Webb Finds Dust-Rich Water Ice Streaming From Interstellar Comet 3I/ATLAS

The third object ever identified as arriving from beyond the solar system has now had the ice in its coma analysed. A Southwest Research Institute-led team used the James Webb Space Telescope to examine grains streaming from comet 3I/ATLAS, finding particles about a micron or smaller made of water ice mixed with dust — a combination that sets the visitor apart from most comets previously studied. Within the study of interstellar visitors, the work supplies a detailed spectral account of what such an object releases as the Sun warms it, and a basis for comparing that material with icy bodies native to our own system.

The team observed 3I/ATLAS with Webb's Near-Infrared Spectrograph three times, in August and December 2025 and again in April 2026, bracketing the comet's closest approach to the Sun. Water ice leaves a deep absorption band near 3 microns and weaker bands near 1.5 and 2.0 microns. In 3I/ATLAS the 3-micron band is strong, but the shorter-wavelength features are very weak or missing, unlike most icy comets, which show all three. "That combination was a puzzle," said Silvia Protopapa of SwRI, the paper's lead author. Modelling of how light scatters off grains of differing size and composition favours aggregates at or below a micron in which ice and dust are intimately mixed, so that light passes only a short distance through the ice.

The comparison that followed gives the finding its wider purchase. The team set the 3I/ATLAS spectrum against Webb spectra of trans-Neptunian objects, icy bodies orbiting beyond Neptune, and found that the shape of its 3-micron band resembles several groups of them, a similarity that lies in the water-ice feature, not in any carbon dioxide signature. "When we placed it alongside distant icy worlds in our own solar system, it no longer looked out of place," Protopapa said. Martin Cordiner of NASA's Goddard Space Flight Center and the Catholic University of America, principal investigator of the Webb programme, said it was designed to be ready the moment an active interstellar comet was found and to follow it over time.

Uncertainties remain, and the authors are explicit about them. By the final observation in April 2026, as the comet moved away from the Sun, the data favoured an additional population of ice-rich grains, larger than those inferred earlier but still around a micron. A weak spectral feature at 3.1 microns is consistent with crystalline water ice, though the data cannot yet show whether the ice structure itself changed or only grain size and dust content. Michael Kelley of the University of Maryland, the second author, said the team cannot yet tell whether the changes reflect the Sun heating grains differently at each observation or new material exposed as the comet's surface eroded. The modelling carries a further caveat: effective grain sizes derived from light need not correspond to the original sizes of ice within the comet. The next test is further Webb observation of solar system comets and newly discovered interstellar visitors, to establish whether 3I/ATLAS is typical.