¹²C/ ¹³C Ratio in ethane on titan and implications for methane's replenishment

The 12C/13C abundance ratio in ethane in the atmosphere of Titan has been measured at 822 cm-1 from high spectral resolution ground-based observations. The value, 89(8), coincides with the telluric standard and also agrees with the ratio seen in the outer planets. It is almost identical to the result for ethane on Titan found by the composite infrared spectrometer (CIRS) on Cassini. The 12C/13C ratio for ethane is higher than the ratio measured in atmospheric methane by Cassini/Huygens GCMS, 82.3(1), representing an enrichment of 12C in the ethane that might be explained by a kinetic isotope effect of approximately 1.1 in the formation of methyl radicals. If methane is being continuously resupplied to balance photochemical destruction, then we expect the isotopic composition in the ethane product to equilibrate at close to the same 12C/13C ratio as that in the supply. The telluric value of the ratio in ethane then implies that the methane reservoir is primordial.