Fig. 21.

Chemical tracers in Jupiter’s atmosphere at 5 mbar from Cassini CIRS. Both acetylene and ethane are formed at low latitudes by solar UV. In photochemical equilibrium their abundances would be greatest at pressures less than 0.1 mbar. C₂H₂ has a short chemical time constant, ~ 3 × 10⁷ s, so its abundance reflects photochemical equilibrium with lower sunlight toward the poles. C₂H₆ has a long lifetime, ~ 3 × 10¹⁰ s, so its abundance at 5 mbar reflects transport by the meridional circulation. The time constant of this circulation is between the lifetimes of the two molecules (Nixon et al. 2007)