16 - Lunar meteorites

Summary

Introduction

It is well nigh impossible to compile a chapter about lunar meteorites without frequent reference to information obtained from the materials returned directly to Earth from the Apollo and Luna missions. We will, however, attempt to confine ourselves as much as practicable to observations arising directly from studies of lunar meteorites. (Note that the term ‘lunaite’ is not accepted (or acceptable) for lunar meteorites.) There is a wealth of data from lunar studies, much of which has been collected in review articles, either of Apollo and Luna data [16.1–16.3] or specific to lunar meteorites [16.4, 16.5]. Other valuable resources for material about lunar meteorites can be found at specialist websites [16.6a, b]. These sources have been drawn on heavily to produce this chapter.

No lunar meteorites have, so far, been observed to fall, and all lunar meteorites have (as of June 2014) been collected from desert regions, mainly Antarctica and Northern Africa [16.7]. The first meteorite to be recognized as lunar was collected in Antarctica in 1981 [16.8]. Since then, not taking account of pairing, there are currently 181 meteorites from the Earth's Moon [16.4, 16.6a, 16.7]. Lunar meteorites account for fewer than 0.4% of all known meteorites. Kalahari 009, at 13.5 kg, is the largest so far found [16.8].

Lunar meteorites are ejected from the Moon's surface by excavation during impact. They are a more random sampling of the Moon than the Apollo and Luna specimens, which sampled material from an area that covered <5% of the Moon's surface (Figure 16.1) [16.2, 16.3]. Spectral reflectance measurements taken by the Clementine orbiter (1994) and data from gamma-ray and neutron spectrometers on the Lunar Prospector mission (1998–99) revealed that Apollo sites were not completely representative of the Moon, since most of them were collected from a radioactive ‘hot spot’ close to Mare Imbrium [16.9, 16.10]. This region (characterized by intermediate iron concentration (Figure 16.1) and high concentrations of K, Th and U) is also known as the Procellarum KREEP Terrane, or PKT.

Over the past 25 years, since lunar meteorites were first recognized [16.8], they have yielded important data that are complementary to information derived from Apollo and Luna samples, and have helped to improve our knowledge of the Moon [16.5].