Major-element abundances on the surface of Mercury:
Results from the MESSENGER Gamma-Ray Spectrometer
Larry G. Evans,1 Patrick N. Peplowski,2 Edgar A. Rhodes,2 David J. Lawrence,2
Timothy J. McCoy,3 Larry R. Nittler,4 Sean C. Solomon,4,5 Ann L. Sprague,6
Karen R. Stockstill-Cahill,3 Richard D. Starr,7 Shoshana Z. Weider,4 William V. Boynton,6
David K. Hamara,6 and John O. Goldsten2
Received 2 July 2012; revised 7 September 2012; accepted 9 September 2012; published 2 November 2012.
[1] Orbital gamma-ray measurements obtained by the MESSENGER spacecraft have been
analyzed to determine the abundances of the major elements Al, Ca, S, Fe, and Na on
the surface of Mercury. The Si abundance was determined and used to normalize those
of the other reported elements. The Na analysis provides the first abundance estimate of
2.9 
 0.1 wt% for this element on Mercury?s surface. The other elemental results
(S/Si = 0.092
 0.015, Ca/Si = 0.24
 0.05, and Fe/Si = 0.077
 0.013) are consistent with
those previously obtained by the MESSENGER X-Ray Spectrometer, including the high
sulfur and low iron abundances. Because of different sampling depths for the two
techniques, this agreement indicates that Mercury?s regolith is, on average, homogenous to
a depth of tens of centimeters. The elemental results from gamma-ray and X-ray
spectrometry are most consistent with petrologic models suggesting that Mercury?s surface
is dominated by Mg-rich silicates. We also compare the results with those obtained during
the MESSENGER flybys and with ground-based observations of Mercury?s surface and
exosphere.
Citation: Evans, L. G., et al. (2012), Major-element abundances on the surface of Mercury: Results from the MESSENGER
Gamma-Ray Spectrometer, J. Geophys. Res., 117, E00L07, doi:10.1029/2012JE004178.
1. Introduction
[2] The MErcury Surface, Space ENvironment, GEo-
chemistry, and Ranging (MESSENGER) spacecraft was
launched on 3 August 2004, made three flybys of Mercury in
2008?2009, and achieved orbit around Mercury on 18 March
2011 to begin a primary orbital mission of 1 (Earth) year. One
of the measurement objectives of the mission is to charac-
terize the surface composition of the planet, and to that end
the spacecraft carries a suite of geochemical remote-sensing
instruments [Solomon et al., 2007] that includes a Gamma-
Ray and Neutron Spectrometer (GRNS). The Gamma-Ray
Spectrometer (GRS) subsystem is designed to measure
gamma rays in the energy range 60 keV to 9 MeV [Goldsten
et al., 2007]. Unlike other geochemical remote-sensing
measurements that are sensitive only to the uppermost sur-
face layer (<100 mm), orbital gamma-ray spectroscopy can
measure elemental abundances in the top tens of centimeters
beneath a planet?s surface. The gamma rays are emitted from
the decay of long-lived radioactive species and from the
nuclear interactions of galactic cosmic ray (GCR) particles
with the planetary materials. The interactions of secondary
neutrons with atomic nuclei produce a majority of the gamma
rays. The number of neutrons made by GCR particles,
averaged over the solar cycle, is about 9 neutrons per incident
GCR particle [Evans et al., 1993]. The primary modes of
gamma-ray production are inelastic-scatter reactions with fast
(>1 MeV) neutrons, neutron capture reactions with thermal
(0.02 eV) neutrons, and activation by both proton and
neutron interactions.
[3] Neutrons with sufficient energy can excite a nucleus
to a higher energy state and exit with reduced energy, a
process known as inelastic scattering. The excited nucleus
decays with a characteristic half-life (typically a very small
fraction of a second, e.g., 1012 s) to a lower state, usually
through the emission of a gamma ray. Inelastic scattering
emission is abbreviated as AX(n,n?g)AX, where n and n? are
the initial and final energies of the neutron, A is the atomic
mass, and X the element. Another type of scatter reaction
1Computer Sciences Corporation, Lanham-Seabrook, Maryland, USA.
2The Johns Hopkins University Applied Physics Laboratory, Laurel,
Maryland, USA.
3Department of Mineral Sciences, National Museum of Natural History,
Smithsonian Institution, Washington, D. C., USA.
4Department of TerrestrialMagnetism, Carnegie Institution ofWashington,
Washington, D. C., USA.
5Lamont-Doherty Earth Observatory, Columbia University, Palisades,
New York, USA.
6Lunar and Planetary Laboratory, University of Arizona, Tucson,
Arizona, USA.
7Department of Physics, Catholic University of America, Washington,
D. C., USA.
Corresponding author: L. G. Evans, Computer Sciences Corporation,
7900 Harkins Rd., Lanham-Seabrook, MD 20706, USA.
(larry.g.evans@nasa.gov)
?2012. American Geophysical Union. All Rights Reserved.
0148-0227/12/2012JE004178
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00L07, doi:10.1029/2012JE004178, 2012
E00L07 1 of 14
that includes the emission of an alpha particle (a) is denoted
as AXZ(n,n?ag)
A4XZ2, where Z is the atomic number,
e.g., 16O8(n,n?ag)
12C6. The bulk composition of the planet?s
surface does not strongly affect the relative production rates
for inelastic scatter reactions, but does strongly affect the
moderation of neutrons from MeV to thermal (eV) energies.
Neutrons with thermal energies can be captured by a nucleus,
which puts the new isotope into an excited state that can
rapidly decay to a lower level with the emission of a gamma
ray. This reaction is abbreviated as AX(n,g)A+1X, and the
new isotope can also be radioactive and decay with a char-
acteristic half-life that is typically much longer than the first
decay. This process is termed neutron activation. The gamma
rays emitted by these interactions have characteristic energies
that can be used to identify the nuclides involved, and their
fluxes can be related to the concentrations. Gamma-ray
emission also occurs as a result of nuclear interactions
with the gamma-ray detector and materials surrounding the
detector, and from material in the spacecraft [Peplowski et al.,
2012b]. The elements that produce these background emis-
sions can include the same elements as those on the surface
of Mercury. An understanding of the sources and intensities
of background gamma rays is needed to subtract their
contribution to the measurements correctly and to determine
accurately the surface composition of Mercury. Here we
discuss the GRS instrument, measurements, analysis, and
result for the abundances of the major elements Si, Al, Ca,
S, Fe, and Na.
2. MESSENGER Gamma-Ray Spectrometer
[4] TheGRS sensor has been described in detail byGoldsten
et al. [2007], and its characteristics will only be summarized
here. It consists of a high-purity n-type germanium crystal
that is actively cooled to cryo-temperatures (<90 K) by a
miniature Stirling-cycle cooler. The crystal is contained in an
aluminum cryostat that is suspended by Kevlar strings. It
measures gamma-ray photons in the energy range 60 keV to
9 MeV. The Ge crystal is a closed-end coaxial detector
approximately 5 cm in diameter and 5 cm in length. It has an
intrinsic efficiency of 9% at 1332 keV. The gamma-ray
measurements are digitized by a 14-bit analog-to-digital
converter into 16,384 pulse-height channels. The mechanical
cryocooler is a Ricor K508 rotary Stirling-cycle unit with a
mass of 450 g, including motor drive electronics. In order to
provide a lifetime margin for the planned one Earth-year orbit
of Mercury, a He fill pressure was chosen to yield a nominal
mean time to failure of 12,000 h.
[5] The energy resolution of a GRS is one of the most
important parameters for scientific investigations. This quan-
tity is usually expressed as the full width at half-maximum
(FWHM) of the 1332 keV peak from the decay of 60Co.
