How Do We Know That It's a Rock From the Moon?
Many people have approached us over the years wanting to know if a rock that they possess is a Moon rock. The most common story we hear is that the rock was given to a relative in the 1970's by an astronaut, a military person, or a NASA security guard. We have chemically tested five such rocks and none has been a moon rock. Other people suspect that they have found a lunar meteorite. None of the many samples that we have been sent has been a lunar meteorite, except those from meteorite dealers or persons who bought lunar meteorites from a dealer.
No lunar meteorite has yet been found in North America, South America, or Europe. They undoubtedly exist, but the probability of finding a lunar meteorite in a temperate environment is incredibly low. Many experienced meteorite collectors have been looking and none have yet succeeded. Realistically, the probability that an amateur will find a lunar meteorite is so low that I cannot raise much enthusiasm to examine the many rocks that I have been asked to examine. If I wanted to find a lunar meteorite myself, I would not scour the Mojave Desert. I'd look through rock collections at colleges and universities. It's not unreasonable that a lunar meteorite exists in an old drawer somewhere because a sharp-eyed geology student or professor found a funny-looking rock years ago in a place it didn't belong. It would not surprise me to learn that some "expert" proclaimed that the rock was not a meteorite because it didn't look like an ordinary chondrite, it wasn't magnetic or it didn't contain a high concentration of nickel. Both visually and compositionally, lunar meteorites "look" more like terrestrial (Earth) rocks than do "normal" meteorites (ordinary chondrites). It would be easy to overlook a lunar meteorite. A weathered lunar meteorite would look remarkably unremarkable.
Here I discuss some aspects of lunar geology, mineralogy, and chemistry that guide us in our attempts to identify lunar material.
Only four minerals - plagioclase feldspar, pyroxene, olivine, and ilmenite - account for 98-99% of the crystalline material of the lunar crust. [Material at the lunar surface contains a high proportion of non-crystalline material, but most of this material is glass that formed from melting of rocks containing the four major minerals.] The remaining 1-2% is largely potassium feldspar, oxide minerals such as chromite, pleonaste, and rutile, calcium phosphates, zircon, troilite, and iron metal. Many other minerals have been identified, but most are rare and occur only as very small grains interstitial to the four major minerals.
Some of the most common minerals at the surface of the Earth are rare or have never been found in lunar samples. These include quartz, calcite, magnetite, hematite, micas, amphiboles, and most sulfide minerals. Many terrestrial minerals contain water as part of their crystal structure. Micas and amphiboles are common examples. Hydrous (water containing) minerals have not been found on the Moon. The simplicity of lunar mineralogy often makes it very easy for me to say with great confidence "This is not a moon rock." A rock that contains quartz, calcite, or mica as a primary mineral is not from the Moon. Some lunar meteorites do, in fact, contain calcite. However, the calcite was formed on Earth from exposure of the meteorite to air and water after it landed. The calcite occurs as a secondary mineral, one that fills cracks and voids. Secondary minerals are easy to recognize when the meteorite is studied with a microscope.
Most of the lunar crust, that part called the Feldspathic Highlands Terrane or simply the feldspathic highlands, consists of rocks that are rich in a particular variety of plagioclase feldspar known as anorthite. As a consequence, rocks of the lunar crust are said to be anorthositic because they are plagioclase-rich rocks with names like anorthosite, noritic anorthosite, or anorthositic troctolite (see table below). The ratio of iron-bearing minerals to plagioclase probably increases with depth in the feldspathic highlands at most places. For example, rocks exposed in the giant South Pole - Aitken impact basin on the far side are richer in pyroxene than typical feldspathic highlands.
In much of the northwest quadrant of the nearside of the Moon, in the region known as the Procellarum KREEP Terrane, the crust contains less plagioclase and more pyroxene. The original rocks of this anomalous crust were probably mostly norites and gabbros.
