Before 1969, lunar scientists were unable to recognize any known meteorite as being from the Moon. No one knew what chemical characteristics would positively identify a lunar meteorite. After 1969, when Apollo astronauts brought back samples from the Moon for laboratory analysis, a lunar meteorite could actually be positively identified by its unique chemistry. But now that lunar scientists could recognize a meteorite as definitely being from the Moon, there were no unclassified meteorites to examine.
Lunar meteorites are difficult to find visually or by typical meteorite hunting techniques. They look quite similar to some Earth rocks and in the field there is no way to determine if they are chemically different. Lunar meteorites have very little metallic iron so they are not attracted to a magnet and not located by metal detectors. In the 1970s, several national governments began a systematic exploration of Antarctic glaciers for meteorites, and in 1983 the first paper was published that positively identified an Antarctic meteorite as being from the Moon.
It turns out desert areas are an ideal place to search for all types of meteorites. Dark colored meteorite rocks stand out against a light colored background of ice or sand. Since meteorite collecting in Antarctica requires the resources of a national government, individual meteorite collectors began seriously exploring the desert areas of Australia and northern Africa. Searching in these vast desert areas produced immediate results and continues to provide additional lunar meteorites for study today.
The number and random variety of lunar meteorites available to researchers is extremely important because the Apollo and Luna samples returned to Earth from the Moon do not tell the complete story of lunar chemistry or how the Moon formed. As the map above shows, those samples were from only nine locations on the Moon's near side and provide no information about the very different far side. But, lunar meteorites are from random locations and provide information from a variety of rocks from both sides of the Moon. SEE FIGURE 1
With many more lunar samples from a variety of locations from around the Moon available for laboratory analysis, lunar scientists were able to use this data to put together a comprehensive theory of how the Moon formed and evolved. Surprisingly, the resulting theory was immediately accepted and embraced by an overwhelming number of lunar scientists as the best possible explanation of how the Moon formed and evolved. Some knowledge of the "BIG SPLAT" theory is important for understanding lunar meteorites.
The Moon formed about 4.52 billion years ago when Earth was a still accreting, mostly differentiated, and molten planet with a metallic core. Another Mars size body tangentially impacted Earth with such cataclysmic force it blew off a large amount of Earth's less dense upper mantle as splattered liquid blobs orbiting the Earth. The incredible energy of that impact completely vaporized this mantle material, leaving it totally devoid of any water or other volatiles. Much of the impactor's mantle material plus its metallic core were absorbed into the still molten Earth. SEE FIGURE 2
These numerous liquid splatters of mostly Earth mantle material that were completely dehydrated, totally gasless, and containing very little metallic iron quickly coalesced into a molten proto-Moon orbiting the Earth. While the proto-Moon was in this nearly liquid molten state, the lighter ANORTHOSITE minerals floated above the heavier BASALT minerals and soon cooled to create a differentiated Moon with no atmosphere, a solid anorthosite crust, a still molten basalt mantle, and essentially no metallic core. PLAGIOCLASE was the primary anorthosite mineral and PYROXENE was the primary basalt mineral, with OLIVINE being an important secondary basalt mineral.
Almost as soon as a solid lunar crust formed about 4.5 billion years ago, a Period of Intense Heavy Bombardment by asteroids began. During this period, the lunar crust was constantly pounded and pulverized by impacts from a mind boggling number of asteroids of varying sizes for about 700 million years. These unrelenting impacts broke up surface rocks into ever smaller and smaller pieces creating countless BRECCIA rocks, which are impact products of tiny broken bits of various rock material fused together in a matrix of fine rock and mineral dust during an impact event.
This asteroid assault left the Moon with a whitish anorthosite surface of crushed rock, breccias, tiny broken rock pieces, glass bits, and lots of rock dust that comprise a deep REGOLITH of these very small impact products. Lunar regolith is analogous to Earth soil. However, the word soil implies the presence of biotic material which is not found on the Moon. The lunar surface was also dotted with many large ringed impact craters and huge multiple ring basins created from the shock of enormous asteroid impacts. SEE FIGURE 3
As the Period of Intense Heavy Bombardment subsided about 3.8 billion years ago to a less intense but still continual asteroid bombardment, a Period of Lunar Volcanism began on mostly the Moon's near side. Still molten basalt magma erupted through fissures in the thinner near side crust. Some of this erupted basalt pooled in the lower elevations of large craters, and some flowed out on top of large surface areas of older low elevations on the Moon's near side forming ocean size areas of erupted basalt.
The Period of Lunar Volcanism essentially ceased about 3.2 billion years ago, and all lunar volcanism had completely ended by about 1.2 billion years ago when the energy from radioactive element decay no longer produced enough heat to keep any basalt magma molten. SEE FIGURE 4
These newly exposed basalt areas quickly cooled, solidified, and were then cratered and pummeled by asteroids for billions of years which created newer blackish basalt impact products in a deep regolith. The HIGHLAND areas are the older anorthosite and mainly higher surface elevations, while the MARE areas are the newer basalt and mainly lower surface elevations.
Every large asteroid impact on the Moon spews huge amounts of ejecta over very long distances. A large impact in a mare area can throw mare ejecta into highland areas, and a large impact in a highland area can hurl highland ejecta into mare areas. When this happens continuously for billions of years, the result is a considerable mixing of surface materials. Billions of years of surface material mixing has even resulted in some near side areas that appear grayish. SEE FIGURE 5
Lunar meteorites bring samples of all these rocks, minerals, and impact products to Earth for anyone to examine and study. These meteorites allow everyone to see the results of the Moon's tortured history and see the evidence that led lunar scientists to embrace the "Big Splat" theory. Lunar meteorites also allow a person who simply wants to hold a rock from the Moon in their hand the opportunity to do exactly that.
Most lunar meteorites offered for sale have been cut so their interiors are visible. This makes it easy to examine and enjoy a Moon rock with simply a magnifying glass. With a few online references, most of the minerals, crystals, glass, and bits of rock visible in a particular specimen can be identified. These bits and pieces will reveal the meteorite's story to anyone willing to investigate what it has to say. The history of a Moon rock is just waiting to be discovered and it's a fascinating experience to follow the clues.