Inherited argon complicates estimates of rock inside Uranus and Neptune
Argon could help reveal how much rock lies inside Uranus and Neptune, but its origin complicates the calculation. Peer-reviewed modelling separates gas inherited during planet formation from argon formed through radioactive decay. Depending on formation conditions, the inherited component can dominate; other argon isotopes and atmospheric carbon could help distinguish the contributions.
Science··Midday
Argon has two origins inside ice giants
Researchers modelled how argon isotopes could reveal the interiors of Uranus and Neptune, the ice giants. The peer-reviewed theoretical study considers both gas inherited during planet formation and gas formed later inside rocky material.[1], [2]
Argon-40 can form through radioactive decay of potassium-40 in a rocky interior. If it reaches the atmosphere, its abundance could constrain the amount of rock. But primordial argon-40, acquired while the planet grew, adds to the same inventory. Calculations combined radioactive production with the changing chemistry and temperature of the protosolar nebula, the early gas and dust cloud.[1]
Formation conditions produce a wide range
Argon can be trapped as pure ice or within cages of ice. Where and when planetary building blocks formed therefore changes the predicted primordial inventory. Combining models with observed atmospheric carbon abundances yielded enrichments of roughly one to 610 times the protosolar value for Uranus and one to 1,220 times for Neptune. These are conditional model ranges, rather than new atmospheric measurements.[1]
At low primordial enrichment, argon-40 formed through decay can constrain the rocky interior. At high enrichment, the same isotope mainly reflects formation conditions. Results also depend on the compounds carrying carbon, planetary migration during growth and transport between the deep interior and atmosphere.[1]
Other isotopes could separate the contributions
The nonradiogenic argon isotope could help distinguish inherited gas from the decay contribution. The initial isotope ratio remains poorly known: The Galileo probe's descent into Jupiter did not detect argon-40. Measurements at Jupiter, Saturn or comets could narrow that reference inventory. Extremely high argon enrichment might also produce upper-atmosphere clouds, although deeper entry probes could still sample bulk abundance.[1]
