Recent studies reveal that Uranus houses significantly more frozen water ice than previously believed, potentially resolving longstanding questions regarding its formation compared to its neighbor, Neptune.

As ice giants, Uranus and Neptune possess thick gaseous atmospheres, complicating our understanding of their internal structures and formation history. However, scientists can analyze atmospheric gases to glean insights into deeper planetary processes and composition.

Typically, the presence of carbon monoxide in a planet’s atmosphere indicates a core abundant in water and ice. Neptune exhibits ample carbon monoxide, suggesting it has an ice-rich core; in contrast, the lack of carbon monoxide in Uranus has led some researchers to posit that it has a rocky interior. If accurate, this implies a divergent formation history for these two ice giants.

Recently, Thibault Cavalier and colleagues at the University of Bordeaux, France, detected carbon monoxide in Uranus’ lower atmosphere for the first time. This discovery indicates that Uranus is likely more water-rich than previously assumed.

“Our findings suggest that Uranus aligns more with an ice giant profile than a rocky one,” stated Cavalier. “While model variations may influence our interpretations, this debate appears to have concluded.”

Utilizing the Atacama Large Millimeter/Submillimeter Array Telescope in Chile, Cavalier’s team observed Uranus three times between 2022 and 2024, identifying substantial carbon monoxide amounts in the lower atmosphere. They employed various models with different rock-to-ice ratios to replicate the detected carbon monoxide levels and found that only the ice-rich model was consistent with their observations.

The study also detected carbon monoxide in Uranus’ upper atmosphere, implying it may originate from an external source, such as a comet colliding with the planet centuries ago, according to Cavalier.

This landmark discovery of carbon monoxide is pivotal for understanding Uranus’ internal structure, yet the origin of this gas remains uncertain. Vanesa Ramirez from Leiden University in the Netherlands cautioned, “Interpreting atmospheric composition involves numerous assumptions regarding chemistry, mixing, and internal structures, all of which are still uncertain for Uranus.”

These uncertainties, combined with the varying models for simulating Uranus’ interior, suggest that multiple rock-to-ice ratios might align with the available data. Ramirez asserted, “This finding alone does not definitively resolve whether Uranus is primarily an ice-rich or rock-rich giant.”