Interstellar Comet 3I/ATLAS Shows Unusual Chemical Makeup

Recent studies of interstellar comet 3I/ATLAS have provided new data on its chemical composition, using observations from NASA’s James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array. These findings are relevant for understanding differences between interstellar objects and comets originating within the solar system.

The James Webb Space Telescope observed 3I/ATLAS on August 6, 2025, using its Near-Infrared Spectrograph to collect spectroscopic data. Analysis of this data revealed that the comet’s coma contains a notably high proportion of carbon dioxide compared to water, with a CO₂/H₂O mixing ratio among the highest recorded for any comet.

Further measurements by JWST in December 2025 determined the water ortho-to-para ratio in the coma to be approximately 2.7 ± 0.1. This ratio provides additional information about the physical conditions in which the comet’s water ice formed.

Observations by ALMA indicated that 3I/ATLAS has a concentration of deuterated water about 30 times greater than that found in typical comets from the solar system. ALMA also detected that methanol is present in higher amounts relative to hydrogen cyanide than is usual for solar system comets.

What the numbers show

• JWST observed 3I/ATLAS on August 6, 2025
• The water ortho-to-para ratio measured was 2.7 ± 0.1
• D/H ratio in comet water was about 0.95 ± 0.06 %
• Deuterated water levels are 30 times higher than solar system comets

Isotopic analysis by JWST found that the water in 3I/ATLAS has a deuterium-to-hydrogen (D/H) ratio of approximately 0.95 ± 0.06 %, which is over ten times higher than values measured in solar system comets. The same study reported elevated ¹²C/¹³C ratios in both carbon dioxide and carbon monoxide, with values for CO₂ ranging from 141 to 191 and for CO from 123 to 172, both above typical solar system levels.

These compositional differences have led researchers to suggest that 3I/ATLAS may have originated in a very cold, metal-poor region of the early Milky Way, possibly 10 to 12 billion years ago. This interpretation is based on the unique isotopic and molecular characteristics identified in the comet’s coma.

NASA’s open-science policy ensures that all data collected from observations of 3I/ATLAS will be made publicly available in designated repositories. This approach supports ongoing and future research into the properties of interstellar objects and their implications for planetary science.

Continued analysis of 3I/ATLAS and similar interstellar comets may provide further insights into the diversity of materials present in the galaxy and the processes that shaped them in the distant past. The availability of these datasets is intended to facilitate collaborative research across the scientific community.

* This article is based on publicly available information at the time of writing.