Pluto's Haze Abundance and Size Distribution from Limb Scatter Observations by MVIC

Abstract

The New Horizons spacecraft observed Pluto and Charon at solar-phase angles between 16° and 169°. In this work, we use the Multispectral Visible Imaging Camera (MVIC) observations to construct multiwavelength phase curves of Pluto's atmosphere, using the limb scatter technique. Observational artifacts and biases were removed using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering, indicative of wavelength-scale particles, and the non-negligible backscattering indicative of particles much smaller than the wavelength. Instead, we find it necessary to use bimodal or power-law distributions, especially below ~200 km, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodisperse, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. As compared to the results of previously published articles, we find that Pluto's atmosphere must contain haze particle number densities an order of magnitude greater for small (~10 nm) and large (~1 μm) radii, and relatively fewer intermediate sizes (~100 nm). These conclusions support a lower aggregate aerosol growth rate than that found by Gao et al., indicating a higher charge-to-radius ratio, upwards of 60e⁻ μm⁻¹. In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (<~0.01 m s⁻¹ at 200 km), which is possible with a fractal dimension of less than 2.

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