W. C. Feldman,
*
S. Maurice,
A.
B. Binder,
B. L. Barraclough,
R. C. Elphic,
D. J. Lawrence
Maps of epithermal- and fast-neutron fluxes measured by Lunar
Prospector were used to search for deposits enriched in hydrogen at
both lunar poles. Depressions in epithermal fluxes were observed close
to permanently shaded areas at both poles. The peak depression at the
North Pole is 4.6 percent below the average epithermal flux intensity
at lower latitudes, and that at the South Pole is 3.0 percent below the
low-latitude average. No measurable depression in fast neutrons is seen
at either pole. These data are consistent with deposits of hydrogen in
the form of water ice that are covered by as much as 40 centimeters of
desiccated regolith within permanently shaded craters near both poles.
W. C. Feldman, B. L. Barraclough, R. C. Elphic,
D. J. Lawrence, Los Alamos National Laboratory, MS D-466, Los
Alamos, NM 87545, USA. S. Maurice, Observatoire Midi-Pyrenees, 14 avenue Edouard Belin, 31400 Toulouse, France. A. B. Binder, Lunar
Research Institute, 1180 Sunrise Drive, Gilroy, CA 95020, USA.
*
To whom correspondence should be addressed. E-mail:
wfeldman@lanl.gov
The moon is depleted in all volatile
elements compared with Earth (1). However, water was brought
to the moon by comets and asteroids and was formed by the reduction of
FeO in lunar materials by solar wind hydrogen, and some juvenile water may have been released from the lunar interior over billions of years
(2, 3). Studies of the transport of such water
over the lunar surface after its release indicate that 20 to 50%
should be retained as frozen water ice within permanently shaded
craters near both poles (2-6). However, losses
due to meteoritic bombardment (3) and erosion due to
particle sputtering (7), or photodissociation by
interstellar hydrogen Lyman-
(8), may exceed the accretion rate preventing the development and retention of permanent ice deposits at the poles.
The interpretation of anomalously large intensities of same-sense
polarized radar echoes that are localized to permanently shaded craters
near the poles of Mercury (9-11) as caused by
deposits of nearly pure water ice suggests that similar deposits should
also exist on the moon. Although a report of a possible detection of
water ice on the moon with Clementine data (12) supports
this suggestion, it is not universally accepted (13,
14). We address the question of lunar water ice using
epithermal- and fast-neutron data measured using the Lunar Prospector
(LP) neutron spectrometer (NS).
Expected signature of H. A unique identification of chemical
species enriched in hydrogen and a characterization of their spatial
distribution are possible through measurement of neutron flux spectra
(15, 16). The magnitude of this effect at 100-km
altitude is illustrated with simulated neutron flux spectra (Fig.
1). The different curves give neutron
lethargy, L(E), as a function of energy,
E, for ferroan-anorthosite (FAN, a major type of soil or
regolith in the lunar highlands) containing various admixtures of
H2O (17). There are three general energy ranges
that correspond to low or thermal, intermediate or epithermal, and high
or fast neutrons, which reflect the neutron production and energy
moderation process. Fast neutrons are formed at high energies as the
result of interactions between galactic cosmic rays and the nuclear
constituents of the regolith. They transfer energy to nuclei in the
regolith at a nearly constant fractional rate during subsequent,
primarily elastic scattering collisions. This process results in an
epithermal flux spectrum that is proportional to
E
1 [or constant L(E)].
As their energy approaches that of the thermal motion of regolith
nuclei, they begin to absorb energy as fast as they lose it and thereby
develop a Maxwellian velocity distribution (the peaks at the lowest
energies in Fig. 1). The thermal-neutron population builds in amplitude
until the rate of injection from the fast-neutron population equals the
rate of absorption due to thermal capture reactions plus the rate of
loss to space.
Fig. 1.
Simulated neutron lethargy-energy spectra for a
FAN regolith containing various weight percent admixtures of
H2O (w). These spectra were generated with the
ONEDANT part of the DANTSYS code system (27) as discussed in
(16). From top to bottom, the spectra correspond to mass
fractions of H2O (w) of 0, 0.01, 0.1, 1, 3, 10, 30, and 100%, respectively. R, altitude of spacecraft.
