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Lunar Sample CompendiumIntroduction |
DRAFT |
Disclaimers
Introduction to Compendium
The purpose of the Lunar Sample Compendium will be to inform scientists, astronauts and the public about the various lunar samples that have been returned from the Moon. This Compendium will be organized rock by rock in the manor of a catalog, but will not be as comprehensive, nor as complete, as the various lunar sample catalogs that are available. Likewise, this Compendium will not duplicate the various excellent books and reviews on the subject of lunar samples (Cadogen 1981, Heiken et al. 1991, Papike et al. 1998, Warren 2003, Eugster 2003). However, it is thought that an online Compendium, such as this, will prove useful to scientists proposing to study individual lunar samples and should help provide backup information for lunar sample displays.
This Compendium will allow easy access to the scientific literature by briefly summarizing the significant findings of each rock along with the documentation of where the detailed scientific data are to be found. In general, discussion and interpretation of the results is left to the formal reviews found in the scientific literature. An advantage of this Compendium will be that it can be updated, expanded and corrected as need be.
The Apollo Program
The Apollo Program was remarkably successful. Between 1969 and 1972, twelve men
walked on the surface of the Moon and carefully collected 2,196 documented samples
of soils and rocks during about 80 hours of exploration. Altogether, these samples
weigh 382 kilograms. Only a small portion has been consumed during analysis (as
described herein). A large number are on public display. The largest portion of
each sample is available for future studies.
The lunar samples were collected by the astronauts at great personal risk.
Actually, twenty-four astronauts traveled to the moon and back, during nine
trips (three astronauts went twice). Some missions, like Apollo 8, did not land.
The explosion of the fuel cell during the Apollo 13 mission emphasized the dangers
of space travel. There was also the unseen danger of radiation from potentially
fatal solar flares (Reedy 1977; Rancitelli et al. 1974a). Today, watching films
of the astronauts working on the lunar surface, it is hard to realize that they
were working in a complete vacuum, where any tear in the space suit, or
micrometeorite impact, would have been catastrophic. Some of the tasks, such as
the withdrawal of the Apollo 15 core, were extremely difficult to accomplish.
On the last three missions, there was the potential need for a long walk back to
the Lunar Module in case the rover broke down. For geoscientists, the legacy of
Apollo is a complicated task to unravel the secrets hidden in the collection of
samples obtained during this extraordinary adventure.
Samples from the early missions were returned in sealed rock boxes; however,
other samples were exposed to the atmosphere of the Lunar Module, the Command
Module and even (briefly) the atmosphere of the equatorial Pacific Ocean.
However, once the samples reached the curatorial laboratory in Houston, they
were stored and processed in pure nitrogen. Originally they were quarantined
to make sure there were no extraterrestrial life forms, and oriented by means
of artificial lighting to match shadow patterns on the documentation photos
taken by the astronauts. Indeed, no life forms, organic molecules nor water
was found; so the quarantine was discontinued.
Immediately after each
of the six Apollo mission, the samples were examined by an international team
of investigators in what was call PET (for preliminary examination team):
http://curator.jsc.nasa.gov/lunar/PETScience.pdf.
This led to the set of initial lunar sample information catalogs. Some samples
were immediately sent to the radiation counting laboratory to determine the
effects of cosmic ray and solar flare exposure. As the years have gone by, this
collection has been re-catalogued in various publications; but the lunar sample
literature is the best source of information. Most of this literature is
published, in peer-review manner, in the Proceedings of the Lunar and Planetary
Science Conferences (1970-1992). However, some of this literature is spread out
in various scientific journals; some of it is only published in abstracts which
are getting harder and harder to access.
The Luna Program
The USSR was successful in returning lunar samples automatically and operating a large Lunakod. The Luna samples were collected as core tubes and returned by wrapping the core liners around a drum. They were also studied intensively by international efforts and it was learned that modern analytical techniques could extract a lot of information from small samples. Luna 16 collected samples from the eastern part of Tranquility, Luna 20 from the highlands and Luna 24 from Mare Crisium (Vinogradov 1971, 1973, Barsukov 1977).
The Consortium Approach
Some lunar samples were studied in “consortium mode”. The allocation
committees (LSAPT, LAPST and CAPTEM) often encouraged consortium studies and the
consortium reports, where they exist, are the best reference to individual sample
studies. The consortium approach still remains the best way to study a sample,
when it involves close cooperation of scientists with different backgrounds.
