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An introduction
to the Gamma Ray Spectrometer
The Lunar Prospector Gamma-Ray Spectrometer (GRS) has acquired the first global
measurements of gamma-ray spectra from the lunar surface. Since gamma-rays coming
from the lunar surface carry information about lunar elemental composition, this
data set comprises the first direct elemental composition measurements that have
been made for the entire lunar surface.
It has long been known that a full understanding of the surface elemental composition
of the Moon will significantly improve our understanding of lunar formation and evolution.
For example, one long-standing issue of lunar formation that can be addressed with
global composition data concerns the elements Al, U, and Th (refractory elements)
and FeO content of the Moon. There are suggestions from Apollo, Galileo, and Clementine
data that the Moon is enriched, that is has greater abundances of these refractory
elements and FeO compared to the Earth. If the Moon indeed has such enrichments,
then lunar origin models which assume that most of the Moon's material comes from
the Earth's mantle (such as the giant impact hypothesis) would be incorrect. Another
issue that can be addressed using composition data concerns the variability and evolution
of the lunar highlands as traced by the material KREEP. KREEP, associated with thorium,
is a material thought to have formed between the lunar crust-mantle boundary, so
its distribution on the lunar surface can give information about how the lunar surface
has evolved over time. The following two sets of images show LP thorium (Th) data
side by side with Clementine images of the Moon's near and far sides. White outlines
on the data half of the image show major topographical features such as the Mare
Imbrium impact basin near the center of the nearside images. The LP data images shows
a clear picture of increased thorium deposits sprayed out around the impact basin,
suggesting that the basin was formed by an impactor large enough to breach the thin
nearside crust, reaching the KREEP/Thorium layer and shooting out these elements
in the ejecta.
A number of other lunar science issues can be addressed using GRS data; these
include:
1) Identifying and delineating basaltic units in the maria;
2) Determining the composition of ancient or "Cryptic" mare units found
in the highlands using Clementine data, and searching for more of these units using
mainly the Fe and Ti data;
3) Identifying and delineating highland petrological units;
4) Searching for anomalous areas with unusual elemental compositions that might
be indicative of deposits with resource potential.
When this preliminary map is compared to data obtained by both the LP neutron
detector and earlier Clementine data, it is seen that all of the known regions of
high iron concentrations are identified with this LP GRS data. For example, high
iron concentrations are seen in the near side mare, the south pole Aitken Basin,
and Mare Australe. Interestingly, we also see high counts in regions not known for
having high iron content. There are indications from LP thermal neutron data that
some of these discrepancies with the Clementine data may be the result of iron deposits
in these regions having a mineralogical form not observable with the Clementine infrared
spectra.
This graph shows abundances of the ten elements that LP is mapping. The upper
graph shows the counts averaged for the entire Moon. The lower graph shows the distinctions
between a sample mare region (Imbrium) and a sample highlands regions (Joule).
There also appear, however, to be regions of high iron count rates on the lunar
farside that are not seen in either the thermal neutron data or the Clementine data.
Most of this region is lunar highlands thought to be relatively high in aluminum
abundance. Since the aluminum gamma-ray line (7.72 MeV ) is one of the only gamma-ray
lines that can produce an interference with the iron lines (7.6 MeV), this suggests
that some of these high count rate regions may be due to high aluminum abundances.
It should be stressed that this is only a preliminary conclusion. A full analysis
of the entire gamma-ray spectrum needs to be completed final conclusions can be drawn.
 GRS Data showing iron distribution
on the Moon
 GRS Data showing potassium
distribution on the Moon
 GRS Data showing thorium distribution
on the Moon
To summarize, the data presented here show that the quality of Lunar Prospector
GRS data is very high and it contains a wealth of information about lunar composition.
Yet, while the data presented here is very exciting, over half of the GRS data is
yet to be collected. In addition, much work needs to be done in careful analysis
of this data so that all the information it contains is fully revealed.
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