Left: Herschel-HIFI observation of the CO(8−7) line in Uranus on June 15, 2012, expressed in terms of line-to-continuum ratio (l/c, black line). For each source, the models that best fit the emission core are displayed: an internal source yielding a mole fraction of 1.9 × 10^-8 in the upper troposphere (red line), a steady external flux (due to IDP or a local source) of 2.2 × 10⁵  cm^-2 s^-1 (blue line), and a comet with a diameter of 640 m depositing 3.4 × 10¹³ g of CO above the 0.1 mbar level ~300 years ago (green line). These models were computed with the thermal profile of Feuchtgruber et al. (2013). The internal source model overestimates the line core width and produces a broad absorption that is not observed in the data. The external source models can barely be differentiated. Right: vertical profiles associated with the spectra.