Millimeter spectroscopy of Uranus and Neptune: Constraints on CO and PH3 tropospheric abundances

Spectra of Uranus and Neptune have been obtained in the 190-310 GHz frequency range (i.e., 6.3-10.3 cm(-1), wavelength range 1.0-1.5 mm), with a Fourier transform spectrometer at the Caltech Submillimeter Observatory, The data, with a spectral resolution of 7.2 GHz, were analyzed in a search for CO and PH3. Neither the CO (2-1) transition at 230 GHz nor the PH3 (1-0) transition at 267 GHz were detected. The absence of a detectable absorption feature at the position of the CO (2-1) line favors a CO tropospheric mixing ratio lower than 5 x 10(-7) in Uranus and 1.0 x 10(-6) in Neptune (3 sigma limits). In the case of Neptune, a tropospheric mixing ratio of 6 x 10(-7) seems to be in best agreement with all the existing low resolution data for CO. In the case of Uranus, a PH3 enrichment by more than a factor of 4 can be excluded, There is no evidence for strong PH3 supersaturation in either Uranus or Neptune. (C) 1996 Academic Press, Inc.