13C-Ethane in the Atmospheres of Jupiter and Saturn

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.