19.03

The Solar Reflected Component in Jupiter's 5 Micron Spectra from NIMS/Galileo Observations

P. Drossart, M. Roos, Th. Encrenaz, E. Lellouch (Paris-Meudon Obs.), R.W. Carlson, K.H. Baines (JPL), F.W. Taylor, S. Calcutt (Oxford Univ.)

Observations of Jupiter with the Galileo/NIMS experiment have been analyzed in the 4-5  m spectral range to search for the relative contribution of the thermal emission and of the solar reflected component in Jupiter's spectrum. Cold spectra within the equatorial zone are chosen as they provide very low thermal emission. NIMS spectra of Galileo G2, C3, E4 and G7 orbits are used, with an average over homogeneous cold equatorial regions to enhance the signal to noise ratio. The coldest spectra of the night side have a brightness temperature around 165 K at 5  m, and a flux at only one third of the coldest day side spectra, which are at about 180 K in equivalent brightness temperature. The difference with the night side spectra can be interpreted as due to the solar reflected component on the cloud top, with an equivalent reflection coefficient of a few percent. The temperature of the absorbing cloud in cold regions must be about 160 K or colder to fit the observed spectra, significantly lower than previous estimates from Voyager/IRIS observations, which were limited, because of S/N limitations, to spectra with brightness temperatures of about 180 K or higher (Drossart et al., 1981; Bjoraker et al., 1986.) The detection of a spectral feature due to H O implies that the origin of the spectra comes from the deep atmosphere, with strong cloud attenuation, if the atmospheric structure is assumed to be homogeneous within the NIMS field of view. An alternative explanation is that the thermal emission comes from small locally cloud-free regions, with a filling factor of the order of a few percent only.