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

Abstract

We aim at investigating whether the chemical composition of the outer region of the protosolar nebula can be consistent with current estimates of the elemental abundances in the ice giants. To do so, we use a self-consistent evolutionary disc and transport model to investigate the time and radial distributions of H 2 O, CO, CO 2 , CH 3 OH, CH 4 , N 2 and H 2 S, i.e. the main O-, C-, N and S-bearing volatiles in the outer disc. We show that it is impossible to accrete a mixture composed of gas and solids from the disc with a C/H ratio presenting enrichments comparable to the measurements (approx. 70 times protosolar). We also find that the C/N and C/S ratios measured in Uranus and Neptune are compatible with those acquired by building blocks agglomerated from solids condensed in the 10–20 AU region of the protosolar nebula. By contrast, the presence of protosolar C/N and C/S ratios in Uranus and Neptune would imply that their building blocks agglomerated from particles condensed at larger heliocentric distances. Our study outlines the importance of measuring the elemental abundances in the ice giant atmospheres, as they can be used to trace the planetary formation location, the origin of their building blocks and/or the chemical and physical conditions of the protosolar nebula. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.

Neptune and Uranus: ice or rock giants?

“…If Neptune formed at the ice line, then these CO-rich pebbles could explain Neptune’s large internal CO source without requiring excessive overall O/H enrichment or H 2 O abundance [93]. However, the CO ice line location is very sensitive to rapidly evolving disc conditions [94], with a general tendency to move inwards as the solar nebula cools [95], so considering disc evolution timescales is essential [96]. Recent modelling of condensation rates in the protosolar nebula also shows that production of solids near the ice line is less efficient than previously thought [96], making it difficult to form Neptune at this location effectively.…”

“…However, the CO ice line location is very sensitive to rapidly evolving disc conditions [94], with a general tendency to move inwards as the solar nebula cools [95], so considering disc evolution timescales is essential [96]. Recent modelling of condensation rates in the protosolar nebula also shows that production of solids near the ice line is less efficient than previously thought [96], making it difficult to form Neptune at this location effectively. Furthermore, it would be very difficult to form both Neptune and Uranus this way because the ice line would have to migrate from one planet forming location to the other, but remain stable for long enough to build planets in each location, making this scenario even less likely if Neptune and Uranus have similar internal compositions.…”

Sulfur Ice Astrochemistry: A Review of Laboratory Studies

“…On Mercury, for instance, solid phase elemental sulfur has been detected within the permanently shadowed polar regions (Sprague et al 1995). Oxides of sulfur and H 2 S have been observed in the atmospheres of Venus (Vandaele et al 2017a(Vandaele et al , 2017b and the ice giants Uranus and Neptune (Irwin et al 2018;Mousis et al 2020), respectively. On Mars, sulfur is found across the planetary surface in the form of sulfate minerals (King and McLennan 2010) and in organic molecules such as thiophenes (Eigenbrode et al 2018;Heinz and Schulze-Makuch 2020).…”

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

“…Öberg et al [26]). A difference of 10 AU could lead to substantial differences in the heavy element enrichments of the planets (see Mousis et al [27] and references therein). In addition, since the solid-surface densities at the two locations is different, the planetary growth history is also expected to differ (e.g.…”

“…At the same time, measuring the atmospheric composition from an entry probe can be used to better understand the connection between the atmosphere and the deep interior (e.g. Guillot [11]) as well as the origin of the planets (e.g.Mousis et al [27]). In addition, determining the rotation rates of the planets helps to tighten constraints on interior models.…”