Comet 3I/ATLAS streaks across a dense star field in this image captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab. This image is composed of exposures taken through four filters — red, green, blue and ultraviolet. As exposures are taken, the comet remains fixed in the center of the telescope’s field of view. However, the positions of the background stars change relative to the comet, causing them to appear as colorful streaks in the final image. See a version of the image where the stars have been “frozen” here. These observations of Comet 3I/ATLAS were conducted during a Shadow the Scientists program hosted by NSF NOIRLab. A full recording of the session can be found here.
Research using observations from the James Webb Space Telescope (JWST) has revealed that comet 3I/ATLAS has undergone significant transformation due to billions of years of exposure to cosmic radiation. This extensive irradiation has led to the development of a deep, irradiated crust on the comet, which no longer resembles the materials that originated from its home star system.
The study, which was posted on the preprint server arXiv on October 31, 2023, indicates that the comet’s extreme levels of carbon dioxide (CO2) enrichment result from galactic cosmic rays absorbed over its estimated 7 billion years of existence. Although the findings have not yet undergone peer review, they provide crucial insights into the effects of space radiation on interstellar objects.
Cosmic Ray Impact and Comet Composition
Galactic cosmic rays, a type of high-energy radiation originating from outside the solar system, have a transformative effect on the comet’s composition. These cosmic rays interact with carbon monoxide (CO) in space, converting it into carbon dioxide (CO2). Within our solar system, the heliosphere, which is a protective bubble of radiation created by the sun, shields planets from much of this cosmic radiation. However, in the vastness of interstellar space, where 3I/ATLAS has spent the majority of its life, such protection is absent.
The research led by Romain Maggiolo, a scientist at the Royal Belgian Institute for Space Aeronomy, suggests that these cosmic rays have altered the physical state of the comet’s ice to a depth of approximately 15 to 20 meters (50 to 65 feet). “It’s very slow, but over billions of years, it’s a very strong effect,” Maggiolo explained in an interview with Live Science.
This study represents a significant shift in how scientists understand interstellar objects. It posits that bodies like comet 3I/ATLAS are primarily comprised of materials processed by galactic cosmic rays, rather than pristine material reflective of their initial formation environments.
Observations and Future Implications
Currently, comet 3I/ATLAS is navigating its orbit around the sun, having reached perihelion, its closest approach to the sun, on October 29, 2023. As comets approach stars, their surfaces heat up, leading to the sublimation of ices into gas. The new findings suggest that the gases released by the comet prior to perihelion were primarily from its irradiated outer layer. This pattern is expected to continue after perihelion, although Maggiolo noted that solar erosion may expose more pristine materials locked within the comet’s nucleus.
“It will be very interesting to compare observations made before and after perihelion,” Maggiolo remarked. “We may gain insights into its initial composition by examining these differences.”
Since its discovery in July 2023, researchers have utilized various telescopes to gather data on 3I/ATLAS. Initial findings indicate that the comet is traveling through our solar system at speeds exceeding 130,000 mph (210,000 km/h) along an unusually flat trajectory. Some studies suggest that it may be the oldest comet ever observed, potentially around 3 billion years older than our 4.6 billion-year-old solar system.
The latest research builds upon previous observations that highlighted the comet’s rich carbon dioxide content, based on JWST’s initial images in August and data from NASA’s SPHEREx orbiter. The team adapted their models from earlier studies on a domestic comet, 67P, to assess the effects of cosmic rays on 3I/ATLAS.
In their simulations, researchers modeled the cumulative impact of galactic cosmic ray exposure on the comet’s ice structure and chemical composition over a billion years. While laboratory experiments may not perfectly replicate interstellar conditions, they provide valuable indicators of the experiences comets like 3I/ATLAS undergo during their extensive journeys through space.
Maggiolo emphasized that although comet 3I/ATLAS has significantly aged and transformed, it still offers a wealth of information for scientists. “We must be cautious and consider aging processes in our analyses, but 3I/ATLAS remains a fascinating subject of study,” he concluded.
