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Observations of Io in eclipse demonstrate conclusively that Io emits substantial amounts of radiation at 4.8 and 3.8 micrometers and a measurable amount at 2.2 micrometers. Color temperatures derived from the observations fit blackbody emission at 560 K. The required source area to yield the observed 4.8-micrometer flux is approximately 5 x 10(-5) of the disk of Io and is most likely comprised of small hot spots in the vicinity of the volcanoes.
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Sinton et al. (1980) studied this question.
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