Interstellar Comet 3I/ATLAS Shows Mysterious Spectroscopic Activity

Introduction

In July 2025, astronomers observed interstellar comet 3I/ATLAS (C/2025 N1), the third confirmed interstellar object to ever enter our solar system. Discovered by the ATLAS survey in Chile, this comet has provided a unique opportunity to study materials from beyond our solar system. Recent observations have shown surprising characteristics, including unexpected activity levels and a composition that challenges many prior assumptions about interstellar objects (1–3).

3I/ATLAS interstellar comet, asteroid in space © Batibati -chronicles-stock.adobe.com

Strange Comet Activity

Between July 2 and 29, 2025, a team of astronomers conducted extensive photometric and spectroscopic observations of 3I/ATLAS using various ground-based telescopes. These observations discovered that the comet exhibited a spin period of 16.16 ± 0.01 hours, with a light curve amplitude of approximately 0.3 magnitudes. This suggests that the nucleus of 3I/ATLAS is rotating in a manner consistent with other active comets observed in our solar system (1).

Additionally, the comet's dust activity increased over the observation period, with estimated dust mass-loss rates ranging from 0.3 to 4.2 kg/s. This level of activity is comparable to that of weakly active distant comets, indicating that 3I/ATLAS is more active than initially expected for an interstellar object (1).

Spectroscopic Insights into Composition

Spectroscopic data obtained from the Southern African Large Telescope (SALT) and the Nordic Optical Telescope (NOT) provided further insights into the comet's composition. The reflectance spectra indicated a slight reddening in the optical range 0.4–0.7 µm (400–700 nm), with a spectral slope of 22.8 ± 0.1%/µm, suggesting the presence of organic-rich materials similar to those comets found with origins in the outer Solar System (1–3).

Moreover, observations from the James Webb Space Telescope (JWST) revealed an unusually high carbon dioxide (CO₂) to water (H₂O) ratio in the comet's coma, approximately 8:1, one of the highest ever observed in any comet. This finding suggests that 3I/ATLAS may have originated in a region with conditions unique from those present in our solar system, possibly near a CO₂ ice line or in an unusual radiation-rich environment (2).

Morphological Characteristics

Despite its increased activity, 3I/ATLAS did not exhibit a visible tail during the observation period. This absence is likely due to the comet's viewing geometry and low dust production, which resulted in a coma that remained predominantly asymmetric. The coma's morphology and the lack of fragmentation or rapid changes further suggest that the comet is composed of relatively large dust grains, which are less affected by solar radiation pressure and remain near the nucleus (1).

Implications for Interstellar Material Studies

The observations of 3I/ATLAS have significant implications for our understanding of interstellar material. The comet's composition, characterized by a high CO₂ to H₂O ratio and the presence of organic-rich materials, indicates that interstellar objects may share more similarities with Solar System comets than previously thought. This challenges the notion that interstellar objects are entirely distinct from those in our solar system and suggests that they may be composed of materials that are more familiar to us (1–2).

Summary

Observations of the interstellar comet 3I/ATLAS revealed unexpected levels of activity, with a measured spin period of about 16.16 hours and dust mass-loss rates ranging from 0.3 to 4.2 kg per second. Its reflectance spectra showed slight reddening, and the coma’s unusually high CO₂-to-H₂O ratio of roughly 8:1 points to a composition unlike typical Solar System comets. Morphologically, the absence of a visible tail and the asymmetric structure of the coma suggest the presence of large dust grains that are less affected by solar radiation pressure. Together, these findings challenge previous assumptions about interstellar objects, suggesting that they may share more characteristics with comets originating in our own Solar System than once thought (1–3).

References

(1) Santana-Ros, T.; Ivanova, O.; Mykhailova, S.; Erasmus, N.; Kaminski, K.; Oszkiewicz, D.; Kwiatkowski, T.; Husárik, M.; Ngwane, T. S.; Penttilä, A. Temporal Evolution of the Third Interstellar Comet 3I/ATLAS: Spin, Color, Spectra, and Dust Activity. arXiv 2025, arXiv:2508.00808. https://www.aanda.org/articles/aa/pdf/forth/aa56717-25.pdf.

(2) Garcia, A. New Comet from Another Star System Has Unheard-of Chemistry. Chron, August 28, 2025. https://www.chron.com/news/space/article/interstellar-comet-3i-atlas-nasa-21019864.php?utm_source=chatgpt.com (accessed 2025-09-25).

(3) Lisse, C. M.; Bach, Y. P.; Bryan, S.; Crill, B. P.; Cukierman, A.; Doré, O.; Fabinsky, B.; Faisst, A.; Korngut, P. M.; Melnick, G.; Rustamkulov, Z. SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS. Res. Notes AAS 2025, 9 (9), 242. DOI: 10.3847/2515-5172/ae0293.