Abstract:
During its 2015 flyby of Pluto, NASA’s New Horizons spacecraft investigated Pluto’s atmosphere with in-situ plasma instruments [SWAP and PEPSSI], UV airglow [Alice], UV and radio occultations [Alice and REX], and visible and near-IR imaging of the haze [LORRI, MVIC, LEISA], augmented by observations of Pluto’s surface. Additionally, ground-based observations [ALMA, stellar occultation] measured CO, HCN, and HNC, and constrained the thermal structure. Together, these reveal an atmosphere with a low CH4 mixing ratio and escape rate, and even slower N2 escape, a cool upper atmosphere, the detection of C2 hydrocarbons and methylacetylene, global layers of hazes that are primarily forward scattering and blue in color, a small eddy diffusion coefficient, a large near-surface positive thermal gradient that overlies a tropopause in one place and connects directly to the surface in another, and a surface pressure of around 12 µbar, varying by several µbar with the altitude of the local terrain. Imagery, topography, colors, and spectroscopy [LORRI, MVIC, LEISA] reveal a surface modified by sublimation, deposition, and atmospheric transport on a range of timescales..
Some aspects of Pluto’s atmosphere are familiar, some have an odd Plutonian twist, and some of our new knowledge only raises more questions. While Pluto’s surface pressure is in vapor-pressure equilibrium with surface N2 ice, the CH4 is out of thermodynamic equilibrium in both the solid and gaseous phases. Pluto’s atmospheric dynamics is impacted by the volatile transport of N2 across Pluto, but also by Pluto’s size and rotation rate (being an intermediate case between fast rotators like Earth and Mars and slow rotators like Venus and Titan), a near-surface stratosphere, and relatively long radiative timescales. Like Pluto, other worlds show surfaces affected by sublimation/deposition, or seasonal volatile cycles dominated by vapor pressure equilibrium. What makes Pluto’s seasonal volatile cycle unique is the fact that it is strongly controlled by the high obliquity and orbital eccentricity of Pluto, and by a large and permanent near-equatorial reservoir of N2 ice (in Sputnik Planitia) and CH4 ice (in the bladed terrains). Additionally, how low Pluto’s atmospheric pressure will get as it orbits the sun depends on the hidden Pluto–the areas in the southern hemisphere that were not imaged, and the thermal properties of its subsurface–and can be studied with ongoing stellar occultations. Pluto’s haze formation is probably intermediate between Titan-like molecular growth and Triton-like condensation, and may be the key species for radiative heating and cooling in Pluto’s atmosphere. Since the CH4 escape is low due to the Hunten limiting flux, and the N2 escape rate is low due to a cool and compact upper atmosphere, understanding the haze properties in the infrared is important for Pluto’s escape rate.