MAUNAKEA, Hawaii, May 6, 2026, 12:04 HST
- Subaru Telescope observations suggest 3I/ATLAS changed chemistry after its closest pass by the Sun.
- The results add to ALMA findings that the comet’s water carries unusually high deuterium, a marker of a cold birthplace.
- The object is only the third confirmed interstellar visitor, after ‘Oumuamua and 2I/Borisov.
Astronomers using the Subaru Telescope atop Maunakea have found new signs that 3I/ATLAS, a rare interstellar comet, changed chemically after passing near the Sun, pointing to material beneath its surface that may not match its outer layers. A Big Island Now report on Wednesday cited a team led by Yoshiharu Shinnaka of Kyoto Sangyo University.
The finding matters because 3I/ATLAS is moving out of the solar system, leaving researchers to mine a short window of observations for evidence of how bodies formed around other stars. The Subaru result, published April 22 in The Astronomical Journal, is part of a quickening set of post-passage studies on the comet’s gases, ice and dust.
3I/ATLAS is the third known object from outside the solar system to pass through this neighborhood, NASA says. It follows ‘Oumuamua, first detected in 2017, and 2I/Borisov; unlike the rocky-looking ‘Oumuamua, 3I/ATLAS has behaved clearly like a comet, with a coma, the gas-and-dust cloud around its frozen nucleus.
The Subaru team observed 3I/ATLAS on Jan. 7, after perihelion — the comet’s closest approach to the Sun. By studying light from its coma, the group estimated the ratio of carbon dioxide to water in gas escaping from the nucleus.
That carbon dioxide-to-water ratio was lower than values inferred from earlier space-telescope observations, Subaru said. The shift suggests solar heating may have opened up different parts of the nucleus, with inner and outer layers releasing different gases.
Shinnaka said the work lets researchers “directly compare comets hailing from both inside and outside the Solar System,” a rare comparison because so few interstellar objects have been caught in time for detailed study. Subaru Telescope
A separate Nature Astronomy paper published April 23 sharpened the broader picture. Using ALMA observations, researchers constrained the water deuterium-to-hydrogen ratio in 3I/ATLAS at more than 6.6 × 10^-3, more than about 40 times Earth’s ocean value and more than about 30 times typical solar-system comet values. Deuterated water, or HDO, is water in which one hydrogen atom is replaced by deuterium, a heavier form of hydrogen.
Luis E. Salazar Manzano of the University of Michigan, who led the ALMA work, said such chemistry is “really sensitive to temperature” and usually points to conditions colder than about 30 Kelvin. That is about minus 406 Fahrenheit, far colder than the setting that formed many familiar solar-system comets. ALMA Observatory
Teresa Paneque-Carreño, also at the University of Michigan, said ALMA had an edge because “most instruments can’t point toward the Sun.” Its radio antennas observed the comet days after perihelion, when many optical telescopes had a poor line of sight. National Radio Astronomy Observatory
There are caveats. The Nature Astronomy team said the deuterium finding comes from a single epoch, and ALMA did not directly detect ordinary water above its threshold in that observing run; the water production rate was inferred through modeling tied partly to methanol lines. That leaves room for the exact ratios to move as teams compare datasets from different dates and instruments.
NASA says more than a dozen of its science missions observed 3I/ATLAS, including TESS, MAVEN, Webb and SPHEREx, and their data will remain in public archives. For Subaru, ALMA and other observatories, the race is less about one comet now than about the next one: the methods tested on 3I/ATLAS may become the baseline when more interstellar visitors are found.
