Reference

Methane and other hydrocarbons are found in the atmospheres of Jupiter and Saturn due to several factors. According to the search results, these gases are remnants from the early solar system and are also produced through various chemical reactions. The upper atmospheres of these gas giants contain trace amounts of simple hydrocarbons like ethane, acetylene, and diacetylene, which form from methane under the influence of sunlight and other energetic particles [1][3].

At the temperatures and pressures found in the upper atmospheres of Jupiter and Saturn, methane remains a gas while other gases like ammonia can condense and freeze [2]. The photochemical processes in the upper atmosphere of Jupiter produce higher hydrocarbons from methane [1]. The generally accepted idea is that these hydrocarbons derive mainly from ancient plant life photosynthesis [4].

The differences in methane content between the gas giants and the ice giants, Uranus and Neptune, are due to a combination of factors including equations of state (EOS), serpentinization, and other geological processes [5]. The atmosphere of Saturn contains methane, which does not condense in the upper troposphere but vaporizes at deeper levels when the temperature increases [6].

In summary, the presence of methane and other hydrocarbons in the atmospheres of Jupiter and Saturn is a result of their formation from ancient solar system materials, ongoing chemical reactions driven by solar radiation, and the specific temperature and pressure conditions that allow these gases to remain in a gaseous state in the upper atmospheres of these planets.