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Teanby, N. A., Irwin, P. G. J., Moses, J. I., & Helled, R. (2020). Neptune and Uranus: ice or rock giants? Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 378(2187). https://doi.org/10.1098/rsta.2019.0489 Peer reviewed version Link to published version (if available): 10.1098/rsta.2019.0489 Link to publication record in Explore Bristol Research PDF-document This is the author accepted manuscript (AAM). The final published version (version of record) is available online via The Royal Society at https://royalsocietypublishing.org/doi/10.1098/rsta.2019.0489 . Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Submitted to Phil. Trans. R. Soc. A - Issue Page 2 of 19 1 2 3 4 5 Neptune and Uranus: ice or 6 7 rock giants? 8 rsta.royalsocietypublishing.org 1 2 3 9 N. A. Teanby , P. G. J. Irwin , J. I. Moses 10 4 11 and R. Helled 12 1 13 Research School of Earth Sciences, University of Bristol, Wills 14 Memorial Building, Queens Road, Bristol, BS8 1RJ, UK 15 2Atmospheric, Oceanic & Planetary Physics, University Article submitted to journal 16 of Oxford, Clarendon Laboratory, Parks Road, Oxford, 17 18 OX1 3PU. UK. 3 19 Subject Areas: Space Science Institute, 4750 Walnut Street, Suite 20 Solar System, Planetary Interiors,For Review205, Boulder, Only CO 80301, USA. 21 Planetary Atmospheres, Planet 4Institute for Computational Science, Center for 22 Formation 23 Theoretical Astrophysics & Cosmology, University of 24 Zurich, Winterthurerstr. 190, 8057, Zurich, Switzerland Keywords: 25 Neptune, interior, atmosphere, 26 Existing observations of Uranus and Neptune’s composition 27 fundamental physical properties can be fitted with 28 a wide range of interior models. A key parameter 29 Author for correspondence: in these models is the bulk rock:ice ratio and 30 N. A. Teanby models broadly fall into ice-dominated (ice giant) 31 e-mail: [email protected] and rock-dominated (rock giant) categories. Here we 32 consider how observations of Neptune’s atmospheric 33 temperature and composition (H2, He, D/H, CO, 34 CH4,H2O, and CS) can provide further constraints. 35 The tropospheric CO profile in particular is highly 36 diagnostic of interior ice content, but is also 37 controversial, with deep values ranging from zero 38 to 0.5 parts per million. Most existing CO profiles 39 imply extreme O/H enrichments of >250 times solar 40 composition, thus favouring an ice giant. However, 41 such high O/H enrichment is not consistent with 42 D/H observations for a fully mixed and equilibrated 43 Neptune. CO and D/H measurements can be 44 reconciled if there is incomplete interior mixing (ice 45 giant) or if tropospheric CO has a solely external 46 source and only exists in the upper troposphere 47 (rock giant). An interior with more rock than ice 48 is also more compatible with likely outer solar 49 system ice sources. We primarily consider Neptune, 50 but similar arguments apply to Uranus, which has 51 comparable C/H and D/H enrichment, but no 52 observed tropospheric CO. While both ice and rock 53 dominated models are viable, we suggest a rock 54 giant provides a more consistent match to available 55 atmospheric observations. 56 57 c The Author(s) Published by the Royal Society. All rights reserved. 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Page 3 of 19 Submitted to Phil. Trans. R. Soc. A - Issue 1 2 3 4 5 6 7 1 1. Introduction 2 8 2 The internal structure of giant planets is key to understanding how the solar system formed rsta.ro .................................................................. 9 3 and evolved over the last 4.6 billion years. While multiple spacecraft have visited Jupiter and 10 4 Saturn - including the Galileo, Juno, and Cassini orbiters - the Voyager 2 flybys of Uranus yalsocietypublishing.org Phil. Trans. R. Soc. A 0000000 11 5 in 1986 and Neptune in 1989 remain the only spacecraft visits to the icy giants. Information 12 6 from a single spacecraft flyby combined with subsequent remote observations provide a fairly 13 7 limited dataset in terms of constraining a planet’s internal structure, but some interpretations can 14 8 be inferred from Uranus and Neptune’s fundamental physical properties. Voyager 2 provided 15 9 the first accurate mass, radius, bulk density, and low-order gravitational harmonics for Uranus 3 16 10 and Neptune [1–4]. Early interpretations of planetary bulk densities of 1.3 kg m− for Uranus 3 17 11 and 1.6 kg m− for Neptune suggested interiors dominated by ices [5,6], although the non- 18 12 uniqueness of constraints on the interior was considerable, and rock-dominated interiors are 