|Discovered by||E. Barnard|
|Mean radius||181,400 km|
|Revolution period||11h 57.4m|
|Is a satellite of||Jupiter|
|Equatorial diameter||270(×168×150) km|
|Mean density||1.8 g/cm3|
|Surface gravity||0.066 m/s2|
|Rotation period||11h 57.4m|
|Atmospheric pressure||0 kPa|
Amalthea (pronounced "am al THEE uh") is the third of Jupiter's known moonss. It was discovered on September 9 1892 by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope at Lick Observatory. Amalthea was the last moon to be discovered by direct visual observation as opposed to photography, and the first since Galileo Galilei discovered the Galilean moons in 1610. It is named after the nymph of Greek legend who nursed the infant Jupiter with goat's milk.
Amalthea is the reddest object in the solar system, even redder than the planet Mars. The reddish color is apparently due to sulfur originating from Io. Bright patches of green appear on the major slopes of Amalthea, but the nature of this color is currently unknown.
Amalthea is irregularly shaped and heavily scarred by craters, some of which are extremely large relative to the size of the moon. Pan, the largest crater, measures 100 kilometers across and is at least 8 kilometers deep. Another crater, Gaea, measures 80 kilometers across and is probably twice as deep as Pan. Amalthea has two known mountains, Mons Lyctas and Mons Ida with local relief reaching up to 20 kilometers.
The combination of Amalthea's irregular shape and large size implies that Amalthea is a fairly strong, rigid body; if it were composed of ices or other weak materials its own gravity would have pulled it into a more sphereical shape. Like all of Jupiter's moons it is tidally locked with the planet, its long axis pointing towards Jupiter at all times. Its composition is probably more like an asteroid's than like the Galilean moons, and it may be captured. Like Io, Amalthea radiates more heat than it receives from the Sun. This is probably due to the electrical currentss induced within it by its orbit through Jupiter's magnetic field.