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Title: Uranus evolution models with simple thermal boundary layers

Abstract

The strikingly low luminosity of Uranus (T eff ≃ T eq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Here, our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2–3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of 1 × solar. Finally, we allow for an equilibrium evolution (T eff ≃ T eq) that begun prior to the present day, which would therefore no longer require the current era to bemore » a ”special time” in Uranus’ evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When T eff ≃ T eq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.« less

Authors:
 [1]; ORCiD logo [2];  [3]; ORCiD logo [4]; ORCiD logo [5];  [6];  [7]
  1. Univ. of Rostock (Germany). Inst. of Physics; Univ. of California, Santa Cruz, CA (United States). Dept. of Astronomy and Astrophysics
  2. Castilleja High School, Palo Alto, CA (United States); Univ. of California, Santa Cruz, CA (United States). Science Internship Program
  3. Univ. of Rostock (Germany). Inst. of Physics
  4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  5. Saratoga High School, Saratoga, CA (United States); Univ. of California, Santa Cruz, CA (United States). Science Internship Program
  6. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Univ. of California, Santa Cruz, CA (United States). Dept. of Astronomy and Astrophysics
  7. Univ. of California, Santa Cruz, CA (United States). Dept. of Astronomy and Astrophysics
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1497277
Report Number(s):
LLNL-JRNL-738448
Journal ID: ISSN 0019-1035; 891299
Grant/Contract Number:  
AC52-07NA27344
Resource Type:
Accepted Manuscript
Journal Name:
Icarus
Additional Journal Information:
Journal Volume: 275; Journal Issue: C; Journal ID: ISSN 0019-1035
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astronomy and AstroPhysics; Uranus; Neptune; Planetary Evolution

Citation Formats

Nettelmann, N., Wang, K., Fortney, J. J., Hamel, S., Yellamilli, S., Bethkenhagen, M., and Redmer, R. Uranus evolution models with simple thermal boundary layers. United States: N. p., 2016. Web. doi:10.1016/j.icarus.2016.04.008.
Nettelmann, N., Wang, K., Fortney, J. J., Hamel, S., Yellamilli, S., Bethkenhagen, M., & Redmer, R. Uranus evolution models with simple thermal boundary layers. United States. doi:10.1016/j.icarus.2016.04.008.
Nettelmann, N., Wang, K., Fortney, J. J., Hamel, S., Yellamilli, S., Bethkenhagen, M., and Redmer, R. Tue . "Uranus evolution models with simple thermal boundary layers". United States. doi:10.1016/j.icarus.2016.04.008. https://www.osti.gov/servlets/purl/1497277.
@article{osti_1497277,
title = {Uranus evolution models with simple thermal boundary layers},
author = {Nettelmann, N. and Wang, K. and Fortney, J. J. and Hamel, S. and Yellamilli, S. and Bethkenhagen, M. and Redmer, R.},
abstractNote = {The strikingly low luminosity of Uranus (Teff ≃ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Here, our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2–3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of 1 × solar. Finally, we allow for an equilibrium evolution (Teff ≃ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a ”special time” in Uranus’ evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ≃ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.},
doi = {10.1016/j.icarus.2016.04.008},
journal = {Icarus},
number = C,
volume = 275,
place = {United States},
year = {2016},
month = {4}
}

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