Proc. Radiochim. Acta 1, 349–355 (2011) / DOI 10.1524/rcpr.2011.0062
© by Oldenbourg Wissenschaftsverlag, München
Planetary gamma ray spectrometry: remote sensing of elemental
abundances
By P. A. J. Englert∗
Hawai’i Institute for Geophysics and Planetology, University of Hawai’i at Mânoa, 1680 East West Road, Honolulu, HI 96822, USA
(Received January 3, 2010; accepted in revised form November 11, 2010)
Summary. Planetary gamma ray spectrometry is a form of
nuclear spectroscopy applied remotely to provide geochemical maps of planetary bodies. From early developments it
has by now become a standard modality of planetary exploration. Basic and applied nuclear science has made significant
contributions to the advancement of planetary gamma ray
spectrometry, as outlined in this methodological and historical
assessment.
spectrometry and to demonstrate its developments from genuinely nuclear chemical and physical domains to the integration of the modality into the new knowledge compact of
remote geochemical analysis.
1. Introduction
Of the key elements of planetary gamma ray spectrometry
only a few will be discussed here in form of an overview.
These include the origin of gamma rays from planetary surfaces, their transport to a detector above the surface, their
measurement and interpretation.
Gamma ray spectroscopy, since its inception has been
widely used as a tool to determine elemental abundances
of geological samples in the laboratory and in the field. Diagnostic gamma rays of naturally radioactive elements and
those of isotopes excited or activated by neutrons are predominantly used in terrestrial laboratory and field settings,
including all forms of neutron activation analysis.
The power of gamma ray spectroscopy for the determination of elemental abundances of distant planetary surfaces
has been recognized very early in the history of planetary exploration. While at that time the focus of planetary gamma
ray spectrometry was on determining the overall abundances
of naturally radioactive elements K, Th, and U, it was soon
recognized that a wider range of elements, including major rock-forming ones such as O, Si, and Fe were accessible
through gamma rays produced by interaction of planetary
surfaces with galactic cosmic radiation. Advancements in
instrument and space vehicle development soon supported
the concept of orbital mapping of elemental abundances of
terrestrial planets. Major targets for planetary gamma ray
spectroscopy were the Moon, Mars, Venus and one Asteroid
with missions to further asteroids and Mercury underway.
Compared to terrestrial applications of e.g. prompt
gamma activation analysis with neutron beams, planetary
gamma ray spectroscopy is comparatively complex and resource intensive. Therefore nations with major space exploration programs dominated the development of the modality
in its early development phases. To date five nations have
successfully completed such experiments, beginning with
the United States and USSR/Russia, and recently Japan,
China, and India.
The scope of this paper is to provide a brief methodological and historical overview over planetary gamma ray
*E-mail: [email protected].
2. Principles of planetary gamma ray
spectrometry
2.1 Cosmic radiation and planetary gamma rays
While planetary surface gamma rays are produced by decay of naturally radioactive elements, the majority of gamma
rays in planetary surfaces are produced by the interaction
of galactic cosmic radiation. Galactic cosmic radiation consists of charged particles, predominantly protons and alpha
particles of energies between several hundred MeV to several GeV per nucleon. Their flux is about 4 particles per cm2
and s, with protons being by far the most abundant. Galactic cosmic ray fluxes are modulated by solar activity but do
not originate from the Sun. These high-energy particles are
stopped in the planetary surface through multiple interactions with atomic nuclei, transferring their energy, among
other mechanisms, through an intranuclear cascade to nucleons of the surface producing directly (knock-on) or through
subsequent evaporation secondary particles. These include
neutrons which through further interactions including scattering, moderation, and capture produce gamma rays diagnostic for the elements/isotopes they interact with [1, 2].
Conceptually, therefore, planetary gamma ray spectroscopy
can also be considered a special form of prompt gamma activation analysis with neutron beams.
