The Scientific Context for Exploration of the Moon (2007) / Chapter Skim
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2 Current Understanding of Early Earth and the Moon
2 Current Understanding of Early Earth and the Moon
Pages 10-19
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From page 10... ...
(An artist's conception of a giant impact is shown in Figure 2.1.) • The lunar magma ocean hypothesis governs understanding of the formation of lunar rocks following lunar formation, and suggests that the outer portions of the Moon (several hundred kilometers in depth)
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From page 11... ...
CURRENT UNDERSTANDING OF EARLY EARTH AND THE MOON FIGURE 2.1 An artist's conception of a giant impact. The Moon is hypothesized to have been formed at ~4.5 Ga when a Mars-size body called Thea struck Earth.
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From page 12... ...
A highly important revelation during this period of contemplation was the formulation of the giant impact hypothesis for lunar origin. The fundamental question of lunar origin persisted beyond Apollo, and prevailing hypotheses all suffered from one or more serious shortcomings.
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From page 13... ...
These meteorites raise questions about the notion that the Moon can be understood solely in terms of the samples from Apollo, since the meteorites exhibit subtle differences that may ultimately challenge one or more of the prevailing hypotheses and lend additional impetus to the need for new lunar samples. The fifth key factor enabling the formulation of the unifying hypotheses is the three post-Apollo missions that returned global lunar remote sensing data.
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From page 14... ...
and on the Moon. At present, straightforward compelling tests do not exist; however, illumination of the giant impact hypothesis may be possible with new sampling, more abundant data, better analytical techniques, or conceptual breakthroughs.
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From page 15... ...
A major enigma is the dramatic asymmetric distribution of thorium, a significant heat-producing element. According to the lunar magma ocean hypothesis, as the globe-encircling magma body cooled and minerals precipitated, a lunar mantle formed from dense olivine and pyroxene, and a crust accumulated composed of buoyant plagioclase.
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From page 16... ...
At this early stage, the efforts to model mantle overturn require sensitive and unsatisfying adjustment of viscosity in order to enable this type of global overturn while also preventing breakup into small convection cells; thus, at present this fundamental problem is far from solved, but it is certain that the magma ocean model as it existed prior to Lunar Prospector will require substantial revision and enhancement to take into account the new constraints. Of the above-named hypotheses, the most vulnerable, because of the existence of well-defined tests aimed at its overturn, is the terminal cataclysm hypothesis.
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From page 17... ...
As a consequence, the giant planets pass through highly disruptive resonances that ought to scatter small objects formed in the outer solar system both outward and inward. The excited orbits of the Kuiper Belt objects are testimony to a scattering event or events that may also have sprayed the inner solar system with icy objects large and small long after planetary accretion had waned and background impact fluxes had drastically dropped.
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From page 18... ...
The mildly elevated values of δ18O in these zircons support the proposal that, during this period, chemical weathering and erosion had occurred, both of which require liquid water to alter the protoliths (original rocks) of subsequently formed granitic rocks.
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From page 19... ...
But this era has also revealed troubling defects in that lens owing to the lack of key data in time, space, and physical characteristics. Building on the foundation of the Apollo program and over 30 years of contemplation and technical progress, a well-formulated new era of lunar exploration will enable key tests of hypotheses major and minor that may revolutionize the understanding of the origin and evolution of the Moon, Earth, and the planets.
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