[meteorite-list] Gamma Rays, Meteorites, Lunar Samples, and the Composition of the Moon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Nov 22 22:02:39 2005
Message-ID: <200511230142.jAN1gRC29998_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Nov05/MoonComposition.html

Gamma Rays, Meteorites, Lunar Samples, and the Composition of the Moon
Planetary Science Research Discoveries
November 22, 2005

--- Lunar meteorites provide ground truth to help calibrate orbital
geochemical data, allowing an estimate of the composition of the entire
Moon.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

A gamma-ray spectrometer built at Los Alamos National Laboratory and
carried on the Lunar Prospector orbiter in 1997-1998 allowed scientists
to measure the concentrations of several elements on the entire lunar
surface. The data have been widely used by planetary scientists to
determine the chemical composition of the Moon and infer something about
the processes operating when it formed. However, specialists in the
study of lunar samples have been a bit uneasy about the details of the
elemental compositions and have offered modest, but significant,
corrections to the gamma ray data to make them more in line with what we
know from samples.

The latest of these approaches to correcting the gamma-ray data has been
done by Paul Warren (University of California, Los Angeles), a renowned
lunar sample specialist. He concentrated on correcting the analysis for
the element thorium (Th), whose natural radioactive decay releases
characteristic gamma rays. Thorium is an important element because we
understand its behavior during the formation and subsequent evolution of
magma, and because it is a refractory element-that is, it condenses at a
high temperature from a gas. This means that if you know the thorium
concentration, you also know the concentrations of all other refractory
elements with similar geochemical behavior, which includes the rare
earth elements, uranium, zirconium, titanium, calcium, and aluminum.
Using his revised global thorium concentration as a springboard, Warren
then estimated the concentration of numerous elements in the entire
rocky portion of the Moon, which makes up more than 95% of the orb that
graces the night sky. His estimates do not agree with those produced by
others, which will lead to continued debate and refinement of the Moon's
chemical composition.

References:

    * Warren, Paul H. (2005) "New" Lunar Meteorites: Implications for
      composition of the global lunar surface, lunar crust, and the bulk
      Moon. Meteoritics and Planetary Science v. 40, p. 477-506.
    * Warren, Paul H. (2001) Compositional structure within the lunar
      crust as constrained by Lunar Prospector thorium data. Geophysical
      Research Letters, v. 28, p. 2565-2568.

------------------------------------------------------------------------

The Composition of the Moon and Planet Formation

Most cosmochemists subscribe to the hypothesis that the Moon formed when
a huge, Mars-sized object slammed into the Earth near the end of its
construction (see the computer-simulation movie of the impact, below).
Understanding this event, including the origin of the impactor, is
central to testing ideas about how the inner (rocky) planets formed.
Determining the chemical composition of the Moon is a crucial link in
our chain of evidence.

The current consensus is that the Moon formed as the result of the
impact of a Mars-sized object with the young Earth. Events like this
were probably common during formation of the planets, so it is important
to understand the processes operating in the hot cloud of vaporized gas
and molten rock. The record of those processes is contained in the
chemical composition of the Moon, so it is important to figure out what
the Moon is made of. (Movie courtesy of Alfred G. W. Cameron,
Harvard-Smithsonian Center for Astrophysics.)

Earth building involved the accretion of large objects from smaller
ones. Experts in the physics of planetary accretion define three major
stages in the planet construction process. The first is not well
defined, but involves dust grains clumping together, like those dust
bunnies that accumulate under our beds (unless you are a fanatical
housekeeper!). In the planet construction process the dust bunnies
continued to gather more dust until there were thousands or millions of
objects the size of asteroids (1 to a few hundred kilometers in
diameter). Heating by the decay of short-lived isotopes such as
aluminum-26 caused these planetesimals to sinter into hard rocks and
even to melt. This stage lasted no more than a few million years.

painting of planetesimal formation by accretion
This painting by James Garry illustrates the initial stage of accretion
that led to the formation of asteroid-sized objects from a cloud of dust.

The asteroid-sized planetesimals were strewn about the solar system, in
circular orbits. They started to interact gravitationally, attracting
each other. This led to an episode of what the experts call "runaway
growth." The swarm of asteroid-sized planetesimals evolved to a group of
perhaps a couple of hundred Moon to Mars-sized objects named "planetary
embryos" by V.S. Safranov, a Russian theoretician and pioneer in the
study of planet formation. The stage of runaway growth lasted between a
hundred thousand and a million years.

