When the Voyager 2 spacecraft whizzed past Uranus in January 1986, it revealed the planet's dark, delicate rings and its pale cyan atmosphere. Precisely what lies beneath the ice giant's thick atmosphere is unknown, though researchers expect that the planet's core is made of rock and ice.

But just how much of the core is made of rock is unknown, and it's likely to be challenging to measure. Space-based measurements of gravitational pull are are often used to infer a planet's interior density structure. However, if some of Uranus's atmospheric gas is mixed into the rock, the mixture will have a density similar to that of ice, making it impossible to differentiate between rock and ice using gravity measurements. How, then, can we tell how much rock is in Uranus's core?

Noble Gas Method
Francis Nimmo (University of California, Santa Cruz) and collaborators proposed that the amount of rock in Uranus's core could be calculated by measuring the concentration of argon-40 in its atmosphere. Argon-40, a form of the noble gas argon containing 40 neutrons, is the most common type of argon in Earth's atmosphere.

Argon can be produced through the radioactive decay of potassium, which clings to silicate-rich materials like the rocks thought to be present in Uranus's core. As potassium slowly decays to argon with a 1.25-billion-year half-life, the newly produced argon diffuses into the planet's atmosphere. By measuring the amount of argon in Uranus's atmosphere, Nimmo's team suggests, researchers can infer the amount of potassium - and rock, by extension - in the planet's core.

A Complex Measurement
Nimmo and coauthors find that if the transport of argon from Uranus's core to its atmosphere is efficient, an atmospheric probe could easily measure the concentration of argon-40. But because there appears to be a trade-off between the mass of the rock core and how efficiently it propels argon-40 into the atmosphere, a spacecraft would need to measure the total mass of the core through gravity measurements or seismology to get a final estimate of how much rock is in the core.

There are other possible complications: some argon in Uranus's atmosphere was likely already present when the planet was swirled together from the nebula that birthed the Sun and the planets. To disentangle the argon produced in the core from the argon present since the planet's birth, a visiting spacecraft would need to measure the ratio of argon-40 to argon-36, a form of the element that is produced in supernovae. This ratio would then need to be compared to the primordial ratio of the two forms of argon, which is not known precisely.

The opportunity to test the authors' theory may lie ahead: a Uranian orbiter and probe was the top priority in the 2023-2032 Planetary Science and Astrobiology Decadal Survey. With two decades or more until the possible arrival of such a spacecraft, scientists have time to contemplate how to measure the makeup of Uranus's core.

Research Report:Probing the Rock Mass Fraction and Transport Efficiency Inside Uranus Using 40Ar Measurements

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