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An introduction to the Neutron
Spectrometer
As reported at the March 5, 1998 press conference, based upon telltale dips in
the epithermal neutron energy spectra sent back to Earth by ProspectorÕs 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 exist no precise models describing exactly how neutrons on the lunar
surface behave. Further extensive analysis of the NSÕs reams of data -- with
the help of newly constructed computer algorithms -- have allowed scientists to advance
the accuracy of the amount of water buried in the lunar polar regions.
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.
To hear more information about water on the Moon, please select one of the following:
At the current point in the primary mission, there seems to be a ten fold increase
in the detected amount of water ice at the poles. 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
the "fast" neutron data which indicates "confined", that is discrete
deposits of pure water ice buried beneath roughly 50 cm of dry regolith.
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.
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 Prospector's 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, Prospector's instruments will be able to gather
extremely high resolution data.
To see an animation about how the Neutron Spectromter works, please select one
of the following:
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