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.
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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)."
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A road sign in Newfoundland.
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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.
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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.
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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 130 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 70. Pairing has not
yet been established or rejected for the most 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.
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Pairing and NamingAlthough 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 the 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 489 et al. lunar meteorite had
been found, for example, in the U.S., they would likely all been
given the same name.
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Do All Lunar Meteorites Come from One Big Impact on the Moon?
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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.
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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
missions, so the meteorites must come from various 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.
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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 "YQE" meteorites, Yamato 793274/981031, QUE 94281 and EET
87521/96008 (Arai and Warren, 1999,
Korotev et al., 2003) 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?
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.
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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.
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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.
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Schematic map of lunar impact basins on the nearside and farside of the
Moon.
(Based on Figure 2.3 of The Lunar
Sourcebook.)
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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 a recent 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 derives from
the near side.
How Big Are They?
The largest single stones are Kalahari
009 at 13.5 kg (30 lbs) and Northwest Africa
5000 at 11.5 kg (25 lbs). The next biggest are the eight stones of NWA 2995 et al. at 2.82 kg (6.2 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
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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.
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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 month), of the ~32,000 meteorites listed in the
Meteoritical Bulletin
Database, only 1 in 320 are lunar meteorite stones and only 1 in 700
are distinct lunar meteorites.
For comparison, of the ~17000 meteorite stones found by ANSMET in Antarctica (1976-2007),
1 in about 900 stones is lunar (19 stones representing 11 meteorites; for
Mars, it's 9 stones representing 8 meteorites).
Another measure of rarity is mass. The total mass of all known lunar
meteorites is only about 50 kg (110 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).
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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 $20 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 (see alleged lunar
meteorites). 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.
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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, 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. A
number of lunar meteorites have been found in the Sahara Desert of northern
Africa. About half of all lunar meteorites stones have been found in Oman -
all since 2000. Meteorites from hot deserts are almost exclusively found by
private collectors or local people.
Allan Hills 81005 (ALHA81005), 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.
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ANSMET 1988-89 field team searching
for meteorites in "Meteorite Moraine" near Lewis Cliff.
Photo by Robbie Score.
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ANSMETANSMET (Antarctic Search for
Meteorites) is a program 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).
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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 upper plot includes 15 stones that we
know to be lunar but which, in fact, do not yet have official names at
this writing.
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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 PCA02007. Some lunar meteorites (the basalts) do not
have vesicular fusion crusts, however, and the fusion crust of some 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 they are at best only weakly attracted to a
magnet. Also, they have densities similar
to terrestrial rocks; they're not heavy for their
size, as are most meteorites. Although he has studied Apollo lunar rocks for
more than 40 years, the writer of this article did not
recognize the MAC88105 lunar meteorite as a
Moon rock when another member of the 1988 ANSMET team handed it to him 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. Only expensive and time-consuming tests can prove that a rock is a
lunar (or martian) meteorite.
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.)
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What's the Difference Between a Lunar Meteorite and a
Tektite?
A lunar meteorite is a meteorite from the Moon. A tektite is not a meteorite (it
never orbited the sun or Earth) and it's 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.
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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 (70-99%) 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 lava 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 or comets
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.
NWA
2995 is a fragmental breccia (2-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). Lunar
meteorites from hot deserts are sometimes 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).
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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
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).
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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 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.
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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."
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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.
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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 boundary between the maria and the highlands 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 moderately 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.
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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 and the NWA 032/479 meteorite
count as 1 source crater because both 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 they 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).
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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:
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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.
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About half of the lunar meteorites have 3—6% FeO, thus these meteorites
are entirely consistent with derivation from typical feldspathic
highlands:
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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).
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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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