Ethane Abundance on Neptune

Ethane mole fractions on Neptune were determined from infrared heterodyne spectroscopic measurements of the C2H6RR (4,5) emission line at 840.9764 cm−1. The measured line was fit using existing atmospheric models and C2H6 mixing ratio profiles for Neptune. The value of the ethane mole fraction retrieved, using a profile consisting of a constant mixing ratio with altitude above the saturation region, was lower than that obtained by G. S. Orton et al. (1987, Icarus 70, 1–12), but was within the 2σ error bounds of their results. Nonconstant mixing ratio profiles derived from the photochemical model of P. N. Romani and S. K. Atreya (1989, Geophys. Res. Lett. 16, 941–944) were also tested. Our results require 2.0 to 5.8 times more ethane in the 0.02–2 mbar pressure region than the model predicts. Ethane mole fractions ∼3 × 10−6 were retrieved near the 0.27-mbar pressure level (line center contribution function peak) for all models tested. The cumulative uncertainty on the retrievals, excluding that associated with the temperature profile, is 30%. Better agreement with the model can be achieved if the eddy mixing is reduced in the lower stratosphere and/or the stratospheric temperature is increased by 10 K above the 6-mbar level.