Interstellar Comet 3I/ATLAS Has Dusty Ice Grains Unlike Those in Most Comets, but Similar to Frozen Worlds Beyond Neptune

Comet 3I/ATLAS is an interstellar visitor with unique characteristics in its icy tail. The Southwest Research Institute (SwRI) utilized NASA's James Webb Space Telescope (JWST) to analyze the dust and water ice grains expelled by the comet. The findings indicate that the ice grains from 3I/ATLAS display distinct spectral features compared to many previously studied comets.

Lead author Silvia Protopapa explains that as the comet warms up, the ice and dust particles carried by the gas escaping from its interior contribute to the coma. These grains provide valuable insights into the conditions under which the comet originated from its formation in the disks of gas and dust around other stars. While the ice grains from 3I/ATLAS differ from those found in most comets, they bear a resemblance to those found in icy bodies located beyond Neptune, known as trans-Neptunian objects.

This similarity suggests that planets around other stars may have similarly constructed building blocks. The research team utilized the JWST's Near-Infrared Spectrograph to measure the comet's composition over time, capturing data between August 2025 and April 2026. The infrared light spectrum revealed a strong 3-micron band, indicating the presence of water ice, but the shorter-wavelength bands were either weak or absent.

This unique combination suggests that 3I/ATLAS' ice is a mixture of water ice and dust. The study also noted that the size of the ice aggregates decreased over time, with the final observation in April 2026 revealing larger particles. Comparing the data from 3I/ATLAS with models of light scattering from grains of different compositions and sizes, the team concluded that the dust content of 3I/ATLAS' grains is greater than in most studied comets.

This composition difference likely contributes to the weakening of the 1.5- and 2.0-micron bands while maintaining the strong 3-micron band. The findings suggest that the icy building blocks of planets, both in our solar system and around other stars, may share similar characteristics, with 3I/ATLAS representing a dusty end of the spectrum.