Atmospheric Physics and Chemistry of Pluto's Haze Layers



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Observations made by the NASA New Horizons (NH) spacecraft mission's flyby of Pluto on July 14, 2015 provided a vast amount of new information on Pluto’s surface and atmosphere. The New Horizons observations showed a planet that is geologically active with a diverse surface covered by ices of nitrogen, methane, carbon monoxide, as well as water. We now know these ices buffer its atmosphere through sublimation and drive winds over highly variable terrain containing mountain ranges and a large basin. Pluto’s atmosphere supports an extensive circumplanetary haze with embedded layers, suggesting several possible microphysical and/or dynamical excitation processes. The photochemistry leading to the formation of the haze is outlined, as well as timescales for multiple growth processes to understand haze particle lifetime and processing in the context of possible haze layer formation mechanisms. Many haze layers exist in an altitude range where Pluto’s atmosphere is highly subsaturated, making local regions of rapid particle growth from supersaturation unlikely. Particles likely grow gradually by coagulation as they fall through a large altitude region, which also cannot explain the formation of layers with the observed thicknesses. The brightness of the haze and embedded layers is proportional to the line of sight column particle number density—posing the possibility that haze layers form and are made visible by perturbations in haze particle number density. The haze particle sedimentation timescale to traverse the measured average haze layer spacing is much greater than the buoyancy or wave oscillation period in Pluto’s atmosphere, supporting the explanation that atmospheric waves are the most likely formation mechanism behind Pluto’s complex haze layer structure by wave action imparted on the background sedimenting particles to cause layers of particle compaction and rarefaction. Images of Pluto’s limb taken by NH/Long Range Reconnaissance Imager (LORRI) were analyzed here. Several haze layer characteristics were extracted, namely—slope, amplitude, waveform, and the associated power spectral densities (PSDs); and their variations with local geography. These were then explored in the context of possible wave types in Pluto’s atmosphere, such as tidal and orographically driven inertia-gravity (buoyancy) waves. A single-scattering model was also adapted to Pluto’s atmospheric scattering and LORRI’s observing characteristics to simulate images. The scattering model was used to directly compare observations of layers to haze layering generated by an orographic gravity wave model. Qualitative and quantitative comparisons informed which wave types are likely to cause most of the layers observed by NH. Observations, models, and theory are then considered in the context future work for better understanding waves in Pluto’s atmosphere is proposed.