Three different gases condense in Jupiter's troposphere, resulting in a complex vertical distribution of clouds. Water forms the deepest of these clouds, but water clouds are usually obscured by overlying cloud layers. The layer above, probably composed of ammonium hydrosulfide, results when highly toxic ammonia and hydrogen sulfide gases react to form a solid. The chemistry of this strange cloud layer is poorly understood, partly because of the hazards of working with these gases in the laboratory. Ammonia itself condenses in Jupiter's cold upper troposphere and may contribute to a thin haze that blankets the planet above the clouds. Since cloud heights, cloud thicknesses, and ammonia gas concentration are variable on Jupiter, we use them to trace dynamic processes in the atmosphere.
The elements N and S (and probably O) in Jupiter's cloud-forming gases are about three times more enriched (with respect to hydrogen) than in a protosolar composition gas. Carbon and noble gases are also enriched. It is generally believed that these elements were enriched when Jupiter accumulated icy planetesimals during its formation, but planetesimals with the necessary abundance ratios have never been observed. The origin of these planetesimals (and therefore Jupiter itself) is still a mystery.