Pluto’s haze abundance and size distribution from limb scatter observations by MVIC

The New Horizons spacecraft observed the Pluto system at solar phase angles between 16° and 169°. We used Multispectral Visible Imaging Camera (MVIC) observations to construct multi-wavelength phase curves of Pluto’s atmosphere using the limb scatter technique. Observational artifacts and biases were removed by using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function [Tomasko et al. 2008]. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering phase curve, indicative of wavelength-sized particles, and the non-negligible back scattering indicative of particles much smaller than the wavelength. Instead, we require a bimodal or power-law distribution, especially below ~200 km altitude, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodispersed, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. Compared to previously published distributions [e.g., Gao et al. 2017], we find that Pluto’s atmosphere must have an order of magnitude more small (~10 nm) and large (~1 μm) scatterers, and relatively fewer intermediate size scatterers (~100 nm). These conclusions support a lower aggregate aerosol growth rate than proposed by Gao et al., indicating a higher charge to radius ratio upwards of 60 e^-/μm. In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (≲0.01 m/s at 200 km), which is possible with a fractal dimension less than 2. The production of haze is closely linked with condensation of “C₂H_x”, UV photolysis of CH₄, and changes in temperature.