The geometric albedos of Uranus and Neptune between 2100 and 3350 Å

Abstract

Absolutely calibrated spectra of Uranus, Neptune, and the solar analog stars 16 Cyg A and B between 2100 and 3350 Å are reported. The geometric albedos of both planets are close to the curve expected for a semi-infinite Rayleigh-Raman scattering atmosphere between 2100 and 2800 Å. Longward of 2800 Å the albedos fall below the Rayleigh-Raman values and connect smoothly to the ground-based photometry of J.S. Neff, D.C. Humm, J.T. Bergstralh, A.L. Cochran, W.D. Cochran, E.S. Barker, and R.G. Tull (1984, Icarus 60, 221–235). Neptune is about 5.5% brighter than Uranus and shows slightly stronger Raman scattering signatures in the MgII lines at 2800 Å in accordance with the results of Neff et al. for the CaII H and K lines. This means that the stratospheric haze on Neptune is thinner than on Uranus. The fact that the Neptunian geometric albedo between 2100 and 2800 Å is so close to the ideal semi-infinite Rayleigh-Raman scattering atmosphere could be exploited for future absolute calibrations of other Solar System objects in this wavelength region.