is 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%).
is a Lunar Meteorite?
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.
Did Lunar Meteorites Get Here?
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.
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
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)."
sign in Newfoundland
Do We Know That They Are Meteorites?
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.
Do We Know That They Come From the Moon?
compositions, isotope ratios, mineralogy, 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.
"How Do We Know That It's a Rock From the Moon?"
Many Are There?
depends upon how one counts. More than 200 named stones have been
described in the scientific literature that are 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 115. 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.
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.
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.
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
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 all the lunar meteorites.
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
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).
It Take a Big Impact to Launch a Lunar Meteoroid?
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.
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.
on the Moon Did They Come From? Are Any from the Far Side of the Moon?
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.
Schematic map of lunar impact basins on the nearside
and farside of the Moon. (Based on Figure 2.3 of The Lunar
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.
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
"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).
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, Northwest
Africa 4472/4485 and Northwest Africa 6687
must derive from the near side.
Big Are They?
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). At the other extreme,
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 64-128-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. Based on data through 2014.
How Rare Are They?
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 ~51,600 meteorites listed in the Meteoritical Bulletin
Database, only 1 in 247 are lunar meteorite stones.
comparison, of the ~40,900 meteorite stones found in Antarctica, where
record keeping has been superb, (1976-2013), 1 in 1200 meteorite stones is
lunar (34 stones representing ~21 meteorites).
measure of rarity is mass. The total mass of all known lunar meteorites is
only about 103 kg (227 lbs.). By comparison, the Allende
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,
mass of all known lunar meteorites is now about 27% the mass of the Apollo
lunar sample collection.
Meteorites for Sale
Meteorites, including lunar
and martian meteorites, are easily available for purchase. Samples (end
cuts, slices, chips, crumbs, dust) of the lunar meteorites sell on the
Internet (e.g., e-Bay)
for between about $600 and $4,000 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. Prices have declined as the number of meteorites has
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 than 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.
I've been contacted by seven men who have wanted to buy a lunar meteorite
to mount in a piece of jewelry for their girl friend, fiancée, or
wife. Be aware that compared to many gem stones, lunar meteorites are not
"hard" rocks and most have fractures from meteorite impacts on
the Moon. And although I love lunar meteorites, they are not all that
attractive compared to most gem stones. Get her a diamond, emerald, opal,
How, and When Are They Found?
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,
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.
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, or where meteorites do not weather away
quickly from exposure to water.
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
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
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.
Shişr 166 was found at night with a
flashlight. Oued Awlitis 001 was found
embedded in roots of a dead tree during a search for firewood.
1988-89 field team searching for meteorites in "Meteorite
Moraine" near Lewis Cliff. Photo by Robbie Score.
for meteorites in Morocco. Photo courtesy of Hasnaa
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 (Arabian Peninsula). Data
through the end of 2014.
How Do I Recognize a Lunar Meteorite?
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 or volcaniclastic
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. People have sent me photos of broken concrete that they
claim "looks like" some of the photos of lunar meteorites on my
"How Do We Know That It's a Rock From the Moon?"
Are They Named?
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.)
the Difference Between a Lunar Meteorite and a Tektite?
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.
Are Lunar Meteorites Classified?
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).
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.
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.
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 are rocks made
up of bits and pieces of other rocks (clasts) in a matrix of
finer-grained rock fragments, glass, or crystallized melt.
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.
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.
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.
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
Most brecciated lunar meteorites are regolith
breccias. Some kinds of terrestrial rocks strongly resemble
lunar regolith breccias (e.g., meteorwrong
anorthosites are rare in the lunar highlands, but some were found on the
Apollo missions. Impacts of asteroidal meteorites on the Moon both break
rocks of the lunar crust apart and glue them back together. All lunar
meteorites 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.
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
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
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
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 intermediate 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.
Detail: See "Chemical Classification of Lunar Meteorites"
Are Lunar Meteorites Important?
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.
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.)
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.
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
PKT (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 PKT 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).
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
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.
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 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
~19% FeO corresponds to nearside maria (see map). The lunar meteorite data
are updated (end of 2014) 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).
various factors lead to the ironic circumstance that the feldspathic lunar
meteorites 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.
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).
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July 25-31, 2004
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