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The second closet moon to Jupiter is Adrastea. . Ganymede is the largest. Europa is the 6th farthest moon from Jupiter and Io is the first known body with volcanic activity besides earth. Io's volcanoes spew ash and lava hundred's of miles before it settles to the planet.  

                   From left to right, you see, Ganymede, Callisto Io and Europa

FAST FACT: Visit a site with information about Io and some of it's hotspots.  dsc.discovery.com/news/briefs/20010226/io.html