Neptune and Uranus: ice or rock giants?

Abstract

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 (H 2 , He, D/H, CO, CH 4 , H 2 O 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’.

The role of ice lines in the formation of Uranus and Neptune

“…However, such a large enrichment of O/H is not compatible with D/H measurements, which suggest more modest O/H enrichments of approximately 50−150 [25] if Neptune’s internal water was sourced from protoplanetary ices with D/H comparable to present day comets. These considerations point toward how challenging it is to derive the O/H ratio from CO [13,26]. Given those difficulties, the present work only takes into account the measurements of C, N and S in the envelopes of Uranus and Neptune (table 1).…”

The interiors of Uranus and Neptune: current understanding and open questions

“…However, we actually do not know if the compositions of Uranus and Neptune are dominated by these materials, and even Pluto seems to consist of more rock than ice [84]. Indeed it was shown that the observed properties of the planets can be fit also with a rock-dominated composition [63], and recently, it has been suggested that Neptune could be a ‘rock-giant’ based on measured atmospheric abundances (see Teanby et al [85] and references therein for further details). Also, although the argument that the planets must consist of large fractions of water to have high enough electrical conductivities (ionic/super-ionic water) to generate their magnetic fields is convincing (e.g.…”

Neptune’s HCl upper limit from Herschel/HIFI

“…The exact magnitude of elemental enrichment in Neptune is currently uncertain, but can be estimated using observations of trace species in Neptune's observable atmosphere. This is challenging because the observable atmospheric composition may not be directly related to the interior composition due to many atmospheric and interior processes (Atreya et al, 2020;Cavalié et al, 2020;Teanby et al, 2020). These processes include: exter-nal flux from micrometeorites and interplanetary dust particles (Moses and Poppe, 2017); comet impacts (Moreno et al, 2017;Lellouch et al, 2005); photochemical processing (Moses, 1992;Moses et al, 2018;Dobrijevic et al, 2020); internal thermochemical reactions (Lodders and Fegley, 1994;Cavalié et al, 2020;Venot et al, 2020); and uncertainty in mixing and equilibration processes in the deep interior (Helled and Stevenson, 2017;Helled et al, 2020).…”

“…Nevertheless, with caution some inferences from atmospheric observations can be made, assuming that the interior is well mixed. These are reviewed in detail by Teanby et al (2020), with the key observations summarised briefly here. Observations of CH 4 abundances of 2-4% by volume, at pressures higher than the 1-2 bar condensation level, provide the most reliable indicator of internal enrichment currently available (Atreya et al, 2020;Cavalié et al, 2020;Mousis et al, 2020), and suggest a C/H enrichment of ∼50-100 (Baines et al, 1995;Karkoschka and Tomasko, 2011;Tollefson et al, 2019;Irwin et al, 2019a).…”

“…CO observations imply even more extreme enrichments might be possible for O/H, with enrichments as high as 250-650 being suggested to explain to explain the presence of possibly internally-sourced CO in Neptune's troposphere (Lellouch et al, 2005;Luszcz-Cook and de Pater, 2013;Cavalié et al, 2017;Venot et al, 2020). Alternatively, if CO is externally sourced and only present in Neptune's stratosphere and upper troposphere because of downward mixing, O/H enrichments of ∼30-130 may be more reasonable (Teanby et al, 2019;Teanby et al, 2020).…”