[meteorite-list] Composition of the Moon's Crust

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun Dec 12 23:18:03 2004
Message-ID: <200412130417.UAA07882_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Dec04/LunarCrust.html

Composition of the Moon's Crust
Planetary Science Research Discoveries
December 10, 2004

--- New empirical calibrations of Lunar Prospector and Clementine data
yield improved global maps of Th, K, and FeO. The movie of the Moon,
shown to the right, consists of compositional data for Th, K, FeO, and
the Clementine 750 nm albedo image played in that order. (Courtesy of
J.J. Gillis.)

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

In 1997, PSRD first reported on the trailblazing efforts to map the
abundance and distribution of titanium and iron on the entire lunar
surface based on Clementine orbital remote sensing data [see PSRD
article: Moonbeams and Elements <http://www.psrd.hawaii.edu/Oct97/MoonFeO.html>].
Researchers calibrated
the remote sensing data with the best ground-truth standards available:
lunar soil and rock samples. Since the initial mapping, planetary
scientists have been striving to improve the calibration of the remote
sensing data to correct for over or under estimates of the global
concentrations of primary elements. This work is important because it
prevents us from getting erroneous ideas about the Moon's composition
and origin. New calibrations to Lunar Prospector and Clementine data by
Jeff Gillis (previously at Washington University in St. Louis and now at
the University of Hawai'i), Brad Jolliff, and Randy Korotev (both at
Washington University in St. Louis) have resulted in updated global maps
for thorium (Th), potassium (K), and iron oxide (FeO) that are more
consistent with the compositions of lunar samples and lunar meteorites,
and allow a better understanding of the Moon's formation and evolution.

Reference:

Gillis, J. J., Jolliff, B. L., and Korotev, R. L. (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.

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

Why refinements in the Calibrations Were Needed

Deriving the abundance of various elements on the surface of the Moon
from orbital remote sensing data is a complicated task. It involves
understanding instrument performance and resolution (how much surface
area is in one pixel of data) as well as the geology and how to relate
the energy reflected by the surface to actual elemental abundance
values. Calibrating the remote sensing data to ground truth is a
critical step: the derived elemental abundances must make sense at every
point on the surface where we are certain of the soil compositions, such
as at the Apollo landing sites on the nearside. Ground truth for the
farside highlands comes indirectly in the form of feldspar-rich lunar
meteorites. Only when we have derived accurate measures of the global
abundance and distribution of elements do we gain a realistic
understanding of the crustal and bulk Moon compositions.

The difficulty has been that the elemental concentrations derived from
Lunar Prospector gamma-ray spectrometer data and Clementine five-band
UVVIS (ultraviolet-visible) spectral reflectance data have not quite
matched the ground truth. For example, the graph below compares Th and K
values from previous calibrations of Lunar Prospector gamma-ray data to
the Apollo sample and lunar meteorite data.

K, Th data

The data points on this graph (where the 0,0 origin is marked with a
cross) are concentrations of K and Th obtained from the Lunar Prospector
gamma-ray spectrometer data by previous (as of 2002) theoretical-based
calibration techniques (by David Lawrence and Tom Prettyman, both at Los
Alamos National Laboratory, and others). The black line is a best-fit
linear regression to these data points. Small black triangles denote
ground truth data points (Apollo samples and lunar meteorites) and the
red line is a linear least squares fit to these data. The lines do not
match. The black line and most of the data points lie above the red line
indicating a systematic overestimation of both Th and K.

The talented scientists mapping the composition of the Moon's crust are
improving the quality, and their understanding, of the remote sensing
data and they are continually upgrading the calibration techniques.
After correcting the uncertainties and systematic errors they are
successfully minimizing discrepancies between the derived data and lunar
sample data.

Jeff Gillis and colleagues focused their recent recalibration work on
Th, K, and FeO because these elements are important for determining
geologic processes, as well as understanding the crustal and bulk
compositions of the Moon. For example, high-Th concentration can be used
to infer high concentrations of other incompatible trace elements and is
a useful marker for KREEP materials--late
crystallizing igneous rocks that relate to the lunar magma ocean
and theories of crustal formation.
Also, Th and K (and by correlation uranium) are naturally radioactive
elements, so a better understanding of their distribution at the surface
allows the researchers to extrapolate their concentrations to depth,
thereby allowing them to work out the thermal evolution of the Moon's
crust and mantle. Global FeO abundance (iron is expressed as an oxide
because it is chemically bonded to oxygen inside minerals) is used to
estimate Al2O3 abundance, and to determine rock types and regolith
mixing caused by impacts. Calibration methods previously used to derive
FeO concentrations from Clementine UVVIS spectral reflectance data
showed a slight but systematic underestimation of FeO values in
locations with low-titanium mare soils.