Both natural and galactic cosmic ray produced gamma
rays are formed at depths that include or exceed those where
galactic cosmic ray particles are completely stopped. Their
production and attenuation are dependent on factors such
as gamma ray energy, density and average atomic number
Z of the surface, and the presence ofUnauthenticated
neutron moderators
and absorbers. The effective
depths
from
which
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gamma rays are emitted covers only a few decimeters. And it
is important that as many factors that influence the behavior
of neutrons and gamma ray absorption are known or obtainable from other measurements [2]. The advantage over
optical and X-ray spectroscopic methods in depth coverage
of chemical information makes planetary gamma ray spectrometry a valuable complement.
2.2 Measurement and interpretation
Gamma rays meeting a detector interact with that detector
through different mechanisms. The most important effect in
a detector able to record the full energy of a gamma ray is
the photoelectric effect in which the gamma photon transfers its entire energy to an electron of the detector material.
This energetic electron is stopped in the detector material
by ionization or other processes, providing an electrical signal in a solid-state detector and a light yield in a scintillation detector directly proportional to the gamma energy
deposited. These signals can be recorded in event/energy
multi-channel analyzers as so-called pulse height spectra
recording the number events/gamma rays over a defined
energy range and time interval for the target observed [4].
Gamma ray spectra obtained can be used for elemental abundance determination.
For space based gamma ray experiments the choice of
detector is a question of detection technology available at
the time of mission and experimental boundary conditions
provided by spacecraft and flight parameters. The earliest
detectors used were scintillation detectors such as NaI(Tl),
Sodium Iodide, scintillation detectors, and CsI(Tl), Cesium
Iodide, scintillation detectors, and later also BGO, Bismuth
Germanate, scintillation detectors. When high volume solid
state detectors became available, especially HPGe, High Purity Germanium, they were used in space based gamma ray
measurements, including planetary gamma ray spectrometry. These detectors need to be cooled to 80–100 K for operation. While the ultimate choice of detector types depends
on the scientific goals to be achieved, practical considerations determine the choice of flight instrumentation. The key
weighing has to occur between the following parameters:
Overall gamma ray detection efficiency is higher in scintillation detectors, a choice when measurement time is short.
Overall energy resolution is better in solid-state detectors,
a choice when improving detection opportunity for more
major and some minor elements. Scintillation detectors do
not show degradation of resolution under radiation exposure, but solid state detectors do. Hence for longer flights
solid-state detector radiation damage needs to be repeatedly annealed in flight. Cooling of solid-state detectors can
be active through mechanical coolers, which caused in the
past some technical, but mostly energy use issues. Passive
space based cooling places constraints on orbit and pointing accuracy of solid-state detector passive coolers into dark
space [5].
Gamma ray spectrometers on landers, on spacecraft in
fly-by mode or highly elliptical orbits tend to integrate all
gamma rays measured during active counting. Spacecraft
in circular equatorial and especially those in circular polar
orbits have the opportunity to obtain data in a mode that
allows establishing partial or complete geochemical maps
P. A. J. Englert
of the target planet, respectively. The spatial resolution for
such maps depends predominantly on the angular response
of the spectrometer and the altitude of the spacecraft orbit. Unless collimated by anti-coincidence techniques, the
angular response of most detector arrangements used is omnidirectional. For gamma rays from the target surface the
effective geometric counting efficiency for an area on the
surface is in first order dependent on the spacecraft altitude.
This means that the footprint for spatial resolution can be
large [2]. For the Apollo Missions to the Moon at an orbital
altitude of 100 km the spatial resolution was about 150 km
and for Mars Odyssey at 400 km it was 600 km, respectively.
An additional concern of omnidirectional detector response is gamma radiation induced in spacecraft materials
by cosmic radiation. Reduction of this background can be
achieved by positioning the detector away from the spacecraft on a boom and measurement of this background before
reaching the planetary target. If for technical reasons the
detector is integrated into the spacecraft, gamma ray anticoincidence shielding towards the spacecraft source can be
effective; this can also be used to reduce background resulting from gamma ray–detector interactions. For gamma
radiation produced in the detector assembly itself careful selection of the construction materials can be helpful. Signals
produced by charged particle interaction with the detector
itself need to be rejected electronically.
