, since the vertical variation will provide important clues as to how the winds are generated. The probe will be tracked during its descent through the atmosphere by the Galileo Orbiter travelling over the probe entry site, and in this way the winds can be measured.

Jupiter, Saturn, and Neptune radiate about twice as much energy to space as they receive from the sun. This is in contrast to the terrestrial planets Mercury, Venus, Earth, and Mars, which all radiate to space almost exactly the amount of energy received from the sun. In other words, the terrestrial planets are in energy equilibrium with the solar energy input, but the outer planets, at least Jupiter, Saturn, and Neptune, are not.

It is theorized that the excess energy being radiated from the outer planets results from heat trapped in the interior of the planet during the time when the planet was accreting material and condensing early in the formation history of the solar system. Some gravitational contraction is continuing even at present and contributes to the energy being released. How this interior energy flow, as well as sunlight, gets distributed in the atmosphere is an important question, which relates to how and where the clouds are formed and what drives the winds.

The Galileo Probe will measure the total radiative energy transmission through the atmosphere and determine in what atmospheric regions sunlight and heat from the interior get absorbed, thereby producing higher temperatures, which in turn affect cloud formation and winds.

We know from photographs of the night side of Jupiter taken by the Voyager spacecraft (which flew by Jupiter in 1979) that lightning occurs on Jupiter. The lightning events observed by Voyager indicated very intense lightning, about 100 times more energetic than a typical lightning flash on Earth.

It is likely that the lightning is generated in the water clouds that we expect to be in Jupiter's atmosphere. However, it appears that most of the lightning occurs near 49 degrees North latitude, and not much occurs elsewhere. If this is actually the case, it is very puzzling.

The Galileo Probe will detect lightning if it occurs within about 10,000 km of the probe as it is descending through the atmosphere.