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Thermodynamically Governed Interior Models of Uranus and Neptune

Bailey, Elizabeth and Stevenson, David J. (2021) Thermodynamically Governed Interior Models of Uranus and Neptune. Planetary Science Journal, 2 (2). Art. No. 64. ISSN 2632-3338. doi:10.3847/PSJ/abd1e0.

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Interior models of Uranus and Neptune often assume discrete layers, but sharp interfaces are expected only if major constituents are immiscible. Diffuse interfaces could arise if accretion favored a central concentration of the least volatile constituents (also, incidentally, the most dense); compositional gradients arising in such a structure would likely inhibit convection. Currently, two lines of evidence suggest possible hydrogen–water immiscibility in ice giant interiors. The first arises from crude extrapolation of the experimental H₂–H₂O critical curve to ~3 GPa. The data are obtained for an impure system containing silicates, though Uranus and Neptune could also be "dirty." Current ab initio models disagree (Soubiran & Militzer 2015), though hydrogen and water are difficult to model from first-principles quantum mechanics with the necessary precision. The second argument for H₂–H₂O immiscibility in ice giants, outlined herein, invokes reasoning about the gravitational and magnetic fields. While consensus remains lacking, here we examine the immiscible case. Applying the resulting thermodynamic constraints, we find that Neptune models with envelopes containing a substantial water mole fraction, as much as χ'_(env) ≳ 0.1 relative to hydrogen, can satisfy observations. In contrast, Uranus models appear to require χ'_(env) ≾ 0.01, potentially suggestive of fully demixed hydrogen and water. Enough gravitational potential energy would be available from gradual hydrogen–water demixing to supply Neptune's present-day heat flow for roughly 10 solar system lifetimes. Hydrogen–water demixing could slow Neptune's cooling rate by an order of magnitude; different hydrogen–water demixing states could account for the different heat flows of Uranus and Neptune.

Item Type:Article
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URLURL TypeDescription Paper
Bailey, Elizabeth0000-0002-4769-8253
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2021. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2020 June 5; revised 2020 December 3; accepted 2020 December 7; published 2021 March 29. This work was funded in part by NASA FINESST and the Heising-Simons Foundation 51 Pegasi b Fellowship (E.B.). The authors thank Yayaati Chachan, Ravit Helled, Imke de Pater, and the attendees of the 2020 Bern ISSI Ice Giants meeting, for thoughtful discussions. We thank the anonymous reviewers for their careful review and thoughtful comments which guided the direction of this manuscript.
Funding AgencyGrant Number
NASA Earth and Space Science FellowshipUNSPECIFIED
Heising-Simons Foundation51 Pegasi b Fellowship
Subject Keywords:Uranus; Planetary interior; Neptune; Planetary structure
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Uranus (1751); Planetary interior (1248); Neptune (1096); Planetary structure (1256)
Record Number:CaltechAUTHORS:20210202-095241215
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Official Citation:Elizabeth Bailey and David J. Stevenson 2021 Planet. Sci. J. 2 64
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107859
Deposited By: George Porter
Deposited On:03 Feb 2021 21:38
Last Modified:22 Apr 2021 21:33

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