This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The interstellar comet 3I/ATLAS brought material from another planetary system within reach of our telescopes, but reading its chemistry poses a problem. Energetic particles encountered during its journey through space may have changed its outer layers, so how much of what astronomers detect still reflects the comet's birthplace?
The sun may have helped reveal a clearer record. Using published measurements of escaping gas only, researchers estimate that solar heating removed the equivalent of a surface layer about 1.7–5.2 meters (5.6–17 feet) thick. Including plausible amounts of lost dust pushes some estimates beyond 10 meters (33 feet).
The calculations, made by Tessa Frincke and Darryl Seligman of Michigan State University, appear in an accepted manuscript in Monthly Notices of the Royal Astronomical Society. Whether that erosion reached material relatively untouched by the interstellar journey depends on how much dust escaped alongside the gas. Discovered in July 2025, 3I/ATLAS is the third interstellar object detected passing through our solar system.
As sunlight heated it, frozen substances turned into gas, contributing to the cloud surrounding its solid nucleus. Frincke and Seligman assembled published measurements of how quickly different substances escaped before and after the comet's closest approach to the sun. They modeled how those rates changed along its trajectory, then added up the resulting mass loss.
Their estimates put the gas loss at roughly 18–58 billion kilograms (40–128 billion pounds), mostly water, carbon dioxide and carbon monoxide. To translate that loss into a depth, the researchers modeled the nucleus as a sphere initially 1.3 kilometers (0.8 miles) in radius, with a density typical of solar system comets. They calculated how much its radius would shrink as those amounts of material escaped.
The resulting thickness estimate describes erosion spread evenly across the surface, however. It does not show how deeply individual patches actually wore away. Gas is only part of what a comet sheds.
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