The ISO spectra of Uranus and Neptune between 2.5 and 4.2 mu m: constraints on albedos and H-3(+)

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 CH4 absorption is co nsistent with a reflection over the 3 bar-level cloud level, presumably due to H2S; in the case of Neptune, the reflection seems to take place above t he CH4 cloud at about 0.3 bar. Two ISO-SWS spectra of Uranus, recorded arou nd 3.3 mu m in April and May 1998, show the presence of four H-3(+) emissio n lines. From the May 1998 data, the inferred H-3(+) column density is 0.1 - 40 x 10(12) cm(-2), and the rotational temperature is 600 +/- 200 K. The observed H-3(+) emission is significantly stronger than previous ground-bas ed determinations, obtained in 1992 and 1995.