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# Iron and nickel atoms in cometary atmospheres even far from the Sun

## Abstract

In comets, iron and nickel are found in refractory dust particles or in metallic and sulfide grains1. So far, no iron- or nickel-bearing molecules have been observed in the gaseous coma of comets2. Iron and a few other heavy atoms, such as copper and cobalt, have been observed only in two exceptional objects: the Great Comet of 18823 and, almost a century later, C/1965 S1 (Ikeya–Seki)4,5,6,7,8,9. These sungrazing comets approached the Sun so closely that refractory materials sublimated, and their relative abundance of nickel to iron was similar to that of the Sun and meteorites7. More recently, the presence of iron vapour was inferred from the properties of a faint tail in comet C/2006 P1 (McNaught) at perihelion10, but neither iron nor nickel was reported in the gaseous coma of comet 67P/Churyumov–Gerasimenko by the in situ Rosetta mission11. Here we report that neutral Fe i and Ni i emission lines are ubiquitous in cometary atmospheres, even far from the Sun, as revealed by high-resolution ultraviolet–optical spectra of a large sample of comets of various compositions and dynamical origins. The abundances of both species appear to be of the same order of magnitude, contrasting the typical Solar System abundance ratio.

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Experimental Astronomy Open Access 13 April 2022

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## Data availability

The datasets analysed during the current study are available at the ESO Science Archive Facility at http://archive.eso.org/eso/eso_archive_main.html, under programme numbers 073.C-0525, 075.C- 0355(A), 080.C-0615, 086.C-0958, 087.C-0929, 270.C-5043, 274.C-5015, 2100.C-5035(A), 280.C-5053 and 2101.C-5051.

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## Acknowledgements

We thank P. van Hoof for discussions on the iron atomic data and their uncertainties. We thank R. Hewins and R. Warin for discussions about various Fe- and Ni-rich compounds in meteorites. We thank C. Arpigny, D. Bockelée-Morvan, A. Decock, C. Opitom, H. Rauer, P. Rousselot and B. Yang for leading some UVES proposals, and the ESO staff for service mode observations. J.M., D.H. and E.J. are Honorary Research Director, Research Director and Senior Research Associate at the Fonds de la Recherche Scientifique (F.R.S-FNRS), respectively.

## Author information

Authors

### Contributions

J.M. analysed the spectra and the coma profiles and wrote the main text. D.H. contributed to the proposals and observations, reduced and calibrated the spectra, built the fluorescence model, computed the carbonyl sublimation properties and wrote the Supplementary Information. E.J. led the UVES proposals and made most of the observations. All authors contributed to the discussion and the final text.

### Corresponding author

Correspondence to J. Manfroid.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Dennis Bodewits and Ryan Fortenberry for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Examples of UVES comet spectra.

Comet spectra obtained with the UVES spectrograph at ESO VLT, showing many Fe i and Ni i lines in the selected wavelength region (3,425–3,530 Å). a, Spectrum of the water-poor and CO-rich long-period comet C/2016 R2 (PanSTARRS) at 3 au. b, Spectrum of the Jupiter-family comet 88P/Howell at 1.4 au. c, Spectrum of the new comet C/2002 T7 (LINEAR) at 0.68 au, with lines from the OH(1-2) band. d, Spectrum of the long-period comet C/2020 X5 (Kudo−Fujikawa). Fe i and Ni i lines are indicated by red and blue marks, respectively.

### Extended Data Fig. 2 Comparisons of Fe i, Ni i and dust production rates.

ad, The production rates of Fe i and Ni i are compared to Afρ (which is the product of the reflectivity of the grains, their filling factor and the radius of the coma; used as a proxy for the dust production rate) and to the production rates of OH, CN and CO2+, as determined from our spectra. ef, The production rates of Fe i and Ni i are compared to those of H2O and CO measured in various previous studies for comets 8P, 9P, 21P, 73P, 103P, C/2000 WM1, C/2001 Q4, C/2002 T7, C/2009 P1, C/2012 F6 and C/2016 R2 at about the same epochs as our observations41,42,43,44,45,46,47,48,49,50,51,52. The various cometary types are colour-coded according to their dynamical classification (see Extended Data Table 1). The OH and H2O values relative to comet C/2016 R2 are upper limits. The Pearson correlation coefficients calculated without (with) the C/2016 R2 data are ρOH = 0.844 (0.531), ρAfρ = 0.644 (0.616), ρCN = 0.892 (0.518), $${\rho }_{{{{\rm{CO}}}_{2}}^{+}}$$ = 0.755 (0.804), $${\rho }_{{{\rm{H}}}_{2}{\rm{O}}}$$ = 0.849 (0.627) and ρCO = 0.752 (0.770).

### Extended Data Fig. 3 Iron and nickel carbonyl sublimation properties.

a, Sublimation rates (Z; in molecules cm−2 s−1) of Fe and Ni carbonyls as a function of temperature, compared to those of the main ices in comets. The carbonyl rates are intermediate between those of H2O and CO2. b, The ratio of the sublimation rate of Ni(CO)4 over that of Fe(CO)5 shows that the former is considerably higher than the latter. These quantities were computed as follows. As in refs. 53,54, we estimate the condensation or sublimation temperature Ts of these compounds by solving the equation fxnkTs = Pv,x(Ts) where fx is the relative abundance of species x, n is the number density of the gas, k is the Boltzmann constant, and Pv,x is the vapour pressure, given by the relation log[Pv,x(T)] = −(A/T) + B. The constants A and B for Fe(CO)5 and Ni(CO)4 are obtained from refs. 55,56: A = 2,097 K and B = 11.62 for Fe(CO)5, A = 1,534 K and B = 10.87 for Ni(CO)4, with Pv,x in dyn cm−2. We consider relative abundances fx of 10−3–10−5 × fx(H2O) for both Fe(CO)5 and Ni(CO)4, and we adopt n = 1013 cm−3 as in ref. 54. The resulting sublimation temperatures of the iron and nickel carbonyls (97–108 K and 74–82 K, respectively, depending on fx) are between the sublimation temperatures of H2O and CO2 (152 K and 72 K), whereas CO sublimates at 25 K (ref. 54). The sublimation rate (in molecules cm−2 s−1) from the surface of a pure ice into vacuum can be expressed as57: Zx(T= Pv,x(T)(2πmxkT)−1/2, where T is the ice temperature and mx the mass of species x.

## Supplementary information

### Supplementary Information

This file contains details regarding the FeI and NiI fluorescence models and abundance measurements, Supplementary Figures 1–6 and Supplementary References.

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Manfroid, J., Hutsemékers, D. & Jehin, E. Iron and nickel atoms in cometary atmospheres even far from the Sun. Nature 593, 372–374 (2021). https://doi.org/10.1038/s41586-021-03435-0

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• ### Cometary science with CUBES

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Experimental Astronomy (2022)