MILLIMETER-WAVE OBSERVATIONS OF SATURN, URANUS, AND NEPTUNE - CO AND HCN ON NEPTUNE

Saturn, Uranus, and Neptune were observed at millimeter wavelengths with the IRAM 30 m telescope. T major result is the detection of CO and HCN in Neptune's stratosphere, with respective mixing ratios of (6.5 +/- 3.5) x 10(-7) and (3 +/- 1.5) x 10(-10). CO seems to be present in Neptune's troposphere as well and to slowly decrease with altitude (scale height approximately 200 km). HCN is probably formed from reactions between CH3 and N, which can be supplied in sufficient amounts by escape from Triton's atmosphere. The origin of CO, however, is more problematic, because (i) thermochemical models fail to reproduce the observed abundance by a factor approximately 1000 and (ii) an external source would require a very large flux of oxygen. CO appears to be at least 15 times less abundant on Uranus than on Neptune. Finally, an upper limit of 10(-7) for CO in Saturn's stratosphere suggests an internal origin for Saturnian CO.