A massive early atmosphere on Triton

We present models of a massive early atmosphere on Triton powered by tidal evolution of an early eccentric orbit toward the present-day, circular retrograde orbit. For this first suite of models we use the currently present volatiles on Triton's surface-methane, nitrogen, and carbon monoxide-together with a trace amount of molecular hydrogen which could have been present either as a photochemical product or as a residuum of the solar or circum-Neptunian nebula. These species are assumed to be outgassed onto the surface and to create an atmosphere in vapor pressure equilibrium as the crust is warmed by tidal heating. Atmospheric opacity sources are binary collisions among nitrogen (or carbon monoxide), methane, and hydrogen, using the collision-induced absorption coefficients of Courtin (1988, Icarus 75, 245-254). These are frequency averaged to construct gray-atmosphere profiles as a function of tidal heating rate, hydrogen abundance, and thermodynamic behavior of the volatile species. For zero hydrogen abundance, the atmosphere raised by tidal heating remains optically thin for all values of the tidal heating flux. For (constant) hydrogen atmospheric fractions in excess of 0.01% a massive atmosphere is raised by modest tidal heating fluxes, the amount of gas ultimately limited by the supply from the interior. The lifetime of such an atmosphere is limited by the rate of escape of infrared-active gases and could have exceeded a billion years. Atmospheric escape powered by solar extreme ultraviolet heating, and enhanced by the massive nature of the atmosphere, could have removed the equivalent of a layer of methane and nitrogen hundreds of meters deep. The thick atmosphere would have directly impeded impactors up to a kilometer in size, as well as encouraged volcanism and resurfacing, thereby contributing to the youthfulness of Triton's surface.