Once the inner solar system was populated by planetary embryos, the
planet-formation calculation wizards think that the objects interacted
gravitationally with each other. Every so often two objects collided,
creating a larger object. This continued until there were only a few,
separated objects--the inner planets. Because the planetary embryos were
so large, the impacts were extremely powerful, no doubt causing
considerable melting on each growing object, and in one case forming a
large object that ended up orbiting the target-Earth's Moon. Natural
satellites (moons) did not usually form because that apparently required
both enough mass in the two colliding bodies and an off-centered impact
to give ejected debris enough sideways velocity to stay in orbit.
Isotope studies of rocks from Earth, Moon, and Mars indicate that this
final stage of planetary formation took no longer than about 30 million
years.

[painting of two body collision]
Impact between planetary embryos would have been highly energetic, as
depicted in this painting by James Garry. Substantial regions of both
bodies would have melted, resulting in formation of metallic cores in
the new, combined, larger embryo.

There are some significant questions in the process of going from
embryos to planets. The central one is the extent of mixing among
planetary embryos. Computer calculations of the accreting set of
planetary embryos follow the paths of each of a couple of hundred
objects as the inner planets form, while keeping track of their
locations at the beginning of the process. The calculations suggest that
a given planet was assembled from objects throughout the inner solar
system, although most come from relatively nearby. However, some
compositional data hint that the feeding zone for each planet was
relatively small. A good test of the extent of mixing is to determine
the composition of the Moon-forming impactor (itself a planetary embryo)
and compare it to that of the Earth. To do this, we must know the
composition of the Moon. (We already know the composition of the Earth
reasonably well from many years of geochemical study and seismology.)

[planetary embryo mixing diagram]

Calculations by John Chambers (NASA Ames Research Center) indicate that
there was some mixing of planetary embryos as they attracted each other
by their gravity and were scattered by the gravitational field of
Jupiter. This diagram shows the results of four of Chambers' computer
runs. The pie diagrams show the percentage of material in each of the
final inner planets that came from the region of the solar nebula shown
by the colors. Although most of the material for a given planet comes
from relatively nearby, quite a bit comes from much farther away. The
extent of mixing needs to be determined to understand planet formation,
and the Moon's composition gives clues to the composition of the
planetary embryo that hit the Earth to form the Moon.

------------------------------------------------------------------------

Understanding the Big Impact

The problem is that we do not know how the giant impact affected the
material from which the Moon formed. It must have been a monumentally
hot event--up to thousands of degrees Celsius. Earth and impactor
(mostly the latter according to computer simulations) materials ended up
orbiting the Earth as molten rock and silicate vapor. Volatile elements
might have been lost, refractory elements (those that boil at a high
temperature) might have preferentially condensed, and droplets of
metallic iron could have been oxidized. We do not have a quantitative
knowledge of the geochemical environment during the giant impact event.
Understanding the composition of the Moon will help us get a handle on
what cosmochemical processes operated, although we have to disentangle
those effects from differences in composition between Earth and the
impactor. Cosmochemistry is a tricky business.

[orbiting cloud of molten rock]

Some of the processes operating in the post-impact cloud surrounding the
Earth are shown in this diagram based on ideas by Dave Stevenson
(Caltech). An orbiting cloud of molten and vaporized rock surrounded the
Earth. The hot, orbiting lunar birthplace might have been in chemical
communication with the molten material inside the Earth, it might have
lost volatile elements and preferentially condensed refractory elements,
and it would have begun to accrete rapidly to form the Moon.
Cosmochemists do not understand all the processes that could have
operated during this violent, important event.

------------------------------------------------------------------------

Gamma Rays and Elements

The Lunar Prospector spacecraft carried a gamma-ray spectrometer. The
instrument, built at Los Alamos National Laboratory, counted the number
of gamma-rays as a function of their energies. The gamma rays are
produced by either radioactive decay or by nuclear interactions
triggered by cosmic rays with the lunar surface. This allowed
cosmochemists to determine the concentrations of several elements,
including potassium and thorium (radioactive decay), and iron and
titanium (other nuclear processes).

[Lunar Prospector spacecraft and instrument]

[Left] The Lunar Prospector spacecraft orbited the Moon in 1998. It
weighed only 295 kg, small by the standards of most planetary missions.
The gamma ray spectrometer sits on one of the booms. [Right] The Gamma
Ray Spectrometer was a small cylinder that was mounted at the end of one
of the 2.5-meter booms extending from the Lunar Prospector spacecraft.
It weighed 8.6 kilograms and was 16.7 centimeters in diameter and 55
centimeters long. The gamma rays are detected by a bismuth germinate
crystal that is surrounded by a plastic shield that detected and
eliminated particles other than gamma rays. The instrument was build at
Los Alamos National Laboroatory.