The new calibrations and techniques by Gillis and his coauthors,
summarized in the next section, have corrected the estimates that were
too high for Th and K and too low for FeO compared to the ground truth.

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

New Ground-Truth Calibrations

* Lunar Prospector gamma-ray spectrometer K and Th Data

Gillis, Jolliff, and Korotev used a mathematical regression technique
(or empirical technique) to fit a straight line to the standards (Apollo
samples and lunar meteorites) that takes into account the uncertainties
in each data point. This allowed them to relate Lunar Prospector
elemental concentrations to sample elemental concentrations (see the
graph below). Apollo soil compositions span much of the range of Th and
K observed over the Moon's surface, but Apollo 15 and 17 data were
specifically excluded because both sites were located at a mare and
highlands boundary and the soil compositions vary too much across the
site at the resolution of the Lunar Prospector pixel (a 2o area or 60
km/pixel). Feldspar-rich lunar meteorites were included in their
calibrations to serve as proxies for the low-Th highlands rocks and as
anchors at the low-concentration ends of the Th and K correlations.

recalibrated K, Th data

This graph is similar to the one above except that now the black points
are the new, ground-truth calibrated Lunar Prospector gamma-ray data
calculated by Gillis and colleagues. The gold line is the best-fit line
to these data points. As in the graph above, the red line is the best
fit line to ground truth from Apollo samples and lunar meteorites. These
two lines match more closely than in previous calibration studies
(indicated by the light gray points and their black best-fit line). A
few of the new data points have negative values. These are geochemically
impossible, but Gillis and coauthors suggest they are warranted on the
basis of remaining uncertainties in the detection and calibration of the
gamma-ray data. They suggest that all negative values should be treated
as near-zero concentrations.

The results of the new empirical calibrations by Gillis and team yield
lower concentrations of Th and K than reported previously. The global
surface mean Th concentration decreases from 2.4 to 1.6 parts per
million. The global surface mean K concentration decreases from 1480 to
700 parts per million.

* Clementine spectral reflectance FeO Data

The method for deriving FeO content on the lunar surface from the
intensity of light reflected at two wavelengths (Clementine UVVIS
spectral reflectance data) was invented by Paul Lucey (University of
Hawai'i) and incorporated the contemporary understanding of space
weathering. In this technique, the spectral parameter angle theta
(theta) increases as FeO increases (see PSRD article: Moonbeams and
Elements for diagrams
and details). Gillis and coauthors observed the FeO concentrations
calculated for the Apollo 15 site using the Lucey method were too low
when compared to the samples, which are low-titanium and high-FeO mare
soils. Fortunately, Clementine had better resolution (~125 meters/pixel)
than Lunar Prospector, so the Apollo 15 data were included. The other
high-FeO site, Apollo 17, has high-titanium, high-FeO mare soils. Gillis
and colleagues concluded that the mineral ilmenite affects the spectral
reflectance at the two wavelengths used in the Lucey method to calculate
FeO. As a result, they made a small adjustment in the FeO method to
account for the effects of TiO2 in the mineral ilmenite on the UVVIS
spectra of basalts. They called it the TiO2-sensitive FeO derivation,
which changes the slope and offset values in the FeO algorithm of
Lucey's original method. The recalibrated FeO concentration matches the
low-titanium and high-iron mare basalts better than any previous
derivation. The new calibrations also produce FeO concentrations for the
northern farside highlands that are consistent with the feldspar-rich
lunar meteorite compositions. The surface mean FeO concentration
reported by Gillis and coauthors for the maria is 16 wt%, and 5.7 wt%
for non-mare areas.

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

New Global Maps of Th, K, and FeO

Gillis, Jolliff, and Korotev produced empirical ground-truth
calibrations of the Lunar Prospector gamma-ray spectrometer data for Th
and K abundances and a TiO2-sensitive, modified algorithm of the
Clementine UVVIS spectral reflectance data for FeO abundance. Their
recalibrated global maps are shown below.

