Constraints on the Long-term Existence of Dilute Cores in Giant Planets

IF 3.8 Q2 ASTRONOMY & ASTROPHYSICS The Planetary Science Journal Pub Date : 2024-08-29 DOI:10.3847/psj/ad6571
A. Tulekeyev, P. Garaud, B. Idini, J. J. Fortney
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Abstract

Post-Cassini ring seismology analysis suggests the existence of a stable stratification inside Saturn that extends from the center to ∼60% of its radius, in what is recognized today as Saturn’s dilute core. Similarly, gravity measurements on Jupiter suggest the existence of a dilute core of weekly constrained radial extent. These cores are likely in a double-diffusive regime, which prompts the question of their long-term stability. Indeed, previous direct numerical simulation (DNS) studies in triply periodic domains have shown that, in some regimes, double-diffusive convection tends to spontaneously form shallow convective layers, which coarsen until the region becomes fully convective. In this paper, we study the conditions for layering in double-diffusive convection using different boundary conditions, in which temperature and composition fluxes are fixed at the domain boundaries. We run a suite of DNSs varying microscopic diffusivities of the fluid and the strength of the initial stratification. We find that convective layers still form as a result of the previously discovered γ-instability, which takes place whenever the local stratification drops below a critical threshold that only depends on the fluid diffusivities. We also find that the layers grow once formed, eventually occupying the entire domain. Our work thus recovers the results of previous studies, despite the new boundary conditions, suggesting that this behavior is universal. The existence of Saturn’s stably stratified core, today, therefore suggests that this threshold has never been reached, which places a new constraint on scenarios for the planet’s formation and evolution.
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巨行星稀释内核长期存在的制约因素
卡西尼环后地震学分析表明,土星内部存在一个稳定的分层,从中心一直延伸到半径的 60%,即今天公认的土星稀核。同样,木星上的重力测量结果表明,木星内部也存在一个稀释内核,其径向范围每周都受到限制。这些内核很可能处于双扩散状态,这就提出了它们的长期稳定性问题。事实上,之前在三周期域中进行的直接数值模拟(DNS)研究表明,在某些情况下,双扩散对流倾向于自发形成浅对流层,这些对流层会逐渐变粗,直到该区域变得完全对流。在本文中,我们使用不同的边界条件研究了双扩散对流中的分层条件,其中域边界的温度和成分通量是固定的。我们运行了一套 DNS,改变流体的微观扩散性和初始分层的强度。我们发现,由于之前发现的γ-不稳定性,对流层仍然会形成,每当局部分层下降到临界阈值以下时,γ-不稳定性就会发生,而临界阈值只取决于流体的扩散量。我们还发现,层一旦形成就会增长,最终占据整个域。因此,尽管采用了新的边界条件,我们的研究还是恢复了之前的研究结果,表明这种行为具有普遍性。因此,土星稳定分层内核的存在表明,这一阈值从未达到过,这为土星的形成和演化提供了新的约束条件。
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来源期刊
The Planetary Science Journal
The Planetary Science Journal Earth and Planetary Sciences-Geophysics
CiteScore
5.20
自引率
0.00%
发文量
249
审稿时长
15 weeks
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