19 13 also possible [7–10]. The Voyager 2 data have since been reanalysed and augmented with stellar 20 14 occultations and observations of the orbits of Uranus and Neptune’s moons and rings to provide 21 15 more stringent constraintsFor (reviewed Review in [9]). While it is clearOnly that Uranus and Neptune have a high 22 16 fraction of heavy elements, with an overall metallicity mass fraction in the range 0.7–0.9 [9], it is 23 17 still unclear whether rock or ice is the dominant component. Therefore, Uranus and Neptune’s 24 18 internal structure remains elusive despite decades of observation and modelling. Uranus and 25 19 Neptune could either be rock giants or ice giants, which has important implications for both their 26 20 formation and the formation of the solar system as a whole. 27 21 Voyager 2 observations of Uranus and Neptune’s magnetic fields indicate highly non-dipolar 28 22 structures [11–14]. Such fields may suggest dynamo action is limited to a conducting near-surface 29 23 layer [15]. This additionally constrains Uranus and Neptune’s interiors by requiring some kind 30 24 of conducting fluid layer to support dynamo action [16]. In the case of an ice giant this could be a 31 25 superionic form of water ice that exists at high temperature and pressure [17]. In the case of a rock 32 26 giant, magma dynamos such as those hypothesised on super-earths, might be a possibility [18]. 33 27 However, even though silica is conductive at high pressure, it is most likely to be solid at the 34 28 high pressures of a planetary core [19]. Mixtures of hydrogen and silicate present in some models 35 29 [20] might have lower melting points, but it is unknown if they could support dynamo action in 36 30 Uranus or Neptune’s interior because of the wide range of possible conditions and limited high 37 31 pressure and temperature laboratory data. 38 32 Many studies have combined observations of fundamental physical properties and high- 39 33 pressure thermodynamic equations of state to infer Uranus and Neptune’s internal structures 40 34 [6–8,21–23]. These model-data fits are highly non-unique, but can be split into two main types 41 35 representing either ice giant or rock giant scenarios [8]. Here we define ice and rock giants as 42 36 having a rock:ice ratio less than or greater than unity respectively. Ice giant models typically 43 37 have an outer gas envelope of hydrogen and helium, an intermediate region dominated by 44 38 ices, and a small core of silicates, iron, and nickel. Such models are up to 90% ice. Rock giant 45 39 models also have an outer hydrogen/helium gas envelope, but have a bulk interior dominated 46 40 by silicates mixed with hydrogen/helium along with some ices. These models are up to 70% rock. 47 41 An ice giant interpretation is currently favoured in the literature. However, an important source 48 42 of uncertainty is that interiors dominated by rock mixed with light elements such as hydrogen 49 43 have a similar density to ice mixtures, so could also fit the observed physical properties [5,8]. 50 44 An additional complication is that, while simple models with distinct layers are appealing, they 51 45 are not required to fit the observations. Recent work shows there may be significant mixing near 52 46 internal interfaces, leading to more continuous density profiles [9,20,22]. This widens the range 53 47 of possible internal structures significantly and leaves open the question of whether silicate-gas 54 48 mixtures or ice mixtures dominate Uranus and Neptune’s interiors [9,24–26]. 55 49 In summary, there are a wide suite of interior models for Uranus and Neptune, both ice- 56 50 and rock-dominated, with sharp or gradual layer transitions, but none are definitively preferred 57 51 by observations of fundamental physical properties. This is severely unconstrained - even by 58 59 60 http://mc.manuscriptcentral.com/issue-ptrsa Submitted to Phil. Trans. R. Soc. A - Issue Page 4 of 19 1 2 3 4 5 6 52 planetary science standards - and leaves both Uranus and Neptune’s origin and the evolution 7 3 53 of the outer solar system open to rampant speculation. This uncertainty is one of the major 8 rsta.ro 54 motivations for a new ice giant mission [27–30]. In this review we take an alternative approach by .................................................................. 9 55 considering if atmospheric observations can provide further insight into internal structure. For 10 56 Uranus, the atmospheric observations provide less of a constraint than on Neptune as there are yalsocietypublishing.org Phil.

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