Disentangling Primordial and Radiogenic Argon in Uranus and Neptune

Understanding the bulk compositions of Uranus and Neptune remains a major challenge because their interiors cannot be directly sampled and their atmospheric compositions are only weakly constrained by remote sensing. In particular, the relative proportions of rocky, icy, and gaseous material accreted during their formation remain uncertain, limiting our understanding of ice giant formation in the protosolar nebula (PSN). Noble gases provide powerful diagnostics because they are chemically inert and preserve the signature of their formation environment. Among them, argon isotopes are especially promising because they can distinguish primordial material inherited from the PSN from radiogenic components released from rocks in planetary interiors. In particular, 40Ar is produced by the radioactive decay of 40K, a potassium isotope hosted in rocky material. Measuring atmospheric 40Ar in Uranus and Neptune could therefore constrain the amount of rocky material accreted by the ice giants.

A recent study [1] used chondritic potassium abundances and plausible degassing efficiencies to estimate the atmospheric 40Ar/H2​ ratios expected in Uranus and Neptune as a function of their rocky mass fraction. That study showed that radiogenic argon may contribute significantly to the present-day atmospheric argon inventory of the ice giants. Depending on the assumed rocky fraction and degassing efficiency, the predicted 40Ar abundances span several orders of magnitude and could be detectable by future entry probe mass spectrometers. The results further suggested that even moderate-precision measurements of 40Ar would provide meaningful constraints on the amount of rocky material incorporated into the planets.

A major challenge, however, is that atmospheric 40Ar is not expected to be purely radiogenic because a primordial component inherited from the PSN was likely incorporated during planet formation. The goal of the present study is therefore to evaluate the contribution of primordial argon delivered to Uranus and Neptune. This component depends on volatile trapping and transport processes operating in the PSN, including condensation of pure argon ice at very low temperatures (~20 K) and trapping within clathrate hydrates formed from crystalline water ice. We investigate a range of plausible primordial argon enrichments associated with volatile condensation fronts and argon icelines in the PSN, where local enhancements may substantially increase the amount of primordial argon incorporated into the solids accreted by the forming planets.

Specific attention is paid to the evolution of the 40Ar/C abundance ratio in the PSN as a function of time and heliocentric distance within the giant planet formation region. Near argon condensation fronts and icelines, this ratio can exceed the protosolar value by more than an order of magnitude [2, 3]. Combined with the strong carbon enrichment measured in Uranus’ atmosphere (50-100× protosolar), these conditions suggest that substantial amounts of primordial 40Ar may have been incorporated into the planet’s envelope during its formation. However, the presence of this primordial component introduces significant degeneracies when interpreting atmospheric 40Ar measurements. For example, a planet with a relatively small rocky fraction but strong primordial argon enrichment could exhibit atmospheric 40Ar abundances comparable to those of a planet with a larger rocky inventory but weaker primordial enrichment. To quantify these effects, we calculate the evolution of atmospheric 40Ar as a function of the rocky mass fraction under different assumptions for the primordial contribution. The resulting trends demonstrate that the interpretation of future measurements critically depends on disentangling primordial and radiogenic sources of argon.

Fortunately, argon isotopic measurements provide a powerful way to break these degeneracies. Primordial argon preserves the isotopic composition inherited from the PSN, whereas radioactive decay selectively enriches 40Ar. Simultaneous measurements of multiple argon isotopes can therefore disentangle primordial and radiogenic contributions. In particular, isotopic ratios involving 36Ar, 38Ar, and 40Ar provide a direct diagnostic of the radiogenic excess and, consequently, of the rocky component incorporated into the planets. In parallel, the total argon enrichment may constrain the formation location of the planetary building blocks within the PSN, because argon incorporation strongly depends on volatile trapping processes operating near argon icelines and condensation fronts. Together, these measurements would place strong constraints on the interiors, accretion histories, and formation pathways of Uranus and Neptune.

The study highlights the importance of future in situ atmospheric measurements at Uranus and Neptune. Because noble gases lack strong infrared and microwave spectroscopic signatures, their abundances and isotopic ratios cannot be reliably determined by remote sensing. Entry probes equipped with high-resolution mass spectrometers are therefore required to measure argon isotopes with sufficient sensitivity and precision, while also enabling measurements of other noble gases and isotopic systems. Such measurements would provide key constraints on the rocky fractions, accretion histories, migration pathways, and volatile delivery mechanisms of the ice giants, thereby helping reconstruct where and when they formed within the evolving PSN.

More broadly, our study confirms that argon isotopes constitute a promising and largely unexplored diagnostic of the rocky component of ice giants, with implications extending to the interpretation of extrasolar ice giants and sub-Neptune planets, which appear to be common outcomes of planet formation around other stars.

References

[1] Nimmo, F., Lunine, J., Zahnle, K., Stixrude, L. 2024. Probing the Rock Mass Fraction and Transport Efficiency inside Uranus Using 40Ar Measurements. The Planetary Science Journal 5. doi:10.3847/PSJ/ad3b93

[2] Mousis, O. and 8 colleagues 2024. Insights on the Formation Conditions of Uranus and Neptune from Their Deep Elemental Compositions. The Planetary Science Journal 5. doi:10.3847/PSJ/ad58d8

[3] Schneeberger, A., Mousis, O., Aguichine, A., Lunine, J.I. 2023. Evolution of the reservoirs of volatiles in the protosolar nebula. Astronomy and Astrophysics 670. doi:10.1051/0004-6361/202244670