By Ron Cowen
Bombarded by charged particles and exposed to large magnetic fields, the Galileo spacecraft recently braved several passes through Jupiter’s radiation belts to get a closer look at the planet’s volcanically active moon Io. As it neared Io, the craft also captured portraits of Metis, Amalthea, and Thebe, three of Jupiter’s small, innermost moons.
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The new images, released April 24, resolve features as small as 2 kilometers, half the size of the tiniest structures previously visible.
The pictures show that a bright feature on Amalthea, which had appeared to be round in older images taken at other viewing angles, is in fact a 50-km-long streak. Known as Ida, the streak could be material thrown upward when an impactor gouged a nearby crater, or it could mark the crest of a ridge, says Galileo researcher Damon Simonelli of Cornell University.
The craft found that a bright patch near Amalthea’s south pole, discovered in 1979, is the brightest spot on any of the three moons. The region sits astride a large crater and could be material melted into a glass, pulverized, or excavated by the asteroid or comet that struck there. That’s bright only in the relative sense, notes Simonelli, since each moon is quite dark.
On Thebe, Galileo found a 40-km-wide crater that scientists named Zethus, after Thebe’s husband in Greek mythology.