The Lunar Prospector Gamma-Ray Spectrometer Team used its decades of
experience in gamma-ray spectroscopy and nuclear physics to determine
the concentrations of these elements. Many cosmochemists focused on the
concentration of thorium on the lunar surface because of its importance
in understanding the Moon's composition and because of its heterogeneous
distribution on the Moon. However, cosmochemists who work on samples
from the Moon, Mars, Earth, asteroids, and comets are accustomed to
highly accurate analyses because we do them in well-equipped
laboratories. The remote gamma-ray data are amazingly good considering
that they were measured from a spacecraft orbiting the Moon and provide
a database of the entire planet. The data did not match perfectly with
what we knew from analyses of Apollo lunar samples or lunar meteorites,
however. They needed to be tweaked if we were to do things like
determine the composition of the lunar crust. We needed ground truth.

------------------------------------------------------------------------

Ground Truth

Remote sensing observations of planetary surfaces are enhanced if we
know the composition (chemical or mineralogical) in specific places.
This allows us to test instrument calibrations or even to use the ground
truth in the calibration procedure. In the case of thorium on the Moon,
cosmochemists were concerned that the theoretical corrections were
giving concentrations a bit too high in areas thought to have quite low
thorium, namely in the lunar highlands on the farside. The difference
was not really very large, but it adds up when determining the total
amount of thorium in the crust or in the entire Moon. Besides,
cosmochemists are a bit anal-retentive.

[plot of Th vs. K]

This is a plot of thorium (Th) versus potassium (K) for Lunar Prospector
gamma-ray data produced by Tom Prettyman and his co-workers at Los
Alamos National Laboratory (red dots). Lunar meteorites are also plotted
and form a well-defined line that falls to the low side of the Prettyman
dataset. This suggests that the calibration is shifted to high Th
values. (For the meteorite data, filled diamonds have compositions
characteristic of the lunar maria (e.g., high FeO), open diamonds have
compositions like those of the highlands (low FeO, high Al2O3). Similar
discrepancies are found between the gamma-ray data and Apollo rock and
regolith samples. The differences led Jeffrey Gillis-Davis (formerly at
Washington University in St. Louis, now at the University of Hawaii) and
his co-workers and Paul Warren to investigate ways to correct the data.

One approach is to use the compositions of regolith samples (the debris
pile that makes up the lunar surface) from the Apollo landing sites. We
have lots of samples from these sites, so we can determine a reasonable
average concentration.

[Apollo astronaut collecting samples]

Apollo astronauts collected 382 kilograms of samples from the Moon.
Lunar samples, besides yielding scientific treasure through analysis in
terrestrial laboratories, provide excellent ground truth for calibrating
observations made from orbit. Sample data were used by Jeff Gillis-Davis
and his colleagues and by Paul Warren to improve the calibration of the
Lunar Prospector gamma ray data.

The down side is that the Apollo samples were collected in a relatively
small area, but each gamma-ray point (or pixel) is about 60 kilometers
across. Nevertheless, this is a useful approach and was the one taken by
Jeffrey Gillis-Davis and his colleagues at Washington University in St.
Louis. They made synergistic use of gamma-ray data and a dataset with
much higher spatial resolution obtained by the Clementine mission in
1994. Clementine produced a global dataset of the amount of reflected
light at several wavelengths. This allowed Paul Lucey (University of
Hawaii) to develop a method for determining the amount of iron oxide
(FeO) and titanium oxide (TiO2) in spots only 0.1 kilometers across.

Using Clementine data, Gillis-Davis and his co-workers examined the
areas surrounding the Apollo landing sites. They found that in general
the surrounding 120 kilometers were fairly uniform in FeO and TiO2
concentration, suggesting that the Th and K concentrations were also
likely to be uniform. This allowed them to use the average
concentrations of Th and K at the Apollo landing sites to check the
measurements of K and Th reported by the Lunar Prospector gamma-ray
team. They also used the compositions of feldspar-rich lunar meteorites
as proxies for the composition of the farside lunar highlands. By
plotting the landing site (and feldspathic meteorite) data against the
Lunar Prospector data they could determine if some corrections were in
order. They found that the thorium concentration reported by the Lunar
Prospector team ought to be lower by a small amount and gave an equation
for the correction.

[Apollo 14 landing site FeO concentrations]

On the left is the geologic map of the Apollo 14 landing site (marked by
the center "+"). On the right is the same region showing FeO
concentration based on Clementine data as calculated by Gillis-Davis and
colleagues. Average FeO of ~12 wt% occurs in the Fra Mauro Formation
(blue-purple area), whereas the mare basalts (green) have a higher
average FeO of ~16 wt%. The boxes simply show where FeO and TiO2 data
were collected and analyzed. The circle indicates the 2o area observed
by the Lunar Prospector Gamma Ray spectrometer.

