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What is a Meteorite?

A meteorite is a rock that was formed elsewhere in the Solar System, was orbiting the sun or a planet for a long time, was eventually captured by Earth's gravitational field, and fell to Earth as a solid object. A meteoroid is what we call the rock while it is in orbit and before it is decelerated by the Earth's atmosphere. A meteor is the visible streak of light that occurs as the rock passes through the atmosphere and exterior of the rock is heated to incandescence.  Most (~99.8%) meteorites are pieces of asteroids. A few rare meteorites come from the Moon (0.1%) and Mars (0.1%).

What is a Lunar Meteorite?

Lunar meteorites, or lunaites, are meteorites from the Moon.  In other words, they are rocks found on Earth that were ejected from the Moon by the impact of an asteroidal meteoroid or possibly a comet. 

How Did Lunar Meteorites Get Here?

Meteoroids strike the Moon every day. Lunar escape velocity averages 2.38 km/s (1.48 miles per second), only a few times the muzzle velocity of a rifle (0.7-1.0 km/s). Any rock on the lunar surface that is accelerated by the impact of a meteoroid to lunar escape velocity or greater will leave the Moon's gravitational influence. Most rocks ejected from the Moon become captured by the gravitational field of either the Earth or the Sun and go into orbit around these bodies. Over a period of a few years to tens of thousands of years, those orbiting the Earth eventually fall to Earth. Those in orbit around the Sun may also eventually strike the Earth up to a few tens of millions of years after they were launched from the Moon.   
  

 

Words That Confuse People

  • asteroid – A big (>1 meter) rock or aggregation of rocks orbiting the sun
  • meteoroid – A small (<1 meter) rock orbiting the sun
  • meteor – The visible light that occurs when a meteoroid passes through the Earth's atmosphere
  • meteorite – A rock found on Earth that was once a meteoroid.
These are simple definitions. A more technical but accurate definition of a meteorite is given by Alan E. Rubin and Jeffrey N. Grossman (2010):

"A meteorite is a natural, solid object larger than 10 µm in size, derived from a celestial body, that was transported by natural means from the body on which it formed to a region outside the dominant gravitational influence of that body and that later collided with a natural or artificial body larger than itself (even if it was the same body from which it was launched)."
A road sign in Newfoundland.
 

How Do We Know That They Are Meteorites?

On a broken or sawn face, all lunar meteorites look like some kinds of Earth rocks, even to an experienced lunar scientist. We can often tell that they came from space, however, because many lunar meteorites have fusion crusts (the olive-green crust on the photo above) from the melting of the exterior that occurs during their passage through Earth's atmosphere. On meteorites found in hot deserts, the fusion crusts sometimes have weathered away. However, as explained in more detail below, all meteorites contain certain isotopes (nuclides) that can only be produced by reactions with penetrating cosmic rays while outside the Earth's atmosphere. The presence of "cosmogenic nuclides" is the ultimate test of whether or not a rock is a meteorite. All lunar meteorites that have been tested show evidence of cosmic-ray exposure.

How Do We Know That They Come From the Moon?

Chemical compositions, isotope ratios, mineralalogy, and textures of the lunar meteorites are all similar to those of samples collected on the Moon during the Apollo missions.  Taken together, these various characteristics are different from those of any type of terrestrial rock or other type of meteorite.  For example, all of those meteorites in the List that are classified as feldspathic breccias are rich in the mineral anorthite, which is a plagioclase feldspar, mineralogically, and a calcium aluminum silicate, chemically.  Consequently, these meteorites all have high concentrations of aluminum and calcium.  Because of some unique aspects about how the Moon formed, the lunar highlands are composed predominantly of anorthite.  Anorthite is much less common on asteroids and, to the best of our knowledge, on the surface of any other planet or planetary satellite.

 
moon photo

More Detail:   See "How Do We Know That It's a Rock From the Moon?"

How Many Are There?

It depends upon how one counts.  More than 180 named stones have been described in the scientific literature that appear to be lunar meteorites.  Other rocks that have not yet been described in the scientific literature but which might be lunar meteorites are being sold by reputable dealers.  The complication is that some to many of these stones are "paired," that is, two or more of the stones are different fragments of a single meteoroid that made the Moon-Earth trip. When confirmed or strongly suspected cases of pairing are taken into account, the number of actual meteoroids reduces to about 90.  Pairing has not yet been established or rejected for the many recently found meteorites, so the actual number is not known with certainty.  In the List, known or strongly suspected paired stones are listed on a single line separated by slashes.  In most cases, the stones were found close together because a meteoroid broke apart upon encountering the Earth's atmosphere, hitting the ground or ice, or while traveling within the ice in Antarctica. (In the other cases, all from northern Africa, we don't know for sure where they were found.) The six LaPaz Icefield stones all have fusion crusts and the broken edges don't fit together, thus the LAP meteoroid likely broke up in the atmosphere. Among the numerous Dhofar lunar meteorite stones, about 15 appear to all be pieces of a single meteorite.

