Amalthea was discovered by the exceptionally keen eyesight of American astronomer Edward Barnard in 1892 using the 36 inch (91 cm) refractor telescope at Lick Observatory. This was the first discovery of a Jovian moon since Galileo's discovery in 1610. Barnard's discovery of Amalthea was also the last moon of any planet to be discovered by direct visual observation.
Amalthea is the largest of Jupiter's smaller moons. It is one of Jupiter's four inner moons that orbits inside the orbit of Io. With its irregular shape, measuring 270x170x150 km, Amalthea has a striking resemblence to the potato-shaped Phobos, the well-known moon orbiting Mars, but it is ten times larger. Like most satellites of Jupiter, Amalthea is in a synchronous rotation, with the same blunt end always pointing towards Jupiter.
Amalthea is heavily cratered, with two large craters. The larger crater, Pan, is 90 km long and lies in the "northern hemisphere", and the other large crater, Gaea, is 75 km long and straddles the south pole of the moon. Pan is at least 8 km deep and Gaea is probably twice as deep. The local relief on Amalthea reaches 20 km, and two mountains are known, Mons Ida and Mons Lyctos.
Amalthea is a dark object and reddish in color. In fact, Amalthea is the reddest object in the solar system. The red color is probably caused by sulfur from Io's volcanoes spiraling down to Jupiter and impacting on Amalthea. Bright patches observed on the major slopes of Amalthea are green in color. The nature of the green color is unknown, but observations from the Galileo spacecraft may shed some light on the composition of the bright spots.
Amalthea's orbit is only 181,300 km from Jupiter, or only 2.5 Jupiter radii. Being this close to Jupiter, Amalthea is exposed to the intense radiation field of Jupiter, and is exposed to high dosages of energetic ions, protons and electons in the Jovian magnetosphere. Additionally, Amalthea is bombarded with high-velocity micrometeorites, as well as heavy ions - mostly sulfur, oxygen and sodium diffusing away from Io.
Discovery: Sep 9, 1892 by Edward Barnard Diameter (km): 270x170x150 Mass (kg): 7.17e+18 Mass (Earth = 1): 1.1988e-06 Surface Gravity (Earth = 1): 0.0055 - 0.0085 Mean Distance from Jupiter (km): 181,300 Mean Distance From Jupiter (Rj): 2.539 Mean Distance from Sun (AU): 5.203 Orbital period (days): 0.498179 Rotational period (days): 0.498179 Density (gm/cm?3) 1.8 Orbit Eccentricity: 0.003 Orbit Inclination (degrees): 0.40 Orbit Speed (km/sec): 26.47 Escape velocity (km/sec): 0.0842 Visual Albedo: 0.06 Surface Composition: Rock/Sulfur?
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