| Summary: | A comparison between low-flux dayside and nightside spectra of Jupiter recorded by the Galileo near-infrared mapping spectrometer (NIMS) experiment gives the first accurate estimate of the solar reflected component at 5 μm, in the equatorial zone of Jupiter. A minimum flux level of about 0.6 μW cm-2 sr-1V/μm is found on the dayside, compared with 0.1 /μW cm-2 sr-1/μm on the nightside. These fluxes are 100-800 times lower respectively than the bright 5-μm thermal emission in the north equatorial belt (NEB) hot spots. The day/night difference can be interpreted as a solar reflected component from a cloud, presumably the ammonia cloud, with an albedo of the order of 15%, located at a pressure level of 0.79 bar or at higher altitudes (corresponding to cloud temperature of 160 K or lower). Compared to the measurements in hot spots made at other wavelengths from ground-based observations and from NIMS real time spectra, they imply a high cloud opacity in cold regions at atmospheric levels where the cloud optical depth in the hot spots is very low. The residual flux on the nightside arises from (1) a very small cloud transparency giving some access to deeper thermal emission or (2) as high-resolution solid-state imaging (SSI) images of Galileo suggest, to cloud inhomogeneities, with clearer regions of medium brightness temperatures, mixed with dark regions of much lower thermal emission. If the former have the same brightness as a typical hot spot, a filling factor of a few percent is sufficient to explain the observed flux level on the nightside cold regions. Copyright 1998 by the American Geophysical Union.
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