The feldspathic crust of the Moon began to form about 4.5 billion years ago. While it was forming and for some time afterwards, it experienced intense bombardment from meteoroids and asteroids. The rocks of the lunar crust have been repeatedly broken apart by some impacts and glued back together by other impacts. As a consequence, most rocks from the lunar highlands are breccias (brech'-chee-uz), a word for a rock composed of fragments of older rocks. Breccias occur on Earth, but they are much less common than on the Moon. Also, most terrestrial breccias were not formed by meteoroid impacts but by faulting. Lunar breccias are subdivided into a variety of categories such as impact-melt, granulitic, glassy, fragmental, and regolith breccias. In impact-melt and glassy breccias, rock fragments called clasts are suspended in a solidified (crystalline or glassy) melt matrix formed by meteorite impact.
In fragmental and regolith breccias, there is little or no molten portion, just fragmental debris that was lithified (formed into a rock) by the shock pressure of an impact. Because breccia refers to texture and anorthositic or feldspathic refers to mineralogy, rocks from the lunar highlands are variously called anorthositic breccias, feldspathic breccias, or highlands breccias. Because the lunar crust has been battered so intensely, there were very few hand-sized rocks collected on the Apollo missions that are unbrecciated remnants of the early igneous crust of the Moon. Thus it is no surprise that all of the lunar meteorites from the Feldspathic Highlands Terrane and the Procellarum KREEP Terrane are breccias.
Starting about the time of the period of intense bombardment, the lunar mantle partially melted. The resulting magmas rose through the crust to the surface, ponding in low spots. These low spots were mainly the huge craters, called basins, that were left by impacts of the largest meteorites. Lunar volcanism continued for about 2 billion years.
On Earth, volcanic rocks solidify from molten lava (magma). The most common type of volcanic rock is basalt. The ancient astronomers called the round, dark areas on the surface of the Moon seas because they were smooth dark areas surrounded by areas of higher elevation. The features were given Latin names like Mare Serenitatis for Sea of Serenity. We now know that the lunar maria are basalt flows, so we call the rocks of the maria mare basalts. Mare basalts are composed mainly of clinopyroxene, but all also contain plagioclase and ilmenite, and some contain olivine. The maria are darker than the highlands because (1) mare basalts are rich in iron-bearing minerals, (2) iron-bearing minerals are dark colored, and (3) plagioclase is light colored. In contrast to the highlands, most of the rocks collected on maria by the Apollo astronauts are actual basalts, not breccias composed of fragments of basalt. This is one of several reasons why we know that the basalts mostly formed after the time of intense bombardment. Mare basalts cover about 17% of the surface of the Moon, but it is estimated that they account for only about 1% of the volume of the crust.
Because lunar meteorites are samples from randomly distributed locations on the surface of the Moon and because most of the lunar surface is feldspathic, most of the lunar meteorites are feldspathic breccias. Some are crystalline mare basalts, breccias composed of mare basalt, or breccias composed of both mare and highlands material (like QUE 94281, above). A few are dominated by noritic material of the Procellarum KREEP Terrane.
Lunar mare basalts, as well as basaltic meteorites from Mars, bear a strong resemblance to basalts from Earth. In the absence of a fusion crust, there is little about a lunar mare basalt that would provoke much interest in a geologist handed the rock by someone asking "what's this?" Careful examination under the microscope might reveal some suspicious features - the lack of certain minerals and abundance of others (ilmenite) or the low sodium content of the feldspar. However, chemical tests would be required to prove a lunar or martian origin.
Fragmental and regolith breccias are the closest lunar analogs to terrestrial sedimentary rocks, and they bear some textural resemblance. However, there are numerous differences, nearly all associated with the lack of water and wind on the Moon. As noted above, lunar rocks don't contain carbonate minerals or abundant quartz, as do most terrestrial sedimentary rocks. There is no effective sorting mechanism on the Moon, so the lithic components of lunar breccias come in a wide variety of grain sizes, with no preferred size or orientation. Lunar breccias are largely fractal objects that look similar in cross section regardless of the scale at which they are viewed. (See ALHA 81005) There is no known lunar rock that has any feature that resembles the layers that are characteristic of terrestrial sedimentary rocks. Terrestrial sedimentary rocks have layers because the Earth has gravity, so particles settle in water or in the atmosphere. The Moon has only weak gravity and no water or atmosphere.
