Carbon Isotope Abundances in Comets

Rotational lines of ¹³C¹⁴N have been identified in high-resolution (λ/Δλ ~ 60,000) echelle spectra of the CN B²Σ⁺-X²Σ⁺ (0-0) band in three comets. The ¹²C/¹³C abundance ratios determined using a full fluorescence excitation model for comets Levy (C/1990 K1), Austin (C/1989 X1), and Okazaki-Levy-Rudenko (C/1989 XIX) are 90 ± 10, 85 ± 20, and 93 ± 20, respectively, consistent with the solar system ratio, 90. A lower limit for the nitrogen isotope ratio, ¹²C¹⁴N/¹²C¹⁵N ≳ 200, found for comet Levy is consistent with previous comet measurements and the solar system value, 272. The mean CN carbon isotope ratio in the five comets measured to date is ¹²C¹⁴N/¹²C¹⁵N = 90 ± 10, and the mean for the three molecular species C₂, HCN, and CN measured in nine comets is 101 ± 15. Consistency of the cometary carbon isotope ratios with the bulk solar system value indicates (1) chemical homogeneity in the outer protosolar nebula, (2) minimal isotopic fractionation in the protosolar precursor molecular cloud, and (3) that comets formed coevally with the solar system. The 14% difference between the solar system (90) and the present solar neighborhood interstellar ¹²C/¹³C ratio (77 ± 7) may be indicative of significant Galactic ¹³C enrichment over the past 4.6 Gyr. However, even though models can match to within a factor of 2 the solar system abundances, including the carbon isotope ratio, other evidence suggests that simple homogeneous Galactic evolution models may not be adequate to explain detailed stellar and interstellar abundances in the Galaxy.