A dual origin for Neptune's carbon monoxide?

Heterodyne observations of Neptune have provided a measurement of the CO( 2 - 1) line profile with a total bandpass of almost 8 GHz and a resolution of 4 MHz. The lineshape indicates that the CO mole fraction in Neptune's atmosphere is not uniform, but increases by a factor of similar to 2 from the troposphere/lower stratosphere (0.5 ppm at p > 20 mbar) to the upper stratosphere ( 1 ppm at p < 20 mbar). This indicates the existence of both external and internal sources of CO. The equivalent flux associated with the external source is ∼ 1 x 10(8) cm(-2) s(-1). We propose that the stratospheric CO results from a large (2 km) cometary impact that occurred ∼ 200 years ago, although there remains problems with this hypothesis.