Webb observations show an unusual absorption pattern in the interstellar comet. A study favors grains of ice mixed with dust, about a micron or smaller, as an explanation.
A study led by the Southwest Research Institute interprets James Webb Space Telescope observations of 3I/ATLAS as evidence of water ice mixed with dust in the comet’s coma, the cloud surrounding it. Models of how light scatters favor grains about a micron or smaller. The study appeared in The Astrophysical Journal Letters.
Discovered in July 2025, 3I/ATLAS is the third identified object from beyond the solar system. Lead author Silvia Protopapa of the institute describes interstellar comets as icy bodies formed in disks around other stars that are passing through our solar system.
The spectrum showed a strong absorption band—a dip in light—near 3 microns, but bands near 1.5 and 2.0 microns were very weak or absent. It also showed a blue near-infrared continuum across 1.15–2.50 microns. Most previously studied icy comets show all three absorption bands.
To explain that pattern, the team compared the measurements with models of grains of different sizes and compositions. In the favored dusty grains, light travels only a short distance through the ice, weakening the shorter-wavelength bands while leaving the longer-wavelength band prominent. The grain composition and size are therefore model-based interpretations of the measurements.
The modeled mixtures included water ice and dark material such as pyroxene and carbon. Small grains scatter shorter wavelengths more effectively, which may explain the blue continuum.
The institute described the study on 26 October 2026. It said grains in other comets are thought to be similarly small but relatively pure; Protopapa identified the amount of dust mixed into the ice as the apparent difference in 3I/ATLAS.
Webb’s Near-Infrared Spectrograph observed the comet three times between August 2025 and April 2026, before and after its closest approach to the Sun. Martin Cordiner of NASA’s Goddard Space Flight Center and the Catholic University of America leads the observing program and co-authored the paper.
By the final observation, the data favored an additional group of ice-rich grains, larger than those inferred earlier but still around a micron in size. Co-author Michael Kelley of the University of Maryland said the team could not yet tell whether the change reflected different solar heating or new material exposed as the comet’s surface eroded near the Sun.
After the comet’s closest approach to the Sun, a feature known as a Fresnel reflection appeared near 3.1 microns and was interpreted as evidence of crystalline water ice. The ice may have crystallized as sunlight warmed the surface, or previously hidden crystalline material may have been exposed as outer dust was blown away; neither explanation is established.
The researchers found that the comet’s water-ice absorption resembles that of some trans-Neptunian objects, icy bodies orbiting beyond Neptune. Protopapa said the comparison suggests that planet-building material here and around other stars may share a range of grain properties, from relatively pure ice to dusty ice—not that all such ice is identical.
The researchers say future Webb observations of solar-system comets and newly discovered interstellar visitors will help test their interpretation and whether 3I/ATLAS is typical.
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