Earth-Moon System, Planetary Science, and Lessons Learned

Abstract

The origin and evolution of the Moon is now well understood. Many bizarre theories were proposed to account for our satellite before these were swept away by the data from the Apollo missions. We now understand with the benefit of hindsight that we are looking at a very ancient object whose surface has changed little for the past 3.5 b.y. Nevertheless the Moon remains a unique body in the solar system. The Moon formed as the result of a glancing collision with the Earth of a Mars-sized impactor late in the accretional history of the Earth when both bodies had differentiated into metallic cores and silicate mantles. The impactor is derived from the neighborhood of the Earth, as shown by similarities in O and Cr isotopes. Our satellite is mostly derived from the mantle of the impactor rather than from the mantle of the Earth. The Moon melted, perhaps entirely and a thick (50 km?) feldspathic crust, dated at 4460 ± 40 Ma floated on the ocean of magma. The closest analogue may be the crust of Mercury. The interior crystallized into cumulate zones of differing mineralogy from which the mare basalts were later derived by partial melting. The impact history of the lunar surface has provided insights into planetary accretion, the planetesimal hypothesis and the bombardment history in the inner solar system. Debate continues whether the concentration of impact-produced basins constitutes a “cataclysm” or the tail end of accretion. A notable example and a target for future missions is the 2500 km diameter South Pole-Aitken basin that may provide insights into the nature of the deep lunar crust. Remaining significant problems include whether the Moon has a small metallic core, while the deep interior structure is not well understood. New seismic and heat flow data are an urgent priority. …