Caption: 
An SwRI-led study used NASA’s James Webb Space Telescope to compare interstellar comet 3I/ATLAS with icy bodies in our solar system. The comparison offers clues to the early formation of planetary building blocks. At top right, both 3I/ATLAS and the icy solar system comet C/2024 E1 show a strong water-ice absorption band near 3 microns, but the interstellar comet has very weak or missing bands near 1.5 and 2.0 microns. Its spectrum more closely resembles the average spectrum of a class of icy worlds beyond Neptune whose spectra also show carbon dioxide ice and organics (bottom right). The artist’s concept (left, not to scale) compares the relatively pure ice grains with separate dust, as interpreted for most solar system comets, with the dust-rich ice aggregates the study inferred for 3I/ATLAS. Both grain types are around a micron or smaller.