For orbital planetary gamma ray experiments statistically
sufficient spectra for elemental abundance determination
over a point or region of interest, spectra need to be accumulated from “snapshots” (about 20 s) of repeated passes of the
spacecraft over that point or region. Depending on gamma
ray yields for a given element accumulation times of several
ten to a few thousand hours are needed for elemental abundance uncertainties below 10% for e.g. the Mars Odyssey
mission.
As in neutron activation analysis comparator methods are
best suited to provide reliable elemental abundance data.
In the case of Moon and Mars “ground truth” standards
are available through sample return missions and or in-situ
elemental analysis through robotic missions. Complexities
arising from the presence of neutron moderators and minor
and trace elements with high neutron capture cross sections
can be addressed through intrinsic properties of the gamma
ray spectra, through concurrent and complementary measurement of neutron leakage spectra, and model calculations.
3. Planetary gamma ray spectrometry:
from concept to success
Planetary gamma ray spectrometry as tool for comparative
planetology has been proposed by J. R. Arnold, University
of California, San Diego in 1958 [6]. Beginning with the
Ranger missions of the United States in 1962 many missions
with the goal of obtaining planetary gamma ray spectra have
been completed to date. At present, two missions are on trajectory to their targets.
A frequency diagram of all successful planetary gamma
ray missions to date is provided in Fig. 1. Success for the
purpose of this study is defined in terms
of a successful
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Planetary gamma ray spectrometry: remote sensing of elemental abundances
Fig. 1. Frequency diagram of successful planetary gamma ray experiments. The development of planetary gamma ray spectroscopy has
progressed from a trial, proof of concept, and investigation phase into
a standard modality for geological exploration of planets.
with the gamma ray instrument on board. For some experiments it does not include the measurement of planetary
gamma ray spectra as for example in the case of the Mars
Observer mission, where multiple cruise phase data were
obtained but where the spacecraft did not reach its planetary mapping orbit. Parameters of importance for discussion
are given in Table 1 for each of the missions represented in
Fig. 1.
Based on the frequency diagram in Fig. 1 the following
phases of planetary gamma ray development and application can be derived. The early phase including United States
Ranger and Russian Luna missions can be termed the trial
phase from 1962 to 1966, followed by the proof of concept
phase that consists in principle of the first successful orbiter
experiments on Apollo 15 and 16 in 1971 and 1972. With
the exception of the Russian Venera 8 mission to Venus in
1974 and the Phobos mission to Mars in 1988 there were
no other planetary gamma ray experiments until the successful launch of the Mars Observer Mission in 1992. This
phase of almost two decades could be called the investigation phase as many experiments including detector development and underpinning nuclear science were undertaken to
improve the planetary gamma ray spectrometry modality. It
was in the transition from this phase to the next that planetary gamma ray spectroscopy became an integral part of
the area of remote geochemical analysis [7]. The geological
exploration phase, that is the phase where geological exploration goals dominated mission design and expectations
begins in 1992, with to date nine missions completed or in
progress by five nations.
3.1 Trial
The trial phase can be associated with the Ranger 3 and 5
missions and the Luna 10 and 12 missions to the Moon, and
under the definition above, Kosmos 60. Several tasks were
accomplished by these missions. All of these missions provided measurements of space background gamma rays, and
all but one, Kosmos 60, provided lunar surface gamma ray
spectra using scintillation detectors. Rangers 3 and 5 can be
351
classified as fly-bys while Luna 10 and 12 were successfully
inserted into elliptical lunar orbits. Gamma ray accumulation times of the missions ranging from several hours to
more than 30 days were too short to provide fine scale geochemical mapping. Gamma energies measured ranged from
about 0.2 MeV to 2.0 and 3.0 MeV, respectively, and were
useful to record natural radioactivity and some inelastic neutron scattering gamma ray lines. Neutron capture gamma
ray lines at higher energies could not be measured. Arnold
et al. (1962) [8], van Dilla et al. (1962) [9], and Metzger
et al. (1964) [10] reported first results of the Ranger missions. Vinogradov et al. (1966 & 1968) [11, 12] reported on
the Luna missions.
