Mixing ratio profiles of the most abundant oxygen species in the Uranus atmosphere assuming an oxygen combined source due to ice grain ablation and cometary impact. Upper panel: mixing ratio profiles resulting from assuming H₂ O and CO influx of 6.0 × 10⁴ and 2.9 × 10⁴ cm⁻² s⁻¹, respectively. Additionally, an impactor of 0.64 km in diameter hits the planet delivering only CO (see Table 4). Carbon dioxide is produced by chemical reactions. The evolution time of the material delivered by the comet is tuned to produce a CO₂ abundance compatible with Spitzer observations resulting into t_impact = 67 yr. On the other hand, CO largely overestimates the data. Lower panel: as above, but the size of the impactor is reduced to 0.56 km to inject a smaller amount of CO in the atmosphere. The evolution time needed to match the CO₂ observationsis 19 yr giving rise to a CO mixing ratio ~5 × 10⁻⁹ at 0.4 mbar, whereas H₂O largely overestimates ISO data.