3I/ATLAS has water ice that doesn't match our comets
JWST watched the interstellar comet 3I/ATLAS three times across eight months, and its water ice doesn't behave like the ice in comets from our solar system.
The spectra show a broad 3 um water-ice absorption, plus H2O, CO2 and CO gas. What isn't: the 1.5 and 2.0 um water-ice bands that normally ride along are weak or missing.
Protopapa and colleagues fit all three epochs (Aug 2025, Dec 2025, Apr 2026). The best match is submicron- to micron-sized ice grains carrying a lot of rocky material. By the last epoch larger ice-rich grains fit better, and a faint feature near 3.1 um hints at crystalline ice. Two stories fit: crystalline ice throughout with grain size and dust mixing changing, or ice going from amorphous to crystalline.
The bigger claim is comparative. 3I's spectrum sits between water-ice-rich solar-system comae and several classes of mid-sized trans-Neptunian objects. Icy planetesimals formed in different disks may span a continuum from nearly pure ice to ice heavily mixed with rock, and 3I sits at the rocky end.
Caveats: accepted for ApJL (arXiv 2609.19286), not final. The grain picture is a model fit, one object in three snapshots.