Recalibrated lunar Th abundance maps

Recalibrated lunar K abundance maps

Recalibrated lunar FeO abundance maps

Diamonds on the three Nearside maps show the locations of the six Apollo
and three Luna landing sites. The pink
outline on the Farside FeO map defines the region of the northern
feldspathic highlands (including the Feldspathic Highlands Terrane) from
which Gillis and colleagues took data for comparison to feldspar-rich
lunar meteorites. The white, circular outline on the Farside FeO map
shows the South Pole-Aitken Terrane. The white outline on the Nearside
FeO map indicates the Procellarum KREEP Terrane.

These maps agree with the compositional data of soil samples in the
lunar collection (i.e., KREEP-rich materials; Fe-poor and Th-poor and
feldspar-rich materials; Fe-rich mare volcanics). But the new
calibrations for Th, K, and FeO do show some differences from previous
work in the three crustal terranes, defined previously by Jolliff and
colleagues: the Procellarum KREEP Terrane, Feldspathic Highlands
Terrane, and the South Pole-Aitken Terrane (see outlines on the FeO maps
above). In particular, the Procellarum KREEP Terrane, which is the
Th-rich nearside highlands, shows higher Th values in the new map than
in previous maps--Jolliff used an earlier calibration by David Lawrence
than the one discussed herein. Conversely, the new calibration map shows
lower Th values for the Feldspathic Highlands Terrane than previous
studies. The biggest differences between the revised FeO data and
previous calibrations occur in the SPA Terrane and the Procellarum KREEP
Terrane, where the TiO2-sensitive algorithm reports higher FeO values.
Gillis suggests these differences in FeO reflect an abundance of low-Ti,
high-FeO mare materials that previously were reported as lower FeO due
to their low TiO2 contents.

Gillis and colleagues' ground-truth-calibrated abundances of Th, K, and
FeO provide researchers a better look at the Moon's crust, which bears
on all studies of the Moon's origin, crustal formation, and bulk
composition. Future missions will enhance our knowledge of the chemical
composition of the lunar surface. For instance, the SMART-1
(European Space Agency) and SELENE
(ISAS, NASDA - Japan) missions carry instruments to directly measure
magnesium and aluminum, two other important elements in the
cosmochemist's tool box.

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

ADDITIONAL RESOURCES

Gillis, J. J., Jolliff, B. L., and Korotev, R. L. (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, B.L., Gillis J.J., Haskin L.A., Korotev R.L., and Wieczorek
M.A. (2000) Major lunar crustal terranes: Surface expressions and
crust-mantle origins. Journal of Geophys. Res., v. 105, p. 4197-4216.

Korotev, R.L., Jolliff B.L., Zeigler R.A., Gillis, J.J., and Haskin,
L.A. (2003) Feldspathic lunar meteorites and their implications for
compositional remote sensing of the lunar surface and the composition of
the lunar crust. Geochimica et Cosmochimica Acta, v. 67, p. 4895-4923.

Lawrence D.J., Feldman W.C., Barraclough B.L., Binder A.B., Elphic R.C.,
Maurice S., Miller M.C., and Prettyman T.H. (2000) Thorium abundances on
the lunar surface. Journal of Geophys. Res., v. 105, p. 20307-20331.

Prettyman, T.H., Feldman, W.C., Lawrence, D.J., McKinney, G.W., Binder,
A.G., Elphic, R.C., Gasnault O.M., Maurice S., and Moore, K.R. (2002)
Library least squares analysis of Lunar Prospector gamma ray spectra.
Lunar Planet. Sci. 33,, abstract 2012.

SELENE <http://www.isas.ac.jp/e/enterp/missions/selene/index.shtml> The
ISAS, NASDA - Japan mission scheduled for launch in 2005.

SMART-1 <http://www.esa.int/SPECIALS/SMART-1/> The European Space
Agency's current mission to the Moon. It achieved lunar orbit on
November 15, 2004. Science observations will begin in January, 2005 to
study surface mineralogy and water ice.

Taylor, G.J. (1997) Moonbeams and Elements. Planetary Science Research
Discoveries. http://www.psrd.hawaii.edu/Oct97/MoonFeO.html

Taylor, G.J. (2000) A New Moon for the Twenty-First Century. Planetary
Science Research Discoveries. http://www.psrd.hawaii.edu/Aug00/newMoon.html
Received on Sun 12 Dec 2004 11:17:52 PM PST