Interpretation of NIMS and SSI Images on the Jovian Cloud Structure

We present maps of jovian cloud properties derived from images taken simultaneously by the Galileo solid state imaging system (SSI) and the near-infrared mapping spectrometer (NIMS) at 26 visible and near-infrared wavelengths, ranging from 0.41 to 5.2μm. Three regions—the Great Red Spot (GRS), a 5-micron Hot Spot, and one of the White Ovals—were studied. We perform a principal component analysis (PCA) on the multispectral images. The principal components (PCs), also known as empirical orthogonal functions, depend only on wavelength. The first PC is that spectral function which, when multiplied by an optimally chosen number (amplitude factor) at each pixel location and subtracted from the spectrum there, minimizes the variance for the image as a whole. Succeeding PCs minimize the residual variance after the earlier PCs have been subtracted off. We find that the pixel-to-pixel variations at the different wavelengths are highly correlated, such that the first three PCs explain 91% of the variance in the spectra. Further, one can estimate the amplitudes of the first two PCs using only the four SSI wavelengths and still explain 62% of the variance of the entire spectrum. This can be an advantage when trying to classify features that are resolved in the SSI images but not in the NIMS images. The first PC in all three regions shows negative correlation between 5μm emission and reflected solar light in both atmospheric windows and the methane and ammonia absorption bands. Thus most of the bright, optically thick clouds blocking thermal emission are also extended vertically to the upper troposphere. The first PC at the GRS shows a negative correlation between the violet and all other bands except 5μm for which the correlation is positive. Thus in the GRS there is a red chromophore (absorbing in the violet, reflecting at longer wavelengths) which is associated with clouds that block 5-μm emission. There is no such correlation at the hot spot and white oval regions and therefore no chromophore associated with clouds. The second PC shows a positive correlation between the depth of the methane and ammonia absorption bands and brightness at other visible and near-IR wavelengths; there is also a negative correlation between these quantities and 5-μm emission. Thus some of the bright, optically thick clouds blocking thermal emission are deep and do not extend vertically to the upper troposphere. A color image composed using the first three PCs shows areas of unusual spectra, which appear in distinct colors. An example is the small convective stormlike cloud to the northwest of the GRS. This cloud is highly reflective at long wavelengths (4μm) and might indicate unusually large particles