According to ScienceAlert, a NASA Solar System Exploration Division team publishing in Nature Astronomy analyzed the comet's coma with the James Webb Space Telescope in late December 2025, when the comet was about 2.5 times Earth's distance from the Sun, crossing the asteroid belt between Mars and Jupiter.

The chemistry it released suggests it may have formed more than 7 billion years before our Sun ignited, possibly around one of the Milky Way's oldest stars.

The researchers focused on the deuterium-to-hydrogen ratio in the comet's methane, a signature frozen in when its ices formed.

ScienceAlert reports that 3I/ATLAS held significantly more heavy methane than any body in our Solar System, and 13 times more than comet 67P/Churyumov-Gerasimenko, the only local comet with a secure signature, which required in-situ measurements from the Rosetta spacecraft.

The researchers said the elevated ratio is consistent with ices forming in a cold, low-metallicity interstellar or protostellar environment under high cosmic ray irradiation. Low carbon-13 content further hints at an origin in an ancient Universe not yet enriched by dying stars.

A separate Southwest Research Institute-led study, published in The Astrophysical Journal Letters and reported by phys.org, examined ice grains in 3I/ATLAS's coma using Webb's Near-Infrared Spectrograph three times between August 2025 and April 2026, before and after closest approach to the Sun.

The comet shows a strong water-ice band near 3 microns but very weak or missing bands near 1.5 and 2.0 microns — a puzzle Protopapa's team solved by modeling grains about a micron or smaller made of water ice mixed with dust, unlike the relatively pure ice grains of most solar system comets.

When the SwRI team compared 3I/ATLAS with Webb spectra of trans-Neptunian objects — icy bodies beyond Neptune that preserve early solar system material — the comet no longer looked out of place.

According to phys.org, the shape of its 3-micron water-ice band resembles that of several distant groups, suggesting planet-forming building blocks around other stars share a common range of grain properties, from relatively pure ice to ice mixed with dust, with 3I/ATLAS toward the dusty end.

Grain properties also changed between observations, and researchers say telling whether solar heating or surface erosion caused it is an open question future Webb observations will address.