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The Scientific Context for Exploration of the Moon
Zircons (which are incredibly difficult to alter) with ages as great as 4.4 Ga have been found in the ancient terrestrial Archaean metasediments (metamorphosed but recognizably sedimentary rocks). Because most terrestrial zircons are formed in granitic rocks, these ancient zircons imply the presence of continent-building activity at the time of zircon formation, rather than the presence of a simple basaltic crust. 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. If the high impact flux suggested by lunar chronology could have caused the oceans to vaporize repeatedly, there may be an inconsistency in the lunar and terrestrial observations. More investigations of the lunar record and terrestrial history are indicated.
As on the Moon, terrestrial zircons are the end product of extensive igneous processing, and they host some of the important incompatible elements of the KREEP association (of potassium, rare-earth elements, and phosphorus found in lunar basalts). Zircons generally require an evolved host rock such as granite, and on Earth the presence of large-scale processes giving rise to granite and mountain belts account for most, though not all, zircons. In addition to hosting a range of incompatible elements, zircons are extremely durable and are resistant to melting, abrasion, and weathering. It is possible that these minerals are the end result of differentiation of basaltic magma, but their sheer abundance in Archaean sedimentary rocks on Earth suggests that most were derived from rocks of evolved, possibly granitic, composition. Radiometric dating of these detrital zircons shows that some were formed before 4.0 Ga, with several terrestrial ages near 4.4 Ga. Elevated oxygen isotope ratios in the most ancient zircons appear to require that the protoliths were altered by liquid water as long ago as 4.3 Ga. Although work on these zircons has only begun, they already indicate the presence of buoyant, probably granitic components in the crust shortly after the formation of Earth (while the lunar magma ocean completed crystallizing many of its primary rocks), and the zircon host rock was profoundly affected by the presence of liquid water. Further study of ancient zircons and rocks from Earth may resolve these differences. Relatively few zircons have been studied from the Moon, and further analysis will elucidate differences in magmatic environment. It is also possible that zircons from otherwise-unsampled domains of early Earth will be located on the Moon, delivered as meteorites, if sufficient quantities of regolith are searched.
The terminal cataclysm hypothesis can be definitively tested by measuring the ages of large impact basins that are far from the Apollo sampled zone. The basis for the hypothesis, a cluster of radiometric dates near 3.8 Ga, may all be related to the vast Imbrium basin that dominates all portions of the Moon visited by Apollo. Thus, sample-derived radiometric dates of basins far from Imbrium, and most especially the largest and oldest basin of them all, South Pole-Aitken Basin, will rigorously test the terminal cataclysm hypothesis.
THE LUNAR ATMOSPHERE
The lunar atmosphere links the Sun and solar system volatiles through the lunar surface. The Sun provides one input, the solar wind, to the lunar environment, some of which is entrained in the lunar plasma environment, some of which is implanted into grains of the lunar surface, and some of which serves as a critical loss mechanism for the lunar neutral atmosphere. Hydrogen and helium are the dominant species that saturate the outermost surfaces of lunar soil grains, but the solar wind provides a wide diversity of other components, particularly carbon and nitrogen. Many other sources can provide volatile inputs to the lunar environment: comets, asteroidal meteorites, interplanetary dust particles, the transit of interstellar giant molecular clouds, Earth itself (ions from its atmosphere are continuously shed down the magnetotail or episodically removed as impacts eject atmosphere into space), and even the Moon’s interior (which may occasionally still outgas at high rates, as suggested by some young geologic features). These sources provide volatile species that may then be transported across the lunar surface until either lost or sequestered in lunar sinks. This atmosphere is extremely tenuous, having a total mass of a few tons. Research shows that the lunar atmosphere is generally resilient and can restore its original state even a few weeks or months after a disturbance, but its present state is relatively fragile against extended large-scale lunar activities.
The lunar atmosphere may actually be dominated by dust, although its properties are not well known. The mass of suspended dust may be larger than the mass of the atmosphere. Observations by the Surveyor, Apollo, and Clementine missions showed the presence of high-altitude dust, presumably lofted by electrostatic levitation.