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
These are simple definitions. A more technical but accurate
definition of a meteorite is given by Alan E. Rubin and Jeffrey N.
- 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.
"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.
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?
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
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
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
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
Schematic map of lunar impact basins on the nearside and farside of the
(Based on Figure 2.3 of The Lunar
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
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,
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.,
Campo del Cielo).
Lunar Meteorites for SaleMeteorites, 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
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.
field team searching for meteorites in "Meteorite Moraine"
near Lewis Cliff. Photo by Robbie Score.
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
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.
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
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
terrestrial rocks, not like lunar rocks.
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.
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
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.
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
BrecciasBreccias 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
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.
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
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 breccia. Breccia
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.
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
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.
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
, 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 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:
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
(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
. 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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July 25-31, 2004
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