Argon in the Atmospheres of Uranus and Neptune Reveals Where They Formed, Not What's Inside
A Southwest Research Institute study finds that gas inherited from the early solar system would swamp any signal from rock, changing how future probes should use the measurement.
Scientists hoping to learn whether Uranus and Neptune are mostly ice or mostly rock have been handed an unexpected answer: one of their favorite clues may not say much about the planets' interiors at all. A new study led by the Southwest Research Institute, published in The Planetary Science Journal, finds that atmospheric argon-40 is a better record of where the planets formed than of what lies beneath their cloud tops.
"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, the SwRI planetary scientist who led the work. "It is worth considering whether they may contain much more rock than their ice giant classifications suggest."
The argon-40 in those atmospheres has two possible sources. Some could have been inherited from the protosolar nebula, the cloud of dust and gas that formed the solar system about 4.6 billion years ago. The rest could come from billions of years of radioactive decay of potassium-40 in rocky material deep inside the planets. If researchers could separate the two, they reasoned, the radioactive share would indicate how much rock each planet holds.
To test this, Mousis's team combined calculations of argon-40 production with a numerical model of the protosolar nebula. The model tracked how primordial argon condensed, became trapped in ice particles, and moved around as the nebula evolved, before those particles accumulated into Uranus and Neptune. The result was lopsided. "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," Mousis said. The inherited contribution is more abundant than previously thought.
That means atmospheric argon-40 "probably tell[s] us more about the planets' formation environments and history than about their rocky or icy interiors," he said. The simulations also suggest that the planets' building blocks may have formed near an argon frost line, a boundary near the edge of the nebula where it was cold enough for argon gas to freeze into solid ice. Argon-bearing particles accumulating near that line could have shaped the chemistry of the material that formed the two planets.
"Understanding the composition of the material that formed Uranus and Neptune is essential for reconstructing their formation histories," said co-author Dr. Robin Canup, vice president of SwRI's Solar System Science and Exploration Division. "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 finding is a guide for future missions. Remote observations cannot reliably measure noble gases and isotope ratios, so the authors point to proposed probe missions such as NASA's Uranus Orbiter and Probe concept, which could sample them directly with sensitive mass spectrometers. Measurements of nitrogen, carbon and other atmospheric components may help determine which formation theories hold up. The paper's authors also include Dr. Christopher Glein, Dr. Mark Hofstadter and Dr. Jonathan Lunine, and the study follows an October 2026 SwRI-contributed paper suggesting the chemical origins of the two planets may differ despite their similar size and mass.