James and Blanchard (1976) state: “Most lunar
breccias are extremely complex rocks. They consist of aggregates of materials
broken, melted, transported and recombined by impact processes. Each fragment
has its own unique history. To understand the origin of such a rock and its
constituents requires data from many disciplines and a coordinated approach to
obtaining this data. Coordinated study insures that the various types of data
can be correlated (a problem in such heterogeneous rocks), and that the
investigators can relate their results to a general understanding of rock
genesis and history. This is the rationale that underlies the
“consortium” approach to studies of lunar breccias.”
Geologic History of the Moon
The best reference for the geological history of the Moon remains:
http://ser.sese.asu.edu/GHM/
Stöffler and Ryder (2001) reviewed the stratigraphy of lunar geologic
units and summarized the age dating that has been accomplished. Nyquist et al.
(2001) reviewed the ages and discussed the initial isotopic ratios. Gillis et
al. (2004) provide an excellent review of recent chemical mapping of the moon.
Briefly, the grand sampling strategy for Apollo was to try to sample the interior
of the moon by studying the basalt flows in the maria, and the ejecta blankets
of really large basins (Imbrium, Serentitatis) at different distances. The basalt
flows represent liquids derived by melting the interior of the moon; thus their
composition and ages tell us about the melting and thermal history of the lunar
interior. The ejecta blankets provide shocked materials of the original crust.
Cratering mechanics predicts that samples from different radial distance from
the basin rim will provide materials from different depths beneath the lunar
surface.
Already, in 1893, Gilbert saw that distinctive
textured terrain extended out from the Imbrium Basin. Just prior to Apollo, the
Lunar Orbiter returned pictures of another large basin (Orientale) with distal
ejecta with the same general pattern (figure 1), confirming Gilbert’s
interpretation for Imbrium (Head 1976a). Figure 2 illustrates this material
draping over the ancient crater Fra Mauro; hence the name Fra Mauro Formation.
This, then, was the target for Apollo 14 where numerous breccia sample were
found. The location of Apollo 15 was intended to sample the Apennine Front near
the inner rim of the Imbrium Basin, where deeper material should be found.
Apollo 17 was from the edge of the Serenitatis Formation and Apollo 16 from
the Central Highlands. Indeed, a number of samples of plutonic rocks were found
in these locations, and they have been related to an anorthositic crust of the
moon (prior to basin formation). From a study of these plutonic samples, it is
now thought that the original crust of the moon may have formed by plagioclase
floatation from an early magma ocean (figure 3).
Detailed study of fragments of pristine lunar samples from the non-mare regions,
shows that the belong to two general trends (figure 4). The ferroan anorthosites
(e.g. 60025) have very calcic plagioclase and
variousFe-contents for the mafic minerals. The mg-suite contains troctolite
(e.g 76535), norite (78235)
and trends towards Na-rich plagioclase with Fe-rich mafic minerals (see for
example, clasts in 15405).
Lunar Mineralogy
The mineralogy of lunar samples is rather simple, with only a few major minerals
(plagioclase, pyroxene, olivine and ilmenite). The rocks formed in a completely
dry and very reducing environment, such that the iron is mostly in a plus two
oxidation state with minor metallic iron. Grain boundaries between minerals are
remarkably distinct, with no alteration products. Glass is present in the
mesostasis. Minerals that might have been added by meteorite bombardment have
generally been vaporized.
There are a few unique features
in lunar rocks; plagioclase is almost pure anorthite, maskelynite is common,
rare ternary feldspar (Na, K and Ca) is found. Pyroxene has a wide range of
composition, somewhat characteristic of each rock type. New minerals include
armalcolite, tranquillite, pyroxferroite, and yttrobetafite. Akaganeite (FeOOH)
was found on one Apollo 16 breccia. ZnS coatings were found on volcanic glass
beads.
The surface of lunar rock that were exposed to
space have a thin brown patina of glass splashes and glass-lined micrometeorite
craters (zap pits). Solar flare tracks are abundant beneath these surfaces. Depth
profiles of cosmic ray induced radio-nuclides extend to depths of 10 cm.
Rock Types
The landing sites for Apollo missions were limited to mildly-cratered, flat
spots, which generally turned out to be lava flows. The basalts from these lava
flows were sampled in abundance. Although fresh in appearance, they measured to
be quite old – 3.2 to 3.9 b.y. There are 134 samples of basalt greater
than 40 grams, 42 greater than 500 grams, 24 greater than one kilogram, 11
greater than two kilograms and the largest 9.6 kilograms
(15555). They have
textures of a crystallized liquid – ranging from variolitic to subophitic
to equilgranular. Most are fine-grained with an average about 0.5 mm, but some
have phenocrysts over one cm.