The dominant effect of hydrogen (here assumed to be in the form of
water molecules) is to monotonically decrease the intensity of
epithermal and fast neutrons (Fig. 1). In contrast, the effect of
hydrogen on the thermal population is not monotonic, producing a
maximum between about 3 and 5 weight % of H2O. Also, all
the spectra slope downward toward lower energies in the epithermal range because of the absorption of neutrons by regolith nuclei at the
rate
|
(1)
|
where the sum is over all regolith elements i,
Ni is the number density of element
i, and
ai is the cross
section for absorption of a neutron having velocity
Vn by element i. This rate is
independent of neutron energy because, for most elements, Vn
ai is
constant. The number of neutrons absorbed in the epithermal range then
depends only on the time they spend losing energy between the fast- and
thermal-neutron energy ranges. Because this time decreases with
increasing hydrogen content, the spectra flatten out as the weight
percent of water increases from 0 to 100% (Fig. 1).
Instrumentation and data reduction. Neutrons were
measured with three different spectrometers aboard LP (18). Thermal and epithermal neutrons were detected with two 5.7-cm-diameter by 20-cm-long 3He gas-filled proportional counters
pressurized to 10 atm. One detector was covered by a 0.63-mm-thick
sheet of Cd and was sensitive only to epithermal neutrons [labeled
3He(Cd) in Fig. 1]. The second was covered by a
0.63-mm-thick sheet of Sn. Both sensors were matched so that the
difference in their counting rates provided a measure of the flux of
thermal neutrons (labeled "thermal" in Fig. 1). Fast neutrons were
measured with the anticoincidence shield (ACS) of the gamma-ray spectrometer (GRS). Here, events consisting of a time-correlated pair
of interactions were used to positively identify fast neutrons. The
pulse height of the first of the pair provided a measure of their
energy spectrum between about 0.5 and 8.0 MeV (18).
The trend of decreasing L(E) with increasing
water content at epithermal energies was simulated by integrating the
product of the energy spectra (Fig. 1) and the efficiency of the
3He(Cd) sensor (Fig. 2). A
similar calculation was made for fast neutrons with the efficiency
function estimated for the ACS of the GRS (19,
20). Epithermal and fast-neutron counting rates (epi-counts
and fast-counts, respectively) can be represented by functions of the
form
|
(2)
|
|
(3)
|
where w is the weight fraction of
H2O. These relations illustrate the large measurement
advantage provided by epithermal neutrons in detecting small amounts of
hydrogen. A 1% measurement precision corresponds to a hydrogen weight
fraction of [0.01/(61 × 9)] = 18 × 10-6. We
verified (but do not show here) that these spectra and counting rates
are not affected by a change in the composition from FAN to norite
(another major type of rock abundant in the regolith in the lunar
highlands). The fractional reduction in the epithermal-neutron counting
rate is a factor of 5 higher than that for the fast-neutron counting
rate.
Fig. 2.
Simulated epithermal and fast-neutron counting
rates as a function of mass fraction of H2O (w),
added to a FAN regolith.
The trend shown in Figs. 1 and 2 for an assumed surface
stratigraphy that has infinite horizontal spatial extent does not
represent the case for spatially confined deposits of enhanced
hydrogen. Spatially confined deposits are expected if water ice resides
in permanently shaded craters near both lunar poles. The results of a
series of simulations for circular deposits of pure water ice having a
radius of 24.3 km, which is surrounded by a desiccated FAN regolith and
covered over by a layer of the same regolith of varying thickness
d, are shown in Fig. 3. For
water deposits that are small compared with the spatial response function of the sensor (as chosen for these simulations), the depression in epithermal and fast-neutron counting rates for thin cover
layers (d less than ~10 cm) saturates at a value that is governed by the solid angle of the deposit as seen by the sensor. The
relative solid angle for an assumed circular water deposit having a
radius of 24.3 km is seen to be about 5%, which is consistent with a
foot point size for the epithermal-neutron sensor of 37,100 km2.
Fig. 3.
Simulated epithermal and fast-neutron counting
rates for a desiccated FAN layer of varying thickness d that
overlays a thick, circular deposit of water ice having a radius of 24.3 km, which is surrounded by a desiccated FAN regolith. Here and
throughout this study, a density of 2 g cm-3 is
assumed. Also, inf is an effectively infinite depth.
The percentage of reduction in the epithermal counting rate drops
off slowly with increasing burial depths, whereas that in the
fast-neutron counting rate drops by a factor of 2 for a burial depth of
15 cm (assuming a density of 2 g cm
3) and a factor
of 20 for 50 cm. This difference reflects the decreasing cross section
of hydrogen for elastic neutron scattering as a function of increasing
neutron energy. Beyond about 1 MeV, the cross section for hydrogen
becomes less than that for oxygen. Because of this effect, overlying
desiccated layers of regolith (which contain about 45% by mass of
oxygen) will effectively screen buried hydrogen-rich layers from
detection by fast neutrons.