Most small clasts in lunar breccias are fragments of plagioclase or anorthosite. Most clasts are angular, not rounded. [Exceptions: There are volcanic glass spherules in the lunar regolith (soil). Such spherules are sometimes found in regolith breccias, but they are <0.1 mm in diameter and not easily seen with the unaided eye. Impact-produced spherules occur and may be large, but they are not common compared to rock and mineral fragments. Impact-melt breccias may contain clasts that have been partially melted and which are consequently not angular.] It is rare for the aspect (length to width) ratio of a clast in a lunar breccia to exceed 3.
Brecciated lunar meteorites are sufficiently tough and cohesive that they survived the blast off the Moon and the hard landing on Earth. Many terrestrial sedimentary rocks break apart much easier. Unlike some terrestrial conglomerates, which resemble lunar breccias, the matrix of lunar breccias is as hard as the clasts. (See MAC 88105.) On broken or exterior surfaces of brecciated lunar meteorites, the clasts do not stand out in either negative or positive relief.
Metal and Magnetism
Meteorite collectors know that meteorites are usually magnetic because they contain iron-nickel metal. The most common type of meteorites, the ordinary chondrites, do indeed contain metal as, of course, do iron meteorites. Lunar mare basalts and the original rocks of the lunar highlands contain essentially no iron metal (much, much less than 1%). Brecciated lunar meteorites, however, contain some metal from the asteroidal meteorites that have bombarded the Moon. Among lunar meteorites, Dhofar 1527 contains the most metal, about 1.7%. Most contain much less. In other words, lunar meteorites are not particularly magnetic, except perhaps to very sensitive instruments.
Because of the simplicity of lunar mineralogy, lunar rocks have predictable chemical compositions. Nearly all the aluminum is in plagioclase and nearly all the iron and magnesium are in pyroxene, olivine, and ilmenite. Thus, on a plot of concentrations of iron (FeO) plus magnesium (MgO) versus the concentration of aluminum (Al2O3 in figure), all lunar meteorites and nearly all Apollo lunar rocks plot along a line connecting the composition of plagioclase and the average composition of the three iron-bearing minerals because these are the only four major minerals in the rock.
If the composition of a rock does not plot along this line, the rock is almost certainly not a lunar rock. Meteorites such as ordinary chondrites do not plot on the line because some of the iron is in iron-nickel metal as well as pyroxene and olivine. [To represent meteorites, the average composition of H-group ordinary chondrites is shown on the figure because H chondrites are the most common type of meteorite.] Earth rocks contain many more different kinds and most plot below the lunar line. Some Earth rocks do plot on the lunar line, but they are all rocks composed of plagioclase and pyroxene or olivine. [To represent Earth rocks, the average composition of the terrestrial continental crust and the range of tektites is shown on the figure.]
On Earth, the silica (SiO2) concentration of igneous rocks is used as a first-order chemical classification parameter because it varies widely among different kinds of rocks. On the Moon (1) there are no rocks rich in quartz or other silica polymorphs, (2) in a given rock, particularly breccias, the average concentration of silica in the three main minerals, plagioclase, pyroxene, and olivine, are all about the same, and (3) in highlands rocks ilmenite is usually present only in small amounts (<3%), so silica concentrations of common lunar rocks vary by only a small amount. In lunar meteorites, SiO2 concentrations span the narrow range from 43% to 47%. Because aluminum varies by more than a factor of 3, however, aluminum is more useful as a chemical classification parameter. (Titanium is used in mare basalts.) Similarly, among nearly all common lunar rocks calcium concentrations vary by a factor of 2, from 10% to 20% as calcium oxide (CaO). This is much less than the range in terrestrial rocks. A rock with silica or calcium oxide concentrations substantially outside these ranges is almost certainly not a lunar rock.
In Earth rocks, iron occurs in both the 2+ and 3+ oxidation states. On the Moon, iron occurs in the 0 (metal) and 2+ oxidation states, although in lunar igneous rocks almost all of the iron is in the 2+ oxidation state (in olivine, pyroxene, and ilmenite). On the Moon all manganese is also in the 2+ oxidation state. Because Fe(II) and Mn(II) have very similar chemical behaviors, iron does not fractionate from manganese during lunar geochemical processes, as it does on Earth. As a result, the ratio of iron to manganese in lunar rocks is nearly constant at 70, regardless of whether the rocks are from the maria (high Fe and Mn) or from the highlands (low Fe and Mn). Nonlunar meteorites have different Fe/Mn ratios. Earth rocks have a huge range of Fe/Mn ratios, but for average terrestrial crust the ratio is a bit lower than on the Moon.