Most of the lunar basalts are Fe-rich, often Ti-rich, and have abundant opaque
minerals. Some are very vesicular, with interconnecting vugs and vesicles (15016).
A few lunar basalts are greatly enriched in rare-earth-elements (14310, 15382,
15386). 70215 is the
largest mare basalt (8100 grams) and, perhaps, one of the
best studied. All true basalts were found to have low siderophile (Ni, Ir and Au)
content.
The majority of rocks on the lunar surface are breccias. Most lunar breccias are
the lithified aggregates of clastic debris and melt generated by meteorite
bombardment in the ancient lunar highlands (3.9 b.y. ago). There are 59 lunar
breccias larger than 500 grams, 39 greater than one kilogram and 19 greater than
two kilograms. Many of the breccia samples are ejecta from the giant
basin-forming events. Others are interpreted as melt sheets from the fallback
of hot ejecta into the large lunar basins (figure 5). Some have a fragmental
matrix made up individual mineral fragments, while others have a crystalline
matrix from slow cooling of initially molten matrix. A few lunar breccias are
soil breccias containing glass beads and a component of solar wind. Most breccia
samples are polymict, containing a wide variety of clasts, which are themselves
breccias of an earlier generation. Key to the understanding of soils and breccias
are measurement of otherwise trace element gold and iridium (which indicate the
amount of admixed meteoritic material). Breccia clasts with low levels of gold
or iridium are termed “pristine”, meaning they haven’t been
contaminated by meteoritic materials and must be remnant pieces of the original
lunar crust. Using trace siderophile and volatile element signatures, some
scientists have even assigned breccias to specific lunar craters! (see Moon as
a Target below)
An early discovery of lunar samples was that the lunar highlands must contain an abundance of plagioclase-rich material – termed ANT (for anorthositic, noritic and troctolitic). Scientist have found two major trends in these anothositic materials – some have a high Fe/ Mg ratio and are termed ferroan anorthosites (15415, 60025), and the others are generally termed mg-gabbro norites, trending to alkali norites. These materials are generally quite old, and probably represent the original crust of the moon, presumably formed after differentiation of an original, global magma ocean. One sample of dunite was returned (74215). Late-stage differentiation of the magma ocean presumably led to the rocks such as the quartz-mozodiorite in 15405, the sodic-ferrogabbro in 14306 and the “granite” in 14303, 72275 etc. However, zircons from these rocks indicate continuous magma activity from 4.4 to 3.9 b.y. (figure 6).
Glass
Glass is an important component in lunar samples, and has been studied extensively by many investigators. Glass occurs as mesostasis in basalts, as melt inclusions in minerals, as beads from volcanic eruptions, as agglutinates formed by meteorite bombardment of gas-rich soil and as splash on rock surfaces. Agglutinates are an odd characteristic of lunar soils and a measure of its maturity. They are fragment-laden-vesicular glass that is formed by meteorite bombardment and melting of solar-wind-enriched lunar soil. During agglutinate formation, solar-wind implanted hydrogen reacts with silicate glass, forming minute iron grains with strong magnetic properties (Housley et al. 1975). In addition, about 20 groups of clear glass beads prove to be of volcanic origin (Delano 1986). Two deposits of this material (orange glass in 74220 and green glass in 15425) have been studied extensively. Glass is also found in abundance splashed on the surface of rocks that were returned and even as glass objects (64455). Some of this glass may be from South Ray Crater. Some investigators have measured the compositions of thousands of glass fragments in the soil in the hope of discerning ‘rock types’. Ropy glass fragments, found in the soil at Apollo 12, were related to the crater Copernicus (Meyer et al. 1973).
The Regolith
As the moon is an airless planet, meteorites hit the surface at full force,
creating craters large and small, fragmenting the lunar surface and forming what
is known as the lunar regolith (figure 7). The bulk of the regolith is a fine
gray soil with a density of about 1.5 g/cm3, but the regolith also
includes breccia and rock fragments from the local bedrock. About half the
weight of a lunar soil is less than 60 to 80 microns in size. Calcalong Creek
is a meteorite from the moon that nicely illustrates what the lunar regolith
looks like (figure 8). Samples 15295, 15296, 15297,
15298 and 15299 are closely
comparable.