Key results, as exemplarily expressed in Vinogradov
et al. (1966), are that the ratio of gamma radiation between
lunar maria and lunar highlands is approximately 1.15 to
1.2, that natural radioactivity of the lunar surface indicates
its correspondence to terrestrial basalts, and that only about
10% of the total gamma ray intensity measured on the lunar surface comes from natural radioactivity. These findings
provided valuable information for future lunar gamma ray
missions. Accomodations had to be made to measure cosmic
ray induced inelastic neutron scattering and neutron capture
gamma ray lines.
To some degree the Russian Venera 8, 1972, and Mars 5,
1974, missions also belong into the trial period [13, 14].
3.2 Proof of concept
The opportunity to provide proof of concept for orbital geochemical mapping through planetary gamma ray spectrometry came with the United States manned Apollo missions
to the Moon. Apollo 11 and Apollo 12, both flown in 1969,
landed the first human beings on the Moon and returned
22 kg and 34 kg, respectively, of lunar material from their
lunar equatorial landing sites. This was followed by the Russian robotic return missions Luna 16, 1970, Luna 20, 1974,
and Luna 24, 1976, with 101 g, 30 g, and 170.1 g, respectively. These samples, in particular the Apollo landing site
ones, provided an important standard or „ground truth“ for
absolute elemental abundance determinations with orbital
gamma ray spectrometers or other measurement devices.
Based on missions during the trial phase and knowledge
about celestial body compositions, a fundamental understanding of characteristic gamma ray production became
available [15–17].
Apollo 15, 1971, and Apollo 16, 1972, carried a NaI(Tl)
scintillation detector each attached to the Command Service
Module (CSM) on a 7.6 m long boom. These detectors could
measure lunar surface gamma rays along the 100 km altitude
circular nearly-equatorial orbit of the CSM during lunar surface operations for a combined time of about 10.5 d. Data,
covering energies from 0.2 to 10.0 MeV, the new standard
for planetary gamma ray spectrometry, were stored on the
board computer of the return vehicle so that telemetry was
not needed. The orbits of the CSMs on Apollos 15 and 16
covered about 20% of the lunar surface.
The experiment provided geochemical maps of the areas
covered by the CSM orbit of the naturally radioactive element Th, as well of the elements Fe and
Ti with relatively
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P. A. J. Englert
Table 1. Successfull planetary gamma ray spectrometry experiments.
Year
Mission
name
Object
studied
Country Mode,
orbit
Data
acquisition
time (planet)
Detector Shielding
anti-coincidence
1962
Ranger 3
Moon
USA
Fly-by
4h
CsI(Tl)
Plastic scintillator
1962
Ranger 5
Moon
USA
Fly-bay
4h
CsI(Tl)
1965
Kosmos 60
Moon
USSR
Lander
none
Na(Tl)
1966
Luna 10
Moon
USSR
Elliptical
1966
Luna 12
Moon
USSR
1971
Apollo 15
Moon
1972
Apollo 16
1972
Boom
length
Gamma
energies
Boom
1.8 m
0.2–2.0 MeV
Plastic scintillator
Boom
1.8 m
0.2–2.0 MeV
Plastic scintillator
Integrated
0.2–3.0 MeV
57 d
NaI(Tl) Plastic scintillator
Integrated
0.2–3.0 MeV
Elliptical
85 d
NaI(Tl) Plastic scintillator
Integrated
0.2–3.0 MeV
USA
Equatorial
at 100 km
10.5 d
combined
NaI(Tl) Plastic scintillator
Boom
7.6 m
0.2–10.0 MeV
Moon
USA
Equatorial
at 100 km
10.5 d
combined
NaI(Tl) Plastic scintillator
Boom
7.6 m
0.2–10.0 MeV
Venera 8
Venus
USSR
Lander
50 m
CsI(Tl)
None
Integrated
0.2–3.0 MeV
1974
Mars 5
Mars
USSR
Orbiter
23 d
(22 elliptical
