Argon-40 may tell scientists less about the rocky content of Uranus and Neptune than they once assumed, and more about the environment in which those planets were born. A study led by Olivier Mousis of the Southwest Research Institute, published in The Planetary Science Journal, makes that argument about the two outermost planets, which astronomers classify as ice giants.
The interiors of Uranus and Neptune remain poorly understood, and that gap has made their composition difficult to pin down. Researchers have proposed using argon-40 as a tracer of how much rock the planets contain. The isotope has two possible origins: a primordial share that was present when the solar system formed, and a radiogenic share produced when potassium-40 decays inside rock. The radiogenic portion is the part that would link argon to rocky material.
Mousis and his colleagues argue that the primordial contribution may dominate, which would weaken the link between argon-40 and rock. In that case, the isotope could instead reflect the conditions in the outer protosolar nebula, the disk of gas and dust from which the planets formed. The authors suggest that a snow line for argon may have existed there, a boundary that would have shaped how the gas was captured during formation.
Noble gases such as argon are chemically inert and stay stable through many of the processes that follow a planet's birth, which is why they are considered useful records of early conditions. Measuring them directly, along with their isotopes, would require instruments carried by atmospheric probes. The study therefore points to a proposed Uranus Orbiter and Probe mission as a priority for testing these ideas.
Credit. Based on reporting by Giuseppe Fiasconaro for Media INAF. Is this your work? Write to us.
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