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
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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