CONDENSATION OF METHANE, AMMONIA, AND WATER AND THE INHIBITION OF CONVECTION IN GIANT PLANETS

The condensation of chemical species of high molecular mass such as me thane, ammonia, and water can inhibit convection in the hydrogen-heliu m atmospheres of the giant planets. Convection is inhibited in Uranus and Neptune when methane reaches an abundance of about 15 times the so lar value and in Jupiter and Saturn if the abundance of water is more than about five times the solar value. The temperature gradient conseq uently becomes superadiabatic, which is observed in temperature profil es inferred from radio-occultation measurements. The planetary heat fl ux is then likely to be transported by another mechanism, possibly rad iation in Uranus, or diffusive convection.