Monsters of rock: are Uranus and Neptune rock giants?

Introduction

To understand solar system formation it is critical to know how Uranus and Neptune formed. This requires knowledge of internal composition. Uranus and Neptune are generally referred to as ”ice-giants” in recent literature, as it has been inferred that their interiors are ice-dominated. Physical measurements from the Voyager 2 flybys include mass, radius, oblateness, low order gravity coefficients, moments of inertia, and magnetic field snapshots. One fundamental issue is that high temperature and pressure ice mixtures have similar densities to silicates mixed with hydrogen and helium. Therefore, existing physical constraints cannot by themselves distinguish between ice or rock-dominated interiors and almost any interior model fits the observations [5,10]. Measurements of atmospheric composition and temperature provide a possible complementary window into these planets’ interiors and may provide a way to break the degeneracy [12]. Here we consider the case for ice and rock-dominated interiors and attempt to propose a consistent explanation.

The case for Ice Giants

Internally-generated magnetic fields are observed at Uranus and Neptune [11]. Fields are highly non-dipolar, suggesting a shallow origin. Magnetic field generation requires conducting fluid and the conventional explanation is super-ionic water at high temperature and pressure, implying ice-dominated interiors.

Spectroscopic atmospheric CO observations provide further evidence favouring the ice giant model. CO has higher abundance in Uranus’ and Neptune’s stratospheres than in their tropospheres, indicating an external source [7]. Uranus has ~8 ppb stratospheric CO and <2 ppb tropospheric CO, which can mostly be accounted for with background interplanetary dust particle flux. Conversely, Neptune’s stratospheric CO abundance is the largest of any giant planet at ~1000 ppb. The only way to feasibly explain this is with a kilometre-scale ancient comet impact and shock chemistry, where cometary water reacts with methane in Neptune’s atmosphere to form CO [7,9].

More relevant to the interior is that Neptune appears to have ~100 ppb tropospheric CO. Conventionally, this is explained by quenching CO dredged up from the deep interior by Neptune’s vigorous tropospheric mixing. Thermochemical models predict ~400 x O/H enrichment over solar abundance is required to reproduce this CO amount [2,8]. This extreme enrichment requires ~90% water ice in Neptune’s interior, again implying an ice-dominated interior. It is usually extrapolated that Uranus is also an ice giant with a similarly extreme oxygen and ice abundance, where the lack of CO in Uranus’ troposphere is conveniently explained by more sluggish tropospheric mixing.

Issues with the Ice Giant model

Although ice-dominated interiors can explain many observational aspects of Uranus and Neptune, there are also some worrying discrepancies.

1) Most icy bodies in the outer solar system have rock fractions of ~70%. If Uranus and Neptune formed from similar objects, then we require some explanation of where the missing rock fraction has gone or why the planetesimals that formed Uranus and Neptune are different to anything we observe today.

2) Measurements of atmospheric methane on Uranus and Neptune suggest deep abundances of a few percent [6]. This implies a C/H enrichment of ~50–100 x solar [1], which is much lower than that inferred for O/H from tropospheric CO.

3) D/H is ~4x10^-5 on both Uranus and Neptune [3]. This is much lower than D/H observed in modern solar system icy objects such as comets, which typically have D/H ∼15–60x10^-5. If interiors of Uranus and Neptune are well mixed and equilibrated, this implies only ~15% of the interiors can be ice, suggesting ~50–100 x solar enrichment [12]. Again, much lower than inferred from CO. A way around this is for interiors to only be partially mixed and equilibrated, with more D hiding in the unobservable deep atmosphere. Alternatively, some form of extinct exotic ices with lower D/H could be the source material.

In summary, exotic ices, incomplete interior mixing, and unusually ice-rich planetesimals have all been invoked to make atmospheric observation consistent with the ice giant model. Not impossible, but also not entirely convincing as an explanation.

Rock Giant interiors as a potential solution

The alternative is that Uranus and Neptune’s interiors are rock-dominated. In this case we need to explain magnetic field generation and Neptune’s tropospheric CO.

Recent work shows mixtures of silicates, hydrogen, and helium may be conductive at relevant pressures and temperatures, so super-ionic water is not necessarily required to generate magnetic fields [4]. Alternatively, there is no-doubt some ice in Uranus and Neptune’s interiors, which may form thin shell dynamos and explain non-dipolar field structures.

Recent work also shows tropospheric CO may not actually be present throughout the troposphere and may be limited to the upper troposphere [12,13]. In this case, CO could be entirely sourced externally from comets.

Profiles with CO limited to pressures <1 bar can fit spectroscopic observations very well, but require reduced upper troposphere eddy mixing to allow CO to survive long enough post-comet-impact to still be observable today. This seems plausible, as inspection of the Voyager 2 temperature profile and lapse rate suggest the upper troposphere is relatively stable [12]. Furthermore, Far-IR brightness temperatures suggest the boundary between radiative and convective zones may be ~1 bar.

Conclusion

Recent advances in our understanding of CO profiles on Neptune and high-pressure conductivity of silicate/hydrogen/helium mixtures suggests that rock-dominated interiors for Uranus and Neptune are becoming more plausible than conventional ice giant scenarios. Such a rock giant could be formed from planetesimals with similar rock:ice ratios and D/H ratios to modern-day outer solar system comets, Kuiper belt objects, and icy moons. Interiors could also be well mixed and equilibrated. This opens the possibility of simpler formation mechanisms for Uranus and Neptune, with both planets forming in similar ways, and avoiding any requirements for dubious ice compositions.

References

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[13] Teanby+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..319...86T/abstract