C-13-ethane in the atmospheres of Jupiter and Saturn

High-resolution C-12- and C-13-ethane spectra of Jupiter and Saturn were acquired with the McMath-Pierce 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 C-12/C-13 of 91(-13)(+26) for Jupiter and 99(-23)(+43) 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 C-12/C-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.