deep thinking about the conditions present in the early solar system. The period of contemplation featured many other lesser but still important insights, such as the recognition from detailed crater counts that lunar volcanism extended far later than the youngest dated basalt samples, perhaps being as young as 1 Ga, placing a significant constraint on the lunar thermal history.

A second factor enabling the formulation of the unifying hypotheses has been the exponential improvement in analytical capability that has had a revolutionary impact on the value of lunar samples collected by the Apollo program and Russian Luna missions (1959-1976). These samples have been conserved and very well documented by NASA’s curatorial facility, providing a significant body of data, some of which is still untapped. Analysis at spatial scales and analytical precision inconceivable in 1970—especially more recently developed isotopic systems such as Sm-Nd and Hf-W as well as analysis at the nanoscale by secondary-ion mass spectrometry, transmission electron microscopy, and other methods—has produced new insights in the formation of the Moon, from its core to the regolith. This new capability has enabled the “discovery” (or at least strong inference) about the presence of garnet in the lunar mantle; constraints on the processes attendant on a giant impact origin of the Moon, such as evaporative processes in a silicate vapor cloud; precise refinement of the chronological relationships among ancient lunar rocks; and the recognition of pervasive nanoscale processes involved in regolith evolution.

The improvement in analytical technology also included the patient application of ground-based astronomy. High-performance ground-based telescopic remote sensing, especially infrared spectroscopy, shows that the diversity of the lunar crust revealed at millimeter scale in the samples exists at the kilometer scale, and that rock types unknown in the sample collection exist far from the Apollo landing sites. Ground-based radar has revealed new insights into the nature of the lunar regolith and placed tight constraints on the nature of the permanently shaded portions of the Moon. Astronomical observations also enabled the detection of a tenuous atmosphere of sodium and potassium to supplement the constituents discerned by Apollo surface experiments.

Regarding the third factor, the increase in computational capability, a major beneficiary of this new capability has been the giant impact hypothesis. The ability to apply successful computational tests of this hypothesis using two- and three-dimensional fluid dynamics codes graduated what might have been simply an interesting and competitive notion to the status of a ruling paradigm. Another beneficiary of this new capability is the cataclysm hypothesis according to which models of the time evolution of the large bodies in the outer solar system suggest that planet migration could have instigated the cataclysm. Other fields have benefited as well, such as the reanalysis of Apollo seismic data using modern and computationally intensive techniques and analysis and the integration of remote sensing and new spacecraft geophysical data.

With respect to the fourth factor, the recognition of meteorites from the Moon, many of these meteorites have characteristics which suggest that they originate far from the Apollo zone, being extremely poor in incompatible elements—known collectively by the acronym KREEP (for enrichment of some of the first recognized incompatibles: potassium [K], rare-earth elements [REE], and phosphorus [P])—that are characteristic of the Apollo samples. 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. The pioneer was Galileo, which on its way to Jupiter flew by the Moon twice, in 1990 and 1992; its multispectral image data first drove home the compositional distinctiveness of the vast South Pole-Aitken Basin. Next, in 1994, was the Clementine polar lunar orbiter, a joint NASA/Department of Defense technology demonstration mission run in part by lunar enthusiasts who allowed aspects of its sensor payload to be tailored to lunar issues. That 2-month mission resulted in the first unified global remote sensing data set; revealed the large-scale topography of the Moon, including the extent and depth of the South Pole-Aitken Basin; allowed near-global estimates of crustal thickness; provided the first global evaluation of selected elemental and mineral abundances; and brought the importance of the lunar poles into sharp focus with innovative, if controversial, measurements of polar radar properties aimed at the detection of water ice. The third and arguably most scientifically significant mission was the NASA Discovery mission, Lunar Prospector, in 1998. This mission initiated a possible paradigm shift, as it showed the unanticipated extent to which the Moon is asymmetric in composition, in particular for the heat-producing elements. The profound asymmetry, well out of the scope of the



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