 

Pairing and Naming

Although it is often confusing, meteorite scientists refer to all found pieces of a meteoroid as a single meteorite, ideally with a single name. Thus, Allende refers to hundreds of fragments of a single 2-ton meteoroid that broke apart over Mexico in 1969. All the pieces are paired stones of a fall and they are all called Allende. With finds (meteorites not observed to fall) different stones are often given different names because they are found at different times. If later studies show two stones to be paired, then one of the names is officially discarded. With the Antarctic and hot-desert meteorites, however, all the stones are originally given different designations because so many meteorites are found in a small area. This problem leads to the awkward combination names like Yamato 82192/82193/86032 when one is referring to "the meteorite," in the accepted sense, as opposed to the individual stones. If the 15 stones of the Dhofar 303 et al. lunar meteorite had been found, for example, in the U.S., they would likely all been given the same name.

Do All Lunar Meteorites Come from One Big Impact on the Moon?

crater Daedalus image
The lunar crater Daedalus, about 93 kilometers (58 miles) in diameter, was photographed by the crew of Apollo 11 as they circled the Moon in 1969. NASA photo AS11-44-6611.

For several reasons, we know that the lunar meteorites derive from many different impacts on the Moon. The textural and compositional variety spans, and in some ways exceeds, that of rocks collected on the six Apollo landing missions, so the meteorites must come from many locations. More importantly, it is possible to determine how long ago a rock left the Moon using cosmic-ray exposure ages. Small rocks on the surface of the Moon and in orbit around the Sun or Earth are exposed to cosmic rays. The cosmic rays are so energetic that they cause nuclear reactions in the meteoroids that change one nuclide (isotope) into another. Some of those nuclides produced are radioactive. As soon as they fall to Earth, production stops because the Earth's atmosphere absorbs nearly all cosmic rays. The radionuclides decay on Earth with no further production. The most well-know such isotope is 14C (carbon 14), which is produced from oxygen atoms in the meteoroid. Other important radionuclides produced by cosmic-ray exposure are 10Be, 26Al, 36Cl, and 41Ca. Because the various radionuclides all have different half-lives, it is often possible to tell how long a rock was exposed on or near the surface of the Moon, how long it took to travel to Earth, and how long ago it fell. For example, cosmic-ray exposure data for Kalahari 008/009 suggest that the meteorite left the Moon only a few hundred years ago. At the other extreme, Dhofar 025 took 13-20 million years to get here (Nishiizumi & Caffee, 2001). Because there is a wide range in the Earth-Moon transit times, we know that many impacts on the Moon were required to launch the lunar meteorites.

There are persuasive arguments (cosmic-ray exposure ages, chemical and mineral composition) that the "YAMM" meteorites, Yamato 793169, Asuka 881757, MIL 05035, and MET 01210 are source-cratered paired or launch paired, that is, the four meteorites were ejected from the Moon as separate rocks by a single impact, the rocks traveled to Earth separately, and that they fell to Earth at different places (Warren, 1994; Arai et al., 2005; Zeigler et al., 2007). Other likely cases of launch pairing are the "YQEN" meteorites, Yamato 793274/981031, QUE 94281 EET 87521/96008 (Arai and Warren, 1999, Korotev et al., 2003), and NWA 4884 (Korotev et al., 2009) and the "NNL" meteorites, NWA 032/479, NWA 4734, and LAP (Zeigler et al., 2005). Almost certainly, some of the numerous feldspathic lunar meteorites are source-crater paired. So, the lunar meteorites represent somewhat fewer impact sites on the Moon than the number of meteorites (list).   

Does It Take a Big Impact to Launch
a Lunar Meteoroid?