------------------------------------------------------------------------

[Th comparison]

The graph above shows thorium (Th) concentration (in parts per million,
ppm) at the Apollo landing sites versus Th concentration as reported by
the Lunar Prospector team. Feldspar-rich lunar meteorites are also used.
The line is the best fit to the data and is used to correct the
Prospector gamma ray data.

Paul Warren took a different approach. He made extensive use of lunar
meteorites to establish ground truth. The tricky part of this task is
that we do not know the specific location on the Moon a given meteorite
comes from. What kind of ground truth is that? Warren's clever
innovation was to use a plot of the reported Th concentration versus the
fraction of the lunar surface with that concentration or lower, assuming
that the lunar meteorites from the maria constitute no more than 17% of
the surface (the percentage occupied by visible maria on the Moon), that
the highland meteorites make up the remaining 83% of the surface, and
that each meteorite represents the same surface are as others in its
class (mare or highland). This approach works because the lunar
meteorite suite approximates a random sampling of the lunar surface.
Warren also used Apollo regolith samples in his calibration. The
important contribution is Warren's emphasis on the parts of the surface
that are low in Th. These regions make up a significant percentage of
the surface, so contribute a lot of Th to our calculations of the total
concentration in the crust.

[Lunar Meteorite SaU169]
<http://epsc.wustl.edu/admin/resources/meteorites/sau169.html> Luanr
[Meteorite MET01210]
<http://epsc.wustl.edu/admin/resources/meteorites/met01210.html>

The pictures above show only two examples of lunar meteorites. Less than
1 in 1000 of all known meteorites are from the Moon. As Randy Korotev
says on his excellent Lunar Meteorites
<http://epsc.wustl.edu/admin/resources/meteorites/what_to_do.html> web
page, "You've got a better chance of winning the lottery than finding a
lunar meteorite." They are that rare. All lunar meteorites have been
found in deserts, for example SaU169 [LEFT] found in Oman and MET01210
[RIGHT] found in Meteorite Hills, Antarctica. Though extraterrestrial
materials fall randomly on Earth it is simply easier to find them in
deserts where they are well preserved (due to lack of weathering) and
concentrated on a plain background so that they are easily recognized.
Click on the photos for more information about the meteorites.

Warren's new calibration for Th gives very similar results to
Gillis-Davis' calibration. It gives lower Th concentrations in the lunar
highlands, changing the mean feldspar-rich highlands concentration from
about 0.8 parts per million (ppm) to about 0.5 ppm. Because Th resides
mostly in the lunar crust, this revision lowers the estimate of Th in
the entire Moon.

[Th recalibration by Warren]

This graph shows a comparison of Warren's proposed Th recalibration (red
curves) to the calibration made by the Lunar Prospector team (solid
black line) and to lunar meteorites (green dots) and to Apollo regolith
samples (dark dots). The sample data lies beneath the Lunar Prospector
curve, indicating that the Prospector data may over estimate the Th
concentration on the surface of the Moon.

------------------------------------------------------------------------

Thorium in the Crust

To determine the Th concentration in the entire Moon, we must first
extrapolate the Th content at the surface to the entire crust, and then
estimate the amount of Th in the mantle (which makes up most of the
Moon). This is not easy! Thorium is not distributed uniformly on the
surface. It is concentrated on the nearside (the side that always faces
Earth), centered on the Imbrium-Procellarum region. Thorium abundance is
very low in the highlands, especially in the central highlands on the
Farside. The South Pole-Aitken basin on the farside is slightly elevated
in Th. Th correlates in samples with lots of other elements with the
same geochemical behavior (e.g., uranium, zirconium, rare earth
elements), so this variation in its abundance indicates a significant
chemical variation on the Moon. What caused it? Paul Warren suggests
that this region is the site of an ancient, huge impact that created a
basin, the Procellarum Basin, 3200 kilometers across. Although the
existence of this basin is controversial among lunar geologists,
geophysical measurements show that this region is the site of low
topography and thin crust. In 1998, Warren and his colleague Greg
Kallemeyn proposed that as the original globe-encompassing magma ocean
was nearing the end of its crystallization all the elements that did not
go into the main minerals (thorium, rare earth elements, etc.)
concentrated in a layer near the base of the crust. If the Procellarum
basin formed by a huge impact at that time, it would become a site for
concentration of the leftover magma, creating the great thorium hot spot.

