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Published May 1, 2017 | Submitted
Journal Article Open

Constraints on the microphysics of Pluto's photochemical haze from New Horizons observations


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 (indicative of Rayleigh scattering), suggest that the haze particles are fractal aggregates, perhaps 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 downward transport and coagulation of particles originating from photochemistry. Hazes composed of both purely spherical and purely fractal aggregate particles are considered. General agreement between model results and solar occultation observations is obtained with aggregate particles when the downward mass flux of photochemical products is equal to the column-integrated methane destruction rate ∼1.2 × 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 effects, and a particle charge to radius ratio of 30 e − /µm is necessary to produce ∼0.1–0.2 µm 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 at Pluto's surface. These results further suggest that the haze particles are fractal aggregates. We also consider the effect of condensation of HCN, C_2H_2, C_2H_4, and C_2H_6 on the haze particles, which may play an important role in shaping their altitude and size distributions.

Additional Information

© 2016 Elsevier Inc. Received 16 May 2016; Revised 27 August 2016; Accepted 20 September 2016; Available online 22 September 2016. This research was supported in part by a grant from the New Horizons Mission. YLY and RLS were supported in part by the Cassini UVIS program via NASA Grant JPL.1459109, NASA NNX09AB72G grant to the California Institute of Technology. PG was supported in part by an RTD grant from JPL.

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