Vogt et al. (1991) estimated that the frequency of impacts on the Moon large enough to eject lunar meteorites is greater than 5 per million years. On the basis of impact probability and the known size distribution of lunar craters, Paul Warren (1994) makes a persuasive case that lunar meteorites come from relatively small craters — those of only a few kilometers in diameter.  The main thrust of his argument is that all the lunar meteorites were blasted off the Moon in the last ~20 million years (most in the last few hundred thousand years) and that there haven't been enough "big" impacts on the Moon in that time to account for all the different lunar meteorites.  As new lunar meteorites are found each year, Warren's argument becomes more valid. James Head (2001) calculates on a theoretical basis that impacts causing craters as small as 450 m (about a quarter of a mile) in diameter can launch lunar meteorites. More recently, Basilevsky et al. (2010) argue on the basis of the known number of lunar meteorites and the frequency of impacts on the Moon that "a significant part of the lunar meteorite source craters are not larger than a few hundreds of meters in diameter." (That's big if it happens in your backyard, but it's not so big for the whole Moon.) If lunar meteorites come from such small craters, it would be especially difficult to locate the actual source crater of a particular lunar meteorite.

 

Prediction 19 Years Before the First Lunar Meteorite Was Recognized

"The occurrence of secondary craters in the rays extending as much as 500 km from some large craters on the moon shows that fragments of considerable size are ejected at speeds nearly half the escape velocity from the moon (2.4 km/sec). At least a small amount of material from the lunar surface and perhaps as much or more than the impacting mass is probably ejected at speeds exceeding the escape velocity by impacting objects moving in asteroidal orbits. Some small part of this material may follow direct trajectories to the earth, some will go into orbit around the earth, and the rest will go into independent orbit around the sun. Much of it is probably ultimately swept up by earth."
...
"There is also a possibility that fragments can be ejected at escape velocity from Mars by asteroidal impact, though not as large a fraction as is ejected from the moon. If some small amount of material escapes from Mars from time to time, it seems likely that at least some small fraction of this material would ultimately collide with earth."

Shoemaker E. M., Hackman R. J., and Eggleton R. E. (1963) Interplanetary correlation of geologic time. Advances in Astronautical Sciences, vol. 8, p. 70-89.

Where on the Moon Did They Come From?  Are Any from the Far Side of the Moon?

Although scientists like to speculate that a certain lunar meteorite came from a certain crater or region of the Moon, no one has actually identified with certainty the source crater from which any of the lunar meteorites originated.

map of lunar basins
Schematic map of lunar impact basins on the nearside and farside of the Moon.
(Based on Figure 2.3 of The Lunar Sourcebook.)
 

There is some evidence and model results indicating that asteroidal meteoroids strike the western (leading) hemisphere of the Moon (that is, the "side" with Mare Orientale, which means east because astronomical telescopes see the Moon upside down!) a bit more frequently than the eastern hemisphere (the Mare Marginis "side"). On the other hand, lunar meteoroids leaving the eastern hemisphere may have a slightly better chance of reaching Earth. Overall, however, there's probably little East-West bias in our lunar meteorite collection. There are reasons to expect that asteroidal meteoroids strike the equatorial areas of the Moon a bit (1.28 times) more frequently that the polar regions.

There are no reasons to suspect that lunar meteorites come from the nearside of the Moon preferentially to the farside, or vice versa. So, half of the lunar meteorites come from the far side of the Moon. We just don't know which ones those are. There is no scientific basis for a statement in an advertisement on e-Bay: "The ONLY LUNAR meteorite from the dark side of the moon." (Also, of course, the "dark side" of the Moon keeps changing with lunar phase! Except for some locations at the poles, any place in the dark will be sunlit 14 days later.)

Some great technical reading: Gladman et al. (1995), Le Feuvre and Wieczorek (2008), and Gallant et al. (2009).

For any given lunar meteorite, the probability is not exactly 50-50 that it came from either the near side or the far side. There is more mare basalt on the near side than the far side (FeO map below), so the chance is better than 50-50 that an iron-rich meteorite (mare basalt or basaltic breccia) is from the near side and that an iron-poor meteorite (feldspathic) is from the far side. As explained below, Sayh al Uhaymir 169, Dhofar 1442, and Northwest Africa 4472/4485 almost certainly derive from the near side.

How Big Are They?

The largest single stones are Kalahari 009 at 13.5 kg (30 lbs), Northwest Africa 5000 at 11.5 kg (25 lbs), and Shisr 162 at 5.5 kg (12 lbs). The next biggest lunar meteorites are the numerous stones of NWA 2995 pair group at 2.22 kg (4.9 lbs), NWA 3163 and paired pieces at 2.45 kg (5.4 lbs), and the 6 paired LAP stones at 1.93 kg (4.3 lbs). Several of the lunar meteorite fragments found in Antarctica and Oman only weigh a few grams (a U.S. nickel weighs 5 grams). The smallest named stones are Graves Nunataks 06157 at 0.788 g and Dar al Gani 1048 at 0.801 grams.

