Carbon monoxide has been detected in Uranus’s lower atmosphere for the first time, pointing to an ice-rich interior and overturning arguments that the planet is more rock-dominated. Scientists used observations from the Atacama Large Millimeter/submillimeter A
For years, Uranus has sat in the uncomfortable middle of an argument about what’s actually inside it. Neptune. its neighbour among the so-called ice giants. shows abundant carbon monoxide in its atmosphere—a clue that its interior may be rich in water or ice. Uranus, by contrast, has been stubbornly lacking. That absence fueled competing claims: perhaps Uranus is built differently, with a more rocky interior than Neptune.
Now the missing piece has turned up in the lower atmosphere of Uranus itself.
Thibault Cavalié at the University of Bordeaux in France and his colleagues have found carbon monoxide in Uranus’s lower atmosphere for the first time. The result suggests Uranus is far richer in water ice than previously suspected, shifting the planet from one side of the debate toward the other.
“ We find that Uranus is more on the ice-giant side than on the rock-giant side,” Cavalié says. “It tells us that this controversy is over now. We have to be careful when we say things like that, because things also depend on modelling, but that’s the feeling we have.”
The work rests on three observing runs with the Atacama Large Millimeter/submillimeter Array telescope in Chile. Cavalié and his team observed Uranus three times between 2022 and 2024. In those measurements, they detected significant amounts of carbon monoxide in the planet’s lower atmosphere.
Then came the hard part: making the gas fit a physical picture. The team ran several models using different ratios of rock and ice. aiming to reproduce the carbon monoxide levels they measured. Their modelling effort converged on a clear conclusion—only the ice-rich models could reproduce the observed amount of carbon monoxide.
Carbon monoxide also showed up in Uranus’s upper atmosphere. But that detection does not necessarily trace the planet’s interior. Cavalié says it likely comes from a different source: probably a comet striking Uranus several centuries ago.
The discovery is a major step toward understanding Uranus’s deep interior, because carbon monoxide in a planet’s atmosphere is often tied to what lies in the deepest parts—especially when the interior is rich in water or ice. Still, the science isn’t finished.
Vanesa Ramirez at Leiden University in the Netherlands cautions that atmospheric measurements are never straightforward. “Interpreting atmospheric abundances requires assumptions about chemistry. mixing and internal structure. all of which remain uncertain for Uranus.” She adds that the combination of uncertain assumptions and the wide range of models used to simulate Uranus’s interior means there are many different rock-to-ice ratios that can remain consistent with the available data.
“On its own, it does not settle the question of whether Uranus should be regarded primarily as an ice-rich or rock-rich giant,” Ramirez says.
Even so. the direction of travel is unmistakable: Uranus no longer looks like a planet that lacks the atmospheric fingerprint associated with an ice-rich centre. Whether the final picture is settled by future observations and refined models. the argument that Uranus’s interior must be mostly rocky has lost a key foothold—and the planet’s long mystery just grew a new. ice-heavy shape.
Uranus carbon monoxide ice-giant interior water ice Atacama Large Millimeter/submillimeter Array ALMA astronomy planetary science