[Recalibrated lunar Th abundance maps]

The asymmetry in the concentration of thorium (Th) on the lunar surface
is very clear in the maps, above, of the Th concentration on the
nearside (left) and farside (right) of the Moon. (Diamonds on the maps
show the locations of the six Apollo and three Luna landing sites.)

------------------------------------------------------------------------

[maps of terrane-FeO on Moon]
[maps of terrane-Th on Moon]

Brad Jolliff and his colleagues at Washington University in St. Louis
have defined three major terrranes on the Moon, as shown on the maps
above: (1) The Feldspathic Highlands Terrane (FHT) which includes its
somewhat different outer portion (FHT,O); this terrane has low FeO and
Th. (2) The Procellarum KREEP Terrane (PKT), characterized by high Th.
(3) South Pole Aitken Terrane (SPA Terrane), which has modest FeO and
Th. These do not correspond to the traditional divisions into highlands
and maria. The thicknesses of these distinctive terrains and the nature
of the lowermost lunar crust are not known with certainty.

The difficult part of determining the average amount of Th in the crust
is to determine the thickness of each distinctive terrane, and whether
any of them make up the lowermost crust. Lunar sample and remote sensing
data have suggested to some lunar scientists that the lower crust is
richer in iron oxide (FeO) and Th than the upper crust. They infer this
from the presence of impact melt rocks that contain about 10 wt% FeO in
ejecta from the Imbrium and Serentatis basins, as sampled by the Apollo
15 and 17 missions. Remote sensing data also suggest that deeper
excavation by larger basins or thinner crust to begin with produced
ejecta richer in FeO.

The surface of the lunar highlands is distinctly low in FeO, which also
means that it is high in Al2O3 and the mineral feldspar. The high
feldspar content is a consequence of feldspar floating in the ocean of
magma that surrounded the Moon shortly after it formed. But things are
much more complicated than that simple picture. The rings of most of the
giant multi-ringed impact basins are composed of rocks with FeO even
lower that the typical highlands surface, hence high feldspar (see map
below). Cratering experts suggest that rings are deep material brought
to the surface by the impact.

[map of FeO concentration in Orientale Basin]

This map shows the FeO distribution in and around the Orientale impact
basin. Note the dark ring areas (black) around the center of the basin.
These areas have very low FeO, implying that they are mountains of
anorthosite. The surrounding highlands (blue) are still low in FeO, but
higher than the anorthosite rings. High FeO in the basin interior (red
and white) are locations of mare basalt lava flows.

Thus, the crust in the lunar highlands appears to the layered: a zone
containing more FeO overlies one that is very low in FeO, which may or
may not overlie one that contains a substantial amount of FeO. Most
lunar scientists interpret this pile as representing the product of an
original feldspar floatation crust that was intruded by magma richer in
FeO (and hence contained FeO-bearing minerals such as olivine and
pyroxene). Mixing by large impacts produced a mixed zone on top (the
part of the crust containing 4 to 6 wt% FeO), with an anorthosite zone
below that. The lower crust is a complex mixture of left over stuff from
magma ocean crystallization and intruded magma.

[drawing of possible complexity of lunar crust]

The drawing shown above depicts the complexity of the lunar crust based
on a concept by Paul Spudis (Applied Physics Laboratory, Johns Hopkins
University). The topmost layer of the crust is composed of a mixture of
underlying anorthosite (rock containing more than 90% plagioclase
feldspar) and lower crustal intrusions of Mg-suite magmas. Mg-suite
magmas are slightly younger than anorthosites and may have formed when
magma became trapped inside the anorthosite crust. This complicated
picture is actually simplified from reality, which makes determining the
bulk chemical composition of the lunar crust a difficult business.

------------------------------------------------------------------------

[idealized lunar stratigraphic column ]

The diagram on the left is a simplified version of the stratigraphy of
the lunar crust, based on work by B. Ray Hawke (University of Hawaii)
and his colleagues. The upper zone is a mixture of the anorthosite layer
and the lower crust. The latter is rich in rocks that contain more
iron-bearing minerals than does anorthosite. In terms of Th
concentration, the anorthosite layer is lowest (probably less than 0.05
parts per million), the lower crust highest (between 1 and 5 parts per
million), and the upper mixed zone has about 1-2 parts per million Th.

