Volume 633, January 2020
|Number of page(s)||10|
|Section||Planets and planetary systems|
|Published online||09 January 2020|
Explaining the low luminosity of Uranus: a self-consistent thermal and structural evolution
Institute for Computational Science, Center for Theoretical Astrophysics & Cosmology University of Zurich Winterthurerstr. 190,
2 Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
Accepted: 28 November 2019
The low luminosity of Uranus is a long-standing challenge in planetary science. Simple adiabatic models are inconsistent with the measured luminosity, which indicates that Uranus is non-adiabatic because it has thermal boundary layers and/or conductive regions. A gradual composition distribution acts as a thermal boundary to suppress convection and slow down the internal cooling. Here we investigate whether composition gradients in the deep interior of Uranus can explain its low luminosity, the required composition gradient, and whether it is stable for convective mixing on a timescale of some billion years. We varied the primordial composition distribution and the initial energy budget of the planet, and chose the models that fit the currently measured properties (radius, luminosity, and moment of inertia) of Uranus. We present several alternative non-adiabatic internal structures that fit the Uranus measurements. We found that convective mixing is limited to the interior of Uranus, and a composition gradient is stable and sufficient to explain its current luminosity. As a result, the interior of Uranus might still be very hot, in spite of its low luminosity. The stable composition gradient also indicates that the current internal structure of Uranus is similar to its primordial structure. Moreover, we suggest that the initial energy content of Uranus cannot be greater than 20% of its formation (accretion) energy. We also find that an interior with a mixture of ice and rock, rather than separated ice and rock shells, is consistent with measurements, suggesting that Uranus might not be “differentiated”. Our models can explain the luminosity of Uranus, and they are also consistent with its metal-rich atmosphere and with the predictions for the location where its magnetic field is generated.
Key words: planets and satellites: composition / planets and satellites: interiors / planets and satellites: individual: Uranus / planets and satellites: formation / planets and satellites: physical evolution
© ESO 2020
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