The Analysis at 5 tex2html_wrap_inline15 m of Hot Spots from Galileo/NIMS  Spectra: Results for Cloud Opacity, Water and Ammonia

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

We present the analysis of 3 Galileo/NIMS Jupiter observations in the thermal 5-micron window (4.6 - 5.2 tex2html_wrap_inline15 m) for the Galileo orbits G1, E4 and G7, concentrating on hotspots in these sequences. The data consist of several thousand spectra in the North and South Equatorial Bands at a spatial resolution on the order of 400 km/pixel, and a spectral resolution of 0.025 tex2html_wrap_inline15 m. NIMS spectra in the 5-micron window are sensitive to a cloud opacity above about 3 bar, and to the relative water humidity between 4 and 8 bar in the first place, and to the ammonia abundance between 4 and 8 bar in a lesser extend. We have developed a retrieval method that allows to determine these 3 parameters from any spectrum.

Probe measurements were used as far as possible for the input parameters in our radiative transfer line-by-line model, i.e.  the thermal profile (Seiff et al. 1996), the level of the cloud (1.55 bar, Ragent et al. 1996), the He abundance (Niemann et al. 1996), and we used solar deep abundances for water and 1.6 times solar for ammonia. We note that the deepest sounded level with NIMS  is about 8 bars.

Our findings are the following: (1) the relative water humidity decreases toward the center of a hotspot by factors of up to 100 and we see evidence for a collar like structure around a hotspot, containing more water; (2) relative water humidities are always lower than about 50 % , they are typically several percent outside hotspots; (3) cloud opacities also decrease over hotspots, but there is no simple relationship with the relative water humidity within the hotspots, (4) low cloud opacities also occur in 5-micron dark regions, where a lot of water is present and absorbing; (5) we observe a small gradient in the ammonia abundance, increasing by a factor of 2 toward the center of the biggest hotspots.

Our findings for the atmospheric structure in hotspot regions are in very good agreement with the observations of the probe. They show that hotspots are very dry areas on Jupiter, in constrast with wetter regions in their surroundings.