SAN ANTONIO — November 11, 2026 — Measurements of an argon isotope could help scientists understand whether the interiors of Uranus and Neptune, our solar system's so-called ice giants, are dominated by ice or rock. New research led by Southwest Research Institute (SwRI) and published in The Planetary Science Journal measured the planets' atmospheric argon-40 to estimate the amount of rock found beneath their surfaces and search for clues about the planets' origins, including where and how they formed.
"Despite decades of research, we still have surprisingly little information about what lies beneath the atmospheres of Uranus and Neptune and their physical compositions," said Dr. Olivier Mousis, a planetary scientist in SwRI's Solar System Science and Exploration Division in Boulder, Colorado, who led the study. "It is worth considering whether they may contain much more rock than their ice giant classifications suggest. We investigated whether argon-40 could help us distinguish between these possibilities, or if there are better uses for this unique chemical marker in future research."
The argon-40 found in the atmospheres of Uranus and Neptune have two possible origins. Some of the atmospheric argon-40 could have been inherited from the protosolar nebula, the cloud of dust and gas that formed the solar system approximately 4.6 billion years ago. Additional argon-40 could be explained by billions of years of radioactive decay of the potassium-40 found in rocky materials deep inside the planets. By distinguishing between the ratios of argon-40 that the planets inherited during formation and the argon-40 produced in their interiors that reached their atmospheres, the team believed that they could potentially extrapolate the amount of rock each planet contains.
The main challenge facing the team was determining exactly how to interpret between the two possible origins for the isotope. To address this problem, Mousis and his team combined calculations of argon-40 production with a numerical model of the protosolar nebula. The model tracked the condensation, trapping and transport of "primordial" argon inside ice particles as the nebula evolved and then which accumulated to help form Uranus and Neptune.
"Based on our research, the primordial argon element that made its way to Uranus and Neptune and its isotopes would completely overwhelm any contributions produced by radioactive decay inside the planets," said Mousis, adding that the argon-40 contributions from the protosolar nebula are more abundant than previously thought. "Taking this under consideration, we have concluded that atmospheric argon-40 measurements probably tell us more about the planets' formation environments and history than about their rocky or icy interiors."
The team's simulations revealed that the materials for planets particularly rich in argon may have formed near an argon frost line, a boundary near the edge of the protosolar nebula where low temperatures condense argon gas into solid ice. The accumulation of argon-bearing particles near the frost line could have influenced the chemical composition of the materials that formed Uranus and Neptune.
"Understanding the composition of the material that formed Uranus and Neptune is essential for reconstructing their formation histories," said Dr. Robin Canup, vice president of SwRI's Solar System Science and Exploration Division, and a co-author of the study. "Our results demonstrate how the evolving conditions in the early solar system could leave chemical signatures that persist in planetary atmospheres billions of years later."
The findings may help guide future study of the atmospheres of Uranus and Neptune. According to the researchers, future probe missions, such as the proposed NASA Uranus Orbiter and Probe mission concept, could directly sample noble gases and isotopic ratios that cannot be reliably determined through remote observations. Measurements of nitrogen, carbon and other atmospheric components may help determine the veracity of theories about how the ice giants formed.
"Argon-40 measurements may help us answer two fundamental questions that have long eluded scientists," Mousis said. "What are Uranus and Neptune made of, and where did their building blocks form before they were incorporated into the planets? Future atmospheric probes equipped with sufficiently sensitive mass spectrometers could bring us closer to answering both."