The final preflight measurements of the energy resolution
of the detector system gave a FWHM of 3.5 keV. It was
expected that the energy resolution would degrade over time
due to interactions of high-energy GCRs and solar charged
particles with the detector crystal. A FWHM measurement at
a similar energy (1368 keV) during the cruise phase of the
mission, about three months after launch, gave a value of
3.7 keV [Goldsten et al., 2007]. The detector was annealed
periodically during the mission to help ameliorate the effects
of radiation damage. This procedure removed some, but
not all, of the radiation damage accumulated during the
6.6 year interplanetary cruise to Mercury. The resolution at
1368 keV during the orbital measurements reported here
was FWHM = 4.8 keV.
[6] The GRS is surrounded by an anticoincidence shield
(ACS) of borated plastic scintillator (BC454) coupled to a
7.6 cm photomultiplier tube. The shield allows events that are
detected in both the shield and the Ge crystal to be removed
(vetoed), thus reducing the continuum background from
GCRs, and also shields the detector from thermal neutrons
emitted from the planet. Since both the raw gamma-ray
spectrum and the vetoed spectrum are measured simulta-
neously, the decrease in the continuum can be measured
directly: a factor of three at 3 MeV and a factor of seven at
8 MeV [Goldsten et al., 2007; Peplowski et al., 2011b].
3. Summing of Spectra
[7] The sensitivity of orbiting gamma-ray experiments is
limited by the photon flux from the surface of the planet.
Many hours of accumulated spectra are required in order
to obtain spectra with counting statistics sufficient for peak
analysis. Typical count rates on Mars Odyssey [Boynton
et al., 2007] were about 190 c/s, but less than 4% of
these counts were measured in discrete line peaks, and the
rest were in the continuum [Evans et al., 2006]. Less than 40
of the more than 300 discrete line peaks measured were
gamma rays originating from Mars that could be used to
determine surface elemental abundances. Mars Odyssey
was in a 400 km circular orbit, so all the measurements could
be summed into a single planetary spectrum. In contrast,
MESSENGER is in a highly eccentric orbit, and only those
GRS measurements taken near periapsis in the northern
hemisphere are useful for summing to obtain the planetary
signal [Peplowski et al., 2011a]. For our analysis we used
planetary spectra acquired at altitudes <2000 km, corre-
sponding to a total accumulation time per 12 h orbit of about
45?50 min. A GRS data processing system developed at the
University of Arizona [Boynton et al., 2007] was used to
produce spectral sums of GRS data.
[8] Prior to spectral summing, corrections were applied to
each individual 60 s spectrum. The electronic gain and offset
for each spectrum were adjusted to a common energy scale to
allow the summed spectra to have the best possible energy
resolution. The first-order gain correction was related to
temperature. Calibration spectra taken over a wide range of
temperatures before launch were used to determine the gain
correction as a function of temperature (from 30 to 64C),
which was fit to a fifth-order polynomial whose coefficient
values were incorporated into the GRS data processing
scheme [Peplowski et al., 2012a]. The pulse-processing
electronics design eliminated most of the electronic offset.
All temperature-corrected spectra were adjusted to a common
linear scale of 0.6 keV/channel and a zero offset. Some
residual nonlinearity remained in the data but did not inter-
fere with peak identifications.
[9] Individual gamma-ray spectra can be corrupted for a
number of reasons, including changes in the detector high
voltage, large temperature changes, and solar particle events
(SPEs). As the Sun became more active in late 2011 and early
2012, the latter events became more numerous. Bad data
flags were assigned to individual spectra, and the default for
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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summing spectra was set to exclude these spectra. During
the first three Mercury sidereal days (177 Earth days) of
MESSENGER?s orbital mission, the data flagged as bad
consisted of about 10% of the total. The query tool used to
obtain summed spectra could be set to include bad data dur-
ing the selection process, e.g., to study spectra accumulated
during SPEs.
[10] The GRS data processing system provides a means of
accumulating summed spectra with a variety of different
criteria. Accessible spectra include raw data for both the Ge
detector and the ACS, as well as anticoincidence gamma-
ray data. Data can be either uncalibrated or energy calibrated.
The software allows summing of calibrated spectra for
selected time intervals, latitude?longitude grids, and altitude
ranges. For most of the analyses reported here, standard
spectra were accumulated for the period 29 March 2011
through 24 September 2011. A large SPE occurred on 4 June
2011 and caused some long-lived activation of spacecraft and
local materials that interfered with the analysis of the Fe
847 keV gamma-ray peak. The analysis for this particular
peak was therefore limited to spectra collected over the first
Mercury sidereal day of MESSENGER?s primary mission,
from 29 March 2011 through 27 May 2011. The use of an
integral number of sidereal days of data acquisition ensures
uniform coverage of the surface for all longitudes.
[11] Due to MESSENGER?s highly eccentric orbit, the
choice of a summed spectrum to represent the planetary
signal and one to represent the background is not straight-
forward. We take the background spectrum as an accumu-
lation for altitudes greater than 14,000 km, where the solid
angle subtended by the planet from the spacecraft is less than
0.06 sr. As in previous work [Peplowski et al., 2011b],
data for altitudes less than 2000 km were summed to produce
a low-altitude spectrum containing the planetary signal.
The periapsis of the orbit varied from 200 km to 490 km
[McAdams et al., 2007]. The average altitude for the low-
altitude spectrum was 980 km, and the solid angle at that
altitude was 1.88 sr. The choice of low-altitude cutoff
represents a compromise; at lower altitudes the planetary
signal-to-background ratio is higher, but there is a shorter
accumulation time and higher statistical uncertainty. Analysis
of summed spectra for low-altitude thresholds of 1000 to
2500 km showed that for most peaks of interest, the cutoff
of 2000 km was the best value. For example, Figure 1
shows the variation of the uncertainty in peak count rates
for the Ca(n,g) 1942 keV peak as a function of the low-
altitude threshold. There were some exceptions; weak Fe
neutron capture peaks, in particular, could not be detected
above background with a 2000 km threshold but could be
analyzed with a lower-altitude cutoff.
[12] The accumulation time for the standard low-altitude
spectrum was 9.91 105 s or 11.5 days and was 4.27 106 s
or 49.4 days for the high-altitude spectrum. From the two
time-normalized spectra shown in Figure 2, it can be seen
that most of the peaks in the low-altitude spectrum have
corresponding peaks in the high-altitude spectrum. As dis-
cussed by Peplowski et al. [2011b], the background count
rate for natural radioactive elements present in spacecraft
materials is constant with altitude and can be simply sub-
tracted from the low-altitude count rate. The background for
nonradioactive elements is due to interactions in the space-
craft and local materials both from GCRs and from neutrons
originating in the planet. The GCR-induced background
decreases with decreasing altitude as the planet blocks more
of the GCR flux, whereas the planetary neutron back-
ground increases with decreasing altitude. This dependence
on altitude was used to separate the background contributions
for aluminum [Peplowski et al., 2012b], but the procedure
did not work for other elements. The GRS sensor housing
and structure are constructed mainly of Mg with some Al
and Fe, which limits the ability of the GRS to measure
these elements from Mercury. Determination of abun-
dances was possible for Al [Peplowski et al., 2012b] and
Fe (see below), but not Mg, for which the primary gamma
Figure 1. The variation of the uncertainty to peak count rates as a function of the low-altitude threshold
for one example peak: Ca(n,g) 1942 keV peak.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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rays are subject to interfering peaks from numerous
detector materials.
4. Peaks and Other Features
[13] The vast majority of counts in a GRS spectrum form a
fairly smoothly varying continuum (Figure 2). This contin-
uum has a number of sources, including gamma rays that
are scattered in the planetary regolith and in the Ge detector,
and bremsstrahlung from charged particles. The charged
particles that interact in the shield are vetoed from the
anticoincidence spectrum. The continuum under a peak or a
group of peaks is usually determined by fitting a slowly
varying low-order polynomial to portions of the spectra
around the peaks that contain no noticeable peaks. Super-
imposed on the continuum are a large number of discrete
gamma-ray peaks (Figure 2) most of which have a Gaussian
shape with a low-energy tail due to trapping effects and
charge-collection losses in the detector. The extent of this
tailing increases with time in space because of radiation
damage [Br?ckner et al., 1991]. The tail is fit with an
exponential that joins smoothly to the Gaussian shape.
