Eight months of observations
The interstellar comet 3I/ATLAS was observed with the James Webb Space Telescope's NIRSpec instrument on three dates: August 6, 2025, December 22, 2025 and April 1, 2026. The three points span about eight months around its closest approach to the Sun on October 29, 2025.
All three spectra show a strong water-ice absorption feature, along with gas emitted by water, carbon dioxide and carbon monoxide as the comet warmed and shed material. What is missing is more telling. Weaker ice signatures that usually appear in comets from our own Solar System were faint or absent.
Tiny grains, heavy dust
Silvia Protopapa of the Southwest Research Institute and her colleagues modelled the data. The best explanation, they write in the Astrophysical Journal Letters, is grains smaller than a few micrometres, each one a blend of water ice and refractory, rocky material.
The grains are about the same size as those in Solar System comets. What sets them apart is how much dust is mixed into the ice. The April 2026 spectrum also hints at a second population of larger, micron-scale grains that are richer in ice.
Michael Kelley of the University of Maryland said the team cannot yet tell whether the changes over time come from the Sun heating grains differently at each observation, or from fresh material exposed as the comet's surface eroded near the Sun.
A shared recipe for ice
The researchers compared 3I/ATLAS with comets and with several classes of icy bodies beyond Neptune. Its spectrum sits close to that of a group of distant icy worlds that also show carbon dioxide ice and organics.
The implication is that icy planetesimals forming around different stars may share a continuum of grain properties, from nearly pure ice to ice heavily mixed with dust, with this visitor near the dusty end. If that holds, the physical form of water ice may be set early in planet-forming disks and partly preserved in the bodies that grow there. The team calls it evidence that the raw material of planets is more universal than the Solar System's own comets suggest.