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Pluto's structure and composition suggest origin in the solar, not a planetary, nebula

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Abstract

The mean density of the Pluto-Charon system is now accurately known at 1.99±0.09 g cm–3 (With formal errors five times smaller)1, through observations of total occultations and transits2. Even allowing Charon's density to vary between 1 and 3 g cm–3 constrains Pluto's density between 1.84 and 2.14 g cm–3. Pluto is thus very rock-rich, with a rock/(rock + H2O-ice) mass ratio of ˜0.68–0.80, much greater than those of the icy satellites Ganymede, Callisto or Titan. It is so rock-rich that spontaneous unmixing of rock and ice phases in its convecting interior is just possible, and any significant differentiation during accretion or during Charon's formation, which may have involved a large-body impact3–5, renders Pluto unstable to further melting and differentiation because the resulting multiple thermal boundary layers inhibit heat transport6. Continual loss of methane over the age of the Solar System7,8 requires a near-surface methane reservoir, and implies that non-trivial differentiation has occurred9. Pluto is probably a differentiated object whose icy mantle is entirely in the ice-I stability field; its closest structural cousin is Europa. Of four explanations for Pluto's large rock/ice ratio10—formation in the inner Solar System, volatile loss during accretion, volatile loss during the large-body impact that created Charon, and formation as a large, ice-poor11,12 outer Solar System planetesimal—we show that only the last two are feasible, and that the depletion of water ice in Pluto is so severe that both explanations may be necessary.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, Saint Louis, Missouri, 63130, USA

    William B. McKinnon

  2. Department of Geological Sciences, Southern Methodist University, Dallas, Texas, 75275, USA

    Steve Mueller

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  1. William B. McKinnon
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  2. Steve Mueller
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McKinnon, W., Mueller, S. Pluto's structure and composition suggest origin in the solar, not a planetary, nebula. Nature 335, 240–243 (1988). https://doi.org/10.1038/335240a0

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