The width of these Gaussian-shaped peaks increases slowly
and systematically with increasing energy. Calibration of the
energy dependence of spectral resolution can be used to help
fit weak peaks [Evans et al., 2006]. Gamma rays with
energies greater than 1022 keV can interact in the detector
through a process known as pair production that results in
the production of two 511 keV photons. One or both of these
photons can escape the detector, producing gamma-ray
peaks at the full-energy peak minus 511 keV (termed the
single-escape peak) and at the full-energy peak minus
2  511 keV (termed the double-escape peak).
[14] Some peaks are further broadened because the gamma
ray is emitted while the excited nucleus is recoiling from an
inelastic scatter interaction, a process known as Doppler
broadening. In solid materials, gamma rays that are emitted
from excited levels with lifetimes less than 0.5 ps are
Doppler broadened. Typically, Doppler-broadened peaks are
symmetric in shape, as the broadening is much larger than the
width of the Gaussian peak plus tailing. Example Doppler-
broadened peaks used in this analysis are the 2211 keV
aluminum and 2230 keV sulfur peaks described below.
[15] Interactions of fast (>1 MeV) neutrons with the Ge
nuclei in the detector produce irregularly shaped peaks when
the energy from the de-excitation of an excited level is
summed with some of the recoil energy of the Ge nucleus.
This summation results in a peak that has an approximately
Gaussian shape below the excitation energy and a slowly
decreasing tail at higher energies. These are called sawtooth
peaks because of their shape. One particular sawtooth peak
at 834 keV is important in the analysis of the iron inelastic
peak at 847 keV.
[16] To extract useful information from summed gamma-
ray spectra, the integrated areas beneath important peaks
must be determined. Analysis software originally developed
for the Mars Odyssey mission [Boynton et al., 2007] was
used to determine peaks areas from the GRS spectra. The
software allows fitting of up to 10 peaks simultaneously in a
portion of the spectrum. The ability of this software to fit
regular (Gaussian shaped), Doppler broadened, and sawtooth
peaks simultaneously [Evans et al., 2006] is particularly
useful. Examples of these fits are discussed below for
particular gamma rays used in determining the abundances
of major elements on Mercury?s surface. The integrated peak
areas are corrected for data acquisition time and detector live-
Figure 2. Comparison of the standard time-normalized low- and high-altitude GRS spectra accumulated
from orbit of Mercury acquired between 29 March 2011 and 24 September 2011.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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time to determine the count rate at the detector. The live-time
was determined using a pulser signal that inserted into the
spectrum [Goldsten et al., 2007]. The correction values for
the standard low-altitude and high-altitude spectra are,
respectively, 0.88 and 0.90.
5. Forward Calculations
[17] Due to the complexity of the process that produces
planetary gamma rays from stable elements, it is not possible
to relate directly the measured peak intensities to elemental
abundances on the surface of Mercury. Instead, for gamma
rays produced by neutron interactions, peak intensities
must be compared with those predicted by a forward model
calculation. The forward calculation depends on the flux and
energy spectrum of incident cosmic rays, the assumed
composition of the regolith, and the properties of the gamma-
ray detector. The production of neutrons was modeled with
the Monte Carlo N-Particle eXtended code (MCNPX) for an
influx of GCRs [Waters, 2002; McKinney et al., 2006].
The composition of the regolith used for the calculation is
shown in Table 1 and was selected on the basis of results from
laboratory modeling, MESSENGER X-Ray Spectrometer
(XRS) measurements of major elements on Mercury [Nittler
et al., 2011], GRS measurements of radioactive element
abundances [Peplowski et al., 2011b], and Neutron Spec-
trometer (NS) measurements [Lawrence et al., 2010]. In
formulating a model composition two factors are most
important for the forward calculations: the average atomic
mass of the regolith and the macroscopic thermal neutron
absorption cross section. The former has the greatest effect on
the fast neutron distribution [Gasnault et al., 2001], and
the latter has the greatest effect on the thermal neutron flux
[Feldman et al., 2000]. The elements that have the largest
effect on the neutron spectral and spatial distributions are
hydrogen, elements of high atomic mass, and elements with
large thermal neutron cross sections, such as Fe and Ti.
The model was also constrained by the NS measurements of
the thermal neutron flux and XRSmeasurements of Fe and Ti
[Lawrence et al., 2010; Nittler et al., 2011]. The model
composition used for the forward calculations (Table 1) has
an average atomic mass of 24.4 and a macroscopic thermal
neutron cross section of 53  104 cm2/g. This value is
within the range determined from the NS flyby measurements,
though near the lower end [Lawrence et al., 2010]. There are
some elements in the model included at very low abundances
that do not affect the neutron calculations but may be relevant
to possible future models. The output of MCNPX is the
predicted flux of neutrons as a function of energy, depth, and
emission angle. From this output and the cross sections for
gamma-ray production, the flux of discrete gamma rays at
the surface was calculated as a function of emission angle.
For inelastic scatter reactions the production cross sections as
a function of energy are required. For gamma rays produced
by neutron capture, the cross sections for thermal neutron
capture for all elements are assumed to vary inversely with
velocity of the neutron. Only changes in composition that
would change substantially either the atomic mass or the
macroscopic thermal neutron cross section would change the
neutron distributions and require new forward calculations.
[18] For each individual measurement in a spectral sum,
a vector was determined between the detector and each loca-
tion on the surface of Mercury that is visible to the detector;
that vector defines the emission angle for that location.
The predicted flux at the spacecraft is the sum of all fluxes
from the surface adjusted on the basis of orbital geometry, i.e.,
spacecraft altitude and attitude. Finally, the calculated
gamma-ray flux at the spacecraft was converted to counts in
the detector on the basis of prelaunch measurements and
calculations of the attenuation through parts of the spacecraft
and local material and the detector efficiency as a function of
incident angle and energy [Kim et al., 2006; Boynton et al.,
2007; Peplowski et al., 2012a]. The process of making these
calculations involves many factors, as the detector views a
large area on the surface of the planet that varies with altitude
and as different viewing angles from the detector have dif-
ferent efficiencies. From the forward model the expected
counts in the detector were calculated for each spectrum
(e.g., accumulation times of 60 s near the planet) for every
location on the planetary surface on a grid of cells of
dimension 0.5 latitude by 0.5 longitude. The location of the
spacecraft over the surface at the midpoint of each accumu-
lation time interval was calculated, and the vectors from each
cell on the surface to the spacecraft were ascertained to
determine the appropriate angle for surface gamma-ray flux.
[19] At the end of this process, each accumulated gamma-
ray spectrum had associated with it a model calculation of
the expected counts from each of approximately 100 gamma-
ray lines of interest, not all of which are necessarily mea-
sured in the spectra. The abundance of an element for that
spectrum was then determined by adjusting the model
Table 1. Model Elemental Abundances Used for the Forward
Calculations and Expected Count Rates From the Forward
Calculationsa
Element Concentration C/min
H 50 ppm
C 50 ppm
O 42.30%
Na 0.80% 0.439
Mg 12.50%
Al 5.65%
Si 24.60% I: 3.40
Si 24.60% C: 0.232
P 30 ppm
S 4.00% I: 0.331
S 4.00% C: 0.128
Cl 0.15%
Ar 10 ppm
K 0.12%
Ca 4.80% I: 0.0380
Ca 4.80% C: 0.108
Ti 0.70%
Cr 0.20%
Mn 0.35%
Fe 3.70% I: 0.687
Fe 3.70% C: 0.210
Co 500 ppm
Ni 500 ppm
Zn 50 ppm
Sm 10.3 ppb
Eu 3.9 ppb
Gd 14.1 ppb
Hg 1 ppm
Th 2.2 ppb
U 0.9 ppb
aC/min denotes counts per minute; I denotes inelastic scatter gamma rays,
and C denotes capture gamma rays.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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abundance by the ratio of the observed counts (minus any
background) to the modeled counts for a particular gamma-
ray peak. For elements with multiple gamma-ray lines, an
average was calculated (weighted by the inverse square of
the uncertainties for the individual results).
