EVIDENCE FILE · ev-3i-atlas-chemistry
The Chemistry of 3I/ATLAS — and Its Evolution
The third interstellar visitor released far more carbon dioxide than water
In the research record
3I/ATLAS is the third object confirmed to have entered the Solar System from interstellar space. Unlike ʻOumuamua, it developed a clearly active coma as sunlight heated its surface and released frozen material.
Scientists can identify the gases in a coma without collecting them. A spectrograph separates the object's light into wavelengths. Different molecules leave different infrared fingerprints, allowing researchers to estimate how much of each gas the comet is releasing.
Before ATLAS reached its closest point to the Sun, the James Webb Space Telescope measured an exceptionally carbon-dioxide-rich state. At that observing epoch, the coma was producing about 7.6 units of carbon dioxide for every unit of water. That was among the highest carbon-dioxide-to-water ratios reported for a comet.
Martin Cordiner and collaborators published the measurement in the peer-reviewed Astrophysical Journal Letters. Later observations, including additional work with JWST, found different mixtures of gas as ATLAS travelled through the Solar System. The ratio did not remain fixed.
The initial measurement was therefore not the object's permanent composition. It was a real state observed at a particular time. That changing chemistry also carries information: different ices can begin escaping as sunlight reaches new layers, so the evolution may reflect where the object formed, how it was altered before arrival, and how heat moved through it during passage.
What this adds
ATLAS did not merely contain an unusual amount of carbon dioxide. Its chemistry changed as the Sun reached different material, giving researchers a moving view into an object formed around another star.
Status: peer-reviewed, evolving compositional finding. The unusually carbon-dioxide-rich epoch is established; the ratio changed across the passage and must not be presented as a permanent composition.
In the novel
In Chapter 14, the team uses a specific pre-perihelion epoch, when JWST measured an exceptionally carbon-dioxide-rich coma, and compares that ratio with the Fourth's shell material under equivalent heating. Later observations show the ratio changing as the coma evolves, and that evolution remains in the finding. The novel's weight comes from the laboratory match to one observed state rather than from a fixed composition assigned to the entire passage.
| Source | Status | Role |
|---|---|---|
| Seligman, Darryl Z., et al. “Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS.” arXiv:2507.02757 (2025). Accepted for publication in The Astrophysical Journal Letters.arXiv:2507.02757 | Preprint | finding |
| Cordiner, Martin A., et al. “JWST Detection of a Carbon Dioxide Dominated Gas Coma Surrounding Interstellar Object 3I/ATLAS.” The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/ae0647.DOI: 10.3847/2041-8213/ae0647 | Peer-reviewed | finding |
| Belyakov, Matthew, et al. “The Volatile Inventory of 3I/ATLAS as Seen with JWST/MIRI.” The Astrophysical Journal Letters 1001, L11 (2026). DOI: 10.3847/2041-8213/ae5700.DOI: 10.3847/2041-8213/ae5700 | Peer-reviewed | finding |
| Keto, Eric, and Abraham Loeb. “A Physical Model for the Ice Coma of the Interstellar, Hyperactive Comet 3I/ATLAS.” arXiv:2510.18157 (2025).arXiv:2510.18157 | Preprint | natural model |
| Hibberd, Adam, Adam Crowl, and Abraham Loeb. “Is the Interstellar Object 3I/ATLAS Alien Technology?” arXiv:2507.12213 (2025).arXiv:2507.12213 | Preprint | interpretation |
Developments
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