Search for methane isotope fractionation due to Rayleigh distillation on Titan

•We search for meridional variation in the abundance of CH3D relative to CH4 on Titan.•Our observations can rule out a larger than 10% variation in the column of CH3D below 50 km.•We use a Rayleigh distillation model to calculate fractionation in an ascending parcel of air that is following a moist...

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Bibliographic Details
Published in:Icarus (New York, N.Y. 1962) N.Y. 1962), 2016-09, Vol.275, p.232-238
Main Authors: Ádámkovics, Máté, Mitchell, Jonathan L.
Format: Article
Language:English
Subjects:
Adaptive optics
Atmospheres, evolution
Atmospheres, structure
Condensation
Fractionation
Mathematical models
Methane
Outflow
Parcels
Searching
Signal to noise ratio
Titan, atmosphere
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Summary:•We search for meridional variation in the abundance of CH3D relative to CH4 on Titan.•Our observations can rule out a larger than 10% variation in the column of CH3D below 50 km.•We use a Rayleigh distillation model to calculate fractionation in an ascending parcel of air that is following a moist adiabat.•We find that deep, precipitating convection can enhance the fractionation of the remaining methane vapor by −10 to −40‰.•Observations with a factor of at least 4–6 times larger signal-to-noise are required to detect this amount of fractionation. 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 ∼50. Our observations can rule out a larger than 10% variation in the column of CH3D below 50 km. 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. 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. 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. 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. 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.
ISSN:0019-1035
1090-2643
DOI:10.1016/j.icarus.2016.04.006