稀有气体行星学与天王星的氙云
Noble Gas Planetology and the Xenon Clouds of Uranus
AI总结：
本文探讨稀有气体作为示踪剂在天王星大气探测器中的应用，通过分析氩、氪、氙的比值与同位素特征来揭示天王星的物质来源与行星起源，并指出氙凝结既带来挑战也提供同位素测量机会。
摘要：
稀有气体提供了宇宙来源的示踪剂，可供未来的天王星大气探测器进行探测。氩和氪预计在天王星上与H$_2$和He充分混合，尽管在冬季极地可能发生凝结。Ar/H$_2$和Ar/Kr比值可揭示天王星吸积的物质是类似于Jupiter大气吸积的极冷物质，还是类似于彗星67P/Churyumov-Gerasimenko的较温暖物质，抑或两者皆非。氙在天王星上对流层上部以冰的形式凝结，可能凝结在甲烷冰上。凝结可能使Xe/H$_2$的解读复杂化，但也提供了收集浓缩氙样本以测量同位素的机会。太阳系Xe追踪三个不同的核合成氙储库：一见于Sun和球粒陨石，二见于难熔 presolar grains，三见于彗星67P/C-G和地球大气。第一和第三储库似乎捕获自不同气体云。这两种气体似乎未充分混合；此外，67P/C-G中高$^{129}$Xe/$^{132}$Xe比值意味着气体在初始核合成$^{129}$I（半衰期15.7 Myr）衰变前即被捕获。若在天王星观测到氙的同位素异常，将有助于颠覆我们对行星起源的理解。氪的同位素异常较细微，可能难以测量。氖和氦的分馏有微小的机会可用于约束天王星获取星云包层的方式。
英文摘要：
Noble gases provide tracers of cosmic provenance that are accessible to a future Uranus Atmospheric Probe. Argon and krypton are expected to be well-mixed on Uranus with respect to H$_2$ and He, although condensation at the winter pole may be possible. The Ar/H$_2$ and Ar/Kr ratios address whether the materials accreted by Uranus resembled the extremely cold materials accreted by Jupiter's atmosphere, or whether they were warmer like comet 67P/Churyumov-Gerasimenko, or whether Uranus is like neither. Xenon condenses as an ice, probably on methane ice, in Uranus's upper troposphere. Condensation may complicate interpretation of Xe/H$_2$, but it also presents an opportunity to collect concentrated xenon samples suitable for measuring isotopes. Solar System Xe tracks three distinct nucleosynthetic xenon reservoirs, one evident in the Sun and in chondritic meteorites, a second in refractory presolar grains, and a third evident in comet 67P/C-G and in Earth's air. The first and third reservoirs appear to have been captured from different clouds of gas. The two gases do not appear to have been well-mixed; moreover, the high $^{129}$Xe/$^{132}$Xe ratio in 67P/C-G implies that the gas was captured before the initial nucleosynthetic complement of $^{129}$I (15.7 Myr half-life) had decayed. Xenon's isotopic peculiarities, if seen in Uranus, could usefully upset our understanding of planetary origins. Krypton's isotopic anomalies are more subtle and may prove hard to measure. There is a slight chance that neon and helium fractionations can be used to constrain how Uranus acquired its nebular envelope.
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