What 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
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
What 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.
How 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
Words That Confuse People
– A big (>1 meter) rock or aggregation of rocks orbiting the sun
– A small (<1 meter) rock orbiting the sun
– The visible light that occurs when a meteoroid passes through the Earth's
– A rock found on Earth that was once a meteoroid.
These are simple definitions.
A more technical but accurate definition of a meteorite is given by Alan E.
Rubin and Jeffrey N. Grossman (2010):
“A meteorite is a natural, solid
object larger than 10 µm in size, derived from a celestial body, that was transported
by natural means from the body on which it formed to a region outside the dominant
gravitational influence of that body and that later collided with a natural
or artificial body larger than itself (even if it was the same body from which
it was launched).”
A road sign in Newfoundland
How Do We Know That They Are Meteorites?
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 It's a Rock from the Moon?"
How Many Are There?
depends upon how one counts. More than 300 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 118. 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
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?
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 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-known
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 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?
et al. (1991) estimated that the frequency of impacts on the Moon
large enough to eject lunar meteorites is greater than 5 per million years.
On the basis of impact probability and the known size distribution of lunar
craters, Paul Warren (1994) makes a persuasive case that lunar meteorites come
from relatively small craters — those of only a few kilometers in diameter.
The main thrust of his argument is that all the lunar meteorites were blasted
off the Moon in the last ~20 million years (most in the last few hundred thousand
years) and that there haven't been enough "big" impacts on the Moon
in that time to account for all the different lunar meteorites. As new lunar
meteorites are found each year, Warren's argument becomes more valid. James
Head (2001) calculates on a theoretical basis that impacts causing
craters as small as 450 m (about a quarter of a mile) in diameter can launch
lunar meteorites. More recently, Basilevsky et al. (2010) argue
on the basis of the known number of lunar meteorites and the frequency of impacts
on the Moon that "a significant part of the lunar meteorite source craters
are not larger than a few hundreds of meters in diameter." (That's big
if it happens in your backyard, but it's not so big for the whole Moon.) If
lunar meteorites come from such small craters, it would be especially difficult
to locate the actual source crater of a particular lunar meteorite.
Prediction 19 Years Before the First Lunar Meteorite
"The occurrence of
secondary craters in the rays extending as much as 500 km from some large
craters on the moon shows that fragments of considerable size are ejected
at speeds nearly half the escape velocity from the moon (2.4 km/sec). At
least a small amount of material from the lunar surface and perhaps as much
or more than the impacting mass is probably ejected at speeds exceeding the
escape velocity by impacting objects moving in asteroidal orbits. Some
small part of this material may follow direct trajectories to the earth,
some will go into orbit around the earth, and the rest will go into
independent orbit around the sun. Much of it is probably ultimately swept
up by earth."
"There is also a possibility that fragments can be ejected at escape
velocity from Mars by asteroidal impact, though not as large a fraction as
is ejected from the moon. If some small amount of material escapes from
Mars from time to time, it seems likely that at least some small fraction
of this material would ultimately collide with earth."
Shoemaker E. M.,
Hackman R. J., and Eggleton R. E. (1963) Interplanetary correlation of
geologic time. Advances in Astronautical Sciences, vol.
8, p. 70-89.
Where on the Moon Did They Come From?
Are Any from the Far Side of the Moon?
scientists like to speculate that a certain lunar meteorite came from a certain
crater or region of the Moon, no one has 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
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. It’s that simple. 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
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.
How 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).
The largest named stone, which was found in several pieces, is NWA 10309 at 16.5 kg (36.4 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 lunar meteorite masses
(all stones of a given meteorite). Masses in the 128–256-gram range are most
common. The plot on the right shows relative masses by continent or country.
Botswana is represented by a single, huge meteorite, Kalahari 008/009. Based
on data through February, 2016.
How Rare Are
are very rare rocks; lunar meteorites are exceedingly rare. It difficult to
assess how rare they really are. Of the ~41,100 meteorite stones found in Antarctica,
where record keeping has been superb, (1976-2014), 1 in 1200 meteorite stones
is lunar (35 stones representing ~22 meteorites).
measure of rarity is mass. The total mass of all known lunar meteorites is about
177 kg (390 lbs.). By comparison, the Allende and
(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
mass of all known lunar meteorites is now about 67% of the mass of the rocks
in the Apollo lunar sample collection.
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.
been contacted by seven men who have wanted to buy a lunar meteorite to
mount in a piece of jewelry for their girlfriend, 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, or agate!
Where, 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 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 experienced collectors.
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.
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.
ANSMET 1988-89 field team searching for meteorites in
"Meteorite Moraine" near Lewis Cliff. Photo by Robbie Score.
Searching for meteorites in Morocco. Photo courtesy of
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 February
2016. There are no more than 300 named lunar meteorite stones and nearly all
the most recent ones are from northwestern Africa.
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 website.
"How Do We Know That It's a Rock from the Moon?"
How 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.
Hills 88105 is a lunar meteorite found in Antarctica in January,
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) January, and it is not from the Moon. A
tektite was formed from Earth material during the impact of a meteoroid.
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.
How 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 cause 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
type of rock components).
the time of Galileo, the lunar surface has been divided into two types of terrane,
(pronounced mar'-ay, which is the Latin word for sea) and the terra (land)
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
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
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
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 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
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.
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., pyroclastic rocks).
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
(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
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 erupted.
Apollo 11 astronaut Buzz Aldrin’s footprint in the lunar
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.
See "Chemical Classification of Lunar Meteorites"
Why 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.) The gamma-ray
and neutron spectrometers on the Lunar
Prospector mission (1998-1999) have shown that all of the Apollo sites were
in or near a unique and anomalously radioactive "hot spot" on the lunar
nearside in the vicinity of Mare Imbrium. This existence of this hot spot, sometimes
known as the Procellarum KREEP Terrane or PKT, indicates that the mare-highlands
distinction of the ancient astronomers is not adequate in a geochemical sense.
Many rocks collected on the Apollo missions that likely originated from the PKT
(especially those from Apollos 12, 14, and 15) are neither mare basalts nor feldspathic
breccias. They are rocks (usually impact-melt breccias) of intermediate FeO concentration
(~10%) with high concentrations of the naturally occurring radioactive elements:
K (potassium), Th (thorium), and U (uranium). Such rocks are often called "KREEP"
because, in addition to K, they have high concentrations of other elements that
geochemists call incompatible
elements such as the rare-earth elements (REE, like lanthanum and
cerium) and phosphorus (P). Lunar meteorite Sayh
al Uhaymir 169 with a whopping 30 ppm Th is a "KREEPy" meteorite.
Almost certainly, it derives from the PKT. Other meteorites that have high concentrations
of Th, like NWA 4472/4485 and Dhofar 1442 also likely originated in or near
the PKT. Most of the rest of the lunar meteorites appear to have come from outside
the PKT because they have low concentrations, typically <1 ppm, of Th. This
distribution is reasonable in that we believe that the lunar meteorites are rocks
from randomly distributed locations on the lunar surface, and most locations on
the lunar surface are not high in radioactivity.
The map 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.
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
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).
These 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.
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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