Observations. The LP neutron data analyzed and
reported here cover the period between 16 January and 27 June 1998. Epithermal and thermal counting rates were constructed from sums of 3He(Cd) and 3He(Sn) pulse-height spectra
measured every 32 s (18) over a range of channels that
bracket the 763-keV peaks of the 3He(n,p)t reaction. These
peaks correspond to detection of low-energy neutrons. Fast-neutron
counting rates were generated by summing all background-subtracted,
correlated double-interaction events detected in the ACS of the GRS
[see (18) for details]. Counts were mapped with nearly
equal area pixels, each having a bin size equal to that contained
within a 2° latitude by 2° longitude box at the equator. All pixel
counts were normalized to a constant neutron production rate by
galactic cosmic rays with the intensity of the 6.13-MeV oxygen
gamma-ray line measured by the LP GRS. Corrections were also made for
the latitudinal dependence of instrument-response functions.
Histograms of thermal, epithermal, and fast-neutron counts (Fig.
4) show that the epithermal counting rate
distribution is narrower than those for the thermal and fast neutrons.
This narrow distribution reflects the relatively low sensitivity of epithermal neutrons to composition (21). Their residual sensitivity can be reduced further with Eq. 1, which for all elements
except Gd and Sm applies equally to thermal and epithermal neutrons.
Effects of composition on epithermal counting rates can therefore
be minimized by subtracting a fraction of the measured thermal counting
rate. We found that histograms of epithermal-neutron counts have a
minimum width if this fraction is 6.8%, which we use to define the
population of epithermal* neutrons [epithermal* = epithermal - (0.068 × thermal)]. Comparison of both epithermal histograms reveals that
the width of the epithermal* histogram is narrower, as evidenced
by the increase in peak amplitude (for the same number of epithermal
pixel counts). The effect is more noticeable in the full map of
epithermal counting rates. Whereas the epithermal map (not shown here)
contains shadows that enclose the maria, which are outlined in a map of
the thermal neutrons [figure 2 of (22)], these shadows are
much reduced in the map of epithermal* neutrons (Fig.
5). In addition, two concentrations of
dark pixels are visible at the lunar poles, which are neither circular
nor centered on the poles.
Fig. 4.
Histograms of counts per areal
pixel of thermal, epithermal, and fast-neutron counting rates. The
histogram labeled "epithermal*" is constructed by subtracting 6.8%
of the thermal histogram from the epithermal histogram.
Fig. 5.
A full-moon map of the epithermal* counting
rates. Both globes are tilted by 45° in latitude with the nearside
and North Pole at the left and the farside and South Pole at the right.
The scale given in the middle corresponds to the counts registered by
the Cd-covered 3He gas proportional counter in 32 s.
Longitudinal averages of epithermal* counting rates as a function
of latitude (Fig. 6) show only two
prominent minima, one at the North Pole and one at the South Pole. The
fractional decreases in polar counting rates have slightly different
shapes and are both offset from the poles. Although not demonstrated here, both depressions are broader than the spatial response function of the epithermal-neutron sensor. Whereas the peak of the depression at
the north is 4.6% relative to neighboring latitudes within about
±30° of each minimum, that at the south is 3.0%. An integration of
epithermal counting rates poleward of ±85° yields fractional depressions in epithermal* counting rates of 3.0 and 2.6% relative to
the full moon value, at the North and South Poles, respectively. A
similar calculation poleward of ±80° yields fractional depressions of 2.0 and 1.6%, respectively.
Fig. 6.
Averages and standard
deviations of all epithermal* counts for all equal area pixels in Fig.
5 within successive 2° wide latitude bins that are bounded in
azimuthal angle by planes that are parallel to the lunar polar axis and
separated by 2° in longitude at the equator (EQ).
A map of fast-neutron counting rates [figure 3 of
(22)] shows no evident decrease in the fast-neutron flux at
either pole that is consistent with the decreases in flux observed in the epithermal* map of the moon (Fig. 5). This perception is quantified
by constructing longitudinal averages and their standard deviations of
the fast-neutron counts as a function of latitude (Fig.
7). Although no substantial effect is seen, we can use these data to establish an upper limit for fast neutrons by comparing the lowest counting rate within ±5° of each pole with a smoothed interpolation of counts within a ±30° range of
each pole. This procedure gives upper-limit relative fast-neutron flux
decreases of 0.35% in the north and 0.5% in the south. The statistical precision of averaged counts at both poles is 0.25%. The
effect in fast neutrons is therefore more than about a factor of 10 smaller than that seen in epithermal* neutrons.
Fig. 7.
Averages and standard deviations of
all fast-neutron counts for all equal area pixels in figure 3 of
(22) within successive 2° wide latitude bins that are
bounded in azimuthal angle by planes that are parallel to the lunar
polar axis and separated by 2° in longitude at the equator.