orbits)
CsI(Tl)
None
Integrated
0.2–3.0 MeV
1988
Phobos 2
Mars
Elliptical
equatorial
120 m
(2 elliptical
orbits)
CsI(Tl)
None
Integrated
0.2–4.0 MeV
1992
Mars
Observer
Mars
USA
Circular
polar, at
400 km
none
HPGe
Boron-loaded
plastic scintillator
1996
Near Earth Eros
Asteroid
Rendezvous
(NEAR)
USA
Orbiter
7 weeks
at 35 km,
7d
NaI (Tl) BGO
Integrated
1998
Lunar
Prospector
Moon
USA
Circular polar,
at 100 km
& 30 km
300 d &
220 d
BGO
Boron-loaded
plastic scintillator
Boom
2.5 m
0.2–10.0 MeV
2001
2001 Mars
Odyssey
Mars
USA
Circular
polar, at
400 km
> 6 years
(> 3 Martian
years)
HPGe
Boron-loaded
plastic scintillator
Passive Boom
6.2 m
0.2–10.0 MeV
2004
Messenger
Mercury USA
3 fly-bys
Elliptical
polar
Few hours
planned:
1 year
HPGe
Boron-loaded
plastic scintillator
Active
Integrated
0.2–10.0 MeV
2007
Selene
(Kaguya)
Moon
JPN
Circular polar,
at 100 km
& 50 km
1.5 years
HPGe
BGO. plastic
scintillator
Active
Integrated
0.2–12.0 MeV
2007
Dawn
Vesta
Ceres
USA
High and
low altitude
circular
Planned:
BGO
6 months at
CZT
each asteroid array
Boron-loaded
plastic scintillator
Integrated
0.2–10.0 MeV
2007
Chang’e-1
Moon
CHN
Circular polar,
at 200 km
1 year
CsI(Tl)
CsI(Tl)
Integrated
0.2–9.0 MeV
2008
Chandrayaan-1
Moon
IND
Circular polar,
312 d
at 100 and 200 km
CZT
array
CsI(Tl)
Integrated
< 250 KeV
Lander
Cooling Location
Passive Boom
4.3 m cruise 0.2–10.0 MeV
6.0 m orbit
0.2–10.0 MeV
In coulmun 4 the following abbreviations were used to identify countries: USA – United States of America, USSR – Soviet Union; JPN – Japan, CHN –
China, and IND – India. In column 7 the following conventions were used to describe gamma ray sensors: NaI – Sodium Iodide, CsI – Cesium Iodide,
BGO – Bismuth Germanate, CZT – Cadmium Zink Telluride, HPGe – High Purity Germanium, Tl - Thallium.
nificant differences of elemental abundances between the far
side and near side of the Moon were reported [18, 19].
The Apollo 15 and 16 lunar gamma ray experiment
provided proof that planetary gamma ray spectrometry is
a valuable and by now proven method of elemental abundance measurements for planetary surfaces. The principle
of ground truth or comparator methodology
for absolute
Unauthenticated
abundance determination
was
established,
mapping
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Planetary gamma ray spectrometry: remote sensing of elemental abundances
the flight path completed, and the results provided valuable
source data addressing the origin and geological history of
the Moon.
3.3 Investigation
Despite the success of the Apollo gamma ray experiment
planetary exploration proceeded over almost two decades
with many space and planetary exploration missions not carrying planetary gamma ray spectrometers. Despite of the
spectacular success of Apollo, many fundamental nuclear
science and radiation detection, measurement and evaluation
questions needed to be answered.
Of the many scientific and technical developments that
aided planetary gamma ray spectrometry in becoming
a standard modality in geochemical space exploration, some
were specifically undertaken for its development. Other advances came from developments in other areas of science
and technology, the closest being from gamma ray astrophysics and otherwise from general nuclear science and
space technology. These include general advancements in
solid state detector development, such as the HPGe detectors with gamma ray energy resolutions superior to those
of scintillation detectors, and associated gamma ray spectra
deconvolution techniques.