[20] The absolute GCR flux varies with time and is not
well characterized, so in GRS analyses the results are often
normalized to some known elemental abundance [Boynton
et al., 2007]. For Mars Odyssey, the results were normal-
ized to Si determined by the Mars Pathfinder silicon surface
measurement of 20.95 wt% Si [Boynton et al., 2007]. There
is no comparable ground truth value for Mercury, but Si is a
major element that generally varies less than other major
elements [Peplowski et al., 2012a]. For this reason, ele-
mental abundances determined for Mercury have been nor-
malized to Si. First, the abundance of each element was
determined using the forward calculation. Then, the element
abundance was ratioed to the Si abundance. Silicon has both
inelastic scatter and neutron capture gamma-ray lines that
can be analyzed. Other elements, detected by either inelastic
scatter or capture mode, can be normalized to the result
derived from the corresponding Si mode.
6. Background Gamma Rays
[21] As mentioned above, gamma rays are emitted from
the spacecraft, material surrounding the detector, and the
detector material itself, as well as from the planet. These
background sources, except for natural radioactivity, will
vary during MESSENGER?s orbit around Mercury. Gamma-
ray emission due to activation of spacecraft materials by
GCRs varies as one minus the solid angle of Mercury as
viewed by the spacecraft. Activation by fast neutrons from
Mercury will vary as the solid angle of Mercury. Prompt
interactions that produce gamma rays, by fast or thermal
neutrons, will also vary as the solid angle of Mercury,
though there is a different dependence as the thermal neu-
trons are gravitationally bound to Mercury. For an analytical
description of the altitude dependence of each source of
gamma rays, see Peplowski et al. [2012b]. Since many of
these background gamma rays are at the same energies as
gamma rays from the surface of Mercury, they cannot be
characterized by simple peak analysis but must be deter-
mined by independent methods. An element that produces
gamma rays from both inelastic scatter and neutron capture
reactions and for which the signal is dominated by the
spacecraft and local materials can be used to estimate the
increase in the background for the low-altitude measurements.
Orbital XRS measurements indicate that the Ti abundance
on the surface of Mercury is small [Nittler et al., 2011].
The dominant source of measured Ti gamma rays is thus
GCR- and planetary-neutron-induced background emission.
Therefore, the measured Ti gamma-ray count rates can be
used to determine the amplification of the background signals,
as a function of altitude, for both inelastic and capture gamma
rays. The background amplification factor (ratio of a back-
ground signal in the low-altitude sum to that in the high-
altitude sum) for inelastic low-energy scatter peaks using
the Ti 983 keV peak is equal to 1.49 
 0.10 [Peplowski
et al., 2012a] and from the Ti 1381 keV capture peak,
the value is 3.7. The 983 keV peak was analyzed only
for the first sidereal day. The first major SPE, on 4 June
2011, produced a long-lived activation product, 48V, which
decays with an emission of the same 983 keV gamma ray.
7. Results
7.1. Spatial Coverage
[22] The standard accumulated spectrum covers three full
Mercury sidereal days (177 Earth days) and represents
unbiased coverage of the surface, except for some minor
data gaps due to bad flags. The highly eccentric orbit of
MESSENGER has a periapsis in the northern hemisphere.
The measured gamma-ray spectrum represents the average
value over the sampled regions north of 20S. Any
measurements acquired for regions south of 20S were
collected at altitudes greater than 2000 km and were not
included in the accumulated low-altitude spectrum. Using
measurements from an integral number of sidereal days
insures that the measurements are approximately evenly
distributed in longitude around Mercury.
7.2. Elemental Abundances
7.2.1. Silicon
[23] Silicon produces strong gamma rays from both
inelastic scatter (1779 keV) and neutron capture (3539 keV)
reactions, though normally the capture peaks are much less
intense than the inelastic scatter peaks. The relation between
the fluxes from inelastic scatter gamma rays and neutron
capture gamma rays from the same element depends on the
hydrogen content of the materials. With little or no hydrogen
expected on Mercury (or in the adopted model), the abun-
dances determined for Si from both mechanisms should give
the same result [Evans and Squyres, 1987]. The Si(n,n?g)
1779 keV peak is in a region with a number of other gamma-
ray lines: the fit to this region is shown in Figure 3. The
values of the Si abundance, derived from both the inelastic
scatter peak and a capture peak should be the same. Applying
the amplification factor from the Ti 983 keV inelastic gamma
rays, 1.49, to the Si inelastic peak and normalizing the Si
capture peak result gives an amplification factor for the
capture peak equal to 3.5, close to the 3.7 value obtained from
the Ti capture peak. The peak analysis results for Si and the
other elements discussed below are summarized in Table 2.
The uncertainties given in Table 2 for Si and all other peaks
are 1 standard deviation (s) and reflect a combination of
statistical uncertainties derived from the peak areas and the
background continuum and the goodness of the fit to the
measurements.
7.2.2. Sulfur
[24] A surprising early result from the MESSENGER
XRS orbital measurements was the discovery of abundant
sulfur on the surface of Mercury [Nittler et al., 2011]. Sulfur
is another element that has measurable gamma rays from
both inelastic scatter (2230 keV) and neutron capture
(5420 keV plus the first escape peak). The 2230 keV S peak
is Doppler broadened and is part of a region that also con-
tains an Al Doppler-broadened peak (2211 keV), a uranium
peak (2204 keV), the hydrogen peak (2223 keV), and the
single-escape peak from 24Na (2243 keV). Figure 4 shows
the fit to these peaks for both the low- and high-altitude
spectra. The presence of a 24Na peak in the spectrum cannot
be used directly to measure Na on the surface, since this
activation product can be produced by multiple reactions of
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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many common elements (e.g., Si, Al, Mg). The H peak
analyzed in this region is dominated by neutron interactions
in the spacecraft fuel, which obscures any potential hydro-
gen signal from the planet. As can be seen, this peak is
detectable in the high-altitude spectrum and was detected
during interplanetary transfer [Goldsten et al., 2007]. The
background amplification values derived from Ti for both
the inelastic-scatter peak and the capture peak allow the
determination of the S/Si ratio. The inelastic-scatter gamma-
ray peak gives a S/Si ratios of 0.086 
 0.017 and the capture
gamma-ray line gives a S/Si ratio of 0.11 
 0.03. The two
derived S/Si values are in excellent agreement, within 1 s
of each other. The weighted average for these two results is:
S/Si = 0.090 
 0.015. The abundance results for S and other
elements, relative to Si, are summarized in Table 3, and the
weighted averages are summarized in Table 4.
7.2.3. Calcium
[25] Calcium has measureable gamma-ray lines from both
inelastic scatter (3736 keV) and neutron capture (1942 keV).
For Ca, the capture peak has a higher count rate than the
inelastic scatter peak. Again, using the background amplifi-
cation factors from Ti allows the determination of the Ca/Si
Figure 3. Fit of the Si(n,n?g) 1779 keV peak in (a) the low-altitude spectrum and (b) the high-altitude
spectrum for the standard spectra accumulated between 29 March 2011 and 24 September 2011 (DOY
denotes day of the year). For this and subsequent figures the red curve is the measured spectrum,
the various colored curves are the fits to each peak, and the thicker blue curve is the sum of the individual
peaks and the fit to the measurements. The vertical black dashed lines denote the limits of the fitting region.
The continuum background is fit over a larger region of the spectrum than is shown. The black curve,
labeled Residual, is the difference between the fit and the measurements in units of standard deviations,
as derived from count statistics. The identifiable peaks in the spectra are 1809 keV from activated
26Mg, 1785 keV from Cr(n,g), and 1764 keV from the decay of 238U (214Bi).