Paul Warren also observed layering from a study of remote sensing data.
In his short but information-packed 2001 paper, he showed that the Th
concentration of the most deeply excavated material was lower than in
the target. He did this by using Lunar Prospector data for Th and
examining 50 craters larger than about 60 kilometers in diameter. Using
our knowledge of impact dynamics, he reckoned that the crater floor and
the rim materials lying within 1.4 crater radii of the center would come
from the deepest depth. He then compared those concentrations to the Th
concentration of materials farther from the crater, which he called
"background concentration." This approximates the surface composition
before the impact. When plotting the ratio of Th concentration within
1.4 radii of the center to the background versus crater diameter, he
found a distinct correlation. As crater size increases, Th concentration
inside the crater decreases, indicating that Th is lower at depth than
at the surface. Warren uses this trend to estimate total crustal Th
concentration, by applying what he calls a "compensation factor." He
infers that this factor is about 0.6. This means that to calculate bulk
crustal Th, he took the average surface Th and multiplied by 0.6. He
also estimated another factor that takes into account the thickness of
the crust beneath every Th concentration pixel. That correction comes
out to 0.88. Combining the two factors results in a total compensation
factor of 0.53. This is consistent with observations by others of
crustal layering, but it does not show a lower crust rich in Th compared
to the rest of the crust. That relationship is shown only by large
impact basins, but not all of them.

[thorium ratio of craters]

This chart shows the thorium concentration ratio (inside crater and
nearby ejecta divided by Th in the surrounding area) vs crater diameter.
The larger the crater, the lower the ratio. This means that Th decreases
in depth in the lunar crust, at least in the upper 30 kilometers or so.
Warren uses different symbols for different initial Th concentrations.
They intermingle on the plot, showing that the initial surface
concentration does not correlate with concentration at depth.

Warren estimates an average surface concentration of 1.35 ppm.
Multiplying by the correction factor with depth, he finds that the Th
concentration in the entire crust is about 0.7 ppm. Other estimates are
higher. For example, Brad Jolliff and colleagues at Washington
University in St. Louis estimate a crustal Th concentration of 1.05 ppm.
In a paper submitted for publication, my colleagues and fellow Taylors,
S. Ross Taylor (Australian National University) and Lawrence A. Taylor
(Univ. of Tennessee) and I estimate a crustal Th concentration of 0.75
to 0.9 ppm. Overall, that is remarkable agreement, with the total range
being only from 0.7 to 1.05 ppm. The variation in the estimates is
caused by different approaches to handling how Th varies with depth in
the lunar crust.

Whether 0.7 or 1 ppm, there is a lot of Th in the crust compared to the
mantle, which we'll look at next. How much the crustal Th contributes to
the total Moon inventory depends the thickness, hence the volume, of the
crust. We used to think that the crust was much thicker than we do now.
In the mid-1970s the crust was thought to be between 60 and 70 km thick.
However, the best estimates of the thickness of the crust are from
recent reworking of the old Apollo seismic data and new ways of
analyzing the combination of lunar gravity and topography. These results
suggest that the crust is only about 45 to 52 kilometers thick on
average, though it might be as thin as 30 kilometers in the Procellarum
KREEP Terrane. Paul Warren uses a value of 48 kilometers. All this
combines to give a crust contribution to the bulk Th in the Moon of
about 0.05 ppm in Warren's assessment. Other recent estimates place the
value somewhat higher, around 0.07 on average, assuming a crust only 45
km thick.

------------------------------------------------------------------------

Thorium in the Mantle

Precisely figuring out the average Th concentration in the lunar crust
seems to be nearly impossible--but it is easy compared to figuring out
the Th concentration in the mantle. One way is to use the Th
concentration in lunar basalts to infer the composition of the regions
of the mantle in which they formed. Brad Jolliff and his colleagues did
that, concluding that the mantle contains about 0.04 ppm. I did the same
thing in a couple of papers and concluded that the mantle source regions
for mare basalt magmas contained between 0.03 and 0.1 ppm. Not much, but
if the crust has only 0.7 ppm (which contributes 0.05 ppm to the bulk
Moon Th), then the ratio of Th in the crust to Th in the mantle is
between 7 (for 0.1 ppm Th in the mantle) 23 (for 0.03 ppm in the
mantle). As Warren points out, that seems too small a concentration
considering the extensive differentiation that must have accompanied
crystallization of the magma ocean. If the crust contains 1 ppm of Th,
then the crust/mantle ratio is between 10 and 33, still small. This
suggests either that the magma ocean products were mixed extensively
back into the mantle, or that there is less Th in the mantle than many
of us have inferred. Warren favors the latter idea. He presents data
from terrestrial basalts and the likely mantle source regions in which
they formed to suggest that the crust/mantle ratio should be 50 or more.
He concludes that the lunar mantle contains no more than 0.025 ppm Th.

