Amalthea: Voyager imaging results

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Voyager images of Amalthea reveal a very irregular satellite with dimensions of some 270 × 165 × 150 km, in synchronous rotation relative to Jupiter. Amalthea is nearly as elongated and as large as the Trojan asteroid Hektor. Its surface is scarred by large craters, sharp ridges, and other prominent topography suggestive of a long history of cosmic battering. The largest crater on the satellite, Pan, is about 90 km across. The normal reflectance of the surface is very low (5–6%); the color is very red. We find that Amalthea's mean opposition magnitude is V=+14 and that the magnitude difference between eastern and western elongation does not exceed +0.1. The phase coefficient between phase angles of 0.8° and 42° of 0.042±0.004 mag/deg indicates that the phase integral does not exceed 0.3 and that the Bond Albedo is less than 0.02. Amalthea is redder than typical Trojan asteroids, but not quite as red as parts of Io. Laboratory simulations show that the combination of low albedo and red color probably requires contamination of the surface by sulfur the most likely source of which is Io. A noteworthy mystery concerning Amalthea is the composition of the material in several prominent bright spots. These isolated spots (typically 10–50 km across) occur preferentially on local slopes and ridges, have albedos several times higher than the background, and have a greenish color (the spectrum bends down beyond 0.56 µ m).