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 ~2 from the troposphere/lower stratosphere (0.5 ppm at mbar) to the upper stratosphere (1 ppm at mbar). This indicates the existence of both external and internal sources of CO. The equivalent flux associated with the external source is ~ 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.