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Arrokoth and the Pluto-Charon binary: towards a unified model of formation

Presentation #111.06 in the session “Arrokoth in Context”.

Published onOct 03, 2021
Arrokoth and the Pluto-Charon binary: towards a unified model of formation

The flyby of the New Horizons spacecraft of the Pluto system in 2015 and the cold classical Kuiper Belt object Arrokoth in 2019 have yielded a a huge bounty of new data about the physical and chemical makeup of these two outer worlds of the Solar system. Key highlights from the Arrokoth flyby were the discovery of its flattened bi-lobate structure, its smooth, lightly-cratered and uniformly-red surface, and the strong abundance of methanol (Spencer et al Science 2020 Science 367 eaay3999; Grundy et al 2020 Science 367 eaay3705). The Pluto system is defined by the Pluto-Charon binary (PCB). Not only is the diameter of Charon half that of Pluto but the system-normalized angular momentum density matches the value expected if the PCB had formed by the rotational fission of a single liquid globe.

Here I extend the quantification of the formation of Arrokoth and the PCB within the framework of the gas ring model of planetary formation (Prentice 1978 Moon & Planets 19 341; 2019a AAS Meeting #233 id.467.01; 2019b AGU FM abstract #331-3541; 2021 43rd COSPAR Sci. Assembly abstract B0.01-0034-21). Prentice (2019b) has suggested that Arrokoth’s physical structure might be explained if it was initially a single-lobe object, consisting of a uniform mix of rock & graphite (mass fraction: 0.5432), water ice (0.1845), dry ice (0.2211), pure CH4 ice (0.0226) and hydrated CH4 ice (0.0286). Intense heating due to the decay of radioactive Al26 caused melting & vaporization of much of the internal CH4 ice, so creating an outer shell. This process was followed by the sublimation of the inner 25% of the CO2 ice mass, leading to the explosive disruption of the outer CH4 shell and the cleaving of the proto-Arrokoth into two parts in its equatorial plane. These parts drifted apart under the action of centrifugal force to form the present structure. Because only ~1% of the inner water ice vaporizes, the surface is nearly water-free. Subsequent radiolysis of the CH4 dominated surface may explain the observed methanol (Grundy et al 2020). The attached graphic describes the early internal thermal evolution of proto-Arrokoth. The computations include the latent heats associated with the phase changes of the various ices and liquids. Next, a numerical model of a proto-Pluto object comprised of Pluto and Charon, and having the same bulk composition as Arrokoth, has been constructed. The inner ~98% of the mass is assumed to be liquid, plus rock & graphite. It has a uniform temperature of 300K. The outer 2% is a solid. The model includes the pressure dependence of density. Rotational fission is assumed to take place through the settling out of the rock-graphite particles.

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