A malthea is one of Jupiter's smaller moons. It was named after the nymph who nursed the infant Jupiter with goats milk. It was discovered in 1892 by the American astronomer Edward Emerson Barnard while making observations from the Lick Observatory with a 36 inch (91 centimeter) refractory telescope. Amalthea was the last moon in the solar system to be discovered through direct visual observation. It was also the first moon of Jupiter to be discovered since Galileo's discovery of the four Galilean Moons in 1610.

A malthea is extremely irregular, having dimensions of about 270x165x150 kilometers in diameter. It is 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 surface is dark and reddish in color apparently caused by a dusting of sulfur originating from Io's volcanoes. Bright patches of green appear on the major slopes of Amalthea. The nature of this color is currently unknown.