Jovian Atmospheric Structure as revealed by Galileo/NIMS

A.L. Weir, P.G.J. Irwin, S. Smith, F.W. Taylor, A. Lambert, P.J. Smith (University of Oxford), R. Carlson, K. Baines (JPL), Th. Encrenaz, P. Drossart, M. Roos-Serote (Meudon)

The four real-time near infrared spectra of Jupiter recorded by the Near Infared Mapping Spectrometer aboard the Galileo spectra in June 1996 remain the only spectrally continuous measurement in the range 0.7-5.2 tex2html_wrap_inline11 m. Limited spatial coverage is provided by a second "hotmap" data set, recorded in the same orbit, consisting of 6020 partial spectra covering -30 tex2html_wrap_inline13 to 10 tex2html_wrap_inline13 W longitude, 5 tex2html_wrap_inline13 to 15 tex2html_wrap_inline13 N latitude.

This paper presents retrievals of data from these measurements based on two different radiative transfer models: 1) a simplified reflecting layer model, 2) a multiple scattering, correlated-k model. In both cases, the non-linear retrieval problem is solved utilising a simulated annealing algorithm, a technique that is highly effective in finding global minimum solutions. Information content theory is used to select the most significant wavelengths.

It is found that the main reflecting layers exist at 1.0 and 0.35 bars and that they have similar features for all four RT spectra. Hence the large variation of intensity in the 5 tex2html_wrap_inline11 m part of the spectrum is attributed to a lower, variable opacity cloud whilst the variation in depth of the Near-IR ammonia absorption features is attributed to gaseous abundance variations above 1 bar which correlate with variations in 5 micron brightness.

Identification of the scattering particles themselves is hampered by the limited viewing geometry of these first spectra and also by limitations in available laboratory data. However Henyey-Greenstein parameters have been recovered and are compared to the scattering properties of real possible scatterers.