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The Scientific Context for Exploration of the Moon
PRIORITIZATION OF SCIENCE CONCEPTS
The eight science concepts discussed in Chapter 3 address broad areas of scientific research. Each has multiple components and is linked (see Table 3.1) to different aspects of the overarching themes—early Earth-Moon system, terrestrial planet differentiation and evolution, solar system impact record, and lunar environment—presented in Chapter 1. In addition, there are multiple avenues for implementation (information extraction; orbital measurements, sample return; landed experiments, instruments, and rovers; and human fieldwork; see Table 4.1). In order to provide a sense of the overall importance of each of the eight broad science concepts, the committee evaluated only the scientific merit of each concept to rank order these concepts. They are listed in order throughout this report and in Tables 3.1 and 4.1. It should be noted that all concepts discussed are viewed to be scientifically important and their ordering in this report is simply a relative ranking:
The bombardment history of the inner solar system is uniquely revealed on the Moon.
The structure and composition of the lunar interior provides fundamental information on the evolution of a differentiated planetary body.
Key planetary processes are manifested in the diversity of lunar crustal rocks.
The lunar poles are special environments that may bear witness to the volatile flux over the latter part of solar system history.
Lunar volcanism provides a window into the thermal and compositional evolution of the Moon.
The Moon is an accessible laboratory for studying the impact process on planetary scales.
The Moon is a natural laboratory for regolith processes and weathering on anhydrous airless bodies.
Processes involved with the atmosphere and dust environment of the Moon are accessible for scientific study while the environment remains in a pristine state.
PRIORITIZATION OF SCIENCE GOALS
Within the science concepts, the committee identified 35 specific science goals that can be addressed at least in part during the early phases of the VSE. For these science goals, the committee evaluated their science merit as well as the degree to which they are possible to achieve within the limits of current or near-term technical readiness and practical accessibility. Within their respective science concepts, these goals are listed in order of their overall priority ranking (a-e) in Table 3.1.
The committee also evaluated and rank ordered all 35 specific science goals together, apart from the science concepts with which they are grouped. The 11 highest-ranking lunar science goals are listed below and in Table 5.1 in priority order. To achieve this group of goals, the committee identified possible means of implementation (see Table 5.1).
The committee’s highest-priority science goals are the following:
1a. Test the cataclysm hypothesis by determining the spacing in time of the creation of the lunar basins. The history of impacts in the early Earth-Moon system, in particular around 3.9 Ga, the time that life was emerging on Earth, is a critical chapter in terrestrial planet evolution. Understanding this period is important for several reasons: as tests of our models of the impact rate, planetary accretion, impact frustration of life, magma ocean formation and evolution, and extension and verification of the chronology. In order to answer the question of whether there was a cataclysm at 3.9 Ga, sample returns from the oldest impact basins combined with high-resolution imaging from orbit are required.
1b. Anchor the early Earth-Moon impact flux curve by determining the age of the oldest lunar basin (SouthPole-Aitken Basin). Although the enormous South Pole-Aitken Basin is stratigraphically the oldest basin on the Moon, its absolute age is completely unconstrained. All models of the first few hundred million years of solar system history depend on whether the large basins are part of a decreasing flux of material swept up by growing planet embryos or a later separate pulse of planetesimal-sized bodies. Details of the lunar stratigraphy can be better defined by integrated high-resolution imagery and topography, but it is essential to provide an absolute date for