The plot to the left shows the distribution of meteorite masses (all stones of a given meteorite). Masses in the 128-256-gram range are most common. The plot on the right shows masses by continent or country. Botswana is represented by a single, huge meteorite, Kalahari 008/009.   


How Rare Are They?

Meteorites are very rare rocks; lunar meteorites are exceedingly rare. It difficult to assess how rare they really are. At this writing (the numbers change nearly every day), of the ~44,400 meteorites listed in the Meteoritical Bulletin Database, only 1 in 284 are lunar meteorite stones.

For comparison, of the ~30,700 meteorite stones found in Antarctica, where record keeping has been superb, (1976-2011), 1 in 930 stones is lunar (33 stones representing 19-21 meteorites; for Mars, it's 24 stones representing 14 meteorites).

Another measure of rarity is mass. The total mass of all known lunar meteorites is only about 70 kg (154 lbs.). By comparison, the Allende and Jilin meteorites (both stony) are 2 and 4 metric tons (2000 and 4000 kg) each while several iron meteorites weigh more than 10 tons! (e.g., Hoba, Gibeon, Campo del Cielo).

 

Lunar Meteorites for Sale

Meteorites, including lunar and martian meteorites, are easily available for purchase on the Internet. Samples (end cuts, slices, chips, crumbs, dust) of the lunar meteorites sell on the Internet (e.g., e-Bay) for between about $800 and $40,000 per gram, depending upon rarity (perceived or real!) and demand. By comparison, the price of 24-carat gold is about $60 per gram and gem-quality diamonds start at $1000-2000/gram.

Most rocks advertised on the Internet as lunar meteorites are, in fact, meteorites from the Moon sold by reputable dealers. Some are not, however. Also, on more that one occasion, I have seen samples advertised on e-Bay as one particular lunar meteorite (e.g., Dhofar 081) when the sample in the photo is clearly from a different lunar meteorite (e.g., Dhofar 911). Caveat emptor.
  

No lunar meteorite has yet been found in North America, South America, or Europe. We can reasonably assume that lunar meteorites have fallen on these continents in the past 100,000 years, but if someone has found one, it's not yet been recognized as a lunar meteorite.

Where, How, and When Are They Found?

In the lingo of meteoritics, all lunar meteorites have been "finds;" none are "falls." In other words, no lunar meteorite has been observed as a meteor. This is a curious fact as there are fewer martian meteorites than lunar meteorites yet several of the martian meteorites were observed to fall (Chassigny, Shergotty, Nakhla, Tissint, and Zagami).

Nearly all lunar meteorites have been found in areas that are well known to be good places to find meteorites.  All such places are dry deserts where there are geologic mechanisms for concentrating meteorites, where rocks of terrestrial origin are rare, and where meteorites do not weather away quickly from exposure to water. 

Many lunar meteorites have been found in Antarctica (see "Why Antarctica") by expeditions funded by the U.S. (ANSMET) or Japanese (NIPR) governments. Most of lunar meteorites have been found in the Sahara Desert of northern Africa and in the desert of Oman - all since 1997. Meteorites from hot deserts are almost exclusively found by local people or private collectors.

Allan Hills 81005 (ALHA 81005), the first meteorite to be recognized as originating from the Moon, was found during the 1981-82 ANSMET collection season, on January18, 1982.  The three Yamato 79xxx meteorites were collected earlier, but not recognized to be of lunar origin until after 1982.  The first lunar meteorite to be found appears to be Yamato 791197, on 20 November 1979. However, it is not known when Calcalong Creek was found. The Meteoritical Bulletin says "after 1960," but it was not recognized to be of lunar origin until 1990, so it may well have been collected earlier than Yamato 791197.

 

 

searching for meteorites in Antarctica
ANSMET 1988-89 field team searching for meteorites in "Meteorite Moraine" near Lewis Cliff. Photo by Robbie Score.
  

ANSMET

ANSMET (Antarctic Search for Meteorites) is a program that has been funded by the United States government through the Office of Polar Programs of the National Science Foundation (NSF) and the Solar System Exploration Division of the National Aeronautics and Space Administration (NASA) in cooperation with the National Museum of Natural History (Smithsonian Institution).


