Comet 3I/ATLAS surprises astronomers with carbon dioxide-rich coma discovery

Astronomers study unusual carbon-rich coma of interstellar comet 3I/ATLAS (Image: NASA)

The cosmos has sent another rare visitor our way. Astronomers are watching comet 3I/ATLAS, spotted in July, with great interest. The interstellar object is moving fast and has offered unusual findings.

What did JWST discover about the comet’s coma?
The James Webb Space Telescope observed the comet on 6 August. It used its NIRSpec infrared camera while the comet was 3.32 AU away. Findings revealed the coma is largely made of carbon dioxide. It also has water, carbon monoxide and carbonyl sulphide.

On 6 August 2025, the James Webb Space Telescope observed interstellar comet 3I/ATLAS using its Near-Infrared Spectrograph. (Image: NASA/ESA)

Why is the carbon dioxide ratio so surprising?
The ratio of carbon dioxide to water is 8:1. This is the highest ever detected in a comet. It is six standard deviations above the typical ratio. Carbon monoxide levels, however, appear closer to known values. That ratio stands at 1.4 when compared with water.

The coma contains dust and water scattered in the region. Dust was more concentrated on the sun-facing side. That aligns with higher outgassing where heat is stronger. The carbon isotope ratio was also measured by JWST. Carbon-12 and Carbon-13 levels matched those found on Earth.

What could explain the unusual chemical balance?
Astronomers suggest possible origins for this strange CO₂ ratio. It could have formed under intense ultraviolet radiation exposure. Another idea links it to formation beyond the CO₂ ice line. Heat transfer within the nucleus may also explain the imbalance. Carbon dioxide, with a lower melting point, sublimates before water ice.

This is only the third confirmed interstellar object observed. The first, ‘Oumuamua, did not have a visible coma. The second, 2I/Borisov, had higher carbon monoxide levels. Each interstellar comet so far has shown unique chemical signatures.

What comes next for astronomers studying this comet?
The comet will reach perihelion in early October. During that time, it will remain hidden from Earth. Mars-based probes might catch some activity during this phase. Observations are expected to continue until it disappears from view. When visible again in December, it will already be departing. By then, most of its stored material may be gone.

Such fleeting events give astronomers only limited observation chances. The comet offers fresh insights into planetary system formation. More papers are expected before it finally leaves the solar system.