Overview
Emission lines from neutral nickel (NiI) and neutral iron (FeI) are commonly seen in the comae of solar system comets, even when the Sun’s heat is far from sublimating refractory materials. The interstellar journey of comets preserves chemical fingerprints from their parent systems, and recent work on the interstellar visitor 3I/ATLAS has highlighted an extreme NiI/FeI abundance ratio in its coma. Previously, similar signatures were detected in 2I/Borisov, where the NiI/FeI ratio tended to exceed the solar Ni/Fe value by an order of magnitude. The new observations of 3I/ATLAS push this line of inquiry further, suggesting a distinctive, possibly carbonyl-rich, chemistry in this extrasolar object.
Observations and Methods
A team led by Damien Hutsemékers and colleagues used the ESO Very Large Telescope (VLT) with the UVES spectrograph to monitor 3I/ATLAS at six separate epochs. The spectra spanned heliocentric distances from 3.14 to 2.14 astronomical units (au), allowing a look at how the metal-bearing gas in the coma evolves as the comet approaches the Sun. In all six epochs, NiI emission lines were detected. FeI lines, however, appeared only when the comet was closer than 2.64 au to the Sun, indicating a distance-dependent release mechanism or excitation efficiency for FeI relative to NiI.
To translate line intensities into production rates, the researchers compared the observed NiI and FeI line strengths with a fluorescence model that simulates how these atoms are excited and emit photons in the solar-illuminated coma. This approach yields estimates of how many NiI and FeI atoms are being produced (or released) by the nucleus or its surroundings at each epoch. The method hinges on robust atomic data and realistic coma conditions, ensuring that the derived rates reflect real changes in outgassing rather than observational biases.
Key Findings
The analysis shows that 3I/ATLAS maintains a high production rate of NiI across all six epochs, while FeI is only observable when the comet lies within roughly 2.64 au of the Sun. Most notably, the NiI/FeI ratio is exceptionally high compared with solar system comets and even with 2I/Borisov, making 3I/ATLAS an outlier in the sample of known interstellar visitors. A striking pattern emerges: as 3I approaches the Sun, the NiI/FeI ratio declines rapidly. This trend may reflect differential release or survival of Ni- and Fe-bearing species in the near-Sun environment, or changing excitation conditions in the coma.
Interpretation of these results leans toward the sublimation of nickel and iron carbonyls as a principal source of the observed NiI and FeI, namely Ni(CO)4 and Fe(CO)5. If Ni(CO)4 and Fe(CO)5 are embedded in the nucleus’s material, their preferential release could account for the early prominence of NiI and the later emergence (and subsequent fading) of FeI as the distance to the Sun shrinks. The findings thus suggest the presence of these volatile carbonyl species in the comet’s interior, offering a tangible link to the chemical inventory of the parent system.
Implications for Interstellar Material
The discovery of a pronounced NiI/FeI ratio in 3I/ATLAS adds a new dimension to our understanding of the chemical diversity of extrasolar bodies. If Ni(CO)4 and Fe(CO)5 are indeed carriers for the observed lines, the result implies that complex, metal-containing carbonyls can survive formation and retention in the material that assembles into comets around other stars. This has potential implications for models of planetesimal composition, volatile delivery, and the chemical evolution of planetary systems beyond our own. As 3I/ATLAS continues to evolve and its distance to the Sun decreases, the NiI/FeI signature may converge toward the patterns observed in solar system comets, offering a natural laboratory for comparing interstellar and solar system chemistry.
Comparison with 2I/Borisov
While 2I/Borisov exhibited a NiI/FeI ratio higher than solar values, 3I/ATLAS stands out with an even more pronounced disparity, particularly at larger heliocentric distances. Borisov’s ratio was more in line with solar-system comets, whereas 3I/ATLAS reveals a clearly enhanced nickel-to-iron signature whose evolution with heliocentric distance provides clues about the physical conditions and composition of interstellar material. The rapid change of the ratio with distance in 3I/ATLAS underscores the importance of monitoring interstellar comets across multiple epochs to capture transient processes in their comae.
Future Prospects
Continued spectroscopy of 3I/ATLAS and other interstellar visitors will be essential to test the carbonyl-subsolar hypothesis and to search for additional carriers of nickel and iron. Deeper modeling of fluorescence, including potential non-equilibrium effects, could refine production-rate estimates and clarify the roles of Ni(CO)4 and Fe(CO)5. As more interstellar objects are discovered, comparative studies will illuminate whether the NiI/FeI enrichment is a common feature of extrasolar materials or a peculiarity of certain dynamical histories. The ongoing work by Hutsemékers, Manfroid, Jehin, Opitom and collaborators thus not only characterizes a remarkable object, but also expands our view of chemical diversity in the cosmos.
