Spectrophotometric evidence for a metal-bearing, carbonaceous, and pristine interstellar comet 3I/ATLAS (2511.19112v2)

Abstract: 3I/ATLAS is only the second confirmed cometary object known to enter the Solar System from interstellar space. Cosmogonic considerations suggest that this body may possess relatively high tensile strength and a substantial metal fraction. We present photometric observations along its inbound trajectory toward perihelion, together with a spectroscopic comparison to pristine carbonaceous chondrites from the NASA Antarctic collection. The spectral similarities indicate that 3I/ATLAS may be a primitive carbonaceous object, likely enriched in native metal and undergoing significant aqueous alteration during its approach to the Sun, experiencing cryovolcanism as we could expect for a pristine Trans-Neptunian Object. We propose that the combination of elevated metal abundance and abundant water ice can account for the unusual coma morphology and chemical products reported to date. To do so, corrosion of fine-grained metal grains can originate energetic Fischer-Tropsch reactions, generating specific chemical products into the coma that are not so common in other comets because most of them formed in the outer solar system and didn't inherited so much metal. Interstellar objects such as 3I/ATLAS provide rare opportunities to investigate physical and chemical processes in distant minor bodies of our own Solar System, including trans-Neptunian objects and Oort Cloud comets.

• The paper demonstrates that 3I/ATLAS is a pristine interstellar comet exhibiting active cryovolcanism, confirmed through coordinated photometric and spectrophotometric analyses.
• It employs Gemini S/GMOS data and CR chondrite spectral comparisons to identify a metal-rich, carbonaceous composition with anomalous volatile ratios.
• The findings suggest that interstellar comets may harbor undifferentiated, catalytically active structures, reshaping theories on small body evolution.

Summary of "Spectrophotometric evidence for a metal-bearing, carbonaceous, and pristine interstellar comet 3I/ATLAS"

Introduction

This paper presents a comprehensive photometric and spectrophotometric analysis of 3I/ATLAS, the third confirmed interstellar object and only the second cometary body identified as entering the Solar System from interstellar space. The authors contextualize 3I/ATLAS within planetary dynamical models that predict ejection of primordial bodies from planetary systems, highlighting cosmogonic implications regarding mechanical strength, volatile inventories, and the role of metal content.

Photometric and Spectrophotometric Characterization

Observational campaigns executed over the pre-perihelion phase documented a pronounced increase in cometary activity at a heliocentric distance of 2.53 au. The onset of activity coincided with the activation of water ice sublimation, marked by a sustained ~2 magnitude surge in brightness and escalation of Afρ values from 200–300 cm to above 1500 cm. Such activity is interpreted as the activation of a pristine volatile-rich mantle unmodified by prior perihelion passages. The coma exhibited a broad, diffuse morphology, with the false nucleus resolving into multiple filaments and jets, consistent with volatile-driven outgassing and cryovolcanism.

Spectral analysis leveraged Gemini S/GMOS data and extensive comparison with carbonaceous chondrite spectra, particularly the CR group (EET 92159, GRA 95229, LAP 02342). The reflectance spectra of 3I/ATLAS aligned closely with these samples, characterized by featureless red slopes and a low albedo of ~10%, analogous to D-type asteroids and certain TNOs. The match with CR chondrites supports the hypothesis of a primitive, metal-rich, and aqueously altered object with significant correlation to trans-Neptunian populations.

Chemical and Physical Implications

The compositional modeling of 3I/ATLAS integrates both endogenous and exogenous processes. The authors emphasize the role of implanted cosmic-ray-processed materials and exogenous dust accreted during prolonged interstellar passage. Notably, anomalous volatile ratios (CO2/H2O = 7.6, CO/H2O = 1.65) and a strong red slope in its reflectance spectrum underscore the uniqueness of 3I/ATLAS compared to Solar System comets.

Activation of cryovolcanic mechanisms is inferred from both photometric trends and observed coma morphology. Sustained high H2O production rates (~40 kg s⁻¹) and the detection of OH emission at >2.5 au indicate deep reservoirs responsive to thermal perturbation. The authors propose that water-soaked fine-grained FeNi metal grains within a highly porous matrix catalyze exothermic Fischer-Tropsch-type reactions, facilitating rapid release of CO, CO2, and complex organics. Spectral evidence of Ni enrichment in the coma, including prominent absorption lines, further supports the ongoing corrosion and catalytic transformation of metal grains as a primary driver of activity and coma chemistry.

Meteorite Comparisons, Cosmic Ray Effects, and Evolutionary Scenarios

Correlation with CR chondrites addresses both the metallicity and the inventory of volatile and organic-rich inclusions. For example, LAP 02342 contains trans-Neptunian xenoliths and abundant presolar grains, suggesting that the spectral properties of 3I/ATLAS are consistent with relatively undifferentiated outer-disk materials. Fischer-Tropsch reactions in the presence of Ni-rich metal grains may also explain the observed excess of Ni in its coma and the predicted presence of phosphorous compounds such as PO4³⁻.

The prolonged exposure to galactic cosmic rays is invoked to explain both the surface reddening and possible amorphous carbon crust formation. As the comet approaches the Sun, this crust fractures, exposing reactive interior phases that further drive outgassing and photometric evolution.

Implications for Small Body Evolution and Interstellar Object Research

The paper's findings demonstrate that interstellar comets like 3I/ATLAS challenge existing paradigms regarding primordial small body alteration, volatile retention, and mechanical integrity. The confirmed high metal content, volatile-rich mantle, and sustained cryovolcanic activity imply that substantial compositional diversity exists among extrasolar minor bodies, including the capacity for extensive catalytically driven organic synthesis. The authors argue against differentiation, suggesting that the object remains largely undifferentiated, similar to primitive TNOs.

The paper also draws attention to the importance of ground- and space-based observation platforms for planetary defense and fundamental science, especially within the context of objects on highly inclined, high-velocity trajectories. Missions such as ESA's Comet Interceptor and the International Asteroid Warning Network are highlighted as vital for future interstellar minor body exploration.

Conclusion

The coordinated photometric and spectroscopic campaign reveals that 3I/ATLAS is best interpreted as a virtually pristine, metal-rich, carbonaceous object exhibiting properties closest to CR chondrites and TNOs. The comet’s activation pattern and chemical evolution during heliocentric approach provide robust evidence for a volatile- and metal-rich structure susceptible to catalytic aqueous alteration and cryovolcanism, producing activity and chemical environments distinct from those observed in conventional Solar System comets. The findings underscore the benefit of intercept and sample-return missions targeting future interstellar objects, promising unique insights into the diversity and formation pathways of primitive bodies beyond the Solar System.