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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Table 2. Peak Analysis Resultsa
Element Energy (keV)
Low-Altitude High-Altitude
Counts C/min Uncertainty Counts C/min Uncertainty
Si 1779 33898 2.200 0.99% 10594 0.1966 3.71%
Si 3539 2112.9 0.137 7.02% 195.2 0.0036 84.2%
S 2230 2460.5 0.160 10.9% 2192.9 0.0407 16.0%
S 5420 and 4909 773.4 0.050 19.1% 45.1 0.0008 360%
Ca 3736 628.8 0.041 24.5% 373.9 0.0069 47.6%
Ca 1942 2053.9 0.133 10.9% 984.8 0.0183 30.6%
Fe 847 4664.5 0.739 5.55% 9789.1 0.429 3.85%
Fe (7632 and 7646) escape peaks 1037.7 0.0673 21.1% 300.8 0.0056 220%
Na 440 56840 3.689 1.05% 151586 2.813 0.54%
aC/min denotes counts per minute. For Fe 847, the analysis spanned days 88 to 147 in 2011. For Fe 7632 and 7646, the analysis was for the sum of four
escape peaks and for altitudes <1500 km.
Figure 4. Fit of the S(n,n?g) 2230 keV Doppler-broadened peak in (a) the low-altitude spectrum and
(b) the high-altitude spectrum for the standard spectra accumulated between 29March 2011 and 24 September
2011. The peaks in the spectra are Al(n,n?g) at 2211 keV, H(n,g) at 2223 keV, a peak from the decay of
238U (214Bi) at 2204 keV, and the single escape peak of 24Na at 2243 keV.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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ratio as 0.28 
 0.10 and 0.22 
 0.06 for the inelastic scatter
gamma ray and capture gamma ray, respectively. As for S/Si,
the two values are within 1 s of each other, and a weighted
average for these two results gives Ca/Si = 0.24 
 0.05.
7.2.4. Sodium
[26] Sodium is an important rock-forming element long
known to be present in the weak exosphere surrounding
Mercury [Potter and Morgan, 1985; Sprague et al., 1998].
Unlike Ca and S, Na is not measureable by theMESSENGER
XRS. It has gamma rays from both inelastic scatter (440 keV)
and neutron capture (472 keV). However, both of these
gamma rays are subject to interferences from other gamma
rays at similar energies; from activation of 69mZn (where the
symbol m denotes a metastable state) at 439 keV and from
10B at 478 keV [Evans et al., 2006]. The 10B background
arises from neutron capture within the borated plastic anti-
coincidence shield. This neutron capture is followed by the
emission of an alpha particle to make excited 7Li, and the
peak has a unique broadened flat-top shape that interferes
with the Na capture peak, so the latter cannot be used to
derive a Na abundance. The 69mZn is a spallation product of
GCRs interacting with the detector material. It was detected
early in the interplanetary cruise measurements and in each of
the preflyby measurements [Goldsten et al., 2007]. The
gamma-ray background count rate from 69mZn (half-life of
13 h) can be considered constant over the orbital period of
MESSENGER as the GCR excitation source can be regarded
as constant through most of the orbit. The background from
Na on the spacecraft is negligible compared with the 69mZn
contribution. The abundance of Na can be determined
directly by subtracting the high-altitude measurement of the
440 keV inelastic peak from the low-altitude measurement.
The fits for the low-altitude and high-altitude spectra are
shown in Figure 5. This procedure gives an abundance ratio
for Na/Si of 0.12 
 0.01. As a check on the assumption
of a constant background, spectra were accumulated for low-
altitude cutoffs of 1500 km and 1000 km. Analysis of these
spectra along with the corresponding forward calculation gave
identical results, within the statistical uncertainties. The strong
Na signal offers the potential for achieving some spatial res-
olution that has so far been possible only for gamma-ray
measurements of K and Si [Peplowski et al., 2012a].
7.2.5. Iron
[27] Iron has gamma rays from both inelastic scatter (e.g.,
847 keV) and from neutron capture (e.g., a doublet at 7632
and 7646 keV). However, both the inelastic and capture
peaks are difficult to analyze. As mentioned above, the
Fe(n,n?g) peak at 847 keV was contaminated by the
production of 56Co (half-life of 77 days) caused by an SPE
that occurred on 4 June 2011. Thus the analysis was limited
to a spectrum accumulated over a period of 59 days instead
of the 177 days used for the analysis of the other elements.
The Fe (n,n?g) 847 keV peak is in a region that includes a
sawtooth peak from fast neutron interactions in the Ge
detector, and the fit for this region is shown in Figure 6. The
Fe(n,g) doublet at 7632 keV and 7646 keV has the highest
flux of any iron capture gamma ray, but because of the high
energy of these gamma rays and the small volume of the
detector, the detection efficiency is small. However, the
escape peaks for these gamma rays have greater detection
efficiencies than the corresponding full energy peaks
[Goldsten et al., 2007]. Calibration measurements were made
with radioactive sources with energies up to 6 MeV; at higher
energies, calculations were used to estimate escape peak
efficiencies.
[28] No peaks from the capture gamma rays from iron
were detectable in the standard low-altitude spectrum.
However, when a low-altitude spectrum with a 1500 km
threshold was used, both the single-escape and double-
escape peaks from this doublet, i.e., a total of four peaks,
could be analyzed. Analysis of the inelastic scatter peak
gives an Fe/Si ratio of 0.066 
 0.016, whereas the Fe/Si
ratio derived from a sum of the four individual neutron
capture escape peaks is 0.10 
 0.02. The weighted average
for these two results is Fe/Si = 0.077 
 0.013.
7.2.6. Aluminum
[29] The analysis of two Al inelastic scatter peaks at
different energies (1014 keV and 2211 keV) described by
Table 3. GRS Abundance Ratio Resultsa
Element Mode Energy (keV) Ratio to Si s Mode Energy (keV) Ratio to Si s
Ca (n,n?g) 3736 0.28 0.10 (n,g) 1942 0.22 0.06
S (n,n?g) 2230 0.086 0.017 (n,g) 5420 0.11 0.03
Fe (n,n?g) 847 0.066 0.016 (n,g) (7632 and 7646) 0.10 0.02
Na (n,n?g) 440 0.12 0.01
Alb (n,n?g) 1114 and 2211 0.29 +0.05/0.13
aRadios are by weight, s denotes one standard deviation.
bAl results are from Peplowski et al. [2012b].
Table 4. Absolute Abundances and Abundance Ratios
Element Ratioa Weighted Average Weighted s Element Abundance wt% s wt%
Si 24.6b
Ca/Si 0.24 0.05 Ca 5.9 1.3
S/Si 0.092 0.015 S 2.3 0.4
Fe/Si 0.077 0.013 Fe 1.9 0.3
Na/Si 0.12 0.01 Na 2.9 0.1
Alc 7.1 +0.4/0.9
aRatios are by weight.
bAssumed from bulk composition model.
cFrom Peplowski et al. [2012b].
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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Peplowski et al. [2012b] gives an Al/Si ratio of 0.29 (+0.05,
0.13), consistent with the XRS results of Nittler et al.
[2011] and Weider et al. [2012].
7.2.7. Other Elements
[30] Of the major rock-forming elements that could be
analyzed from the GRS measurements, only oxygen is not
reported here. The GRS derived O/Si abundance ratio
was 1.40 (
0.03), about 20% lower than that from the model
abundances (1.72) and lower than any measured meteorite
values as compiled byNittler et al. [2004]. Comparison of the
forward model prediction for O to previous gamma-ray
calculations [Masarik and Reedy, 1996; Br?ckner and
Masarik, 1997] indicates that these calculated count rates
are higher than expected for the assumed O abundance.
The Mars surface O abundance was not determined by the
Mars Odyssey GRS because of interference from the largely
CO2 atmosphere. Since the forward calculation for O was
not validated during the Mars Odyssey mission, as for the
other elements, there is no validation for accuracy of the
adopted O cross sections and results, so we do not report an
O abundance here.
