Amalthea is named for a nymph who nursed the baby Jupiter. Though not as tiny as little Metis and Adrastea, Amalthea is a small moon, measuring 169 x 103 x 94 miles (270 x 165 x 150 km). Despite its small size, Amalthea was the very last moon in the solar system discovered with a ground-based optical telescope. Edward Barnard found it in 1892 while observing through the Lick Observatory's 36-inch (91-cm) refracting scope. Barnard became something of a star of astronomy, because this was the first of Jupiter's moons discovered since Galileo found the four largest moons in 1610.
Amalthea orbits at 1.5 Jupiter radii, passing around Jupiter once every 12 hours. Amalthea orbits synchronously, with its long axis pointing in toward Jupiter. Its surface is very old and covered with craters.The largest crater, Pan, is 60 miles (100 km) in diameter, and five miles (8 km) deep. Another large crater, Gaea, is twice as deep as Pan but only 50 miles (80 km) in diameter. Amalthea also has at least two mountains, Mons Lyctas and Mons Ida, which approach 13 miles (20 km) high. Imagine a world only 170 miles long at best, not quite the distance from Boston to New York City, with mountains more than twice the height of Mount Everest (which is about 5.5 miles high)! Small bodies have small gravity, and so higher mountains can exist without being pulled down by gravity. Still, there are no good working hypotheses for what could cause such large mountains to form on such a small body.
Amalthea's surface is dark and reddish. The surface may be that color from its own composition combined with a constant bombardment of energetic particles funneled through Jupiter's magnetic field, or it may be coated with sulfur from neighboring Io's incredibly active
Images of Jupiter's four small inner satellites were taken by Galileo. From left to right are Thebe, Amalthea (the largest moon), Adrastea (the smallest), and Metis, presented on the same scale, Adrastea and Metis were first resolved by a spacecraft camera in these images. The bottom panel shows computer models of the moons' shapes. (NASA/JPL)
volcanoes. Even more strange, steep slopes on Amalthea appear bright green. Green is an uncommon color for solar system bodies, and why it appears on Amalthea is unknown.
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