Support for interpretation of the enhanced deposits of hydrogen
evident in epithermal-neutron data in terms of water ice is given by
comparing the locations of observed decreases in epithermal* counting
rates with locations of permanently shaded polar areas (Fig.
8). The largest decreases of counting
rates in the north (shown by the colors between purple and magenta) are seen to cover the north-facing rim of Peary crater and a linear trend
parallel to the +130° meridian that extends to +77° latitude. They
also cover parts of the rims of the Hermite, Rozhdestvenskiy, and
Plaskett craters. The largest decreases in the south (shown in purple)
are close to the pole but on the nearside of the rim of the South
Pole-Aitken basin. Reduced counting rates (shown in blue) follow a
linear trend along the -50° to +130° meridian and also cover
patches of the rim of the Schrodinger crater. Comparison with
Clementine 750-nm images (23) shows that all of these
locations contain many (unresolved by the NS) small areas that appear
to be in permanent shadow.
Fig. 8.
Overlay of epithermal* counting rates in each
2° by 2° equal area pixel poleward of ±70° with surface relief
maps of the lunar poles (28).
Discussion. Comparison of the upper-limit ratio of fast to
epithermal depressions in counting rates at both poles (about a factor
of 10) with the simulations (Fig. 3) shows that LP epithermal and
fast-neutron data are consistent with models of buried water-ice
deposits. If accumulation of more fast-neutron data in succeeding LP
map cycles confirms our present upper-limit estimate as real
depressions, then the measured epithermal- and fast-neutron fluxes are
consistent with theoretical expectations for pure water-ice deposits
buried beneath a dry regolith having a thickness of about 40 cm. These
deposits would then have total effective surface areas of about 1850 km2 at both poles.
This model is not unique because similar results could be obtained with
different combinations of (i) lower water-ice abundances in the buried
deposit, (ii) different surface area and surface distribution of the
deposit, (iii) different burial depth below dry regolith, and (iv) a
multilayered geometry containing alternate layers of water ice and dry
regolith. This last configuration could result from a series of comet
impacts that occurred at different times in the past.
If our interpretation of the LP neutron data is correct, a
partially buried water-ice deposit is consistent with interpretations of existing radar backscatter data (24). These data provide
only questionable evidence for polar water ice on the moon, including
areas that are within the few permanently shaded lunar craters that are
visible to Earth-based radar [see, for example, (24) and
references therein]. These null observations are consistent with the
presence of buried water ice because the Fe and Ti abundance of lunar
regolith at its poles is sufficiently high (25) to absorb
backscatter radar signals from water-ice deposits that are buried at
depths by more than about 20 cm (26).
We can estimate the amount of water ice that may be found in
permanently shadowed polar areas if we assume that (i) the ice deposits
are below a 40-cm-thick layer of dry regolith, (ii) they are pure water
ice, (iii) they reach a depth of 2 m, that is, the depth estimated
to be gardened in 2 billion years (3), and (iv) the surface
area of permanently shadowed crater deposits is 1850 km2 at
each pole as simulated above. Then each polar region could contain as
much as roughly 3 × 109 metric tons of water ice.
This amount is about an order of magnitude less than that predicted as
possibly delivered to and retained on the moon by comets over the past
2 billion years (3).
This estimate may be wrong by a large factor if the actual area in
permanent shadow at both poles differs from our assumed value of 1850 km2. Estimates based on Clementine data (12,
23) yield 530 km2 north of +87.5° and 6361 km2 south of
87.5° latitude. There is a potential
inconsistency here because the measured epithermal-neutron effect is
larger in the north, yet the permanently shaded area appears to be
larger in the south. This discrepancy implies either that all of the excess hydrogen is not in the form of water ice or that the permanently shaded areas estimated from Clementine data do not represent conditions near the poles at latitudes equatorward of ±87.5°. This last
possibility is supported by the fact that the South Pole of the moon
was viewed by Clementine during winter. Areas that appear to be shaded
during the winter months may receive sunlight during other times of the year.
However, it is also possible that some of the excess hydrogen
inferred from epithermal*-neutron data near the lunar poles is
dispersed throughout relatively large areas of the polar regolith, having been delivered to the moon at more equatorial latitudes by the
solar wind but transported to the poles through multiple collisionless
hops through the lunar exosphere (2-6). By the
method of construction, our polar estimates made from Fig. 6 are
additions to the average mass fraction of hydrogen implanted in the
regolith by the solar wind at lower latitudes. We believe a solar wind
for most of the observed excess hydrogen is unlikely because the lack
of a fast-neutron signature rules against a surface deposit.
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