Probing the Rock Mass Fraction and Transport Efficiency inside Uranus Using 40Ar Measurements

IF 3.8 Q2 ASTRONOMY & ASTROPHYSICS The Planetary Science Journal Pub Date : 2024-05-06 DOI:10.3847/psj/ad3b93
Francis Nimmo, Jonathan Lunine, Kevin Zahnle and Lars Stixrude
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Abstract

The bulk of Uranus consists of a rock–ice core, but the relative proportions of rock and ice are unknown. Radioactive decay of potassium in the silicates produces 40Ar. If transport of argon from the core to the gaseous envelope is efficient, a measurement of 40Ar in the envelope will provide a direct constraint on the rock mass present (assuming a chondritic rock composition). The expected 40Ar concentrations in this case would be readily detectable by a mass spectrometer carried by a future atmospheric probe. For a given envelope concentration there is a trade-off between the rock mass present and the transport efficiency; this degeneracy could be overcome by making independent determinations of the rock mass (e.g., by gravity and seismology). Primordial 40Ar is a potential confounding factor, especially if Ar/H2 is significantly enhanced above solar or if degassing of radiogenic 40Ar were inefficient. Unfortunately, the primordial 40Ar/36Ar ratio is very uncertain; better constraints on this ratio through measurement or theory would be very helpful. Pollution of the envelope by silicates is another confounding factor but can be overcome by a measurement of the alkali metals in the envelope.
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利用 40Ar 测量探测天王星内部岩石质量比例和传输效率
天王星的大部分由岩冰核心组成,但岩石和冰的相对比例尚不清楚。硅酸盐中钾的放射性衰变产生 40Ar。如果氩从内核到气态包层的传输是有效的,那么对包层中 40Ar 的测量将对存在的岩石质量提供直接的约束(假设岩石成分为软玉体)。在这种情况下,未来大气探测器携带的质谱仪将很容易检测到预期的 40Ar 浓度。对于给定的包络线浓度,存在的岩石质量和传输效率之间存在权衡;通过对岩石质量进行独立测定(例如,通过重力和地震学),可以克服这种退化。原始 40Ar 是一个潜在的干扰因素,尤其是当 Ar/H2 显著高于太阳时,或者放射性 40Ar 的脱气效率很低时。遗憾的是,原始 40Ar/36Ar 的比率非常不确定;通过测量或理论对这一比率进行更好的约束将非常有帮助。硅酸盐对包层的污染是另一个干扰因素,但可以通过测量包层中的碱金属来克服。
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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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