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GAMMA RAY SPECTROMETER RESULTS

Lunar Prospectorís gamma-ray spectrometer (GRS) is mapping the abundances of ten elements on the Moon's surface:

thorium (Th) silicon (Si)
potassium (K) aluminum (Al)
uranium (U) calcium (Ca)
iron (Fe) magnesium (Mg)
oxygen (O) titanium (Ti)

The GRS is especially sensitive to the heavy, radioactive element thorium and the light element potassium. These are particularly plentiful in the last part of the crust to solidify. Thus, mission scientists are able to determine the global distribution of KREEP (K-potassium, Rare Earth Elements, and P-phosphorous), a chemical "tracer" of sorts which helps to tell the story of the Moonís volcanic and impact history. The data produced by the GRS are helping scientists to understand the origins of the lunar landscape, and may also tell future explorers where to find useful metals like aluminum and titanium. Specific results of this experiment include:

  • 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. Specifically, preliminary information has been gathered on the distribution of key elements, including Iron and Titanium, along with trace elements associated with lunar KREEP (K-potassium, Rare Earth Elements, and P-phosphorous) material. The global distribution of three major lunar rock types (mare basalts, noritic rocks, and anorthositic rocks) has been compiled.
  • This global mapping has provided further insight into the mechanisms underlying the distribution of various elements about the Moon --- in particular the excavation and deposition of KREEP via lunar impact events.

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 aluminum, uranium, thorium (refractory elements) and iron oxide 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 iron oxide compared to the Earth. If the Moon indeed has such enrichments, then lunar origin models which assume that most of the Moonís material comes from the Earthís 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 data side-by-side with Clementine images of the Moonís near and far sides. White outlines on the data half of the image delineate the lunar maria and highlands boundaries. The nearside LP image clearly indicates that most of the thorium is concentrated on the near-side mare in and around Mare Imbrium. In addition, while it is known from Apollo sample returns that some of the thorium south of Mare Imbrium was produced by volcanic activity, these images also appear to show that some part of the thorium was spread on the lunar surface as a result of the impact that produced the Imbrium basin.

THORIUM CONCENTRATION MOON

A Comparison of LP's nearside thorium data and a corresponding image of the Moon.

The far side image makes an interesting comparison to the nearside image. The South-Pole Aitken Basin, which is the largest known impact basin in the solar system, is located on lunar farside (as indicated by the dark area in the farside Clementine image). Because this basin is so big, the impact that produced it must have dug much deeper into the Moon than any of the impacts on the nearside. The LP data, however, only shows a small amount increased thorium in this area. Since the lunar crust is thicker on the farside than on the nearside, it is possible that the impact which produced the SPA basin never dug deep enough to dredge up much thorium.

 

MOON SOUTH POLE ATKING BASIN

A Comparison of LP's farside thorium data and a corresponding image of the Moon.

In addition to mapping thorium and other elements, a number of other lunar science issues can be addressed using GRS data; these include: 1) Identifying and delineating basaltic regions in the lunar maria using maps of iron and titanium composition; 2) Determining the composition of hidden or "Cryptic" mare regions that were originally found in the lunar highlands using Clementine data; 3) Identifying and delineating highland petrological regions; and 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 spectrometer 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.

MAP OF TEN ELEMENTS

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 before final conclusions can be drawn.

iron distribution

GRS Data showing iron distribution on the Moon

 

potassium distribution

GRS Data showing potassium distribution on the Moon

 

Thorium distribution

GRS Data showing thorium distribution on the Moon

To summarize, the data collected by the Lunar Prospector GRS is of very high quality and contains a wealth of information about lunar composition. Yet, while the data presented here are very exciting, over half of the GRS data are 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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