Figure 5. Fit of the Na(n,n?g) 440 keV peak in (a) the low-altitude spectrum and (b) the high-altitude
spectrum for the standard spectra accumulated between 29 March 2011 and 24 September 2011.
The 439 keV label on the high-altitude fit corresponds to the 69mZn peak, though the energy resolution is
not sufficient to distinguish between that and the 440 keV peak.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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[31] There are other elements included in the forward
model composition (Table 1) that have large neutron capture
cross sections and relatively strong gamma rays (e.g., Cl, Cr,
and Mn) that were not detected in the standard summed
spectrum. One of the limitations for detection is the low
detector efficiency of the GRS for high-energy gamma rays,
e.g., Cr at 8884 keV and Mn at 7243 keV. Measurements
continued over the duration of the primary mission as well
as into the extended mission (up to an additional Earth year)
may allow the detection or firm upper limits to be placed
on the abundance of elements that were not detected during
the primary mission. The detection of the H peak in the
2204?2243 keV region (Figure 4) is from the fuel on the
spacecraft. As the mission evolves, the remaining fuel will
decrease, and a point may be reached later in the mission at
which measurement of H is possible.
8. Discussion
8.1. Abundances Calculated for an Assumed Si
Abundance
[32 ] Converting elemental ratios (normalized to Si) to
absolute abundances requires knowledge of the absolute Si
abundance. Nittler et al. [2011] used ratios of XRS-derived
major elements to silicon to estimate a bulk Si abundance for
Mercury of 25 wt%, under the assumption of normal
Figure 6. Fit of the Fe(n,n?g) 847 keV peak in (a) the low-altitude spectrum and (b) the high-altitude
spectrum for the spectra accumulated between 29 March 2011 and 27 May 2011. The curve labeled
834 keV sawtooth is due to fast neutrons interacting in the detector material. The other peaks in the spectra
are from the decay of 54Mn, and possibly some S(n,g), at 841 keV, from Al(n,n?g) at 844 keV, and from
the decay of 56Co at 854 keV.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
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stoichiometry for oxides of major cations in order to estimate
the O abundance followed by normalization of the total
composition to 100%. This Si abundance is similar to that of
the partially melted enstatite chondrite composition of
Burbine et al. [2002], 25.7 wt%, which is also similar to that
derived from petrologic modeling of representative Mercury
crustal compositions (K. R. Stockstill-Cahill et al., Magnesium-
rich crustal compositions on Mercury: Implications for mag-
matism from petrologic modeling, submitted to Journal of
Geophysical Research, 2012), 24.3 wt%. The value used
here for the forward modeling (Table 1) of 24.6 wt% is thus
in line with other estimates and is appropriate to derive
absolute abundances for the other elements measured by
GRS (Table 4).
8.2. Comparison With the Assumed Forward
Model Composition
[33] As discussed above, the bulk composition can affect
the thermal neutron spatial distribution and, as a result, the
measured neutron capture fluxes. High concentrations of
elements with large cross sections for the removal of thermal
and epithermal neutrons decrease the flux of neutron capture
gamma rays. The model composition used for the forward
calculation was based on expected abundances from other
sources and measurements. The low value of Fe and the
nondetection of Ti, Mn, and rare earth elements (e.g., Gd,
Sm) are consistent with the assumed macroscopic cross
section and average atomic mass and validates the abun-
dances assumed in the forward model (Table 1). Although
this model solution is not unique, Boynton et al. [2007]
showed that small variations in these elemental abundances
will not change the results.
8.3. Comparison With GRS Flyby Measurements
[34] During the MESSENGER flybys of Mercury, the GRS
made measurements of gamma rays from the near-equatorial
region. Despite being limited by the low planetary fluxes,
sufficient counts were accumulated to estimate abundances
for some elements with relatively strong gamma-ray peaks,
including Si, Fe, Ti, K, and Th [Rhodes et al., 2011]. Absolute
abundances for the radioactive elements K, Th, and U
obtained from early orbital measurements have been
described by Peplowski et al. [2011b]. Orbital measurements
have also been used to examine the spatial heterogeneity of
the radioactive elements K and Th by Peplowski et al.
[2012a]. Analysis of the GRS flyby results for Fe gave an
Fe/Si ratio of 0.27 
 0.14, higher than, but within 2 s of the
orbital results. This comparison suggests that there is not a
strong latitudinal dependence of the Fe/Si abundance, but this
inference should be tempered by the large uncertainties in the
flyby measurements. Although Ti was not detected unam-
biguously in the flyby measurements, its derived abundance
of 1.2 
 2.9 wt% is consistent with the orbital XRS mea-
surements [Nittler et al., 2011] indicating an abundance
of <1 wt%. The analysis of the orbital GRS measurements of
the Ti inelastic scatter line at 983 keV showed that adopting
the same value for the background amplification factor as that
used for the flybys gives an unphysical negative Ti abundance
[see Peplowski et al., 2012b]. For these reasons, Ti was used
to determine the background amplification factors for the
orbital analysis.
8.4. Comparison With XRS Results
[35] The XRS on MESSENGER measures elemental
composition by detection of X-rays emitted through the
interaction of solar-generated X-rays with surface materials.
There are two major differences between the GRS and XRS
measurements. First, XRS is sensitive to the top tens of
micrometers of the surface, compared with tens of cen-
timeters for GRS. Second, the XRS is collimated, limiting
the field of view (FOV) to 12. This FOV provides relatively
high spatial resolution in the northern hemisphere (down to
<100 km) when the spacecraft is close to the planet and
lower spatial resolution in the southern hemisphere when the
spacecraft is at higher altitudes. Four major-element ratios
can be measured by both XRS and GRS: Al/Si, S/Si, Ca/Si,
and Fe/Si [Nittler et al., 2011; Weider et al., 2012]. Weider
et al. [2012] presented spatially resolved measurements of
Mg/Si, Al/Si, S/Si, and Ca/Si for a number of regions in the
northern hemisphere that broadly encompass the large
expanse of northern volcanic plains [Head et al., 2011] as
well as the surrounding mix of intercrater plains and heavily
cratered terrain. These terrains also correspond to the two
most common geological terrain types sampled by the GRS
measurements, and the data can thus be compared. Table 5
summarizes the elemental ratio values that GRS and XRS
have in common.
[36] The MESSENGER XRS also measures Fe, but only
during solar flares sufficiently energetic to cause the fluo-
rescence of Fe X-rays. The results reported by Nittler et al.
[2011] for the strongest solar flares gave an Fe/Si ratio in
the range 0.01?0.15, all for areas at near-equatorial or
southern latitudes on Mercury. Because of the different
regions sampled, the relatively few XRS Fe measurements,
and the fact that the XRS Fe analysis is more susceptible to
systematic errors than that for the other reported elements, it
is difficult to compare directly the GRS and XRS results for
iron. Nonetheless, the GRS Fe/Si value is within the range
reported for the XRS values, confirming the overall low
abundance of Fe at Mercury?s surface.
8.5. Comparison With Ground-Based Observations
of Mercury?s Surface and Exosphere
[37] As described above, the forward modeling of the GRS
data has been conducted for a Si abundance of 24.6 wt%,
consistent with estimates from major-element abundances
reported from analysis of XRS measurements [Nittler et al.,
2011; Weider et al., 2012]. This value is also consistent
with estimates derived from the location of the transmission
Table 5. Comparison With XRS Resultsa
Ratio
GRS Results
Region
XRS Results
Mean s Mean SD Median
Al/Si 0.29 +0.05/0.13 IcP-HCT 0.22 0.08 0.23
NP 0.26 0.07 0.26
S/Si 0.09 0.02 IcP-HCT 0.09 0.03 0.09
NP 0.06 0.02 0.06
Ca/Si 0.24 0.05 IcP-HCT 0.19 0.04 0.19
NP 0.15 0.04 0.16
aAl results are from Peplowski et al. [2012b]. IcP-HCT denotes intercrater
plains and heavily cratered terrain. NP denotes volcanic northern plains.
