Researchers at Southwest Research Institute led a study measuring argon-40 in the atmospheres of Uranus and Neptune to learn about the planets' interiors and origins. The work appears in The Planetary Science Journal.
Dr. Olivier Mousis and his team investigated whether argon-40 could distinguish between rock-dominated and ice-dominated interiors on these ice giants. Argon-40 in their atmospheres comes from two sources: some inherited from the protosolar nebula 4.6 billion years ago, and some produced by radioactive decay of potassium-40 in rocky materials deep inside the planets.
The team combined calculations of argon-40 production with a numerical model of the protosolar nebula to track how argon condensed and traveled inside ice particles as the nebula evolved. These particles accumulated to 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," Mousis said. The simulations revealed that primordial argon contributions are more abundant than previously thought.
This means atmospheric argon-40 measurements reveal more about where the planets formed than about their rocky or icy composition. The team's simulations showed materials rich in argon may have formed near an argon frost line, a boundary in the protosolar nebula where temperatures condense argon gas into ice.
Dr. Robin Canup, vice president of SwRI's Solar System Science and Exploration Division and a co-author, noted that understanding the material composition is essential for reconstructing formation histories. Future probe missions like NASA's proposed Uranus Orbiter and Probe could directly sample noble gases and isotopic ratios, potentially helping determine how the ice giants formed.