The ISO spectra of Uranus and Neptune between 2.5 and 4.2 mu m: constraints on albedos and H_3⁺

Abstract

Spectra of Uranus and Neptune were recorded in May 1997, between 2.5 and 4.2 mu m, using the ISOPHOT-S instrument. The two planets were detected at 2.75 mu m, with geometric albedos of 0.0015 and 0.0052 for Uranus and Neptune respectively. In the case of Uranus, the shape of the CH_4 absorption is consistent with a reflection over the 3 bar-level cloud level, presumably due to H_2S; in the case of Neptune, the reflection seems to take place above the CH_4 cloud at about 0.3 bar. Two ISO-SWS spectra of Uranus, recorded around 3.3 mu m in April and May 1998, show the presence of four H_3⁺ emission lines. From the May 1998 data, the inferred H_3⁺ column density is 0.1 - 40 x 10¹² cm^-2, and the rotational temperature is 600 +/- 200 K. The observed H_3⁺ emission is significantly stronger than previous ground-based determinations, obtained in 1992 and 1995.