Energy balance and plume dynamics in Triton's lower atmosphere

Consideration of the roles of thermal conduction, eddy mixing, condensation, and radiative heating in the thermal balance of Triton's lower atmosphere results in the conclusion that the temperature gradient is negative in the lower atmosphere but becomes positive at higher altitudes. The negative temperature gradient is caused by eddy mixing, which drives the atmosphere toward the dry adiabat. The positive gradient at higher altitudes is a result of the downward conduction of heat produced in the ionosphere. The low concentrations of thermally active molecules and the small aerosol optical depths imply that radiative processes have a negligible effect on the thermal structure. We show that this temperature profile is reasonably consistent with the data from the radio-occultation experiment. Based on the height of the geyser-like plumes seen by Voyager we suggest that the convective and conductive regions of the atmosphere join at a tropopause near 10 km. We suggest that the eddy diffusion and heat-transport coefficients are about 10⁶ cm²/s below 8 km, dropping to about 300 cm²/sec just above, for a profile that resembles the Earth's. Rather modest geyser action in the subliming nitrogen ice cap triggers moist convective plumes which must have diameters of at least 1 km and may have velocities up to 100 m/sec; they stop within about 1 km of the tropopause.