Extinction layer detected by the 2003 star occultation on Pluto

[1] A star occultation by Pluto was monitored in August 2003 by several telescopes, at several wavelengths between 0.7μm and 2.23μm. Besides information concerning pressure and temperature profiles, this observation revealed a chromatic effect in the light transmitted through the atmosphere at the limb. It was attributed to an extinction layer produced by spherical particles of about 0.2 μm, and it was suggested that such large particle size may be due to fractal particles as on Titan. In this paper, we use an aerosol microphysical model to estimate if such extinction could be due to photochemical aerosols only. We use the case of Titan to scale the haze production rate on Pluto and we show that the chromatic effect is not likely to be produced in this way. We then study how a troposphere in Pluto's atmosphere could generate condensation droplets. We conclude that such an extinction layer is produced by condensation droplets that potentially could grow as large as observed. We find that only N₂ and CO are able to condense easily. CH₄ probably does not condense or, if it does, it needs to reach a very high saturation ratio and, in any case, is supersaturated on Pluto. Noteworthily, a similar layer was observed on Triton by Voyager. Finally, at the end of the paper we discuss which are the conditions to detect a chromatic effect for submicrometer to micrometer particles in future observations.