The interstellar comet 3I/ATLAS, discovered in July 2025 by the ATLAS project in Chile, continues to fascinate astronomers. Unlike comets bound to our sun, 3I/ATLAS originated from beyond our solar system, making it a rare and valuable object of study. As only the third known interstellar object to visit us, and the largest and brightest yet, scientists worldwide are focusing their instruments on unraveling its mysteries.
Unveiling 3I/ATLAS's Composition
NASA's SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer) has made significant strides in analyzing 3I/ATLAS. It detected a substantial amount of carbon dioxide gas in the comet's coma, the fuzzy atmosphere surrounding its nucleus, as well as water ice. This discovery is crucial for comparing comets from other star systems to those formed within our own.
According to Carey Lisse, an astronomer at Johns Hopkins University and a member of the SPHEREx science team, the large quantity of vaporized carbon dioxide suggests that 3I/ATLAS may share similarities with comets from our solar system. Comets typically consist of rocky dust, water, carbon dioxide, and carbon monoxide, with the proportions varying based on their formation location and evolutionary history.
The Speed of an Interstellar Traveler
Before entering our solar system, 3I/ATLAS traveled at approximately 60 kilometers per second, a speed far exceeding that of the fastest racing cars. This high velocity initially led scientists to believe it originated from the thick disk of old stars in the Milky Way galaxy. However, the comet could have been launched at high speed relative to its parent star, meaning its current velocity may not be directly related to the parent star's motion.
JWST's Deep Dive into 3I/ATLAS
The James Webb Space Telescope (JWST) has also contributed significantly to understanding 3I/ATLAS. Observations of the comet's coma revealed a unique composition, primarily carbon dioxide, with a carbon dioxide to water ratio of 8 to 1. This is the highest ratio ever observed in a comet. In contrast, the ratio of carbon monoxide to water was more typical.
Implications for Understanding Cometary Formation
The JWST also found that the ratio of Carbon-12 to Carbon-13 isotopes in 3I/ATLAS is similar to that found on Earth, suggesting a similar origin environment for these carbon species. The unusual CO2/H2O ratio may be due to extremely high levels of something that effects the ratio.
The ongoing study of 3I/ATLAS promises to provide valuable insights into the formation and evolution of comets, both within and beyond our solar system, and offers a glimpse into the diversity of planetary systems throughout the galaxy.