Nonisothermal Pluto atmosphere models

We calculate the thermal profile of a model Pluto atmosphere containing a substantial number fraction of methane molecules, taking into account the heating of the atmosphere by absorption of solar infrared flux and the cooling by conductive transport to the planet's surface. We assume an equilibrium temperature of the high atmosphere equal to 106°K and an atmospheric composition with an appreciable fraction of molecules heavier than methane (such as, perhaps, CO or N₂). For surface pressures of a few microbars, we find that an appreciable temperature gradient must exist close to the surface. The predicted stellar occultation lightcurve for such a Pluto atmosphere agrees with observations, suggesting that an inferred Pluto haze layer may not exist. Instead, similar effects on the lightcurve could be produced by the strong temperature gradient near the surface. Our alternative model implies that Pluto's solid surface probably lies in the vicinity of 1180 km from the planet's center and that the normal and tangential optical depth of the atmosphere is quite negligible. We calculate an upper limit to the escape rate for the model atmosphere deduced here and find that the minimum time for depletion of the atmospheric methane inventory is about 30 Earth years.