Existing observations of Uranus and Neptune’s fundamental physical properties can be fitted with a wide range of interior models. A key parameter in these models is the bulk rock:ice ratio and models broadly fall into ice-dominated (ice giant) and rock-dominated (rock giant) categories. Here we consider how observations of Neptune’s atmospheric temperature and composition (H2, He, D/H, CO, CH4, H2O and CS) can provide further constraints. The tropospheric CO profile in particular is highly diagnostic of interior ice content, but is also controversial, with deep values ranging from zero to 0.5 parts per million. Most existing CO profiles imply extreme O/H enrichments of >250 times solar composition, thus favouring an ice giant. However, such high O/H enrichment is not consistent with D/H observations for a fully mixed and equilibrated Neptune. CO and D/H measurements can be reconciled if there is incomplete interior mixing (ice giant) or if tropospheric CO has a solely external source and only exists in the upper troposphere (rock giant). An interior with more rock than ice is also more compatible with likely outer solar system ice sources. We primarily consider Neptune, but similar arguments apply to Uranus, which has comparable C/H and D/H enrichment, but no observed tropospheric CO. While both ice- and rock-dominated models are viable, we suggest a rock giant provides a more consistent match to available atmospheric observations. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.

中文翻译：

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海王星和天王星：冰巨星还是岩石巨星？

对天王星和海王星基本物理特性的现有观测可以与各种内部模型相拟合。这些模型中的一个关键参数是块体岩石：冰的比例，模型大致分为以冰为主（冰巨星）和以岩石为主（巨岩）的类别。在这里，我们考虑对海王星大气温度和成分（H2、He、D/H、CO、CH4、H2O 和 CS）的观测如何提供进一步的约束。对流层 CO 分布尤其可以高度诊断内部冰含量，但也存在争议，深度值范围从 0 到 0.5 ppm。大多数现有的二氧化碳剖面表明，O/H 的极端富集程度超过太阳成分的 250 倍，因此有利于冰巨星。然而，如此高的 O/H 富集度与完全混合和平衡的海王星的 D/H 观测结果并不一致。如果内部混合不完全（冰巨星）或者对流层 CO 仅有外部来源并且仅存在于对流层上层（岩石巨星），则 CO 和 D/H 测量值可以协调一致。岩石比冰更多的内部也更容易与可能的太阳系外冰源兼容。我们主要考虑海王星，但类似的论点也适用于天王星，天王星具有相当的 C/H 和 D/H 富集度，但没有观测到对流层 CO。虽然冰和岩石主导的模型都是可行的，但我们建议岩石巨星提供了更多与现有的大气观测结果一致。本文是“冰巨系统的未来探索”讨论会议问题的一部分。