Astronomers using the James Webb Space Telescope found that the water ice in interstellar comet 3I/ATLAS is mixed with dark, rocky material in tiny grains. The Southwest Research Institute-led study, published in The Astrophysical Journal Letters, reports that the grains measure a micron or smaller and carry a spectral fingerprint unlike most comets in our solar system.
Instead, the fingerprint closely matches icy worlds beyond Neptune. The researchers say the match suggests the icy building blocks of planets around other stars may share a common range of physical properties with those in our own solar system.
3I/ATLAS is only the third interstellar object ever identified passing through our solar system. Lead author Dr. Silvia Protopapa of the Southwest Research Institute said interstellar comets are small icy bodies that formed in disks of gas and dust around other stars. She said the grains hold clues to the conditions where the comet was born.
The team observed the comet with Webb’s NIRSpec instrument on August 6, 2025, December 22, 2025, and April 1, 2026, spanning about eight months around its closest approach to the Sun on October 29, 2025. As the comet warmed, gas escaping from its interior carried ice and dust grains into the surrounding coma.
The spectra showed a strong water-ice band near 3 microns but weak or missing bands near 1.5 and 2.0 microns. The researchers say that pattern matches trans-Neptunian objects, which also show carbon dioxide ice and organics. By the final observation in April 2026, Webb detected an additional population of larger, ice-rich grains, which the team attributes either to different heating at each stage of the orbit or to fresh material exposed as the Sun eroded the comet’s surface.
Source: Phys.org