Putting the Th concentrations in the crust and mantle together with the
weight percentages of the crust and mantle (7%and 93%, respectively) in
the silicate portion of the Moon, Warren recommends a bulk Th
concentration of 0.073 ppm, indistinguishable from estimates of the Th
concentration in the bulk silicate Earth (crust plus mantle), 0.075 ppm
(plus or minus 0.06 ppm). Brad Jolliff came up with 0.14 ppm in the
Moon, double that of the Earth. The traditional view, argued in a series
of papers over the past three decades by Ross Taylor has been that the
Moon is enriched compared to the Earth by about 50%, in between the
Warren and Jolliff estimates. The exact value is very important as it
may hold the evidence for processes that operated after the giant impact
that formed the Moon. If no enrichment in Th (hence in all the other
refractory elements), then perhaps the Moon's raw materials did not form
simply by condensation from a hot cloud of silicate vapor. If there is
an enrichment in refractory elements compared to Earth, then perhaps
such a hot origin is implicated.

One way to test whether Th is enriched in the Moon compared to Earth is
to determine the concentration of another refractory, but one whose
geochemical behavior is different. Aluminum is particularly useful
because the lunar crust has lots of it. The problem is that we do not
have a clear idea of how much aluminum there is in the mantle of the
Moon. Estimates range from 2.5 to 7 wt% Al2O3, compared to 4.1 wt% in
Earth. (Major--the most abundant--elements are traditionally listed as
oxides, rather than elements, as they are always bound to oxygen in
mineral structures.) Warren estimates Al from the Al/Th ratio in
chondritic meteorites, a perfectly reasonable thing to do because the
refractory elements occur in the same relative abundances in all types
of chondritic meteorites. His estimate is 3.8 wt%, basically the same as
the bulk Earth, but that is not surprising because it is derived from
his estimate for thorium, which he estimates is same as in the bulk
Earth. We can try to estimate Al independently by determining Al2O3 in
the crust and mantle. We have the crust under control. It almost
certainly contains between 24 and 29 wt% Al2O3. In contrast, we are
relatively clueless about the mantle. My Taylor colleagues and I guess
it is between 3 and 4 wt%, but we need more data to pin that down.

------------------------------------------------------------------------

Other Elements

Once you know Th you know a slew of other refractory elements. Volatile
elements are also not too difficult to determine. For example, we can
use the ratio of potassium to thorium (K/Th) or K to any other
refractory element. This gives a global value for K/Th because during
melting and crystallization in magma these two elements behave
similarly, so their ratio stays relatively constant. Data from lunar
samples and Lunar Prospector show that the Moon is clearly depleted in
moderately volatile elements such as potassium. The K/Th ratio is about
360 compared to 2900 in Earth and 5300 in Mars. The Moon is clearly
depleted in volatiles. Highly volatile compounds such as H2O are present
at extremely low levels. In fact, there is essentially no water inside
the Moon. Amazingly, all lunar scientists agree that the Moon is
depleted in volatile elements compared to chondrites, Earth, and Mars.

One of the very important elements that Warren tries to pin down is
magnesium (Mg). We usually go after this by examining the variation in a
somewhat complicated ratio: Mg/(Mg+Fe). Those of us who study how magma
changes in composition as it crystallizes are particularly fond of this
ratio because it tends to systematically decrease with crystallization.
This happens because the first minerals to crystallize tend to take up
more Mg than they do Fe, leading to a decrease in the ratio. It is also
useful because it varies from 0 to 1. Or from 0 to 100 if multiplied by
100 as is often done to give the ratio in mole percent. It is
abbreviated in several ways: mg, mg', mg, Mg#, and mg#.

The standard Moon composition model has an mg# of 79. In contrast, the
bulk silicate Earth (crust+mantle) has an mg# of 89. Clearly a big
difference. However, Warren argues strongly that the lunar mg# is much
higher than our standard model. He says that lunar scientists have been
biasing the estimates by putting too much weight on the origin of mare
basalts, which are exceptionally rich in FeO and have low mg#. In fact,
he is a tad irritated by this suspected bias, as seen from this sentence
in his paper: "The notion that mare basalts can supply more than
marginal constraints on bulk lunar mantle mg# is likewise a facile
approach based on a doubtful premise." There is no doubt that mare
basalts provide only one look at the mantle, and maybe not a very
thorough a look at that. They make up only about 1% of the crust, so
maybe they are telling us about only a few percent of the mantle.

