How does organic ice condensation affect the observable optical properties of Pluto’s haze particles?

The flyby of the New Horizons spacecraft in July 2015 revealed an unexpected cold atmosphere of Pluto and confirmed the existence of its atmospheric haze. The observed and simulated vertical profiles of chemical species and microphysical processes suggest that the haze particles in Pluto’s middle and lower atmosphere may contain organic ice condensation. Such organic ice components can potentially affect Pluto’s haze chemistry and optical properties, as well as its energy budget. This study investigates the influence of the ice components on the scattering properties of Pluto’s haze by comparing New Horizons observations and simulated particle scattering properties. Comprehensive tests are performed for various haze particle parameters, including their size, chemical component, ice content, and morphology. Scattering properties of these ice-bearing haze particles are calculated by a discrete dipole approximation method and compared to multi-spectral observations obtained by four New Horizons instruments in spectral regions ranging from the ultraviolet to the near-infrared. The results indicate that the inclusion of the organic ice component leads to higher ratios of backscattering in the visible and forward scattering in the near-infrared to extinction in the ultraviolet; this provides better agreement with observations, but it alone is not sufficient to explain the observed forward scattering values in the visible. Therefore, other scattering sources and/or mechanisms are still required to explain the full set of scattering observations. Further observations as well as laboratory measurements and numerical tests are anticipated to improve our understanding of the morphology and ice content of Pluto’s haze.