Even months after its closest approach to Earth, observations of the interstellar comet 3I/ATLAS (C/2025 N1) continue to provide new insights into its internal structure and the evolution of its constituent materials.
A research team led by Yoshiharu Shinnaka of the Koyama Space Science Institute at Kyoto Sangyo University utilized the Subaru Telescope to study the object on January 7, 2026, shortly after its perihelion (the point in its orbit closest to the Sun).
By analyzing the colors of the coma, the cloud of gas and dust surrounding the comet’s nucleus, astronomers were able to estimate the ratio of carbon dioxide (CO₂) to water (H₂O). This parameter is important because the gas observed in the coma originates directly from the nucleus and therefore reflects, at least in part, its composition.
The results show a significantly lower CO₂/H₂O ratio compared to previous observations made with space telescopes when the comet was before perihelion. This change suggests that the composition of the material released by the comet is not constant over time.
The study, published in The Astronomical Journal, indicates that heating from its close passage to the Sun can alter how gases are released.
A Core with Variable Composition
Comparing observations made before and after perihelion was crucial for interpreting these findings. As the comet approaches the Sun, increased temperatures cause the ices within the nucleus to sublimate. Not all materials react in the same way; some substances evaporate more readily, while others remain trapped until the heating becomes more intense.
The lower CO₂ to water ratio observed by the Subaru Telescope suggests that, after perihelion, the material emerging from the comet originates from different regions than those previously active. In other words, the surface layers and the deeper interior of the nucleus may have different compositions.
This interpretation aligns with the idea that the nucleus of 3I/ATLAS is not homogeneous. During its close passage to the Sun, erosion of the outer layers can expose deeper materials, thus altering the observed composition of the coma.
New Perspectives for Studying Interstellar Objects
Comet 3I/ATLAS is of particular interest because it originates from outside the Solar System. This allows astronomers to directly compare objects formed around other stars with those born in our planetary system.
The team applied techniques developed for studying Solar System comets, demonstrating that these methods can also be extended to interstellar objects. According to Shinnaka, the increasing observational capabilities in the coming years, thanks to new survey telescopes like the Vera C. Rubin Observatory, will lead to the discovery of a growing number of these objects.
By analyzing the composition and evolution of objects like 3I/ATLAS, it will be possible to gather information on the formation processes of planetesimals and planets in different stellar environments. The observed variations in the coma of 3I/ATLAS, therefore, are not just a local phenomenon but a broader clue to the chemical and physical conditions under which these objects formed.
