With the Herschel Space Observatory, my colleagues and I have detected CO (carbon monoxide) in the stratosphere of Uranus. The analysis and interpretation of the data demonstate that it is supplied by an external source. Is this oxygen delivered by comets, as in Jupiter (see this post), or by interplanetary dust particles? Complementary observations (e.g., with ALMA) are required to disentangle the two origins.
CO emission of Uranus as observed with Herschel. Different source models produce the vertical profiles shown in the right panel. These profiles translate into the emission lines with the corresponding colors in the left panel. Observation/model comparison proves that CO has an external source.
Reference: Cavalié et al. 2014, Astronomy and Astrophysics 562, A33.