High-resolution 12C- and 13C-ethane spectra of Jupiter and Saturn were acquired with the McMathPierce 60 inch (1.5 m) Telescope and Celeste, Goddard Space Flight Center's cryogenic grating spectrometer, in 1995 November and December. A relative abundance ratio 12C/13C of 91+26-13 for Jupiter and 99+43-23 for Saturn was derived from the measurements. These nearly terrestrial values suggest little or no fractionation of carbon isotopes in the atmospheres of Jupiter and Saturn. A weighted average of the available 12C/13 ratios for the outer planets yields 88±7, thus presenting no evidence for change in the carbon isotopic ratio between the presolar nebula and the present atmospheres of the outer planets.
Sada, P. V., McCabe, G. H., Bjoraker, G. L., Jennings, D. E., & Reuter, D. C. (1996). 13C-Ethane in the Atmospheres of Jupiter and Saturn. Astrophysical Journal v.472, 472, 903. https://doi.org/10.1086/178120