Amalthea is one of Jupiter's smaller moons. Amalthea 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.
Amalthea rotates synchronously with its long, blunt axis pointed towards Jupiter. Because of Amalthea's close proximity to Jupiter, it is exposed to the intense Jovian radiation field. It continuously receives high doses of energetic ions, protons, and electrons produced by the Jovian magnetosphere. In addition it is bombarded with micrometeorites, and heavy sulfur, oxygen, and sodium ions that have been striped away from Io.