Editing Lunar Prospector (section)

=== Gamma Ray Spectrometer (GRS) ===
[[File:Lunar prospector instrument lp grs fs lg.gif|thumb|''Lunar Prospector'' [[Gamma Ray Spectrometer]] (GRS) ]]
[[File:Lunar Thorium concentrations.jpg|thumb|[[Thorium]] concentrations on the Moon, as mapped by ''Lunar Prospector'']]
The ''Lunar Prospector'' [[Gamma Ray Spectrometer]] (GRS) produced the first global measurements of [[gamma-ray]] spectra from the lunar surface, from which are derived the first "direct" measurements of the chemical composition for the entire lunar surface. 

The GRS was a small cylinder which was mounted on the end of one of the three {{convert|2.5|m|abbr=on}} radial booms extending from ''Lunar Prospector''. It consisted of a bismuth germanate crystal surrounded by a shield of borated plastic. Gamma rays striking the bismuth atoms produced a flash of light with an intensity proportional to the energy of the gamma ray which was recorded by detectors. The energy of the gamma ray is associated with the element responsible for its emission. Due to a low signal-to-noise ratio, multiple passes were required to generate statistically significant results. At nine passes per month, it was expected to take about three months to confidently estimate abundances of thorium, potassium, and uranium, and 12 months for the other elements. The precision varies according to element measured. For U, Th, and K, the precision is 7% to 15%, for Fe 45%, for Ti 20%, and for the overall distribution of KREEP 15% to 30%. The borated plastic shield was used in the detection of fast neutrons. The GRS was designed to achieve global coverage from an altitude of approximately {{convert|100|km|abbr=on}} and with a surface resolution of {{convert|150|km|abbr=on}}.<ref>{{cite journal | journal = Science | date = 1998 | volume = 281 | issue = 5382 | pages = 1484–1489 | doi = 10.1126/science.281.5382.1484 | title = Global Elemental Maps of the Moon: The ''Lunar Prospector'' Gamma-Ray Spectrometer | author = D. J. Lawrence |author2=W. C. Feldman |author3=B. L. Barraclough |author4=A. B. Binder |author5=R. C. Elphic |author6=S. Maurice |author7=D. R. Thomsen | pmid = 9727970 | bibcode=1998Sci...281.1484L| doi-access =  }}</ref>

The instrument mapped the distribution of various important elements across the Moon. For example, the ''Lunar Prospector'' GRS identified several regions with high iron concentrations.<ref>{{cite web|title=Iron Distribution - ''Lunar Prospector''|url=http://lunar.arc.nasa.gov/results/images/ironmap.jpg|publisher=[[NASA]]|access-date=July 14, 2008|url-status=dead|archive-url=https://web.archive.org/web/20080626081909/http://lunar.arc.nasa.gov/results/images/ironmap.jpg|archive-date=June 26, 2008}}</ref>

The fundamental purpose of the GRS experiment was to provide global maps of elemental abundances on the lunar surface. The GRS was designed to record the spectrum of gamma rays emitted by:
# the radioactive decay of elements contained in the Moon's crust; and
# elements in the crust bombarded by cosmic rays and solar wind particles.
The most important elements detectable by the GRS were uranium (U), thorium (Th), and potassium (K), radioactive elements which generate gamma rays spontaneously, and iron (Fe), titanium (Ti), oxygen (O), silicon (Si), aluminum (Al), magnesium (Mg), and calcium (Ca), elements which emit gamma rays when hit by cosmic rays or solar wind particles. The uranium, thorium, and potassium in particular were used to map the location of [[KREEP]] (potassium, rare-earth element, and phosphorus containing material, which is thought  to have developed late in the formation of the crust and upper mantle, and is therefore important to understanding lunar evolution). The GRS was also capable of detecting fast (epithermal) neutrons, which complemented the neutron spectrometer in the search for water on the Moon.