Mapping Metal and Molecule Mysteries in Interstellar Comet 3I/ATLAS
In their new study, Hoogendam et al. (2025) use the Keck Cosmic Web Imager (KCWI) to investigate the interstellar comet 3I/ATLAS, the third known object from beyond our solar system to pass through it. Unlike the earlier interstellar visitors, ‘Oumuamua and 2I/Borisov, 3I/ATLAS displays vivid cometary activity. This means sunlight heats its icy surface, releasing gases that glow as they interact with solar radiation. Observing these glowing gases allows astronomers to probe what kinds of materials formed in other star systems. The team focuses on two key signatures: light emitted by cyanogen (CN), a molecule commonly found in comets, and nickel (Ni), a metal that has rarely been observed in gas form.
Capturing the Comet in Action
Hoogendam and collaborators observed 3I/ATLAS on August 24, 2025, when it was about 2.75 astronomical units from the Sun, roughly the distance between the Sun and the asteroid belt. Using KCWI’s integral field spectroscopy, which records both spatial and spectral information at once, the researchers collected data across wavelengths between 3400 and 5500 angstroms. This method allowed them to construct two-dimensional maps of the comet’s gaseous emissions without the limitations of a narrow slit, providing a more complete picture of the surrounding coma, the hazy envelope of gas and dust around the nucleus.
Identifying Cometary Signatures
The analysis confirmed the presence of CN and Ni emission lines, both previously reported by other teams. CN emission, typically created when sunlight breaks apart organic molecules, appeared as an extended halo around the comet, while Ni emission was much more tightly concentrated near the nucleus. No clear iron (Fe) emission was detected, and the authors set an upper limit on how strong such a signal could be. Using established cometary models, they estimated that 3I/ATLAS was releasing roughly 9 × 10²² CN molecules per second, a modest but measurable rate of outgassing.
How the Gases Spread
A key advantage of the KCWI data was the ability to study how gas emissions changed with distance from the comet’s core. By mapping the radial profiles of the two emissions, the team found that CN extended farther into the coma than Ni. Quantitatively, the Ni emission had an e-folding radius of about 594 km, while CN’s extended to roughly 841 km. This means Ni’s glow fades faster with distance, indicating it likely comes from molecules that break apart very quickly once released. The researchers also noticed that both CN and the overall brightness of the comet were slightly asymmetric, showing stronger emission in both the sunward and antisolar directions, consistent with previously observed dust plumes and tails.
Where Does the Nickel Come From?
The team explored several possible chemical sources of the nickel gas. One leading idea involves metal carbonyls, such as Ni(CO)₄, which can form when nickel atoms combine with carbon monoxide. These molecules are known to be unstable under sunlight and would produce a compact emission region, matching the observed Ni profile. Another possibility involves nickel bound to complex organic molecules known as polycyclic aromatic hydrocarbons (PAHs). When these Ni–PAH compounds absorb light, they can break apart and release Ni atoms. A third “hybrid” explanation suggests that nickel-bearing sulfide minerals on the comet’s surface could transform into Ni(CO)₄ in the presence of carbon monoxide. Because 3I/ATLAS is not particularly rich in CO compared to CO₂, the first two scenarios may be more plausible.
What It Means for Interstellar Chemistry
Hoogendam et al. highlight how both 2I/Borisov and 3I/ATLAS show unusually high Ni-to-Fe ratios, echoing patterns seen in some solar system comets observed far from the Sun. This may indicate that the nickel-bearing compounds forming in these environments are more stable or more easily released than their iron counterparts. The team’s results also suggest that interstellar comets may be poorer in water but richer in carbon-based and metallic species compared to most solar system comets.
Looking Ahead
This research demonstrates how integral field spectroscopy can unravel the detailed chemistry of cometary comae, even for fleeting interstellar visitors. Future observations, especially after 3I/ATLAS passes closest to the Sun, will test whether nickel and other metal emissions evolve with distance, shedding light on how these exotic ices behave under solar heating. As upcoming surveys like the Legacy Survey of Space and Time (LSST) discover more interstellar comets, astronomers will be able to conduct broader comparisons of their metallic and molecular compositions, revealing, piece by piece, how other planetary systems form and evolve beyond our own.
Source: Hoogendam
