Upper limits on possible photochemical hazes on Pluto

Elliot et al. [1989] invoked a haze layer near the surface of Pluto to explain certain features of a stellar occultation by that planet in June, 1988. The primary requirements for this haze layer were that it achieve unity tangential optical depth at a radius of 1174 km and be essentially transparent above 1189 km. We explore here the possibility that aerosols generated through methane photolysis could be responsible for such a haze layer. A comprehensive model of aerosol production, particle growth, sedimentation and condensation is applied to the atmosphere of Pluto using pressures, temperatures and composition derived from the stellar occultation and other data. We test two atmosphere models proposed in the literature, one from Elliot et al. [1989], and one from Hubbard et al. [1989], as well as a range of optical properties for the particles. In order to produce a haze with unity tangential optical depth at 1174 km, we had to use an aerosol mass production rate equal to twice the total methane dissociation rate due to solar UV expected for Pluto and assume that the particles produced were 10 times more absorbing than those in other hazes in the outer solar system. The possibility of condensation in the lower atmosphere was considered but did not result in distinctly different haze optical depths. If a photochemical haze on Pluto was responsible for the occultation lightcurve measured by Elliot et al., operation of a photochemical system different from those on Titan, Uranus or Neptune is indicated. Alternatives include near surface condensation processes driven by circulation, and the possibility that the occultation light curve can be explained in its entirety by temperature effects as proposed by Hubbard et al. [1989].