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Abstract
The close encounters of the Pluto–Charon system and the Kuiper Belt object Arrokoth (formerly 2014 MU69) by NASA’s New Horizons spacecraft in 2015 and 2019, respectively, have given new perspectives on the most distant planetary bodies yet explored. These bodies are key indicators of the composition, chemistry, and dynamics of the outer regions of the Solar System’s nascent environment. Pluto and Charon reveal characteristics of the largest Kuiper Belt objects formed in the dynamically evolving solar nebula inward of ~30 AU, while the much smaller Arrokoth is a largely undisturbed relic of accretion at ~45 AU. The surfaces of Pluto and Charon are covered with volatile and refractory ices and organic components, and have been shaped by geological activity. On Pluto, N2, CO and CH4 are exchanged between the atmosphere and surface as gaseous and condensed phases on diurnal, seasonal and longer timescales, while Charon’s surface is primarily inert H2O ice with an ammoniated component and a polar region colored with a macromolecular organic deposit. Arrokoth is revealed as a fused binary body in a relatively benign space environment where it originated and has remained for the age of the Solar System. Its surface is a mix of CH3OH ice, a red-orange pigment of presumed complex organic material, and possibly other undetected components.
Highlights
The presence of complex organic molecules, often referred to as COMs, on outer Solar System planetary surfaces and in the atmosphere of Saturn’s satellite Titan were first suspected from their low albedos and reddish colors
Comparisons with terrestrial kerogens and complex refractory organics synthesized by photolysis and radiolysis of gaseous mixtures of CH4, N2, and other simple molecules have gradually led to the recognition that planetary processes in atmospheres and surface ices containing hydrocarbons lead to the formation of colored materials of relevance to both planetary environments and the interstellar medium, e.g., [1]
These results demonstrate that energetic processing of ices and gases containing simple organic molecules, such as CH4, readily transforms the initial carbon-bearing materials into large molecules with complex structures termed “tholin”
Summary
The presence of complex organic molecules, often referred to as COMs, on outer Solar System planetary surfaces and in the atmosphere of Saturn’s satellite Titan were first suspected from their low albedos and reddish colors. The relevance of Pluto, Arrokoth, and other small bodies in the outer Solar System to investigations of the origin and evolution of the early Solar System is closely related to the distinct environments in which they formed in the protoplanetary nebula These diverse origins are reflected today in the complex dynamical structure of the Kuiper belt [15]. Arrokoth samples the nebula beyond the 30-AU break that terminated Neptune’s migration, unlike Pluto, which, like other KBOs on more excited orbits, probably formed in the more densely populated region inside of 30 AU Another important class are the Centaur objects. A few have shown continuous cometary activity (e.g., comet 29P/Schwassmann-Wachmann 1, which is regarded as a Centaur) or episodic activity (e.g., 2060 Chiron)
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