Seeing the Missing Half

The Moon is our closest planetary neighbor and the only extraterrestrial body to which humans have traveled, yet many questions about its origin and early history remain unanswered. Four papers published in this issue by scientific teams of the Japanese SELENE (Kaguya) mission offer a new global view of the Moon that helps to elucidate how the Moon evolved to its present state. The Moon is lopsided: Its visible nearside (tidally locked to face the Earth) is covered with smooth, dark volcanic mare, whereas the farside mainly consists of more heavily cratered, bright highland material. The differences in crustal thickness and density, apparent surface age, composition, and volcanic activity between the two sides are variously ascribed to external causes (such as a giant impact) or to internal causes (such as core formation, mantle convection, and crustal differentiation). The key to resolving these questions will be better data.