Comet 3I/ATLAS Chemistry Unveiled: CO₂-Rich Coma Offers New Star System Clue

A rare interstellar visitor has revealed secrets that could reshape our understanding of distant star systems. Comet 3I/ATLAS, now receding from Earth after its closest approach, has provided scientists with an unprecedented view of its chemical makeup, showing it has a CO₂-rich composition that offers a glimpse into material formed around another star.

This fast-moving comet, discovered by NASA's ATLAS survey telescope in Chile, is only the third confirmed interstellar object to visit our solar system. Its hyperbolic trajectory, exceeding 57 km/s, ensures it will not remain bound to the Sun, making its fleeting passage a rare opportunity for study.

A CO₂-Rich Composition

The most significant finding came from infrared observations from the James Webb Space Telescope (JWST). It detected a pronounced CO₂ signature, suggesting that the comet's composition is strikingly different from most solar-system comets. Unlike typical comets in our neighbourhood, which tend to be water-rich, 3I/ATLAS carries a higher fraction of carbon dioxide, hinting at the diversity of chemical environments in other planetary systems.

The CO₂-rich coma of 3I/ATLAS provides a rare sample of extrasolar material. By analysing its chemical fingerprint, astronomers can infer conditions in the comet's home star system. This could refine models of planetary formation, particularly regarding the distribution of volatile compounds essential for life formation.

Ultraviolet Insights from Europa Clipper

NASA's Europa Clipper captured a unique ultraviolet perspective of 3I/ATLAS with its Ultraviolet Spectrograph (Europa-UVS).

According to ScienceDaily, this view was possible when Earth and Mars observations were limited. Researchers reported that Europa-UVS 'can detect fundamental transitions that show gas release and molecular breakup. "We can see gases come off the comet, and water molecules break apart into hydrogen and oxygen atoms."'

These ultraviolet measurements allowed scientists to map the comet's dust and plasma tails from an unusual angle, revealing outgassing patterns shortly after its perihelion.

The data also helped determine the speed and density of the ejected gases, which appear to interact strongly with solar radiation.

ESA's JUICE mission with complementary ultraviolet capabilities is expected to help reconstruct the comet's tail structure in three dimensions. This further enhances the understanding of its activity and reveals new clues about the forces shaping interstellar comets.

Parker Solar Probe Peeks Through the Sun's Glare

Observing 3I/ATLAS from Earth proved challenging even as the comet passed close to the Sun. NASA's Parker Solar Probe filled the gap, capturing daily images from 18 October 18 to 5 November 2025.

Parker's WISPR camera navigated the Sun's intense glare to track the comet's tail and dust emissions.

NASA notes that these images provide a critical time series when ground-based tracking was impossible, offering unprecedented insight into the comet's behaviour near perihelion.

Observing so close to the Sun required complex spacecraft manoeuvres and sophisticated image processing to filter out coronal brightness.

The resulting data show how the comet's dust and gas interact with solar wind, giving researchers a rare window into processes that may be common in other star systems but are rarely observable in real time.

X-Ray Observations Reveal Solar Wind Interactions

X-ray observatories added another dimension to the study. ESA's XMM-Newton observed 3I/ATLAS on 3 December 2025, followed by Japan's XRISM mission from 26–28 November.

These observations revealed X-ray emissions stretching hundreds of thousands of kilometres from the nucleus, a clear signature of interactions between solar-wind ions and neutral gases in the coma.

Opportunities for Amateur Astronomers

For amateur astronomers, 3I/ATLAS remains a telescope-only target, with an estimated magnitude of 12.8 and currently positioned near the constellation Leo.

While faint, it continues to offer scientific treasure. Its passage also highlights the importance of international space cooperation, as data from multiple spacecraft and observatories are combined to paint a complete picture of this interstellar visitor.