NASA's SPHEREx Mission Reveals Unusual Carbon Dioxide Dominance in Interstellar Comet 3I/ATLAS

In a significant advancement for cometary science, NASA's SPHEREx mission has conducted detailed observations of interstellar comet 3I/ATLAS, revealing an unexpected dominance of carbon dioxide (CO₂) in its composition. These findings offer new insights into the chemical diversity of celestial bodies originating beyond our solar system.

Between August 7 and August 15, 2025, the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) utilized imaging spectrophotometry to analyze 3I/ATLAS across wavelengths from 0.75 to 5.0 microns. The data indicated a bright CO₂ gas coma with a radius of approximately 3 arcminutes, corresponding to a gas production rate of 9.4 × 10²⁶ molecules per second. Additionally, strong water ice absorption features were detected, suggesting a significant presence of water ice in the comet's nucleus. Notably, there was no significant detection of water vapor or carbon monoxide emissions, with upper limits on their production rates set at 1.5 × 10²⁶ and 2.8 × 10²⁶ molecules per second, respectively. This suggests that CO₂ is the dominant volatile being released from 3I/ATLAS, distinguishing it from typical solar system comets.

Comet 3I/ATLAS, the third confirmed interstellar object to traverse our solar system, was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Rio Hurtado, Chile. Its hyperbolic trajectory confirmed its origin from outside the solar system. Traveling at approximately 37 miles per second (over 130,000 mph), 3I/ATLAS is currently about 420 million miles from Earth and poses no threat, as it will remain at least 150 million miles away.

The SPHEREx mission, designed to survey the sky in near-infrared light, aims to map the distribution of galaxies and study the formation of stars and planets. Its wide field of view and multi-spectral sensitivity make it particularly suited for observing transient astronomical events, such as the passage of interstellar objects.

Complementary observations from the James Webb Space Telescope (JWST) on August 6, 2025, corroborated SPHEREx's findings. JWST's Near-Infrared Spectrograph revealed that the comet's coma is strikingly rich in CO₂ relative to water, presenting one of the highest such ratios ever recorded in a comet. This finding suggests 3I/ATLAS may have formed near the CO₂ ice line in a distant protoplanetary disk or been exposed to substantial radiation—both indicators of its extragalactic origins.

The dominance of CO₂ in 3I/ATLAS's coma provides valuable insights into the chemical makeup of interstellar comets and contributes to our understanding of such objects traversing our solar system. It suggests that the comet may have formed in a region of its home star system where CO₂ ice is more prevalent than water ice, or that it has undergone significant processing that depleted its water content. These findings offer a unique opportunity to study material from another star system and potentially understand the formation of exoplanets.

The study of interstellar objects like 3I/ATLAS has profound implications for our understanding of planetary system formation and the distribution of organic compounds in the galaxy. By analyzing the composition and behavior of such objects, scientists can gain insights into the diversity of planetary systems and the potential for life elsewhere in the universe.

The SPHEREx observations of interstellar comet 3I/ATLAS have provided groundbreaking data that challenge existing models of cometary composition and behavior. These findings not only enhance our understanding of interstellar objects but also offer a glimpse into the diverse chemical environments of other planetary systems.