The Small Inner Satellites of Jupiter
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文摘
The images of the four inner small jovian satellites obtained by the Galileo Solid State Imaging (SSI) experiment have much more detailed shape, color, and photometric information than were provided previously by Voyager images. The satellites are in synchronous rotation and show no binary or bifurcated shapes. Thebe and Amalthea have densities of large craters approximately at “empirical equilibrium” levels. The leading sides of Metis, Amalthea, and Thebe are all 25–35 % brighter than their trailing sides; the global-average, clear-filter (λ = 0.64 μm) geometric albedos of these three satellites are 0.063, 0.091, and 0.049, respectively. A definite color gradient is observed, with the satellites closer to Jupiter being redder: the mean violet/green ratio (0.42/0.56 μm) decreases from Thebe to Metis. This ratio also is lower for the trailing sides of Thebe and Amalthea than for their leading sides. Bright spots on Amalthea and Thebe are small (<20 km) patches on ridges or near crater rims and have albedo contrasts with their surroundings of more than 2:1. The colors of the spots are slightly different from the average surrounding surfaces. There are suggestions of absorption features near 1 μm on Thebe and Amalthea. While the colors of the small satellites indicate possible contamination by materials lost from Io, the brighter leading sides of all three well-measured objects, spanning a region from inside to outside the synchronous orbit distance, suggest that macroscopic impacts may have a substantial influence on surface photometric properties.