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 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?
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
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 Moon.
Words That Confuse
– 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
“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
More Detail: See “How Do We Know That It’s a Rock from the
How Many Are There?
depends upon how one counts. More than 350 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 140. Pairing has not yet been
established or rejected for the many recently found meteorites, so the
actual number is not known with certainty. In the List, known or strongly
suspected paired stones are listed on a single line separated by slashes.
In most cases, the stones were found close together because a meteoroid
broke apart upon encountering the Earth’s atmosphere, hitting the ground or
ice, or while traveling within the ice in Antarctica. (In the other cases,
all from northern Africa, we don’t know for sure where they were found.)
The six LaPaz Icefield stones all have
fusion crusts and the broken edges don’t fit together, thus the LAP
meteoroid likely broke up in the atmosphere. Among the numerous Dhofar
lunar meteorite stones, about 15 appear
to all be pieces of a single meteorite.
Pairing and Naming
Although it is often confusing,
meteorite scientists refer to all found pieces of a meteoroid as a single
meteorite, ideally with a single name. Thus, Allende
refers to hundreds of fragments of a single 2-ton meteoroid that broke
apart over Mexico in 1969. All the pieces are paired stones of a fall and
they are all called Allende. With finds
(meteorites not observed to fall) different stones are often given
different names because they are found at different times. If later studies
show two stones to be paired, then one of the names is officially
discarded. With the Antarctic and hot-desert meteorites, however, all the
stones are originally given different designations because so many
meteorites are found in a small area. This problem leads to the awkward
combination names like Yamato
82192/82193/86032 when one is referring to “the meteorite,” in the
accepted sense, as opposed to the individual stones. If the 15 stones of
the Dhofar 303 et al. lunar meteorite
had been found, for example, in the U.S., they would likely all been given
the same name.
Do All Lunar Meteorites Come from One Big Impact on the Moon?
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 Was Recognized
“The occurrence of secondary
craters in the rays extending as much as 500 km from some large craters on
the moon shows that fragments of considerable size are ejected at speeds
nearly half the escape velocity from the moon (2.4 km/sec). At least a
small amount of material from the lunar surface and perhaps as much or more
than the impacting mass is probably ejected at speeds exceeding the escape
velocity by impacting objects moving in asteroidal orbits. Some small part
of this material may follow direct trajectories to the earth, some will go
into orbit around the earth, and the rest will go into independent orbit
around the sun. Much of it is probably ultimately swept up by earth.”
“There is also a possibility that fragments can be ejected at escape
velocity from Mars by asteroidal impact, though not as large a fraction as
is ejected from the moon. If some small amount of material escapes from
Mars from time to time, it seems likely that at least some small fraction
of this material would ultimately collide with earth.”
Shoemaker E. M.,
Hackman R. J., and Eggleton R. E. (1963)
Interplanetary correlation of geologic time. Advances in Astronautical Sciences, vol. 8, p. 70-89.
Where on the Moon Did They Come
From? Are Any from the Far Side of the Moon?
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 on ebay: “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.)
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.
are very rare rocks; lunar meteorites are exceedingly rare. It difficult to
assess how rare they really are. Of the ~37,100 meteorite stones found in
Antarctica, where record keeping has been superb, (1976-2014), 1 in 1050
meteorite stones is lunar (35 stones representing ~22 meteorites).
measure of rarity is mass. The total mass of all known lunar meteorites is
about 212 kg (467 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, Campo del
mass of all known lunar meteorites is now about 80% of the mass of the
rocks >1 cm in size in the Apollo lunar sample collection.
Meteorites for Sale
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., ebay) for between
about $50 and $4,000 per gram, depending upon rarity (perceived or real!)
and demand. By comparison, the price of 24-carat gold is about $40 per gram
and gem-quality diamonds start at $1000-2000/gram. Prices have declined as
the number of lunar meteorites has increased.
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 ebay 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 eight 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
the lingo of meteoritics, all lunar meteorites
have been “finds;” none are “falls.” In other words, no lunar meteorite has
been observed as a meteor. This is a curious fact as there are fewer
martian meteorites than lunar meteorites yet several of the martian meteorites
were observed to fall (Chassigny,
Shergotty, Nakhla, Tissint, Zagami). 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
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
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.
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 Hasnaa Chennaoui Aoudjehane.
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). In the
past 10 years most lunar meteorites have been found in northwestern Africa.
How Do I Recognize a Lunar
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.
See “How Do We Know That It’s a Rock from the
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.
MacAlpine Hills 88105 is a lunar meteorite found in
Antarctica in January, 1989.
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
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, the mare
(pronounced mar’-ay, which is the Latin word for sea) and the terra (land)
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 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).
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
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
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 are breccias
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
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
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
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.
of Lunar Meteorites?
Vladimir Shuvalov and Natalia Artemieva
of the Institute for Dynamics of Geospheres in Moscow reach the following conclusion on the
basis of numerical impact modeling:
interesting consequence may be connected with
83-km-diameter crater Tycho. ~100 Myr ago, the crater was created by 6-7 km-diameter
projectile in an oblique (30-45°) impact. This impact event delivered
25–100 km3 of lunar material to the Earth, i.e. our planet was
uniformly covered by ‘Tycho’ meteorites with
average density 0.1-0.3 kg/m2 (assuming 30% losses in the
atmosphere). These massive deposits may be found in proper stratigraphic
layers similar to the Ordovician meteorites .”
[NASA/GSFC/Arizona State University]
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.
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).
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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