Research ArticlePLANETARY SCIENCE

Krypton isotopes and noble gas abundances in the coma of comet 67P/Churyumov-Gerasimenko

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Science Advances  04 Jul 2018:
Vol. 4, no. 7, eaar6297
DOI: 10.1126/sciadv.aar6297
  • Fig. 1 Isotopic composition of 67P/C-G krypton, normalized to 84Kr and the SW composition [from (18)].

    67P/C-G errors reflect 1-σ SEM and calibration uncertainties for the corresponding averaging periods. In this format, SW-Kr is represented by the horizontal orange line. 83Kr appears to be slightly depleted relative to solar. The red line represents a mix of different nucleosynthetic components [the so-called G-Kr and N-Kr components; (11, 20)]. For the G-Kr composition, we consider the weak s-process composition having low 86Kr/84Kr ratios (20). The best fit was obtained for a proportion of 5% G-Kr in cometary krypton.

  • Fig. 2 Relative abundances of the major isotopes of argon, krypton, and xenon.

    (A) 84Kr versus 36Ar relative abundances obtained from daily averages with SEM error bars for the 14 to 31 May 2016 period (R2 = 0.67). (B) 132Xe versus 84Kr. (C) 132Xe versus 36Ar. The number associated with each point indicates the day (R2 = 0.83). The error of the daily averages includes the statistical errors, whereas for the slopes, a 16% calibration uncertainty for each element is included. A correlation coefficient of 0.76 was found between the 132Xe/36Ar and 84Kr /36Ar slopes (A and C).

  • Fig. 3 Relative abundances of 36Ar, N2, and CO2.

    36Ar to H2O abundances for 9 to 21 March 2016 (red) and 14 to 31 May 2016 (blue) in (A), 36Ar to N2 abundances in (B), and N2 to CO2 in (C). The individual measurements show statistical errors. The error of the slope for the combined measurements (black) includes the statistical error, 18% calibration uncertainty (sensitivity and fragmentation pattern for each species), and 20% gain error (R2 = 0.70 for 36Ar/N2 and R2 = 0.12 for CO2/N2).

  • Fig. 4 Noble gas relative abundances compared to other solar system reservoirs.

    Sources of data: 67P/C-G: production rate ratios with 1-σ errors (SEM and calibration uncertainties) derived from Table 1, 84Kr/36Ar = 0.058 ± 0.013, and 132Xe/36Ar = 0.013 ± 0.003; solar: (18); Earth and Mars: (43); chondritic: CRPG compilation of CI and CM data; amorphous water ice: (24, 25). The blue arrow indicates the presumed composition of the initial atmosphere of Earth before secondary loss of xenon, and possibly krypton, to space through geological periods of time. The two red arrows represent upper limits for the 132Xe/36Ar and 84Kr/36Ar ratios measured in Titan by the Cassini-Huygens probe (23).

  • Table 1 Relative 36Ar, 84Kr, and 132Xe abundances for the 14 to 31 May 2016 period based on approximately 500 spectra.

    The second column gives the density ratios measured at the location of Rosetta, which are equivalent to the comet’s production rate, assuming equal outgassing velocities applicable in a collisional coma. The last column (underlined) represents our best estimate for the bulk abundance where the ratio X1/X2 is multiplied by m2/m1 to account for different outgassing velocities based on a thermal expansion approximation. Errors reflect 1-σ SEM and calibration uncertainties.

    IsotopeDensity ratio at
    Rosetta
    m2/m1Production rate ratio
    of 67P/C-G
    36Ar/132Xe41.7 ± 10.11.91579.8 ± 19.3
    36Ar/84Kr11.2 ± 2.61.52717.1 ± 3.9
    84Kr/132Xe3.7 ± 0.91.2544.7 ± 1.1
  • Table 2 Best-estimate bulk abundances of noble gases in comet 67P/C-G in the second column.

    The third column gives the correction factor applied to the values from the second column in table S3 based on the expected outgassing velocities. We assume that, close to perihelion, N2 and H2O share the same velocity due to collisional coupling and apply no correction. Hence, for the May 2016 period, the reference mass for the 1/m velocity correction becomes that of molecular nitrogen (m1 = 28 u) instead of water (18 u). Errors reflect 1-σ SEM and calibration uncertainties.

    SpeciesProduction rate ratio of 67P/C-Gm2/m1Conditions/notes
    N2/H2O(8.9 ± 2.4) × 10−4(1.000)Collisional, measured near perihelion
    36Ar/N2(5.5 ± 1.5) × 10−30.882Rarefied, measured beyond 3 AU
    36A/H2O(4.9 ± 1.9) × 10−60.882Combination of rarefied and collisional
    Ar/H2O(5.8 ± 2.2) × 10−60.882∙(1.000)Combination of rarefied and collisional
    Kr/H2O(4.9 ± 2.2) × 10−70.577Combination of rarefied and collisional
    Xe/H2O(2.4 ± 1.1) × 10−70.461Combination of rarefied and collisional
    Ne/H2O<5 × 10−81.128Upper limit

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaar6297/DC1

    Fig. S1. Averaged ROSINA DFMS mass spectra containing the major krypton isotopes.

    Fig. S2. Observation geometry for the noble gas observations in May 2016.

    Fig. S3. Observation geometry before perihelion in May 2015 and early June 2015 used to derive the N2/H2O ratio.

    Fig. S4. N2 versus H2O abundances for before perihelion in May 2015 and early June 2015.

    Fig. S5. Mix of exotic X-Xe with normal N-Xe compared to 67P/C-G xenon.

    Table S1. Krypton average counts per spectrum (~20-s integration time) including errors and below the associated isotopic ratios with respect to 84Kr with errors for the three periods.

    Table S2. Ratio of 36Ar to total argon, 84Kr to total krypton, and 132Xe to total xenon.

    Table S3. Bulk abundances of noble gases in the coma of comet 67P/C-G.

    Table S4. Measured mass/charge ratios in the dedicated noble gas mode.

  • Supplementary Materials

  • This PDF file includes:
    • Fig. S1. Averaged ROSINA DFMS mass spectra containing the major krypton isotopes.
    • Fig. S2. Observation geometry for the noble gas observations in May 2016.
    • Fig. S3. Observation geometry before perihelion in May 2015 and early June 2015 used to derive the N2/H2O ratio.
    • Fig. S4. N2 versus H2O abundances for before perihelion in May 2015 and early June 2015.
    • Fig. S5. Mix of exotic X-Xe with normal N-Xe compared to 67P/C-G xenon.
    • Table S1. Krypton average counts per spectrum (~20-s integration time) including errors and below the associated isotopic ratios with respect to 84Kr with errors for the three periods.
    • Table S2. Ratio of 36Ar to total argon, 84Kr to total krypton, and 132Xe to total xenon.
    • Table S3. Bulk abundances of noble gases in the coma of comet 67P/C-G.
    • Table S4. Measured mass/charge ratios in the dedicated noble gas mode.

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