Amalthea is Jupiter 's fifth largest moon, much smaller than the four Galilean satellites Io , Europa , Ganymede , and Callisto . The orbit of Amalthea is inside of these moons, and with its long axis always pointing toward Jupiter . It's dark surface color is probably due to sulfur being expelled from Io . Amalthea is not massive enough for its internal gravity to make it spherical. Amalthea was discovered in 1892 by Edward Barnard.
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Credit: NASA, Voyager 1, Copyright Calvin J. Hamilton