Regolith samples included many soil samples, several drive tubes and three deep
drill cores. It should be noted that the Russians automatically returned three
short cores form three additional places on the moon.
Section on these samples will be added to this Compendium in the future. Meteorites from the moon will be treated separately.
The Moon as a Beam Stop
The Moon is an airless body that acts as a “beam stop” for high
energy cosmic rays and solar flares which penetrate the surface materials
causing nuclear reactions (Lal 1972). Cosmic rays and solar flares are primarily
(>90%) high energy protons (Reedy 1987). The high-energy (~1 GeV)
galactic-cosmic-ray particles produce a cascade of secondary particles,
especially neutrons, that penetrate meters into rocks and soils. The relatively
low-energy (~10 to 100 MeV) particles emitted from the sun (solar cosmic rays)
are rapidly stopped in rock within a few cm. A few percent of cosmic rays are
heavy ions (e.g. Fe) that cause radiation damage in minerals along their final
track.
The neutron flux resulting from this comic ray interaction with the lunar
surface produces measurable variation in the isotopic composition of elements
that have large cross sections for neutrons (Gd, Mn etc.). Some isotopes are
themselves radioactive and decay with time (e.g. 14C, 26Al,
etc.) Lunar samples whose orientation was known from lunar photographs and
matched with photographs in the laboratory have been carefully sawn to provide
samples at different depths for these isotopic studies. These studies have, in
turn, provided data for the models of the cosmic ray energy and flux over time.
Perhaps the most useful information obtained from a study of the cosmic ray
produced nuclides is the measure they give of the length of exposure time at or
near the lunar surface. When several samples from the rim of a young crater give
similar exposure ages, we think we have learned the age of the crater! Reviews by
Arvidson (1975), Eugster (2003) and others, summarize these studies.
The Moon as a Target
Lunar basalts and samples least affected by meteorite bombardment are found to
have very low contents of siderophile elements (generally Ni, Ir, Au, Re, Os)
indicating that these elements were initially generally lacking (< 0.1 ppb).
Lunar rocks with low siderophile element contents have come to be called
pristine (Warren and Wasson 1977). However, lunar soils, breccias and impact
melt rocks are found to have relatively high (~10 ppb), and specific contents
of these elements, which are generally considered to have been added by the
meteorite impacts (figure 10). Indeed, it was found that breccias and impact
melts for each of the large basins had characteristic ratios of these meteoritic
siderophiles (Morgan et al. 1974, 1977).
The Lunar Cataclysm
Many of the impact melt rocks returned from the highlands of the Moon dated at
about 3.9 b.y. This led to the hypothesis that there was a period of late
bombardment of the Moon by large objects that were stored somewhere in the Solar
System for 500 m.y., before colliding with the Moon (and the Earth) in a short
period of time around 3.9 b.y. ago (Tera et al. 1974a; Ryder 1990). Most of
the collection of rocks from the Apollo missions come from the areas around the
big basins Imbrium, Serenitatis and Nectaris, but samples from the Luna missions
around Crisium, and among the meteorite collections, also seem to have an
abundance of ages grouped tightly around 3.9 b.y. (Cohen 2001). This event may
have also have influenced the Earth, because few terrestrial materials (if any)
are older that 3.9 b.y.
Nevertheless, several zircons and some few rock clasts have been
dated to be older than this terminal cataclysm (e.g. Meyer et al. 1996; Norman
et al. 2003).
Lunar Nomenclature
Admittedly confusing; such as it is. However, see Stöffler et al. (1980) and Le Bas (2001). In this Compendium, rocks are simply referred to as they have been in the literature, until such time that some brave sole renames each of these rocks, and/or a consensus is reached on how to name them. Impact processes have greatly influenced lunar rock samples such that terms like breccia, impact melt rock, agglutinate, regolith, etc. are important.
Lunar Controversies
On first reading of the vast literature one might get the impression that
everything has already been done – sometimes multiple times. But
nothing could be further from the truth. Many rocks remain poorly described.
Analyses were generally performed on sample too small (often only 10 mg) to be
representative of the whole. Ages were not concordant. Magnetic data could not
be reproduced from lab to lab. Etc.
Some controversies that are still being argued include:
What did we learn?
Important findings from the Lunar Sample Program are listed at:
/lunar/lunar10.cfm
In addition, we think that we have learned that: