Scattering properties of ice components in Pluto’s haze

The flyby of the New Horizons spacecraft in July 2015 revealed an unexpected cold atmosphere on Pluto and confirmed the existence of haze in its atmosphere. Vertical profiles of chemical species and numerical microphysical simulations suggest that organic ice condensation is likely to take place in Pluto’s middle and lower atmosphere. These ice components affect the scattering properties of Pluto’s haze, which in turn influences the interpretation of its morphology and therefore places constraints on Pluto’s atmospheric chemistry and energy budget. Here, we present an analysis of the scattering properties of the ice components in Pluto’s haze. A number of haze morphologies and organic ice components are tested using the accurate Discrete Dipole Approximation (DDA) scattering model. Sensitivity studies are conducted by investigating the trend of the ratio of scattering intensity to extinction when the haze particle parameters are systematically modified. The results of these simulations provide constraints on the ice content of haze particles, and also on gas condensation in Pluto’s atmosphere. Scattering properties of the ice-bearing haze particles are also compared to observations of Pluto’s haze obtained by instruments onboard New Horizons. In this manner, parameters providing the best fit with observational data are derived. We find that the inclusion of organic ice component alone is not sufficient to explain the observed large backscattering; other scattering sources and/or mechanisms, e.g., a small-size particle population, are required to simulate this backscattering feature. Further observations are also needed for better interpretation of the morphology and constituent components of Pluto’s haze.