This Telescope Just Proved an Alien Comet Is Layered Like an Onion, and It Changes Everything

A new study using Japan’s Subaru Telescope reveals that the comet’s carbon dioxide content relative to water dropped significantly after perihelion, suggesting its surface chemistry differs from deeper interior material.

When the interstellar comet 3I/ATLAS swept past the Sun in late 2025, it left behind a chemical record that researchers are now using to probe its internal structure. Observations obtained in January 2026 indicate that the comet’s ratio of carbon dioxide to water had fallen considerably from pre-perihelion levels, pointing to a body whose layers do not share the same composition.

The comet, formally designated C/2025 N1, was first detected on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS). Only the third confirmed interstellar object ever observed, it attracted immediate scientific attention for the volatiles streaming from its nucleus as it approached the Sun.

A Chemical Fingerprint Written in Oxygen Light

Rather than measuring carbon dioxide and water directly, researchers atKyoto Sangyo University analyzed faint forbidden emission lines of atomic oxygen in the comet’s coma. When solar ultraviolet radiation breaks apart oxygen-bearing molecules such as water and carbon dioxide, it leaves oxygen atoms in metastable excited states that emit light at specific wavelengths, 557.7 nm (green) and 630.0 and 636.4 nm (red). The ratio of green to red intensity shifts depending on which parent molecule contributed more oxygen, making it a useful indirect measure of the CO2-to-water balance.

G/R ratio of 3I vs. Solar System comets (Decock 2013; McKay 2015; Shinnaka 2016, 2020; Opitom 2019, 2021) ©A post-perihelion constraint on the CO2/H2O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines

According to the study, published in The Astronomical Journal, Yoshiharu Shinnaka and colleagues used the High Dispersion Spectrograph on the Subaru Telescope at Maunakea, Hawaii, on January 7, 2026, when 3I/ATLAS was 2.87 astronomical units from the Sun on its outbound path. They measured a green-to-red ratio of 0.339, higher than most Solar System comets at comparable distances but similar to 2I/Borisov, the second interstellar object ever confirmed, observed in 2019.

Applying three standard conversion models to that ratio, the team derived CO2-to-water abundance ratios ranging from roughly 0.34 to 2.12, depending on the photodissociation rates assumed. Though the absolute values differ across models, all three place 3I/ATLAS above the typical range for Solar System comets at similar heliocentric distances. The authors note that the red doublet intensity ratio matched its theoretically expected value of approximately 3, confirming that optical depth effects were small and that the measurements were reliable.

A Sharp Decline From Pre-Perihelion Readings

The contrast with earlier observations is considerable. According to infrared data from the James Webb Space Telescope obtained on August 6, 2025, when the comet was 3.32 astronomical units from the Sun on its inbound trajectory, the CO2-to-water ratio stood at 7.6, placing 3I/ATLAS among the most carbon-dioxide-rich comets ever recorded. Observations from the SPHEREx space telescope around the same period found a ratio above 4.4.

After perihelion, which the comet reached on October 29, 2025, the picture became more complicated. SPHEREx reported a post-perihelion ratio of just 0.21 from December 2025 observations, while separate JWST measurements using the MIRI instrument found values of 3.16 and 7.28 at different outbound distances during the same month. The disparity between instruments likely reflects differences in the spatial area each sampled: JWST’s narrower field of view may capture less of the water distributed across the wider coma, artificially elevating its CO2-to-water estimates.

The Subaru result sits between these competing readings and was obtained at a heliocentric distance not covered by either space telescope, adding an independent reference point to the post-perihelion dataset.

One interpretation consistent across several studies is that cosmic ray processing over interstellar timescales enriched the comet’s outermost layers with CO2 and CO, while less-processed, water-richer material lay deeper. As solar heating eroded successive surface layers through perihelion, the coma composition shifted accordingly. The authors caution that other explanations, including changing active regions on the nucleus or short-lived outbursts, remain plausible, and that the layered interpretation, while supported by current data, is not yet definitive.

Future detections of interstellar objects, which survey telescopes are expected to produce in growing numbers, may eventually show whether compositional evolution of this kind is a defining feature of bodies formed around other stars.