Observations with a factor of at least 4-6 times larger signal-to-noise are required to detect this amount of fractionation, depending on the altitude range over which the outflow from deep convection occurs.ĭepolarized Rayleigh and Raman Scattering from Simple Fluids. By relating fractionation of our reference parcel model to the pressure level where the moist adiabat achieves the required temperature, we argue that the measured methane fractionation constrains the outflow level for a deep convective event. We find that deep, precipitating convection can enhance the fractionation of the remaining methane vapor by -10 to -40‰, depending on the final temperature of the rising parcel. While previous estimates of CH3D fractionation on Titan have estimated an upper limit of -6‰, assuming a solid condensate, we consider more recent laboratory data for the equilibrium fractionation over liquid methane, and use a Rayleigh distillation model to calculate fractionation in an ascending parcel of air that is following a moist adiabat. The preferential condensation of the heavy isotopologues will fractionate methane by reducing CH3D in the remaining vapor, and therefore these observations place limits on the amount of condensation that occurs in the troposphere. Our observations can rule out a larger than 10% variation in the column of CH3D below 50 km. We search for meridional variation in the abundance of CH3D relative to CH4 on Titan using near-IR spectra obtained with NIRSPAO at Keck, which have a photon-limited signal-to-noise ratio of âˆ❅0. Ãdámkovics, Máté Mitchell, Jonathan L. Search for methane isotope fractionation due to Rayleigh distillation on Titan
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