Explaining the low luminosity of Uranus: a self-consistent thermal and structural evolution

Abstract

The low luminosity of Uranus is a longstanding challenge in planetary science. Simple adiabatic models are inconsistent with the measured luminosity, which indicates that Uranus is non-adiabatic due to the existence of 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 Uranus' deep interior can explain its low luminosity, what composition gradient is required, and whether it is stable for convective-mixing for a timescale of billion of years. We vary the primordial composition distribution and the planet initial energy budget, and opt the models that fit Uranus measured properties (radius, luminosity, and moment of inertia) at present time. We present several alternative non-adiabatic internal structures that fit Uranus measurements. It is found that convection-mixing is limited in Uranus interior and a composition gradient is stable and sufficient to explain its current luminosity. As a result, Uranus' interior could still be very hot, in spite of its low luminosity. The stable composition gradient also indicates that Uranus' current-state internal structure is not very different from its primordial one. Moreover, initial energy content of Uranus cannot be greater than 20% of its formation (accretion) energy. We also find that an interior with ice+rock mixture, rather than separated ice and rock shells, is consistent with measurements, suggesting that Uranus might not be "differentiated". Our models can explain Uranus' luminosity and are also consistent with its metal-rich atmosphere, and the predictions for the location where its magnetic field is generated.In previous works we developed a detailed non-adiabatic structure evolution model for gas giants (Vazan et al. 2015(Vazan et al. , 2016. This model also includes the change in the interior structure in time by convective-mixing. The evolution of ice giants may be different than that of the gas giants. The more metalrich interior, lower mass, and high atmospheric metallicity affect the thermodynamic properties and the heat transport mechanism in the interior. Therefore, we expand our model by using the evolution features of metal-rich planets (Vazan et al. 2018b,c). We then apply our method to Uranus, and investigate whether a composition gradient is consistent with the low luminosity and the other measurements. Since we follow the entire evolution of Uranus interior in detail, the initial properties can be derived by its current-state.This paper is structured as follows, in Section 2 we describe the model initial composition and properties (Sec. 2.1, 2.2), the thermal and structural evolution methods (2.3), the nature of non-adiabatic structure evolution (2.4), and the parameters we fit (2.5). In Section 3 we present several examples of Uranus valid models (3.1), emphasis the importance of the non-adiabatic evolution (3.2), and summaries the properties of all valid models in our study (3....

Large H 2 O solubility in dense silica and its implications for the interiors of water-rich planets

“…For example, standard models for the internal structures of Uranus and Neptune, water-rich planets in our solar system, have assumed separate layers of ice and rock (2). However, in order to explain the low luminosity of Uranus, recent models included a compositional gradient in the thick ice layer (3,4). Although rock-forming heavy elements were assumed for the compositional gradient and therefore, the properties of rock were used as a component for a mechanical mixture, the exact state of matter under such H2O-rich high P −T conditions remains unknown.…”

“…If the interior of a water-rich planet is not well differentiated as some models have invoked for Uranus (3,4), mutual solubility of water and silica can even open up a possibility for a single high metallicity layer where H2O ice at shallower depths undergoes a gradual change to hydrous silica at greater depths, assuming increasing mutual solubility of H2O and SiO2 with pressure. Accordingly, we propose that the Uranus models should incorporate the effects of significant mutual solubility between H2O and SiO2 that we report here.…”

The underexplored frontier of ice giant dynamos

“…Double-diffusive convection may also arise from an unstable thermal gradient in the presence of stable compositional stratification (e.g. [29]).…”

Thermal evolution of Uranus and Neptune

“…UN_Evolution_II this setup significantly broadens the range of present luminosities and might even produce higher present luminosity than the adiabatic case, which might help explain the brightness of Neptune. Periods of convective instability erroding the TBL might indeed occur when the thermal buoyancy induced by the TBL overcomes the stabilising effect of the compositional gradient (Vazan & Helled 2020). It is also possible that the inhomogenous structure of the ice giants inferred for present Uranus and Neptune (Helled & Fortney 2020) is the result of more recent phase separation and sedimentation processes.…”

“…However, such a process would require a larger amount of condensibles in the outer envelope of Uranus than consistent with interior model predictions. Vazan & Helled (2020) investigated a primordial composition gradient in the fluid envelope inhibiting convective energy transport. They find such a gradient to be stable and were able to reproduce Uranus' observed luminosity with these models.…”