Mission scientists acknowledge a small chance of success, but they hope the crash will kick up a cloud of debris that includes a small quantity of water buried at the lunar poles. Earth-based telescopes and the Hubble Space Telescope will scan the debris cloud for signs of the water.

Los Alamos scientists built three of the instruments aboard Prospector, including the one whose measurements quantified the amount of water-ice that could reside in numbingly cold lunar craters. The two other instruments measure geologic composition and outgassing from seismic events.

Lunar Prospector, part of NASA's Discovery Program of low-cost, fast-track space missions, was envisioned and is led by Alan Binder of the Lunar Research Institute.

The technology and skills behind Los Alamos' three on-board spectrometers -- one for measuring neutrons, one for gamma rays, and one for alpha particles -- were born in Los Alamos' nuclear weapons mission and honed on various satellites for monitoring compliance with weapons nonproliferation treaties. Astrophysical and solar system studies have been a natural follow-on for these capabilities.

"Lunar Prospector represents the first time neutron spectroscopy has been applied to planetary exploration," said Bill Feldman, leader of the Los Alamos team. Feldman has been ecstatic since the first data came rolling in.

"The data returned have been absolutely beautiful," Feldman said. "This is everything one could have dreamed of."

The neutron measurements rely on high-energy cosmic rays slamming into the lunar soil. The cosmic rays generate a microscopic spray of nuclear particles, including neutrons. The neutrons bounce around in the soil, losing a little energy in each collision, before emerging from the soil and heading upward to where an orbiting spacecraft can detect them.

The Los Alamos spectrometer measures neutrons in three energy ranges: fast, medium and slow. The relative proportion of the three neutron energies is a key indicator of the material in the lunar soil.

In particular, hydrogen is very efficient at moderating the energy of the neutrons; over regions with enhanced hydrogen the detector receives a deficit of medium-energy neutrons. Scientists believe water molecules are the most likely form for any hydrogen resident in the lunar soil.

The Lunar Prospector team has now combined data from the craft's high-altitude and low-altitude orbits. The lower-altitude observations provide higher-resolution maps.

"Both results argue for small, unresolved deposits of hydrogen around the northern pole," Feldman said. "The concentrations extend over a larger area around the southern pole, and these data are consistent with islands of enhanced hydrogen deposits -- which we interpret as water ice -- immersed within regolith evenly spread with enhanced hydrogen."

The scientists estimate a hydrogen abundance equivalent to about two billion tons of water in total near the moon's poles. Soil bearing enhanced hydrogen and perhaps frozen water would allow engineers to contemplate ways to mine this precious resource to support colonization.

In research led by Los Alamos' David Lawrence, scientists have used the gamma-ray data to trace out key elements in the lunar soil that bear on the moon's formation and evolution. Cosmic ray-sparked collisions create many of the gamma rays that emerge from the lunar soil, but one key element, thorium, generates its own distinct gamma ray as a result of radioactive decay.

Thorium is part of a chemical amalgam dubbed "KREEP." KREEP represents the last part of a planet's or moon's molten mix to solidify. When it freezes, the KREEP is sandwiched between the relatively lightweight material that floated to the top of the melt to form the crust and the heavier material that sank to form the mantle.

Thorium or other KREEP constituents seen on the surface are evidence of volcanic processes or large meteor impacts that can punch through the crust and dredge up material from the interior.

"We've conclusively demonstrated that the surface occurrence of KREEP is mainly confined to the nearside region in and around the Imbrium basin," Lawrence said. Imbrium basin, one of the distinctive dark areas that make up the "man in the moon," was created by an ancient meteor impact nearly four billion years ago.

"One possible implication of this result is that KREEP was not uniformly distributed over the entire moon, but mostly confined to the nearside region," Lawrence said.

On the moon's far side, the highest thorium abundance occurs near Imbrium's antipode, the surface point defined by a line drawn from Imbrium, through the moon's center and out the other side. This suggests material ejected by the Imbrium impact traveled around the moon and settled out at the antipode.

"These data will play an important part in refining theories for how the moon formed and evolved," Lawrence said.

Los Alamos' third instrument aboard Lunar Prospector is an alpha particle spectrometer. Alpha particles are emitted in various radioactive decay processes, including that of radon, a gas that accumulates underground and can be released in seismic events. Scientists intend to analyze the data for signs of such seismic activity, but have not yet had time to do so.

The most recent scientific results from the Los Alamos team will appear in the Journal for Geophysical Research and Geophysical Research Letters. Team members include Feldman, Lawrence, Rick Elphic, Bruce Barraclough and Tom Prettyman. Sylvestre Maurice and Isabel Genetay of the Midi-Pyrenees Observatory in France are co-authors.

This work has been supported by the Department of Energy, NASA, Lockheed-Martin and the Observatoire Pic Midi.

A Private Lunar Sampler Mission

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A Dry Moon
Stanford - July 29, 1999 -
In the last few years, measurements made by two spacecraft -- Clementine and Lunar Prospector -- have been interpreted as evidence that large amounts of water ice are present in areas of deep shadow in the Moon's polar regions.