the Moon, especially in the early phases of implementation of the VSE. The exploration of the Moon is a rich and fruitful endeavor with many facets. The scientific context for the lunar science discussed throughout this report encompasses the four following overarching themes (see Figure 1.1), which are fundamentally important to solar system science, including the history of Earth:

  • Early Earth-Moon System: The compositional and thermal histories of both the Moon and Earth were closely linked 4.5 billion years ago, after which each evolved separately. A prevailing scientific hypothesis is that the Moon was formed from debris of a collision of a Mars-sized body with the early Earth. How, when, and why did the two parts of the Earth-Moon system take different paths and how have they influenced one another?

  • Terrestrial Planet Differentiation and Evolution: The Moon is a small planetary body that differentiated into crust, mantle, and core within a few hundred million years after formation. A magma ocean hypothesis describes this early process in terms of fractional differentiation of an initial globe-circling ocean of magma. What are the complexities of this fundamental process, and how can the lunar model be used to understand other rocky planets?

  • Solar System Impact Record: Since its birth 4.5 billion years ago, the Moon has experienced the full force of early and late bombardment of solar system debris. Regarding early bombardment: A terminal cataclysm hypothesis holds that a burst of large impacts occurred on the Moon (and the inner solar system) about 4.0 billion years ago, which, if confirmed, provides important constraints on the evolution of terrestrial planets and the origin and evolution of life on Earth. Regarding late bombardment: After formation of the planets, the frequency of impacts gradually decreased, perhaps punctuated by occasional periods of increased impacts. The early impact record on Earth has been largely destroyed by erosion and plate tectonics, but it is well preserved on the Moon. What is the history of impact events throughout the past 4.0 billion years that is recorded on the Moon?

  • Lunar Environment: The surface of the Moon is accessible and special. The lunar atmosphere, though tenuous, is the nearest example of a surface boundary exosphere, the most common type of satellite atmosphere

FIGURE 1.1 Lunar science encompasses four overarching themes of solar system exploration.

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