The first lunar meteorites were found in Antarctica in 1979. In 1997 the first lunar meteorite was found in the Sahara Desert and since 1999 many have been found in Oman. The plot includes 8 stones that we know to be lunar but which, in fact, do not yet have official names at this writing.
  

How Do I Recognize a Lunar Meteorite?

Although the discovery that there are rocks on Earth that originated from the Moon is relatively new, lunar rocks have surely been dropping from the sky throughout geologic history.  Mikhail Nazarov and colleagues of the Vernadsky Institute in Moscow estimate that "several tens or few hundred kilograms" of lunar rocks in the mass range of 10-1000 g strike the Earth's surface every year. That fact does not make lunar meteorites easy to find or recognize, however.  Under ideal conditions (e.g., Antarctica), some lunar meteorites are almost instantly recognizable as lunaites because they have fusion crusts that are highly vesicular. No Earth rock and no other kind of meteorite has a crust that is as vesicular as that of lunar meteorites QUE 93069 or PCA 02007. Some lunar meteorites (the basalts) do not have such vesicular fusion crusts, however, and the fusion crust of most lunar meteorites found in hot deserts has been ablated away by the wind. In the absence of a fusion crust, a lunar (or martian) meteorite is less likely to be recognized as a meteorite than is an asteroidal meteorite because it more closely resembles terrestrial rocks in mineralogy and density. A weathered lunar meteorite would not be an impressive or suspicious looking rock if found in a cornfield or streambed (see Dar al Gani 400 or QUE94281) and a brecciated lunar meteorite could easily be overlooked in the field as a terrestrial sedimentary rock. Even experienced meteorite collectors admit that Kalahari 009 does not "look like" any kind of meteorite. Lunar meteorites contain a much smaller amount of metal than ordinary chondrites, so most are not attracted to a magnet.  Also, they have densities similar to terrestrial rocks; they're not heavy for their size, as are most meteorites. Although I had been studying Apollo lunar rocks for 18 years, I did not recognize the MAC88105 lunar meteorite as a Moon rock when another member of the 1988 ANSMET team handed it to me in the field and asked "What do you think about this one?" Unfortunately, lunar meteorites and some kinds of Earth rocks strongly resemble each other in hand specimen. Bottom line: Even for an expert it's not usually possible to identify a lunar meteorite just "by looking." Only expensive and time-consuming tests can prove that a rock is a lunar (or martian) meteorite. "Looks like" is not a good test for lunar meteorites.

More Detail:   See "How Do We Know That It's a Rock From the Moon?"

How Are They Named?

By long-standing convention, meteorites are named after the location where they fall or are found.  For example, Calcalong Creek is a place in Australia.  Somewhat contrary to the convention, the Antarctic meteorites in the U.S. collection often go by abbreviated names, where ALHA = Allan Hills, EET = Elephant Moraine, GRA = Graves Nunataks, LAP = LaPaz Icefield, LAR = Larkman Nunatak, MAC = MacAlpine Hills, MET = Meteorite Hills, MIL = Miller Range, PCA = Pecora Escarpment, and QUE = Queen Alexandra Range.  Similarly, the Dar al Gani (Libya), Northeast Africa, Northwest Africa, and Sayh al Uhaymir meteorites are sometimes abbreviated DaG, NEA, NWA, and SaU.  Because hundreds to thousands of meteorites have been found in Antarctica and hot deserts, serial numbers are used in addition to names. For the Antarctic meteorites, the first two digits of the numeric part of the name represents the collection year. (See map of Antarctic meteorite locations for the U.S. collection.)

MAC 88105 image
MacAlpine Hills 88105 is a lunar meteorite found in
Antarctica in 1989

What's the Difference Between a Lunar Meteorite and a Tektite?

A lunar meteorite is a rock from the Moon. A tektite is not a meteorite (it never orbited the sun or Earth) and it is not from the Moon. A tektite was formed from Earth material during the impact of a meteoroid.

Tektites consist of glass and are often shaped like spheres, dumbbells, or teardrops. Lunar meteorites never have such interesting shapes and none are composed entirely of glass. Tektites have compositions like terrestrial rocks, not like lunar rocks.
photo of tektites

How Are Lunar Meteorites Classified?

Lunar rocks are classified by the minerals they contain (mineralogy), how the mineral grains are put together (texture), how the rock formed (petrology), and chemical composition (chemistry).  These different parameters sometimes leads to confusion because a geochemist might call a rock "feldspathic" (dominant mineral) or "aluminum rich" (chemical composition) while a petrologist might call it an "anorthosite" (mineral proportions and implied mode of formation) or "regolith breccia" (texture and and type of rock components).

