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A Short Summary of the Findings of Lunar Prospector

by Eric Douglass

Introduction

There are several areas in which the lunar prospector mission has enriched our understanding of the moon.  In this paper we will attempt to make some of the more important ones easily accessible.  In addition, we will include some of the implications for lunar geology.

A.  Water and the Moon

Without a doubt, the most popularized finding of lunar prospector was the finding of water at the moon's north and south poles. Water was expected to exist in cold traps--permanently shadowed craters in the polar regions.  Its source was from comet impacts on the lunar surface.

Lunar Prospector looked for water by measuring neutrons which were dislodged from oxygen atoms during cosmic ray bombardment of the surface.  Where these oxygen atoms were bonded to hydrogen (water), the energy of these neutrons were decreased much more than where they were bonded in silicates (surface minerals of plagioclase, metal oxides, etc.).  When the energies were measured (in the epithermal range), discrete decreases were found in the north and south polar regions.  This presumptive evidence for water was reinforced by location: they occurred in areas of permanently shadowed craters.  While this data doesn't tell us the geometry of the water deposits, the researchers used a series of assumptions to estimate the amount of water.  These assumptions included that the deposits are buried under 40 cm of regolith and that they are in a single layer.  In addition, they assumed that the depth is limited to 200 cm, which is the estimated amount of regolith gardening and accumulation over the last couple of billion years (before that the accumulation of deposits were either non existent due to surface heating from basin ejecta or they were buried under extensive megaregolith deposits).  Given these assumptions, their estimates of water were in the order of 3 billion tons for each polar region.

B. Lunar Core

Lunar Prospector was able to measure the moment of inertia of the moon.  This is important as it gives us information about the lunar core.  A planetary body with no core has a moment of inertia of 0.400.  However Lunar Prospector found a moment of inertia of 0.3931.  This means that there is a small core of high density, most likely composed of iron or iron sulfide.  If it is iron, scientists estimate its size at around 300 km.  A core this size implies that the moon was formed prior to the complete differentiation of the earth, and so has implications for the 'giant impact' hypothesis of moon formation.

C.  KREEP Material

Another finding of importance was the distribution of KREEP on the lunar surface.  KREEP stands for an assemblage of potassium, rare earth elements, and phosphorus.  As these are not easily incorporated into mineral structure, they were expected to remain in the last dregs of a crystallizing magma ocean.  Thus, they were expected to occur at depth (below an anorthositic crust).  So how did KREEP reach the surface?  The two theories are that it did so by early highland volcanism or by being exhumed by basin impacts. 

Prospector looked at KREEP distribution by counting gamma rays from thorium and potassium.  Examination shows that these elements have peaks in two areas.  The first area is in and around the Imbrium basin.  While the evidence is not conclusive, the distribution pattern is highly suggestive of KREEP being exhumed by the Imbrium impact.  If this were the case, than it is also highly likely that the other large basins reached the KREEP layer.  The reason we don't see evidence for this is that the Imbrium basin's ejecta covered over their ejecta blankets (part of the megaregolith).  The second peak area of KREEP distribution occurred in the region of the South Pole Aitken basin. While the reason for a peak here at first seems obscure (as it is one of the oldest basins, and any of its ejecta should have long ago been covered), it is at the antipode of the Imbrium basin.  This suggests possible mechanisms.

D.  Gravity Information and Basins

The Lunar Prospector also measured the gravity field of the moon using doppler shift from its change in altitude/velocity in low lunar orbit.  As its orbit was lower than other imaging missions of the past, the map had significantly greater resolution.  The most interesting finding from the mission was the finding a several new mascons, including one on the near side (Schiller-Zucchius).  Mascons arise in the area of massive impacts which produce basins.  While several competing models exist to explain mascons, they appear to arise from the loss of mass from the impacts (the crater itself), with subsequent elevation of a tongue of mantle (isostatic compensation vs. actual rebound),  and subsequent lava filling.  As the mantle and lava are both of higher density than the anothositic crust, these produce the gravity anomaly called a mascon.

E.  Magnetic Anomalies

Another area of interest that Prospector examined was the moon's magnetic anomalies.  The moon lacks a global magnetic field in the present, but magnetic anomalies have been found.  These tend to be antipodal to major basins, and are often associated with bright 'swirl like' features (such a gamma reiner).  Unfortunately, before Prospector, the extent of the magnetic field was not known due to limited mapping by the Apollo subsatellites.

Lunar Prospector mapped the full extent  of the magnetic fields, showing that they are indeed antipodal to major basins (eg: Imbrium, Serenitatis, Crisium).  The scientists propose that the moon had a global magnetic field in the ancient past powered by a core dynamo.  The region antipodal to the basins underwent shock at extreme pressures producing shock magnetism (from the original magnetic field).  These fields have sufficient strength to produce a local bow shock, keeping solar radiation (ions in the solar wind) from striking the surface.  This then would be the mechanism for keeping the swirls 'bright', though they are quite ancient and so should have darkened (as rays have from crater ejecta.).

F. Final Note

The Lunar Prospector has now finished its nominal mission, and is beginning its extended mission.  It has dropped its altitude and is now collecting information at higher resolution.  We look forward to seeing more data yet.

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