Orbital Measurement of Bulk Carbon, Hydrogen, Oxygen, and Sulfur of
Carbonaceous Asteroids via High Energy Resolution Gamma-Ray Spectroscopy
2
D. Starr , Larry
3
G. Evans , Ann
Count Rate Uncertainties at FWHM = 4.1 vs. 5.7 keV: Data and Models
FWHM = 4.1 keV
FWHM = 5.7 keV
H 2223 keV (Mars Odyssey Data)
+/- 0.46 %
+/- 1.06 %
H 2223 keV (CI Chondrite Model, 30d)
+/- 0.40 %
+/- 0.50 %
S 5420 keV (CI Chondrite Model, 30d)
+/- 3.0 %
+/- 5.1 %
Count rates
GCR flux typically varies by more than a factor of 3 depending on the 11-year solar
cycle (McKinney et al. 2006). We normalized our model GCR flux to high solar
potential (low GCR flux), comparable to that seen by the Mars Odyssey HPGe during
its first 3 years in orbit. Missions that orbit during solar minimum may thus see
gamma-ray fluxes up to three times higher than those illustrated here.
Count rates in our model spectra compared to those in Mars Odyssey data suffer
from the less favorable geometry (0.84 vs. 3.47 sr solid angle at Mars) but benefit
from the absence of a 𝛾-ray/GCR attenuating atmosphere. We compared the CO
chondrite model count rates to the Mars Odyssey flight count rates geometry,
atmosphere (Masarik and Reedy 1996), and composition:
Friday, November 7, 14
Mars Data
Mars
counts/min atm. corr
1.773
1.077
0.038
0.336
2.97
2.69
1.76
1.49
Mars counts/min
x atm x geom x wt%
ratio
0.95
0.15
0.015
0.21
0n
The hydrogen line at 2223 keV sits on the wing of the Doppler-broadened aluminum line at 2211 keV
which is dominated by spacecraft background. A deck-mounted GRS therefore sees vastly more Al
background than the boom-mounted Mars Odyssey GRS. However, the hydrogen line is strong: we find a
1-sigma value of +/- 0.007 counts per minute after 30 days of integration, or under 1% of the 1.76 counts
per minute retrieved from the CI chondrite model spectrum. (1-sigma error bars on the retrieved count
rate plot above but are smaller than the plot symbols.)
Sulfur
Aste
roid
Orbital Al%tude = 1 asteroid radius
MOGRS Data
9ering
elas%c S
ca
Cap
ture
(30-­‐day 1-­‐sigma error bars)
ac%ve
Decay
𝛾’s from
In
m N
eu
𝛾’s fr
o
We have estimated sulfur sensitivity based on the 5420 keV neutron capture line. A titanium line from the
radiator visible at 5396 keV in the Mars data has not yet been included in our model but should have
minimal impact on the uncertainties in the 5420 keV line.
Carbon
CI Chondrite Model
Ga
la
c%c
Co s
mic
R
ays
𝛾’s fro
m Rad
io
0n
Over the course of the MOGRS experiment, the energy resolution (post-anneal) was
slowly degraded by accumulated radiation damage, from 4.1 keV in June 2002 to 5.7
keV over May 2006-Dec. 2007 (FWHM at 1332 keV) . We therefore investigated the
sensitivity effects of this loss of resolution by analyzing MOGRS data from late in the
mission and by broadening our MCNPX model asteroid spectra to 5.7 keV. We find
that our meteorite-based asteroid models can still be distinguished from each other
within a 4.5-month mission at the degraded resolution.
GRS
MCNPX model of total asteroid 𝛾 spectrum,
including Doppler broadening and GRS detector
response
Radiation, Detector Resolution, and Science Performance
Right: Broadened GRS
spectra (5.7 keV vs. 4.1 keV
FWHM). Left: Odyssey
data from 2006-2007.
Right: Simulated CI
chondrite spectra at the
same resolution.
CI Chondrite Model
(30-­‐day 1-­‐sigma error bars)
(Mounted on deck)
d Ep
Doppler broadening was applied to the spectrum prior to the detector model. The
spectra from the MCNPX output were then broadened to a resolution based on the
in-flight performance of MOGRS, FWHM = 4.1 keV at 1332 keV. Line fluxes were
then extracted from the combined background + asteroid spectrum with the same
techniques that were used for the MO GRS and MESSENGER flight data (Evans et al.
2006, 2012) and statistical uncertainties were evaluated.
Ga
with Mars Odyssey, no
boom was assumed: instead, the
GRS was modeled in a deckmounted position similar to that
of the Mercury MESSENGER GRS.
MCNPX model of total spacecraft 𝛾
background spectrum including GRS
detector response
ithe
rma
l Ne
utro
Right: Application of a flightlike detector
resolution to the MCNPX-generated spectrum.
MCNPX generates a line/continuum spectrum
(blue) which then must be broadened to the
Ody GRS-like energy resolution (red) in order
to model the instrument performance
accurately.
c Co
%
l ac
Mars Odyssey GRS Data
ys
a
R
ic Unlike
sm
𝛾’s from the Spacecraft
tron
A Dawn-like mission scenario was modeled with the altitude equal to the asteroid
radius for a 4.5-month low-orbit phase. The detector model was based on Mars
Odyssey Gamma-Ray Spectrometer (“MOGRS”; Boynton et al. 2004), among the
largest and most sensitive HPGe experiments flown to date.