The element chromium is in greater concentration in lunar rocks than most Earth rocks (not shown). Chromium concentrations in mare basalts range from 0.14% to 0.44% (as Cr). Even the feldspathic lunar meteorites, with 0.05-0.09% Cr, are considerably richer in chromium than is the average terrestrial crust (~0.01%).
The elements samarium and thorium are two of many trace elements, that is, elements which occur in low concentrations. They are also among chemical elements that geochemists call incompatible elements because they do not concentrate in the four major minerals of lunar rocks but exist mainly in minor accessory minerals that occur in low abundance. In samples from the lunar highlands, the ratios of the concentrations of any two incompatible elements are usually about the same, so on a plot of concentrations of thorium against samarium, for example, the data form a linear trend of similar Th/K ratio. This constancy of ratios is true of any pair of incompatible trace elements and provides an excellent test of lunar origin.
Concentrations of the alkali elements (potassium, sodium, rubidium, and cesium) are 10 to 100 times lower in lunar rocks than terrestrial rocks. Terrestrial sedimentary rocks often contain sulfide minerals like pyrite. Sulfide minerals are rare in lunar rocks and elements such as copper, zinc, arsenic, selenium, silver, mercury, and lead which are often found in sulfide minerals occur in very low abundances in lunar rocks. Low concentrations of alkali elements and sulfide-loving (chalcophile) elements are one of the most characteristic features of lunar rocks.
As noted above, there are known exceptions to the generalizations, and we lunatics certainly hope that we haven't discovered all the minerals and rock types that occur on the Moon. However, known samples of unusual composition and mineralogy are rare and usually occur only as small (<1 gram) clasts in breccias or in the soil. We have no reason to suspect, based on data obtained from orbit on the Clementine and Lunar Prospector missions, that any region of the Moon is rich in types of rocks significantly different from those we know about or postulate might exist. Most ore-forming processes on Earth involve water, so we would not expect any hidden ore deposits on the Moon. Keep in mind that if more than 70 lunar meteorites have been blasted off the Moon and found on Earth, then at any given point on the lunar surface there can be rocks from any other point. For this reason, the fact that the lunar surface was "poorly sampled" by the Apollo and Luna missions is in itself not a good reason to suspect that rocks vastly different from those that we have studied exist at unsampled points on the Moon. Tens of thousands of lunar rocks and rocklets have been studied since the Apollo missions. It is highly unlikely that any yet-unfound lunar meteorite will differ substantially in the minerals it contains or in its geochemical character from the Apollo lunar rocks.
They Were Faked
Any geoscientist (and there have been thousands from all over the world) who has studied lunar samples knows that anyone who thinks the Apollo lunar samples were created on Earth as part of government conspiracy doesn't know much about rocks. The Apollo samples are just too good. They tell a self-consistent story with a complexly interwoven plot that's better than any story any conspirator could have conceived. I've studied lunar rocks and soils for 40+ years and I couldn't make even a poor imitation of a lunar breccia, lunar soil, or a mare basalt in the lab. And with all due respect to my clever colleagues in government labs, no one in "the Government" could do it either, even now that we know what lunar rocks are like. Lunar samples show evidence of formation in an extremely dry environment with essentially no free oxygen and little gravity. Some have impact craters on the surface and many display evidence for a suite of unanticipated and complicated effects associated with large and small meteorite impacts. Lunar rocks and soil contain gases (hydrogen, helium, nitrogen, neon, argon, krypton, and xenon) derived from the solar wind with isotope ratios different than Earth forms of the same gases. They contain crystal damage from cosmic rays. Lunar igneous rocks have crystallization ages, determined by techniques involving radioisotopes, that are older than any known Earth rocks. (Anyone who figures out how to fake that is worthy of a Nobel Prize.) It was easier and cheaper to go to the Moon and bring back some rocks then it would have been to create all these fascinating features on Earth. [After writing these words I learned that virtually the same sentiments had already been expressed by some of my lunar sample colleagues.]