THE NEAR-MILLIMETER BRIGHTNESS TEMPERATURE SPECTRA OF URANUS AND NEPTUNE

We present new photometric measurements of Uranus and Neptune between 0.35 and 2.0 mm. Together with our previous results (Orton et al., 1986, Icarus 67, 289-304), these data have allowed us to determine their near-millimeter brightness temperatures with improved accuracy. Both planets exhibit smoothly varying continuum spectra in this wavelength region, with no evidence of strong unresolved spectral structure which could be attributed to particular molecular transitions. The possible depression in the Uranian spectrum at 1.3 mm noted by Orton et al. is not confirmed by the new measurements. Combining the two data sets, and using Mars as the primary flux standard, we derive the following empirical fit to the Uranian temperature spectrum, TU(λ), in the 0.35-3.3-mm region:TU = ao + a1 [Log10(λ)] + a2 [Log10(λ)]2 + a3 [Log10(λ)]3 K, withao = -795.694 a1 = 845.179 a2 = -288.946 a3 = 35.200λ = wavelength in micrometers. We have also measured very accurately the surface brightness ratio of Neptune with respect to Uranus over the same wavelength range. The ratio is remarkably constant, implying that the tropospheres of these planets are very similar in the 1-4-bar regions. The observations are consistent with a wavelength-independent Neptune: Uranus brightness ratio of 0.94, although the possibility of a slightly lower value near 0.8 mm, due to CO absorption of Neptune, cannot be ruled out. The neptunian brightness temperature, TN(λ), between 0.35 and 3.3 mm is given byTN = ao + [Log10(λ)] + a2 [Log10(λ)]2 + a3 [Log10(λ)]3K, withao = -598.901 a1 = 655.681 a2 = -229.545 a3 = 28.994 λ = wavelength in micrometers. We estimate the internal errors in temperatures derived from these fits to be less than 2 K, and the absolute calibration error arising from uncertainty in the martian temperature to be 5%. © 1993 by Academic Press, Inc.