There are lunar igneous rocks with much higher mg# than mare basalts. In
fact, the typical highlands (based on both Apollo samples, lunar
meteorites, and Lunar Prospector data) range in mg# from about 55 up to
94. The low values are from areas dominated by anorthosites, the product
of magma ocean crystallization. There are quite a few samples in the
upper part of that range (88 to 92). These would have formed by partial
melting of a region of the mantle that has an even higher mg#. In short,
there must be high mg# mantle areas (to form the highland samples) and
low mg# areas (to form the mare basalts). The question is how much of each.

Another way of looking at the problem is to try to determine the
concentration of FeO in the lunar mantle. The places where mare basalts
formed in the mantle clearly had high FeO (around 18 wt%), as Warren
points out, but we cannot use just the mare basalt data. The highlands
rocks with high mg# have much lower FeO, so their mantle birthplaces
must also have low FeO. I estimate around 7 wt%. The bulk Moon FeO
estimated from the lunar density and moment of inertia (which measures
the density distribution inside the Moon) indicate a bulk FeO content of
13 wt%, in between the mare and highland rock mantle birthplaces.

Paul Warren favors a high mg# for the Moon--in fact, a mg#
indistinguishable from that of Earth. Others (including me) favor a
somewhat lower value for the Moon (maybe around 80), but the real truth
is that there are enormous uncertainties in any of our estimates. What
we do know is:

    * The lunar mantle is compositionally heterogeneous
    * Partially melting in it gave rise to magmas with mg# ranging from
      more than 90 down to about 70.
    * We need more information!

------------------------------------------------------------------------

More Information, Please

It is not easy to figure out the composition of an entire planetary
body. The fact that we know as much as we do about the bulk composition
of the Earth is a triumph of cosmochemistry. On Earth we have samples of
the mantle brought up by erupting magmas and thrust up onto one large
tectonic plate by the subduction of another. We have seismometers all
over the world that help us understand the properties, hence the
mineralogy, of the interior of the Earth.

The Moon is a different story. We have no mantle samples, though we
might be able to collect some if we were to sample the right places. One
such place is the South Pole-Aitken basin, an impact crater 2500
kilometers across. It should have dug down into the mantle, delivering
chunks of mantle rocks to the surface, or at least incorporating melted
mantle rock into impact melt breccias that make up its floor. A sample
return mission from South Pole-Aitken would bring back a priceless
treasure about the Moon's chemical composition.

[SPA topo ]

This map shows the topography of the South Pole-Aitken basin on the
Moon. The black dashed lines outline the basin and locations of its
mountainous rings. The total elevation difference from the center
(purple tones) to the highest areas outside it (white tones) is about 12
kilometers. The basin should have excavated into the lunar mantle.

Installation of a global seismic array on the Moon would lead to major
advances in our understanding of the structure of the crust and
compositional variations inside the mantle. The velocities of seismic
waves can be used to determine the mineralogy and mineral compositions
at depth in the Moon. In particular, geophysicists will be able to
greatly decrease the uncertainty in our estimates of the mg# and Al2O3
concentration in the interior of the Moon.

The Moon's origin by a giant impact is a dramatic event that does not
necessarily follow conventional cosmochemical processes. Understanding
it is important if we are to understand planet formation. That
understanding will not come until we know the bulk chemical composition
of the Moon as well as we know the composition of Earth.

------------------------------------------------------------------------

ADDITIONAL RESOURCES

    * Chambers, J. E. (2004) Planetary accretion in the inner Solar
      System. Earth and Planetary Science Letters, v. 223, p. 241-252.
    * Gillis-Davis, J. J., Bradley L. Jolliff, and Randy L. Korotev
      (2004) Lunar surface geochemistry: Global concentrations of Th, K,
      and FeO as derived from lunar prospector and Clementine data.
      Geochimica et Cosmochimica Acta, v. 68, p. 3791-3805.
    * Jolliff, Bradley L., Gillis-Davis, Jeffrey J., Haskin, Larry A.,
      Korotev, Randy L., and Wieczorek, Mark A. (2000) Major lunar
      crustal terranes: Surface expressions and crust-mantle origins.
      Journal of Geophysical Research, v. 105, p. 4197-4216.
    * Martel, L.M.V. (2004) Composition of the Moon's Crust. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/Dec04/LunarCrust.html
    * Warren, Paul H. (2001) Compositional structure within the lunar
      crust as constrained by Lunar Prospector thorium data. Geophysical
      Research Letters, v. 28, p. 2565-2568.
    * Warren, Paul H. (2005) "New" Lunar Meteorites: Implications for
      composition of the global lunar surface, lunar crust, and the bulk
      Moon. Meteoritics and Planetary Science, v. 40, p. 477-506.
Received on Tue 22 Nov 2005 08:42:27 PM PST