Lunar Samples


An Introduction by P.H. Benoit

Lunar samples are pieces of our nearest neighbor in space, the Moon. All told, there is about 385 kg (about 840 lbs) of material originally from the Moon in the worldwide collection. Most of this material (about 382 kg) was collected by U.S. astronauts during the Apollo program (1969-1972), but the collection also includes material collected by the Soviet Union's Luna robotic landers (1970-1976) and a small number of meteorites that were ejected from the Moon during impact events.

Lunar samples hold a special place in the science of meteoritics, because they are the only samples quite literally hand-picked on an extraterrestrial body and then returned to Earth. The source or sources of meteorites and IDPs are still the subject of scientific investigation and debate, a problem complicated by the long and largely unknown history of these materials after leaving their parent bodies or source regions.

In the broadest sense, lunar samples can be divided into two types, namely, rocks and soil. At the most simple level, the rocks are either basalts or anorthosites. Basalts are volcanic rocks composed of the minerals pyroxene, olivine, and feldspar, and are common on the Earth as well. Anorthosites are igneous rocks, and are composed almost entirely of large grains of the feldspar mineral anorthite. Anorthosites also occur on Earth, but they are extremely rare.

Again on the simplest level, the Moon's surface can be divided into two basic regional types, visible from the Earth's surface. The lunar highlands are the bright colored areas of the surface and are mostly composed on anorthosite. The lunar mare or 'seas' are composed of basalt. In point of fact, both regions are covered by regolith, produced from the local rocks by the impact of meteoroids, ranging in size from large meteorites to tiny micrometeorites.

Research on lunar materials has concentrated in delineating the history of the Moon. It is now generally accepted that the Moon is as old as the Earth, being about 4500 million (or 4.5 billion in U.S. terminology) years old. It is widely thought, but by no means certain, that the Moon was originally part of the early Earth, but was thrown off as a result of a very large impact event. The anorthosite rocks formed first, apparently as 'islands' of plagioclase feldspar that floated to the surface of the molten body. Eventually the Moon consisted of a solid crust of anorthosite with a molten core. For the first 500 million years or so the Moon was extensively bombarded by debris leftover from the formation of the planets, resulting in a very heavily cratered surface. From about 3900 to 3000 million years ago, some of the molten material in the core reached the surface and poured out to fill some of the largest of the craters. This molten material was basaltic in composition and crystallized to form the dark mares or 'seas'. From 3000 million years ago to the present the Moon has been a fairly quiet place, aside from a few moderate impact events that produced new craters, and the continuous bombardment of the Moon by smaller meteorites that produced the modern lunar regolith. The lunar meteorites were ejected from the Moon during some of the larger of these events.

This synopsis is a gross simplification of many years of lunar research. In fact, there are many fine details in this history and many subvarieties of lunar rocks that we do not cover here. The interested reader should refer to some of the reference works given below.

Research on lunar samples was a major branch of meteoritics during and immediately following the Apollo program and continues, at a somewhat less frenetic pace, to this day. New discoveries, and new interpretations of even the heavily studied lunar materials, continue to be made and are certain to occur in the future. Although most of the collection has been studied to some extent, some portions have been only briefly described, and there are still samples that were deliberately put aside during the Apollo program for study by future generations of scientists. Some of the new results and interpretations are presented at the annual Lunar and Planetary Science Conference, held in March in Houston, Texas.

Most of the world collection of lunar material is stored and curated at Johnson Space Center, although major museums often have small amounts for public display. With the exception of a few lunar meteorites that are in private hands, lunar samples are considered national (or international) treasures and are not available for sale or trade. Samples are released for study and educational purposes only to qualified researchers and teachers.

References
The most recent and certainly one of the most complete reference works is the Lunar Sourcebook, edited by G.H. Heiken, D.T. Vaniman and B.M. French, published in 1991 by Cambridge University Press. Some older but still quite interesting books are: Taylor S.R. (1982) Planetary Science: A Lunar Perspective. Published by the Lunar and Planetary Institute, Houston, Texas. and the Origin of the Moon, edited by W.K. Hartmann, R.J. Phillips, and G.J. Tayor, published by the Lunar and Planetary Institute, Houston, Texas, in 1984.