C-13-ETHANE IN THE ATMOSPHERES OF JUPITER AND SATURN

High-resolution C-12- and C-13-ethane spectra of Jupiter and Saturn we re acquired with the McMath-Pierce 60 inch (1.5 m) Telescope and Celes te, Goddard Space Flight Center's cryogenic grating spectrometer, in 1 995 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 fr actionation of carbon isotopes in the atmospheres of Jupiter and Satur n. 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 c arbon isotopic ratio between the presolar nebula and the present atmos pheres of the outer planets.