Since the time of Galileo, the lunar surface has been divided into two types of terrane, the mare (pronounced mar'-ay, which is the Latin word for sea) and the terra (land) or highlands

Feldspars are some of the most common minerals of the crust of the Earth and Moon.  Rocks of the lunar highlands contain a high proportion (60-99%) of a type of feldspar known as plagioclase.  In particular, the plagioclase of the lunar highlands is the calcium-rich variety known as anorthite (the more sodium-rich varieties are rare on the Moon).  Mineralogically, a rock composed mostly of the anorthite is called an anorthosite, and most rocks of the lunar highlands are, in fact, anorthosites.  Lunar scientists often refer to the highlands crust as "feldspathic," indicating the major mineral, or "anorthositic," indicating the major rock type.  Anorthite, like all forms of feldspar, is rich in aluminum and poor iron

Rocks from the maria are classified as basalts because they are crystalline, igneous rocks (texture) consisting mainly of pyroxene and plagioclase (mineralogy).  Specifically, they are called mare basalts because they formed when magmas from inside the Moon erupted (petrology) into the basins formed by the impacts of small asteroids early in lunar history to form the maria.  Mare basalts are subclassified by chemical composition (chemistry), for example, "low-titanium (Ti) mare basalt." Mare basalts are rich in iron because they contain pyroxene, olivine, and ilmenite, all of which are iron-rich minerals, and the amount of pyroxene + olivine + ilmenite exceeds the amount of iron-poor plagioclase.  

photo of slice of NWA 2995
NWA 2995 is a fragmental breccia (2.5-mm grid in background). Note that in this and other brecciated lunar meteorites, the clasts are not particularly colorful. The "gray-scale" nature of brecciated lunar meteorites distinguishes them from many terrestrial sedimentary rocks (e.g., meteorwrong no. 124) which are reddish because they contain ferric iron (hematite).Some lunar meteorites from hot deserts are more colorful than lunar meteorites from Antarctica because the hot-desert meteorites have suffered a greater degree of chemical alteration from interaction with liquids since landing on Earth. Many lunar meteorites from Oman (e.g., Dhofar 303 and paired stones) are pinkish as a result of terrestrial alteration (hematite staining).
 

Breccias

Breccias are rocks made up of bits and pieces of other rocks (clasts) in a matrix of finer-grained rock fragments, glass, or crystallized melt.

Monomict breccia is a term applied to a breccia that is made up entirely one kind of rock. Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks.

Dimict breccias or dilithologic breccias are made up of only two lithologies. The term is usually applied to a common type of rock collected on the Apollo 16 mission that consists of anorthosite (light color) and mafic (dark, iron rich) crystallized impact melt in a mutually intrusive textural relationship. SaU 169, however, could be regarded as a dilithologic breccia.

Polymict breccia is a general term that encompasses all breccias that aren't either monomict or dimict. Types of polymict breccias are glassy melt breccias, impact-melt breccias, granulitic breccias, regolith breccias, and fragmental breccias. Each of these breccia types has a different texture because the set of conditions that formed them differed.

An impact-melt breccia can be regarded as in igneous rock because it formed from the cooling of a melt. Regolith and fragmental breccias are the closest lunar equivalents to terrestrial sedimentary rocks. Granulitic breccias are metamorphic rocks in that they were some other type of breccia that was metamorphosed (recrystallized) by the heat of an impact.

Most brecciated lunar meteorites are regolith breccias. Some kinds of terrestrial rocks strongly resemble lunar regolith breccias (e.g., meteorwrong no. 118).
  

Igneous anorthosites are rare in the lunar highlands. Impacts of asteroidal meteorites on the Moon both break rocks of the lunar crust apart and glue them back together.  Most rocks from the highlands are breccias (pronounced brech'-chee-uz), a textural term for a rock that is composed of fragments of other rocks and that is held together by shock compaction or by material that was partially or totally molten.  An impact can melt rock, forming impact melt.  The melt usually collects rock fragments called clasts as it is forced away from the point of impact within a crater.  When the melt cools, it forms an impact-melt breccia - clasts suspended in a matrix of solidified (glass or crystalline) impact melt. 

