Abstract
Molecules such as C 2 H 2 and C 2 H 6 have been observed in the atmosphere of Neptune, one of the giant planets composed mostly of hydrogen and helium. Planetary scientists have puzzled over whether these hydrocarbons are formed as ultraviolet light from the sun induces photochemical reactions in atmospheric methane. In his Perspective, Hubbard discusses simulations reported in the same issue by Ancilotto et al . (p. 1288 ) suggesting that C 2 H 6 may be produced by high-pressure chemical reactions deep within the planet.