Constraints on the Microphysics of Pluto's Photochemical Haze from New Horizons Observations

TL;DR

This study models Pluto's atmospheric hazes, suggesting they are fractal aggregates similar to Titan's, and uses observations to constrain their microphysical properties and formation processes.

Contribution

It introduces a detailed microphysical model of Pluto's hazes considering fractal aggregates and compares it with observations, highlighting similarities to Titan's haze production.

Findings

Aggregate particles fit the observations better than spherical ones.

Haze production flux on Pluto is similar to Titan's, about 1.2×10⁻¹⁴ g cm⁻² s⁻¹.

Particle charge to radius ratio influences aggregate size and scattering properties.

Abstract

The New Horizons flyby of Pluto confirmed the existence of hazes in its atmosphere. Observations of a large high- to low- phase brightness ratio, combined with the blue color of the haze, suggest that the haze particles are fractal aggregates, analogous to the photochemical hazes on Titan. Therefore, studying the Pluto hazes can shed light on the similarities and differences between the Pluto and Titan atmospheres. We model the haze distribution using the Community Aerosol and Radiation Model for Atmospheres assuming that the distribution is shaped by sedimentation and coagulation of particles originating from photochemistry. Hazes composed of both purely spherical and purely fractal aggregate particles are considered. Agreement between model results and occultation observations is obtained with aggregate particles when the downward flux of photochemical products is equal to the column-integrated methane destruction rate ~1.2 $\times$ 10$^{-14}$ g cm$^{-2}$ s$^{-1}$, while for spherical particles the mass flux must be 2-3 times greater. This flux is nearly identical to the haze production flux of Titan previously obtained by comparing microphysical model results to Cassini observations. The aggregate particle radius is sensitive to particle charging, and a particle charge to radius ratio of 30 e-/{\mu}m is necessary to produce ~0.1-0.2 {\mu}m aggregates near Pluto's surface, in accordance with forward scattering measurements. Such a particle charge to radius ratio is 2-4 times higher than those previously obtained for Titan. Hazes composed of spheres with the same particle charge to radius ratio have particles that are 4 times smaller. These results further suggest that the haze particles are fractal aggregates. We also consider the effect of condensation of HCN, and C$_{2}$-hydrocarbons on the haze particles, which may play an important role in shaping their distributions.