Figure 2.

(a) Reference Neptune atmosphere at 45° S planetocentric latitude with T_exo = 750 K and CO based on cometary and dust sources (see [24]), plotted in altitude to highlight the upper atmosphere. (b) Variations in CO considered in this work due to T_exo (line colours) and CO source (line types). CO from the ‘dust only’ and ‘comet+dust’ model types are identical above 600 km altitude (i.e. the dotted and solid curves overlap there). Similarly, topside CO influxes of <10⁶ cm⁻² s⁻¹ (not shown) are indistinguishable from the ‘dust only’ model below approximately 1000 km. Note that the axes are scaled differently in each panel. (Online version in colour.)