The Dawn-like spacecraft model for
background estimation includes xenon
(~630 kg) and hydrazine (45.5 kg) fuel
tanks in addition to the aluminum
spacecraft structure and some carbon fiber
components.
Hydrogen
the We have therefore conducted new simulations of the performance of a GRS
experiment in orbit around asteroids with carbonaceous chondritic compositions at
levels of hydration ranging from CI-like (~17 wt% structural water) to CO-like (0.07
wt% structural water). Cosmic-ray interactions with the asteroid surfaces were
modeled using the MCNPX Monte-Carlo radiation transport code.
A spacecraft background (based on a Dawn-like spacecraft model for a solarelectric asteroid orbiter) was also modeled using MCNPX: this included
background due to direct cosmic-ray (GCR)/spacecraft interactions as well as
background due to asteroidal neutron flux on the spacecraft.
Mars wt
CO
CO model
%
chon. wt counts/min
%
Si 1779 21.33
15.90
1.21
H 2223 0.32%
0.07%
0.15
S 5420
2.2%
2.0%
0.022
Fe 7631 14.6%
24.8%
0.23
4
5
6,
1
E. Zolensky , William V. Boynton , Cristina A. Thomas
Monte-Carlo Modeling
ns fr
om Various populations of low-albedo asteroids (e.g. C-complex, P, and D
spectral types) dominate the outer Main Asteroid Belt, Hildas, and Jupiter
Trojans and are thought to be related to carbonaceous meteorites.
However, carbonaceous meteorites are themselves a diverse group and it
remains unclear which types represent which asteroids or asteroid
populations. A high-energy-resolution high-purity Ge (HPGe) gamma-ray
spectroscopy (GRS) experiment on an asteroid orbiter would be
sensitive to many of the elements that differentiate carbonaceous
chondrite subclasses from each other and from the ureilites, including H,
C, O, and S, in the outer ~20-50 cm of the asteroid surface.
keV
1
M. Parsons , Michael
1NASA/GSFC, 2Catholic University of America 3Computer Sciences Corporation, 4NASA/JSC, 5University of Arizona, 6ORAU/NPP
Th e
rma
l an
Lucy
1
F. Lim , Richard
C
c
%
c
a
l
o
ic sm
ys
a
R
Mars Ody GRS Data
Ga
Thermal
Neutron
Capture
(4.5-­‐month 1-­‐sigma error bars)
Co
c
%
c
a
l
ys
a
c R
i
sm
Ga
Radioactive
Decay
0n
Inelastic Scattering
Meteorite-Based Model Asteroid Compositions
Meteorite Type
wt% H
Reference
CI Chondrite
2.02%
Tagish Lake (C2 ungrouped)
1.5%
CM Chondrite
1.4%
CO Chondrite
0.07%
Wasson and Kallemeyn (1988)
Novo Urei (ureilite)
0
Wiik (1969)
Brown et al. (2000)
In the Odyssey data the 4438 peak is dominated by the Mar%an CO2 atmosphere.
The GRS analysis of carbon is unusually complex. The strong 4438 keV line of carbon-12 is both
Doppler-broadened by the nuclear recoil of its production and shared with the alpha-decay product of
oxygen-16. We modeled the statistical uncertainties associated with fitting the area of the Dopplerbroadened line in combination with the several non-broadened overlapping lines produced by other
elements in the asteroid and spacecraft. This uncertainty was then combined (root-sum-squared) with
the uncertainty in oxygen abundance derived from fitting a separate O gamma-ray line at 6129 keV.
Thus, although the C contribution to the 4438 keV peak is similar in count rate to the 5420 keV S peak
(above), the C uncertainties are much higher. However, a 4.5-month integration time in low orbit is
sufficient to characterize the carbon abundances in carbon-rich meteorite compositions such as CI
chondrites (3.6 wt%), CM chondrites, and ureilites (2.2 wt%) and to set meaningful upper limits on the
abundances in carbon-poor compositions.
Chlorine
Chondrites
We evaluated chlorine sensitivity based on the count rate in the 1951 keV neutron capture line.
Chondritic chlorine is only about 700 ppm, but the neutron cross section is so high that it can be
measured to a 1-sigma uncertainty of 6% within 30 days.
For reference, the Mars Odyssey midlatitude count rate in this line was 0.152 +/- 1.6%, from regional
abundances ranging from 0.2 to 0.8 wt% (Boynton et al. 2007).
Above: Bulk compositions of asteroidal materials. Left: O/Si versus Fe/Si ratios for
meteorites from major classes and sub-classes. Most meteorites have O/Si ratios between those
of olivine (1.7) and pyroxene (2.3); exceptions are the CM and CI carbonaceous chondrites which
contain significant water. Constant O/Si ratios and varying Fe/Si ratios reflect metal-silicate mixing
(e.g., pallasites), whereas variable O/Si ratios reflect varying silicate abundances or compositions.
Fig. 8 from Nittler et al. (2004).
Right: Sulfur vs. carbon abundances in carbonaceous chondrites and primitive achondrites.
We find that within 4.5 months the orbital GRS can measure H/Si, O/
Si, C/Si, and S/Si with sufficient precision to distinguish OH-rich CI and
CM chondritic compositions from each other and from drier CO-like
compositions, and Fe/Si and S/Si to distinguish chondrites from
ureilites and other achondrites.