Research ArticleASTRONOMY

Detection of argon in the coma of comet 67P/Churyumov-Gerasimenko

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Science Advances  25 Sep 2015:
Vol. 1, no. 8, e1500377
DOI: 10.1126/sciadv.1500377


  • Fig. 1 DFMS mass spectra in the m/z ranges of 36 and 38.

    The spectra demonstrate the clear identification of the two isotopes 36Ar and 38Ar and of the interfering molecules. The exact m/z locations are given in the text. The spacecraft background spectra were obtained before the cometary signal became apparent (2 August 2014, heliocentric distance of 3.6 AU, almost 800 km from the nucleus).

  • Fig. 2 (A and B) Comparison of argon abundances to water (A) and to molecular nitrogen (B).

    (A and B) Relative abundances of 36Ar versus H2O (A) and N2 (B). 36Ar abundances were measured relative to water and molecular nitrogen during four periods in October 2014, when Rosetta was close to 67P/CG (10 km). Individual measurements cover 20 s. Measured particles per 20 s are plotted. Ratios are molecular ratios. (A) Large spread of the relative abundances due to the high temporal variability and spatial heterogeneity of the coma (7). (B) Good correlation between 36Ar and N2 due to their similar volatility.

  • Fig. 3 D/H versus 36Ar/H2O mixing of 67P/CG-like and asteroidal materials.

    The asteroidal composition is represented by the Orgueil (CI) and Murchison (CM) carbonaceous chondrites. CI/CM chondrites are considered as the best representatives of volatile-rich primitive meteorites (19). Cometary data: this work, Altwegg et al. (10). Meteorite data: Mazor et al., Bogard et al., and Kerridge (2022). Earth data, surface inventory: Lécuyer et al. and Ozima et al. (12, 13). Range of estimates for bulk Earth: Marty and Halliday (17, 18).


  • Table 1 Argon isotopic ratio and relative abundances to H2O and N2.

    The isotopic ratio of the comet’s argon is in agreement with the solar system values.

    36Ar/38Ar5.4 ± 1.4
    36Ar/H2O(0.1 to 2.3) × 10−5
    36Ar/N2(9.1 ± 0.3) × 10−3

    The ratio of argon to water is given as a range because of heterogeneity in the coma (Fig. 2A). 36Ar and N2 are closely correlated owing to their similar volatility; an average ratio of (9.1 ± 0.3) × 10−3 has been determined for the investigated periods (Fig. 2B).

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