Plumes on Io have a blue color, so the plume shadow is reddish. The Prometheus plume can be seen in every Galileo image with the appropriate geometry, as well as every such Voyager image acquired in 1979. It is possible that this plume has been continuously active for more than 18 years. In contrast, a plume has never been seen at Pillan Patera prior to the recent images. Color images from orbit C9 have been merged with a high resolution mosaic of images acquired in various orbits to enhance the surface detail.
North is to the top of the picture.
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