The lunar surface is covered with fine-grained material called soil or regolith.  The shock wave associated with an impact can lithify the regolith - it can turn the fine, powdery material into a coherent rock called a regolith breccia.  At depth, coarser fragments can be lithified to form a fragmental brecciaBreccia is a textural term that applies to rocks of both the maria and highlands.  Most lunar meteorites are feldspathic regolith breccias, that is, rocks consisting of lithified soil from the lunar highlands. Most highlands rocks are breccias because the highlands crust is very old and the impact rate was greater in early lunar history than during the time since the magmas forming mare basalts erupted.

 
Apollo 11 astronaut footprint in lunar soil

full Moon image with rae and highlands

FeO vs Al2O3 in lunar meteorites

The lunar crust is formed mainly from a light-colored, aluminum-rich mineral known as anorthite, a plagioclase feldspar.  Early in lunar history the crust was impacted by small asteroids to form large craters called basins.  Dark, iron-rich magmas generated from melting inside the Moon erupted into the basins.  To ancient astronomers the resulting dark, circular features resembled seas.  They were given Latin names like Mare Serenitatis, the "Sea of Serenity."
Rocks from the lunar highlands are rich in aluminum and poor in iron because they are composed mainly of feldspar.  Rocks from the maria contain some feldspar but consist mostly of pyroxene, olivine, and ilmenite, which are minerals that are rich in iron and poor in aluminum.  Each point represents a lunar meteorite, except that 2 or 3 points are plotted for those meteorites that consist of 2 or 3 rock types, like SaU 169.

The concentration of iron or aluminum serves as a useful chemical classification system in lunar rocks.  Lunar meteorites that are mare basalts (e.g., NWA 032) or breccias composed mainly of mare material (EET 87521/96008) are poor in aluminum and rich in iron.  In contrast, meteorites from the feldspathic highlands are rich in aluminum and poor in iron.  Glass spherules and basalt fragments from the maria have been found as clasts in most of the highlands meteorites and some (e.g., Yamato 791197) contain a higher proportion of mare material than others.  Such meteorites plot on the high-iron end of the range of highlands (feldspathic) lunar meteorites.  Some "mingled" lunar meteorites (e.g., QUE 94281) apparently derive from a place where the mare and the highlands are in close proximity because they are breccias consisting of clasts of both mare and highlands rocks.  (All regolith samples from the Apollo 15 and 17 missions are mixed in this way.)  Such meteorites have intermediate concentrations of iron and aluminum.  We might expect, as more lunar meteorites are found, that the gaps in the aluminum-iron plot above will be filled in.  

More Detail:   See "Chemical Classification of Lunar Meteorites"

Why Are Lunar Meteorites Important?

It may seem, considering that 382 kg of well-documented rock and soil samples were obtained from nine locations by the Apollo and Luna missions, that a few small rocks from unknown points on the lunar surface cannot be very important.  For several reasons, however, the lunar meteorites have provided new and useful information.

The Apollo missions all landed in a small area on the lunar nearside, and some of those missions were deliberately sent to sites known to be geologically "interesting," but atypical of the Moon. (On Earth, Yellowstone National Park is geologically "interesting," but hardly typical.)  The gamma-ray and neutron spectrometers on the Lunar Prospector mission (1998-1999) have shown that all of the Apollo sites were in or near a unique and anomalously radioactive "hot spot" on the lunar nearside in the vicinity of Mare Imbrium.  This existence of this hot spot, sometimes known as the Procellarum KREEP Terrane or PKT, indicates that the mare-highlands distinction of the ancient astronomers is not adequate in a geochemical sense.  Many rocks collected on the Apollo missions that likely originated from the PKT (especially those from Apollos 12, 14, and 15) are neither mare basalts nor feldspathic breccias.  They are rocks (usually impact-melt breccias) of intermediate FeO concentration (~10%) with high concentrations of the naturally occurring radioactive elements: K (potassium), Th (thorium), and U (uranium). Such rocks are often called "KREEP" because, in addition to K, they have high concentrations of other elements that geochemists call incompatible elements such as the rare-earth elements (REE, like lanthanum and cerium) and phosphorus (P).  Lunar meteorite Sayh al Uhaymir 169 with a whopping 30 ppm Th is a "KREEPy" meteorite. Almost certainly, it derives from the PKT. Other meteorites that have high concentrations of Th, like NWA 4472/4485 and Dhofar 1442 also likely originated in or near the PKT. Most of the rest of the lunar meteorites appear to have come from outside the PKT because they have low concentrations, typically <1 ppm, of Th.  This distribution is reasonable in that we believe that the lunar meteorites are rocks from randomly distributed locations on the lunar surface, and most locations on the lunar surface are not high in radioactivity.

