NEUTRON SPECTROMETER RESULTS

As reported at the March 5, 1998 press conference, based upon telltale dips in the epithermal neutron energy spectra sent back to Earth by Prospector's NS, mission scientists estimated that there might be 10 to 300 million metric tons of water ice (2.6 to 26 billion gallons) buried in permanently shadowed craters at the lunar poles. It was stated that the range could possibly be as much as an order of magnitude (factor of 10) too high or too low, because of the fact that Lunar Prospector was the very first interplanetary mission to use neutron spectroscopy to detect water, and thus there existed no precise models describing exactly how neutrons on the lunar surface behave. Further extensive analysis of the NS's reams of data -- with the help of newly constructed computer algorithms -- have allowed scientists to advance the accuracy of their estimates of the amount of water buried in the lunar polar regions. Specifically, the latest findings of the Neutron Spectrometer experiment include:

• A ten-fold increase in the detected amount of water ice at the poles than previously reported. That is, current estimates are 3 billion metric tons of water ice at each pole. The earlier, conscientiously conservative, estimates were based on the detected dips in medium energy (epithermal) neutrons at the two polar regions. These dips of 4.6% for the North pole and 3% for the South pole, remain essentially unchanged. The additional accuracy comes from an analysis of "fast" neutron data which indicates "confined," that is discrete, deposits of pure water ice buried beneath roughly 50 cm of dry regolith.

Medium energy neutron counts (LP data) showing the two polar dips which indicate water ice.

A glance at the current neutron data map clearly reveals the increasing definition of the target areas --- the likely water deposits. The North pole region, in particular, shows evidence of water deposits in permanently shadowed craters.

LP Neutron Spectrometer data from the north pole showing evidence of water ice (dark blue to magenta)

The NS instrument aboard Lunar Prospector can detect water (actually, hydrogen) to a depth of a half-meter (a foot and a half). However, since the lunar soil has been effectively "gardened" to a depth of two meters (six and a half feet) by meteoritic impacts over the past two billion years, the water could theoretically be present to that depth (two meters). If the water is in the form of ice crystals mixed with the regolith, pure water-ice deposits could potentially exist at much greater depths. Lunar Prospector scientists are still determining exactly how many craters at the North and South poles contain the millions of tons of water ice measured by the neutron spectrometer. Further data analyses, as well as data from another of Prospectorís instruments, the gamma ray spectrometer, will help mission scientists sort out the precise distribution of lunar ice. The most informative data is expected to be gleaned in just under a year, when the spacecraft begins its extended mission and dips down into a very low orbit of approximately six miles (10 kilometers) above the lunar surface. At this altitude, Prospectorís instruments will be able to gather extremely high resolution data.

LP Neutron Spectrometer data from the south pole showing evidence of water ice (dark blue to purple)

This image shows the distribution of medium energy (epithermal) neutrons for the entire moon. The left side globe is the near side of the Moon tipped to show the north pole. The dark purple region shows the dip in the neutron signal at the pole which indicates an excess of hydorgen. The "extra" hydrogen is the telltale signature of water ice at the Moon's poles in permanently shadowed craters. The right side image shows the far side tipped to show the south pole.

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