A primary crust: the highland crust of the Moon

Abstract

Comparisons with the Earth's geologic style, though inevitable, have proved to be treacherous guides to the Moon. (Don Wilhelms) Every school child is aware that the Moon is not a planet. So why begin this discussion on planetary crusts with examples from a planetary satellite? The reason is that the two types of crusts on the Moon, that form the lighter highlands and the darker maria, are among our best examples of primary and secondary crusts. Their origin and evolution are better understood than those of any other examples in the Solar System, including the Earth. In addition, the Moon, in contrast to the Earth, forms a classic example of a one-plate planet, that is the norm for our Solar System. The composition of the Moon The mean lunar radius is 1737.1 km, which is intermediate between that of the two jovian satellites of Jupiter, Europa ( r = 1561 km) and Io ( r = 1818 km). The Moon is much smaller than the jovian satellite Ganymede ( r = 2634 km), which in turn is the largest satellite in the Solar System and like the saturnian satellite Titan, is larger than Mercury. Although the jovian satellites and also Titan are comparable in mass, the Moon/Earth ratio is the largest satellite-to-parent ratio in the planetary system, a consequence of a distinctive origin.