The third-closest satellite of Jupiter, distance 181 400 km, orbital period 0.498 days; also known as Jupiter V. Its axial rotation period is the same as its orbital period. Amalthea was discovered in 1892 by E. E. Barnard. It is noticeably elongated, measuring 250 × 146 × 128 km. Its surface is heavily scarred by impact craters, the largest 90 km wide. The surface has a very low albedo, 0.09, and is distinctly reddish, possibly because of sulphur from Io’s volcanoes. Spacecraft measurements yield a mean density of 0.85 g/cm3, indicating a porous structure rich in water ice.