Of the several streams of developments during this time
frame specifically targeted on planetary gamma ray spectrometry, a few examples will illustrate important areas of
fundamental and applied nuclear science to further its advancement. Understanding gamma ray yields, especially for
inelastic neutron scattering reactions, knowledge of production cross sections is necessary. Cross sections for major
rock-forming elements are useful in model calculations, especially for geochemical environments for which no ground
truth standards exist. This was at the time the case for all
planetary bodies but the Moon. A limited set of inelastic
cross section measurements were conducted [20].
Interactions of cosmic radiation with planetary objects
are complex and show variations with major elements, such
as Fe and/or Ti, and minor and trace element composition,
such as Gd, of the location of interest. Model calculations
can reflect this complexity and were, in general, verified in
their applicability through comparisons with complex but
controlled terrestrial simulation experiments. Such experiments were completed in preparation of the Mars Observer
mission. So-called “Thick Targets”, simulating a planetary
surface were exposed to beams of accelerator produced
high-energy protons of known flux and energy while gamma
rays emitted from its surfaces were measured with highresolution HPGe detectors [21]. Variations in Thick Target
composition simulated expected variations in Martian surface composition. Gamma ray spectra for two proton energies and several variations of expected Martian composition
were produced and analyzed. These experiments are useful as their gamma ray spectra are very similar to actual
HPGe spectra from Mars and Moon. They are diagnostic in
that over 270 gamma ray lines comprising more that 95%
of all present were identified (target and background), and
many diagnostic gamma ray lines of elements of interest
were available for quantitative evaluation. Comparisons with
model calculations were also undertaken [22].
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Solid-state detectors, and especially high volume HPGe
detectors, lose resolution due to radiation damage caused by
exposure to cosmic radiation. This becomes an issue when
planetary exploration experiments are designed for extended
durations, e.g. of several years, of cruise to the target and
measurement times to obtain gamma ray spectra providing
reasonable uncertainties for the elements of interest. Therefore a set of radiation damage experiments with high volume
p- and n-type HPGe detectors was conducted with highenergy protons. The experiments showed the superiority of
n-type HPGe detectors for planetary gamma ray spectrometry and provided the experimental basis for the design and
optimization of in-space annealing procedures. Annealing
design and procedures maintained the Mars Odyssey gamma
ray spectrometer operational at acceptable resolution for
over six years [23].
Many other experiments contributing to the performance
and of planetary gamma ray spectrometers in space continuing beyond the time frame called investigation phase
were completed. Among others, the problem of mechanical cooling has been solved recently, extending operational
capabilities of HPGe detector use [24]. However, the few
examples presented demonstrate that basic and applied nuclear science investigations were a key element in preparing
the modality of planetary gamma ray spectrometry for its
application-oriented expansion.
3.4 Geological exploration
Missions that, with some exceptions, completed or intended
to complete orbital geochemical mapping of celestial bodies
characterize the geological exploration phase of planetary
gamma ray spectrometry. Two experiments that led into exploration are the 1997/98 Lunar Prospector Mission [25],
and the Mars Observer Mission of 1992, although the latter did not produce planetary data [26]. Both had functioning
gamma ray spectrometers on board. Mars Observer, for the
first time in planetary exploration was carrying a passively
cooled HPGe crystal as gamma ray sensor. The contemporary1988 Phobos 2 mission had a few close passes of the
Martian surface where gamma ray spectra could be taken by
its scintillation detector before the spacecraft failed [27].
Subsequent exploration missions to asteroid Eros, Moon
and Mars were all completed successfully or are in progress.
Two missions in progress are the United States MESSENGER, 2004, and Dawn, 2007, missions to Mercury, and the
asteroids Vesta and Ceres [28, 29].
Of the completed missions, NEAR (Near Earth Asteroid Rendezvous) functioned as a Lander operating a spacecraft integrated NaI(Tl) spectrometer with a BGO shield
for 7 days after having completed orbital measurements far
from the asteroid Eros [30]. Most important, following Lunar Prospector, was the completion of the Mars Odyssey
Mission in 2009 providing elemental maps for more than ten
major and minor elements [31].
