Lunar Prospector Science Results

Ice on the Moon

The neutron spectrometer data show an enhancement of hydrogen concentrations at the lunar north and south poles, indicating significant amounts of water ice are trapped in permanently shadowed craters. More detail on this is available on the Ice on the Moon page.

Composition of the Surface

The neutron spectrometer is also sensitive to iron (Fe) and titanium (Ti) in the lunar soil and rock. Maps made using this data show three provinces: mare basalts, rich in Fe and Ti; highland areas, poor in Fe and Ti; and the floor of the South Pole-Aitken basin and rims of the circular maria on the nearside which are intermediate in Fe and Ti concentration.

The gamma-ray spectrometer data shows that the rocks rich in potassium, rare earth elements and phosphorus, known as KREEP, are concentrated in the Mare Imbrium rim, the nearside maria and highlands near Imbrium and the Mare Ingenii South Pole-Aitken basin and are distributed at a lower level in the highlands. KREEP is rich in uranium and thorium and is thought to represent the last remaining melt after the lunar crust formed. The distribution seen by Lunar Prospector supports the idea that the impact which formed Mare Imbrium excavated KREEP-rich rocks and ejected them over the Moon and the South Pole-Aitken basin impact also exposed KREEP-rich material.

Gravity Data

Results from S-band tracking of Lunar Prospector and calculations based on the Doppler shift of the signal have given more detailed information on the lunar gravity field. This shows that the Moon has a moment of inertia of 0.3929 to 0.3933, implying a small dense core with a mass of 1% to 4% of the Moon's mass, in agreement with earlier estimates. This translates to a core radius of 220 km to 590 km. The smaller 220 km value holds for a core of pure iron, while the 590 km estimate applies to a core of less dense iron sulfide. This small iron core may lend support to the hypothesis that the Moon was formed by a giant impact on the Earth during its early accretion.

Seven new mascons (mass concentrations - large areas of high density material in or below the lunar crust, usually associated with impact basins) were discovered in the gravity data, three on the near-side and four on the far-side. It has been thought that the higher densities were due simply to heavy basalt filling the basins underlain by a plug of dense mantle material. However, some of the newly discovered mascons are not basalt-filled, suggesting all of the mass anomaly associated with these basins is due to the mantle plug.

Magnetic Fields

The Moon does not have a global magnetic field, but it has been known since Explorer 35 measurements in 1967 that localized magnetic fields exist. The Lunar Prospector magnetometer / electron reflectometer has measured particularly strong fields in the regions antipodal (on the exact opposite side of the Moon) to the large Mare Imbrium and Mare Serentatis basins. The field antipodal to Mare Imbrium is so strong it can deflect solar wind particles and form its own small magnetospheric system. The presence of these strong fields supports the idea that shock heating took place in the presence of a strong magnetic field (shock remnant magnetism), which in turn implies that the Moon had a global magnetic field (and presumably a molten core) at the time of the impacts, about 3.6 to 3.8 billion years ago.

Alpha Particle Spectrometer

The Alpha Particle Spectrometer (APS) is designed to detect radon outgassing events on the surface of the Moon. Analysis was initially complicated by solar energetic particle events which produced large fluxes of alpha particles which had to be corrected for. The alpha-particle count due to radon-222 decay indicates radon gas is currently emanating from the vicinity of craters Aristarchus and Kepler. The LP gamma-ray spectrometer found thorium enrichments at these craters, implying likely uranium enrichment as well. The APS data show only marginal detection of alpha particles resulting from decay of polonium-218, but the polonium-210 (which decays later than the radon-222) peaks do not correllate with radon-222, indicating that lunar gas release events are variable in time and space.

References

Binder, A. B., Lunar Prospector: Overview, Science, 281, 1475-1476, Sept. 1998

Elphic, R. C., D. J. Lawrence, W. C. Feldman, B. L. Barraclough, S. Maurice, A. B. Binder, and P. G. Lucey, Lunar Fe and Ti abundances: Comparison of Lunar Prospector and Clementine data, Science, 281, 1493-1496, Sept. 1998.

Feldman, W. C., B. L. Barraclough, S. Maurice, R. C. Elphic, D. J. Lawrence, D. R. Thomsen, and A. B. Binder, Major compositional units of the Moon: Lunar Prospector thermal and fast neutrons, Science, 281, 1489-1493, Sept. 1998.

Feldman, W. C., S. Maurice, A. B. Binder, B. L. Barraclough, R. C. Elphic, and D. J. Lawrence, Fluxes of fast and epithermal neutrons from Lunar Prospector: Evidence for water ice at the lunar poles, Science, 281, 1496-1500, Sept. 1998.

Konopliv, A. S., A. B. Binder, L. L. Hood, A. B. Kucinskas, W. L. Sjogren, and J. G. Williams, Improved gravity field of the moon from Lunar Prospector, Science, 281, 1476-1480, Sept. 1998.

Lawrence, D. J., W. C. Feldman, B. L. Barraclough, A. B. Binder, R. C. Elphic, S. Maurice, and D. R. Thomsen, Global elemental maps of the moon: The Lunar Prospector gamma-ray spectrometer, Science, 281, 1484-1489, Sept. 1998.

Lawson, S.L., W.C. Feldman, D.J. Lawrence, K.R. Moore, R.D. Belian, S. Maurice, and A.B. Binder Results from the Lunar Prospector Alpha Particle Spectrometer: Detection of Radon-222 Over Craters Aristarchus and Kepler, Abstract, DPS 2001 Meeting, 4.05, Nov. 2001.

Lin, R. P., D. L. Mitchell, D. W. Curtis, K. A. Anderson, C. W. Carlson, J. McFadden, M. H. Acuna, L. L. Hood, and A. Binder, Lunar surface magnetic fields and their interaction with the solar wind: Results from Lunar Prospector, Science, 281, 1480-1484, Sept. 1998.

Lunar Prospector page

Articles on science results - Science magazine

Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258