map of thorium concentration of lunar surface
The map on the top part of the diagram shows the distribution of the concentration of thorium (Th, in parts per million), a naturally occurring radioactive element, on the lunar surface as determined by the gamma-ray spectrometer on Lunar Prospector, which orbited the Moon in 1998 and 1999 (Lawrence et al., 2000 and Gillis et al., 2004). The center of the map shows the nearside and the left and right edges show the far side of the Moon.  The locations of the six Apollo (A) and three Russian Luna (L) landing sites are indicated (all on the nearside). The bottom part of the diagram shows the concentrations of Th in lunar meteorite source craters. (This means, for example, that the LAP meteorite, NWA 4734, and NWA 032/479 count as 1 source crater because all 3 meteorites likely came from a single crater.) Most lunar meteorites have low Th concentrations but a few have high concentrations (see last column of the List).  The figure shows that (1) the Apollo missions all landed in or near a region of the Moon with anomalously high radioactivity (the anomaly, which we call the "Procellarum KREEP Terrane") was not known at the time of Apollo site selection) and (2) most of the lunar meteorites must come from areas of the Moon that are distant from the nearside "hot spot" because most have low Th.  Thus, one of the values of the lunar meteorites is that they are samples from places on the Moon that are more typical of the lunar surface (low radioactivity) than the Apollo samples. The histogram on the bottom assumes that the known lunar meteorites derive from 39 source craters. The impact-melt breccia of SaU 169 plots off scale at 30 ppm; the bar at 9.8 ppm Th represents the regolith-breccia lithology. The figure is an updated (July, 2007) version of Figure 5 of Korotev et al. (2003).

Also, most of the lunar meteorites are breccias composed of fine material from near the surface of the Moon. This fine-grained material has been mixed by many impacts.  As a consequence, the composition and mineralogy of a brecciated lunar meteorite is likely to be more representative of the region from which it came than any single unbrecciated (igneous) rock from the same region. 

We know that over much of the Moon, and most of the far side, the material of the lunar surface has only 3-6% FeO because it is highly feldspathic:

map of FeO concentration on lunar surface
Map of the surface concentration of iron (expressed as FeO) on the lunar nearside (left) and far side (right), based on spectral reflectance measurements taken by the Clementine mission in 1994.  The FeO data, from 70°S to 70°N, overlays a shaded relief map.  High-FeO areas occur where volcanic lavas (mare basalts) filled giant impact craters.  Low-FeO areas correspond to the feldspathic highlands.  Image courtesy of Jeff Gillis.      

About half of the lunar meteorites have 3-6% FeO, thus these meteorites are entirely consistent with derivation from typical feldspathic highlands: 

histograms of FeO concentrations on lunar surface
These diagrams compare the distribution of the concentration of iron, expressed as % FeO, in the lunar meteorites (top) with the lunar surface as measured with the gamma-ray spectrometer on Lunar Prospector (middle) and estimated from spectral reflectance measurements taken by the Clementine (bottom).  Because the distributions have the same shape and because the peak occurs at the same concentration, we can reasonably infer that the lunar meteorites are random samples from the surface of the Moon.  The large peak at ~5% FeO corresponds to far side highlands and the small peak at ~17% FeO corresponds to nearside maria (see map).  The lunar meteorite data are updated from Korotev et al (2003).  Clementine data are from Lucey et al. (2000) and Gillis et al. (2004). The Lunar Prospector data are from Prettyman et al. (2006).

These various factors lead to the ironic circumstance that the feldspathic lunar meteorites ("feldspathic" breccias in the List) together provide us with a better estimate of the composition and mineralogy of the typical highlands surface than we were able to obtain from the Apollo samples.

The lunar meteorites have also provided us with crystalline mare basalts that are different from any collected on the Apollo and Russian Luna missions. In particular, the Northwest Africa 773 stones are different from any rock in the Apollo collection (e.g., Jolliff et al., 2003).

Anagrams for Lunar Meteorite

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Prepared by: Randy L. Korotev
  
Department of Earth and Planetary Sciences
Washington University in St. Louis

  
Please don't contact me about the
meteorite you think you've found until you read this and this.

e-mail
korotev@wustl.edu

Last revised25-Nov-2013

Griffith Observatory Star Award
July 25-31, 2004

This site is not copyrighted, but if you use information that you find here, please credit the source:
http://meteorites.wustl.edu/lunar/moon_meteorites.htm

photo by Randy Korotev


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