XRS results are from Weider et al. [2012], SD is the standard deviation of
multiple measurements.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
12 of 14
minimum seen in midinfrared spectra of Mercury?s surface
obtained with ground-based telescope observations [Sprague
et al., 2007].
[38] The GRS results, along with the elemental abundances
measured by the XRS, have clarified results from ground-
based spectroscopy and multispectral imaging of Mercury?s
surface and composition. One of the most important findings
is the confirmation of low total iron on Mercury?s surface.
Neither Earth-based [Vilas, 1988; Blewett et al., 2002] nor
MESSENGER-derived [McClintock et al., 2008; Robinson
et al., 2008; Denevi et al., 2009] reflectance spectra of
Mercury display a 1 mm absorption feature associated with
ferrous iron in silicates, limiting FeO in the silicates at
Mercury?s surface to less than 2?3% [Vilas, 1988; Blewett
et al., 2009; Riner et al., 2010]. Thermal emission spectra
in the midinfrared also suggest the presence of MgO-rich
orthopyroxene and olivine, with the total FeO abundance in
the range from 2?5% [Sprague et al., 2009]. The inference
of MgO-rich mafic silicates is also consistent with relatively
high Mg/Si ratios found by the MESSENGER XRS [Nittler
et al., 2011; Weider et al., 2012]. Ground-based observa-
tions have found a spectral feature near 5 mm indicative of
clinopyroxene high in Ca and Mg (e.g., CaMgSi2O6) in
some plains regions [Strom and Sprague, 2003]. However,
the XRS results [Nittler et al., 2011; Weider et al., 2012]
indicate that, on the whole, Mercury has a low Ca/Si ratio,
which is borne out by the absence of clinopyroxene in
petrologic models for Mercury surface materials (Stockstill-
Cahill et al., submitted manuscript, 2012). The GRS and XRS
results are consistent with Mg-rich and low-Ca silicates.
[39] The relatively high Na abundance reported here
(compared with that expected from the model abundance
in Table 1) is consistent with evidence for Na-bearing pla-
gioclase feldspar inferred from ground-based spectral mea-
surements. Several measurements described by Sprague
et al. [2007] are consistent with feldspar compositions rang-
ing from albite (NaAlSi3O8), with up to 5 wt% anorthite
(CaAl2Si2O8), to labradorite with 30?50 wt% albite. Thus,
Na-bearing plagioclase may account for much of the Na
measured by the GRS. A high-energy distribution of Ca in
the exosphere [Vervack et al., 2010] has been modeled by
M. H. Burger et al. (Modeling MESSENGER observations
of calcium in Mercury?s exosphere: Flyby observations,
submitted to Journal of Geophysical Research, 2012) and
found to suggest a dawnside source region that does not
appear to be associated with any specific geologic region.
A likely source is sputtering from Ca-bearing plagioclase,
but other possibilities are CaS [Nittler et al., 2011; Weider
et al., 2012] and MgO-rich, low-Ca clinopyroxene, both
of which are consistent with a relatively albitic plagioclase
component. In order to relate directly the Na and K mea-
sured in the exosphere with the GRS results, it may be
helpful to map the Na abundance distribution on Mercury
as has been done for the K abundance [Peplowski et al.,
2012a].
9. Conclusions
[40] Analysis of the MESSENGER orbital GRS measure-
ments has yielded the abundances of several major elements
on Mercury: Al, Ca, S, Fe, and Na. For those elements that
have been measured by both GRS and XRS (Al, Ca, S, Fe),
there are no appreciable differences in the derived abun-
dances. This agreement indicates that Mercury?s regolith is,
on average, vertically homogeneous to a depth of tens of
centimeters. The GRS results confirm the low total Fe abun-
dance on Mercury?s surface previously inferred from spectral
reflectance and XRS measurements. The GRS and XRS ele-
mental results are most consistent with petrologic models
suggesting that Mercury?s surface is dominated by Mg-rich
silicates. The observed Na abundance of 3% suggests that
plagioclase feldspar on Mercury?s surface is relatively rich in
albite. The derivation of Na maps may help to constrain
source processes for the Na in Mercury?s exosphere.
[41] Acknowledgments. We thank the entire MESSENGER team
for their invaluable contributions to the development and operation of the
MESSENGER spacecraft. This work was supported in part by the
MESSENGER Participating Scientist program under contract NNH08CC06C
to Computer Sciences Corporation. The MESSENGER project is supported
by the NASA Discovery Program under contract NAS5?97271 to The Johns
Hopkins University Applied Physics Laboratory and NASW-00002 to the
Carnegie Institution of Washington. We also thank two anonymous
reviewers for their thoughtful suggestions to improve this paper.
References
Blewett, D. T., B. R. Hawke, and P. G. Lucey (2002), Lunar pure anorthosite
as a spectral analog for Mercury, Meteorit. Planet. Sci., 37, 1245?1254,
doi:10.1111/j.1945-5100.2002.tb00893.x.
Blewett, D. T., M. S. Robinson, B. W. Denevi, J. J. Gillis-Davis, J. W.
Head, S. C. Solomon, G. M. Holsclaw, and W. E. McClintock (2009),
Multispectral images of Mercury from the first MESSENGER flyby:
Analysis of global and regional color trends, Earth Planet. Sci. Lett., 285,
272?282, doi:10.1016/j.epsl.2009.02.021.
Boynton, W. V., et al. (2007), Concentration of H, Si, Cl, K, Fe, and Th
in the low- and mid-latitude regions of Mars, J. Geophys. Res., 112,
E12S99, doi:10.1029/2007JE002887.
Br?ckner, J., and J. Masarik (1997), Planetary gamma-ray spectoscopy of
the surface of Mercury, Planet. Space Sci., 45, 39?48, doi:10.1016/
S0032-0633(96)00093-1.
Br?ckner, J., M. Koerfer, H. W?nke, A. N. F. Schroeder, D. Filges,
P. Dragovitsch, P. A. J. Englert, R. Starr, and J. I. Trombka (1991),
Proton-induced radiation damage in germanium detectors, IEEE Trans.
Nucl. Sci., 38, 209?217, doi:10.1109/23.289298.
Burbine, T. H., T. J. McCoy, L. R. Nittler, G. K. Benedix, E. A. Cloutis, and
T. L. Dickinson (2002), Spectra of extremely reduced assemblages: Impli-
cations for Mercury, Meteorit. Planet. Sci., 37, 1233?1244, doi:10.1111/
j.1945-5100.2002.tb00892.x.
Denevi, B. W., et al. (2009), The evolution of Mercury?s crust: A global
perspective from MESSENGER, Science, 324, 613?618, doi:10.1126/
science.1172226.
Evans, L. G., and S. W. Squyres (1987), Investigation of Martian H2O
and CO2 via orbital gamma ray spectroscopy, J. Geophys. Res., 92,
9153?9167, doi:10.1029/JB092iB09p09153.
Evans, L. G., R. C. Reedy, and J. I. Trombka (1993), Introduction to
planetary remote sensing gamma-ray spectroscopy, in Remote Geochemical
Analyses: Elemental and Mineralogical Composition, edited by C. M.
Peters and P. A. J. Englert, pp. 167?198, Cambridge Univ. Press,
New York.
Evans, L. G., R. C. Reedy, R. D. Starr, K. E. Kerry, and W. V. Boynton
(2006), Analysis of gamma ray spectra measured by Mars Odyssey,
J. Geophys. Res., 111, E03S04, doi:10.1029/2005JE002657.
Feldman,W. C., D. J. Lawrence, R. C. Elphic, D. T. Vaniman, D. R. Thomsen,
B. L. Barraclough, S. Maurice, and A. B. Binder (2000), Chemical informa-
tion content of lunar thermal and epithermal neutrons, J. Geophys. Res., 105,
20,347?20,363, doi:10.1029/1999JE001183.
Gasnault, O., W. C. Feldman, S. Maurice, I. Genetay, C. d?Uston, T. H.
