A new detection of carbon monoxide deep in the atmosphere of Uranus is giving scientists a fresh look at the planet’s hidden interior. The finding suggests Uranus contains much more water ice than some researchers had proposed.
Uranus and Neptune are both classified as ice giants, yet their internal compositions remain poorly understood. Thick atmospheres hide their deeper layers from view, forcing scientists to rely on indirect clues to work out what these distant worlds are made of.
One of the biggest questions has been whether the ice giant is fundamentally different from Neptune. While it has long appeared rich in water and ice, the lack of carbon monoxide detected on Uranus had led some astronomers to suggest that the planet might contain a larger proportion of rock. The latest observations point in another direction.
Hidden Gas Finally Found on Uranus
The discovery comes from a team led by Thibault Cavalié of the University of Bordeaux, which observed the distant planet using theAtacama Large Millimeter/submillimeter Array (ALMA)in Chile. The planet was studied three times between 2022 and 2024.
The observations revealedsignificant amounts of carbon monoxide in Uranus’s lower atmosphere, marking the first time the gas has been identified in this region of the planet.
For planetary scientists, this gas is often linked to processes occurring deep inside giant planets and can provide clues about the presence of water-rich material hidden beneath the clouds.
The study noted that Neptune has long displayed abundant carbon monoxide. That evidence has generally supported the idea that the planet possesses an ice-rich interior. Uranus, by contrast, appeared to lack the same signature, creating uncertainty about its composition.
Ice-rich Models Provide The Best Match
After detecting the gas, the researchers compared their measurements with a range of computer models designed to simulate its interior. Each model assumed different proportions of rock and ice.
The results were striking. The team found that only ice-rich models could reproduce the amount of carbon monoxide measured in the atmosphere. Models containing lower amounts of ice failed to match the observations.
The study, published on arXiv, points toward a Uranus that contains substantially more water than some earlier interpretations suggested.
“We find that Uranus is more on the ice-giant side than on the rock-giant side,” Cavalié said.
The finding could have implications for understanding how Uranus formed. If its interior is indeed rich in ice, the planet may be more similar to Neptune than some scientists had argued.
Cavalié believes the results help address the long-running controversy over its composition, though he acknowledged that interpretations remain dependent on modeling.
“We have to be careful when we say things like that, because things also depend on modelling, but that’s the feeling we have,” he said.
Not All Researchers Are Convinced
The study team also identified carbon monoxide in Uranus’s upper atmosphere. Researchers believe this gas likely comes from a different source than the material detected deeper down.
Cavalié suggests that a comet impact several centuries ago may have delivered carbon monoxide into the upper layers of the atmosphere. If confirmed, the gas found at higher altitudes would not necessarily reflect the planet’s internal composition. Not everyone is convinced that the new observations settle the debate. Vanesa Ramirez of Leiden University points out that:
“Interpreting atmospheric abundances requires assumptions about chemistry, mixing and internal structure, all of which remain uncertain for Uranus.”
Ramirez argues that several rock-to-ice ratios remain compatible with the available data. In her view, the range of models capable of explaining Uranus’s interior is still broad enough to leave room for different interpretations.
