THE 12C/13C RATIO ON TITAN FROM CASSINI INMS MEASUREMENTS AND IMPLICATIONS FOR THE EVOLUTION OF METHANE

We have re-evaluated the Cassini Ion Neutral Mass Spectrometer (INMS) C-12/C-13 ratios in the upper atmosphere of Titan based on new calibration sensitivities and an improved model for the NH3 background in the (CH4)-C-13 mass channel. The INMS measurements extrapolated to the surface give a C-12/C-13 in CH4 of 88.5 +/- 1.4. We compare the results to a revised ratio of 91.1 +/- 1.4 provided by the Huygens Gas Chromatograph Mass Spectrometer and 86.5 +/- 7.9 provided by the Cassini Infrared Spectrometer and determine implications of the revised ratios for the evolution of methane in Titan's atmosphere. Because the measured C-12/C-13 is within the probable range of primordial values, we can only determine an upper boundary for the length of time since methane began outgassing from the interior, assuming that outgassing of methane (e.g., cryovolcanic activity) has been continuous ever since. We find that three factors play a crucial role in this timescale: (1) the escape rate of methane, (2) the difference between the current and initial ratios and the rate of methane, and (3) production or resupply due to cryovolcanic activity. We estimate an upper limit for the outgassing timescale of 470 Myr. This duration can be extended to 940 Myr if production rates are large enough to counteract the fractionation due to escape and photochemistry. There is no lower limit to the timescale because the current ratios are within the range of possible primordial values.