Light scattering in the clouds on Jupiter

Abstract

We construct maps of jovian cloud properties 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 5.2 µm. Three regions - the Great Red Spot (GRS), a 5-micron Hot Spot, and one of the White Ovals - are studied. We perform a principal component analysis (PCA) on the multispectral images. PCA shows that the pixel-to-pixel variations at the different wavelengths are highly correlated, and that 91% of the variance in the data can be summarized using only three maps. The three maps are combined into one color map, which indicates different 26-wavelength spectra as different colors. Using the representative spectra for each color we compare different areas on the map qualitatively. We find that in the GRS there is a red chromophore which is associated with clouds that block 5-µm emission. At the hot spot and white oval regions there is no chromophore associated with clouds. Most of the bright, optically thick clouds blocking thermal emission are also extended vertically to the upper troposphere. Some of the bright, optically thick clouds blocking thermal emission are deep and do not extend vertically to the upper troposphere. A small convective stormlike cloud to the northwest of the GRS is unusually reflective at long wavelengths (4µm) and might indicate large particles. We study lightning on Jupiter and the clouds illuminated by the lightning. The Galileo SSI lightning images have a resolution of 25 km/pixel and are able to resolve the diffuse spots of light scattered in the clouds, which have full widths at half maximum in the range 90-160 km. We compare the lightning images with the images produced by our 3D Monte Carlo light scattering model. The model reproduces non isotropic non-conservative scattering of the photons in the non-homogeneous opacity distribution. We derive that some of the observed scattering patterns are produced in a 3D cloud rather than in a plane-parallel cloud layer, suggesting deep convection. For the six flashes studied, the clouds above the lightning are optically thick (Γ > 5). Lightning is as deep as the bottom of the water cloud. Jovian flashes are more regular and circular than the largest terrestrial flashes observed from space.