Prettyman, and K. R. Moore (2001), Composition from fast neutrons:
Application to the Moon, Geophys. Res. Lett., 28, 3797?3800,
doi:10.1029/2001GL013072.
Goldsten, J. O., et al. (2007), The MESSENGER Gamma-Ray and Neutron
Spectrometer, Space Sci. Rev., 131, 339?391, doi:10.1007/s11214-007-
9262-7.
Head, J. W., et al. (2011), Flood volcanism in the northern high latitudes
of Mercury revealed by MESSENGER, Science, 333, 1853?1856,
doi:10.1126/science.1211997.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
13 of 14
Kim, K. J., D. M. Drake, R. C. Reedy, R. M. S. Williams, and W. V.
Boynton (2006), Theoretical fluxes of gamma rays from the Martian sur-
face, J. Geophys. Res., 111, E03S09, doi:10.1029/2005JE002655 [printed
112(E3), 2007].
Lawrence, D. J., W. C. Feldman, J. O. Goldsten, T. J. McCoy, D. T.
Blewett, W. V. Boynton, L. G. Evans, L. R. Nittler, E. A. Rhodes, and
S. C. Solomon (2010), Identification and measurement of neutron-absorbing
elements on Mercury?s surface, Icarus, 209, 195?209, doi:10.1016/
j.icarus.2010.04.005.
Masarik, J., and R. C. Reedy (1996), Gamma ray production and transport
in Mars, J. Geophys. Res., 101, 18,891?18,912, doi:10.1029/96JE01563.
McAdams, J. V., R. W. Farquhar, A. H. Taylor, and B. G. Williams (2007),
MESSENGER mission design and navigation, Space Sci. Rev., 131,
219?246, doi:10.1007/s11214-007-9162-x.
McClintock, W. E., et al. (2008), Spectroscopic observations of Mercury?s
surface reflectance during MESSENGER?s first Mercury flyby, Science,
321, 62?65, doi:10.1126/science.1159933.
McKinney, G. W., D. J. Lawrence, T. H. Prettyman, R. C. Elphic, W. C.
Feldman, and J. J. Hagerty (2006), MCNPX benchmark for cosmic ray
interactions with the Moon, J. Geophys. Res., 111, E06004, doi:10.1029/
2005JE002551.
Nittler, L. R., T. J. McCoy, P. E. Clark, M. E. Murphy, J. I. Trombka, and
E. Jarosewich (2004), Bulk element compositions of meteorites: A guide
for interpreting remote-sensing geochemical measurements of planets and
asteroids, Antarct. Meteorite Res., 17, 233?253.
Nittler, L. R., et al. (2011), The major element composition of Mercury?s
surface from MESSENGER X-ray spectrometry, Science, 333, 1847?1850,
doi:10.1126/science.1211567.
Peplowski, P. N., D. T. Blewett, B. W. Denevi, L. G. Evans, D. J.
Lawrence, L. R. Nittler, E. A. Rhodes, C. M. Selby, and S. C. Solomon
(2011a), Mapping iron abundances on the surface of Mercury: Predicted
spatial resolution of the MESSENGER Gamma-Ray Spectrometer,
Planet. Space Sci., 59, 1654?1658, doi:10.1016/j.pss.2011.06.001.
Peplowski, P. N., et al. (2011b), Radioactive elements on Mercury?s surface
from MESSENGER: Implications for the planet?s formation and evolu-
tion, Science, 333, 1850?1852, doi:10.1126/science.1211576.
Peplowski, P. N., et al. (2012a), Variations in the abundances of potassium
and thorium on the surface of Mercury: Results from the MESSENGER
Gamma-Ray Spectrometer, J. Geophys. Res., 117, E00L04, doi:10.1029/
2012JE004141.
Peplowski, P. N., E. A. Rhodes, D. K. Hamara, D. J. Lawrence, L. G.
Evans, L. R. Nittler, and S. C. Solomon (2012b), Aluminum abundance on
the surface of Mercury: Application of a new background-reduction tech-
nique for the analysis of gamma-ray spectroscopy data, J. Geophys. Res.,
doi:10.1029/2012JE004181, in press.
Potter, A. E., and T. H. Morgan (1985), Discovery of sodium in the atmo-
sphere of Mercury, Science, 229, 651?653, doi:10.1126/science.229.
4714.651.
Rhodes, E. A., et al. (2011), Analysis of MESSENGER Gamma-Ray
Spectrometer data from the Mercury flybys, Planet. Space Sci., 59,
1829?1841, doi:10.1016/j.pss.2011.07.018.
Riner, M. A., F. M. McCubbin, G. J. Taylor, and J. J. Gillis-Davis (2010),
Mercury surface composition: Integrating petrologic modeling and
remote sensing data to place constraints on FeO abundance, Icarus, 209,
301?313, doi:10.1016/j.icarus.2010.05.018.
Robinson, M. S., S. L. Murchie, D. T. Blewett, D. L. Domingue, S. E.
Hawkins, J. W. Head, G. M. Holsclaw, W. E. McClintock, S. C. Solomon,
and T. R. Watters (2008), Reflectance and color variations on Mercury:
Regolith processes and compositional heterogeneity, Science, 321, 66?69,
doi:10.1126/science.1160080.
Solomon, S. C., R. L. McNutt Jr., R. E. Gold, and D. L. Domingue (2007),
MESSENGER mission overview, Space Sci. Rev., 131, 3?39, doi:10.1007/
s11214-007-9247-6.
Sprague, A. L., W. J. Schmitt, and R. E. Hill (1998), Sodium atmospheric
enhancements, radar bright spots, and visible surface features, Icarus, 136,
60?68, doi:10.1006/icar.1998.6009.
Sprague, A. L., A. J. Warell, G. Cremonese, Y. Langevin, J. Helbert,
P. Wurz, I. Veselovsky, S. Orsini, and A. Milillo (2007), Mercury?s
surface composition and character as measured by ground-based observa-
tions, Space Sci. Rev., 132, 399?431, doi:10.1007/s11214-007-9221-3.
Sprague, A. L., K. L. Donaldson Hanna, R. W. H. Kozlowski, J. Helbert,
A.Maturilli, J. B.Warell, and J. L. Hora (2009), Spectral emissivity measure-
ments of Mercury?s surface indicate Mg- and Ca-rich mineralogy, K-spar,
Na-rich plagioclase, rutile, with possible perovskite, and garnet, Planet.
Space Sci., 57, 364?383, doi:10.1016/j.pss.2009.01.006.
Strom, R. G., and A. L. Sprague (2003), Exploring Mercury: The Iron
Planet, 216 pp., Springer-Praxis, Chichester, U.K.
Vervack, R. J., Jr., W. E. McClintock, R. Killen, A. Sprague, B. J. Anderson,
M. H. Burger, T. E. Bradley, N. Mouawad, S. C. Solomon, and
N. R. Izenberg (2010), Mercury?s complex exosphere: Results from
MESSENGER?s third flyby, Science, 329, 672?675, doi:10.1126/science.
1188572.
Vilas, F. (1988), Surface composition of Mercury from reflectance spectro-
photometry, in Mercury, edited by F. Vilas, C. R. Chapman, and M. S.
Mathers, pp. 59?76, Univ. of Ariz. Press, Tucson, Ariz.
Waters, L. S. (Ed.) (2002), MCNPX users? manual version 2.4.0, Rep. LA-
CP-02?408, Los Alamos Natl. Lab., Los Alamos, N.M.
Weider, S. Z., L. R. Nittler, R. D. Starr, T. J. McCoy, K. R. Stockstill-
Cahill, P. K. Byrne, B. W. Denevi, J. W. Head, and S. C. Solomon
(2012), Chemical heterogeneity on Mercury?s surface revealed by the
MESSENGER X-ray Spectrometer, J. Geophys. Res., 117, E00L05,
doi:10.1029/2012JE004153.
EVANS ET AL.: MAJOR-ELEMENT ABUNDANCES ON MERCURY E00L07E00L07
14 of 14