Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae

Abstract

Cometary comae are generally depleted in nitrogen. The main carriers for volatile nitrogen in comets are NH3 and HCN. It is known that ammonia readily combines with many acids, such as HCN, HNCO and HCOOH, encountered in the interstellar medium as well as in cometary ice to form ammonium salts (NH4+X) at low temperatures. Ammonium salts, which can have a substantial role in prebiotic chemistry, are hard to detect in space as they are unstable in the gas phase and their infrared signature is often hidden by thermal radiation or by, for example, OH in minerals. Here we report the presence of all possible sublimation products of five different ammonium salts in the comet 67P/Churyumov–Gerasimenko measured by the ROSINA instrument onboard Rosetta. The relatively high sublimation temperatures of the salts leads to an apparent lack of volatile nitrogen in the coma. This then also explains the observed trend of higher NH3/H2O ratios with decreasing perihelion distances in comets.

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Fig. 1: NH3/H2O abundances from 1 August 2014 to 30 June 2016.
Fig. 2: DFMS mass spectra for m/z 17 and 36.
Fig. 3: DFMS mass spectra before, during and after dust impact.
Fig. 4: Abundance ratios normalized to H2O.
Fig. 5: Relative ammonia and elemental abundances in comets.

Data availability

The datasets analysed during the current study together with a user manual for data analysis are available in the ESA-PSA archive (https://www.cosmos.esa.int/web/psa/rosetta) or the NASA PDS archive (https://pdssbn.astro.umd.edu/data_sb/missions/rosetta/index.shtml).

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Acknowledgements

ROSINA would not have produced such outstanding results without the work of the many engineers, technicians and scientists involved in the mission, in the Rosetta spacecraft team and in the ROSINA instrument team over the past 20 years, whose contributions are gratefully acknowledged. Rosetta is an ESA mission with contributions from its member states and NASA. We acknowledge herewith the work of the whole ESA Rosetta team. Work at the University of Bern was funded by the State of Bern, the Swiss National Science Foundation (SNSF, 200021_165869 and 200020_182418), the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 16.0008- 2, the European Space Agency’s PRODEX programme. S.W. acknowledges the financial support of the SNSF Eccellenza Professorial Fellowship PCEFP2_181150. J.D.K. acknowledges support by the Belgian Science Policy Office via PRODEX/ROSINA PEA 90020. S.A.F. acknowledges JPL contract 1496541. Work at UoM was supported by contracts JPL 1266313 and JPL 1266314 from the US Rosetta Project. H.C. is grateful to M. Powner for discussions about prebiotic chemistry during the preparation of the manuscript.

Author information

K.A. was principal investigator of the ROSINA instrument, analysed the data and wrote part of the paper. H.B., J.-J.B., M.C., J.D.K., B.F., S.A.F. and T.I.G. contributed hardware to the instrument. M.R., F.D., M.S., I.S. and T.S. operated and calibrated the instrument. N.H. did laboratory experiments on salts and added to the chemistry. C.B., H.C. and S.W. contributed the part of the paper about the interstellar and astrobiological consequences. All authors read and commented on the paper.

Correspondence to Kathrin Altwegg.

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Extended data

Extended Data Fig. 1 Sample DFMS spectra for m/z 60.

ROSINA-DFMS mass spectrum, Sept. 5, 2016, 18:34 h. Error bars are 1-σ statistical errors. Blue/Grey curves are the two Gaussians, which describe the peaks, sharing the width across the spectrum.

Extended Data Fig. 2 Total densities during the end of mission ellipses.

Total density from Aug. 26 to Sept. 5, 2016 (upper panel) and a zoom for Sept. 5, 15 h – 22 h UTC, measured by ROSINA-COPS. Also displayed are sub-spacecraft latitude in red and the filament current of DFMS for Sept. 5 in blue.

Extended Data Fig. 3 Ammonia density with time on 5./6. Sept. 2016.

Ammonia density as a function of time on 5./6. September 2016.

Extended Data Fig. 4 Amines and their fragments.

Comparison of methylamine and ethylamine fragments from electron impact ionization according to NIST and from measurements in space (Sept. 5, 2016, 20:19 h).

Supplementary information

Supplementary Information

Supplementary Tables 1–3.

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Altwegg, K., Balsiger, H., Hänni, N. et al. Evidence of ammonium salts in comet 67P as explanation for the nitrogen depletion in cometary comae. Nat Astron (2020). https://doi.org/10.1038/s41550-019-0991-9

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