Of later missions, three had circular polar orbits around
their lunar target, capable of complete geochemical mapping. Two US missions underway, MESSENGER and
Dawn, will arrive at their main or first targets Mercury and
Vesta in 2011,to assume elliptical and Unauthenticated
circular orbits respectively. Both carry advanced
gamma
ray
instrumentation,
an
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354
actively cooled BGO shielded HPGe and BGO and Cadmium Zink Telluride (CZT) detectors shielded with borated
plastic scintillators, respectively.
The first ever complete geochemical mapping of the
Moon by gamma ray and neutron spectrometry, or any
planet for that matter, was achieved by the Lunar Prospector, 1998, which operated instruments on 2.5 m booms at an
altitude of 100 km for 300 d, and at an altitude of 30 km for
220 d [32]. The experiment marks the transition from proof
of concept to the first full application of the capabilities
of gamma ray spectrometry for planetary exploration [33].
2001 Mars Odyssey followed immediately with a complete
coverage of Mars using a passively cooled HPGe detector on
a 6.2 m boom and neutron detectors at an altitude of 400 km.
The experiment took data for over six terrestrial years (three
Martian years) until operation of the ceased to conduct other
experiments with the spacecraft. Complete elemental maps
K, Th, Si, H, Cl, Ca, Al, O, S, and U were obtained [3, 31,
34]. Together, these missions verify the full capability and
scientific value of planetary gamma ray spectrometry.
In a different way, the lunar polar orbiters launched by
Japan, China, and India in 2007 and 2008, establish planetary gamma-ray spectrometry as a routine method of remote
geochemical analysis. Each one of these experiments has additional and interesting features that make them unique. The
SELENE (Kaguya) mission, 2007, used for the first time
an actively cooled HPGe to measure lunar gamma rays at
orbits of 100 km and 50 km altitude, respectively, for combined accumulation time of 1.5 years [35]. The Chang’e-1
mission, 2007, used a CsI(Tl) detector with a CsI(Tl) anticoincidence operating at an altitude of 200 km for more
than one year [36]. Objectives of both missions are similar
to that of the Lunar Prospector. The Chandrayaan-1 mission, 2008, operated at an altitude of 100 km and 200 km
respectively. In contrast to all most of the other gamma ray
experiments which covered gamma ray energies between
0.2 and 10.0 MeV, the Chadrayaan-1 team used a CdZnTe
detector array capable of measuring x rays and gamma rays
below 250 KeV, an interesting complement to other recent
lunar gamma ray experiments, focused on naturally radioactive elements Th and U [37].
With five orbital planetary gamma ray experiments to
Moon and Mars completed in the last decade and two more
complex experiments to asteroids and Mercury under way
planetary gamma ray spectrometry is now established as
a standard modality of remote geochemical analysis.
4. Discussion and conclusions
Five decades after the first proposal in 1958 for the development of planetary gamma ray spectrometry as a unique
nuclear analytical technique for planetary exploration the
experiments launched in the past decade have firmly embedded it as a standard modality of space exploration. Many
technical and scientific problems had to be resolved through
supporting work in basic and applied nuclear sciences. However, the field itself with increasing success is not any
more directly dependent on underpinning nuclear science
research. Its major drivers are now planetary geoscience objectives that can be addressed with the current inventory of
P. A. J. Englert
nuclear science knowledge. This does not mean that all basic and applied nuclear science issues are resolved. It only
means that the driving force for additional nuclear science
investigations has become a function of planetary exploration objectives and will be employed only when these
objectives require it.
The development of planetary gamma ray spectrometry
is only one of many examples of the contributions nuclear
science has made to many other fields of research. The development history of planetary gamma ray spectrometry also
falls into a larger framework of knowledge growth and the
establishment of new areas of research and knowledge at the
intersection between originally separate scientific domains.
Under this framework basic nuclear science has made many
contributions over time to knowledge growth in many different areas, a fact that underlines the importance of basic
nuclear sciences.
Acknowledgment. This work was possible through long-term support
from NASA to the Mars Observer and 2001 Mars Odyssey missions,
and through continued collaboration with the Max Planck Institute for
Chemistry, Mainz, Germany.
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