Recent headlines have announced a raging controversy among
scientists about whether there is actually water ice in the permanently
shadowed craters near the lunar poles. Because these permanently shadowed
regions are extremely cold (~100K) water ice is expected to be stable there –
even in the vacuum of space. If water is present, it will dramatically reduce
the cost of a lunar base. The Lunar Crater Observing and Sensing Satellite
(LCROSS) mission is intended to test for this water by impacting
the lunar surface with its empty rocket upper stage, and looking for water
in the ejected plume.
At issue is the evidence for water ice from neutron counts
and radar observations of the polar regions. In the mid-1990s, Earth-based radar
observations of the polar regions detected areas of higher circular
polarization ratio (CPR). This is the ratio of the reflected radar signal
having the same circular polarization as the incident beam to the reflected
signal having the opposite polarization. High values of CPR tend to indicate
blocky, irregular surfaces such as boulder-strewn fields or crater ejecta, but
can also result from low-loss scattering by water ice entrained in smoother
surface material such as soil or regolith. Large rocks and boulders mixed into
the regolith also can have a distinct signature. The high CPRs detected at the
lunar poles seemed to correlate with crater floors in these regions, where
geometry and orbital mechanics conspire to produce permanently shadowed regions.
This effect has been observed on Mercury, where permanently shadowed craters
exhibit CPRs which resemble those from the icy Galilean satellites. The radar
results for the moon were thus interpreted to be due to similar effects.
The neutron detector on Lunar Prospector subsequently found
low counts of epithermal neutrons over the polar regions as well, which are
typically indicative of hydrogen. However, it is not clear whether this is
hydrogen in the form of pure water (H2O), hydroxyl (OH-, likely bound to minerals), or hydrogen (H+) implanted by the
solar wind. If the hydrogen is in the form of H2O or OH- bound to minerals, it could still be useful, but extraction will be more
difficult and costly.
The current controversy centers around recent radar observations
at higher resolution than the previous studies. These observations show high
CPR's in crater walls and floors, but they are not correlated with the
permanently shadowed regions. They are correlated with the rocky surfaces and
ejecta blankets around young craters, and with the inner walls of some of the
larger craters. They definitely do not indicate large deposits of pure ice —
there are no skating rinks on the Moon, as one scientist put it. And the
neutron studies detect hydrogen, not water — so there are other ways to explain
those results too. The results also depend on how much solar wind implanted
hydrogen is assumed to be present, as well as the exact locations and sizes of
the permanently shadowed regions — which aren't actually very well known, since
we cannot see them all from Earth. The two areas where the shadowed regions are
best constrained are within the craters Shackleton
and Shoemaker.
Water ice remains consistent with the neutron studies, but
only in concentrations of around 0.4 to 1 percent by weight, as small chunks
mixed into the regolith. Or, if the ice is trapped in small pores in the soil
minerals, it could be as much as 10 to 20 percent. Surface roughness and
neutron counts both vary on spatial scales much smaller than the instruments
can resolve, creating additional uncertainty. So the interpretation depends on
certain assumptions, and can vary — it is still consistent with water ice mixed
into the soil, and possibly even large amounts, but more likely very small
amounts, if any at all.
The LCROSS mission is being optimized for 0.5 to 2 percent
by weight of water in the soil — consistent with the neutron results. The
general consensus at present is that the radar polarization in the polar
regions is consistent with soil containing small amounts of ice, or could be
due to surface roughness effects. Because the polarization is not actually
correlated with the permanently shadowed regions, many are now leaning toward
the surface roughness hypothesis, but there are insufficient data at present to
conclusively determine whether there is ice. A dry regolith is virtually
indistinguishable from regolith containing tiny amounts of ice within and
between mineral grains. This is why the radar experiments on Chandrayan and Lunar Reconnaissance
Orbiter (LRO) are being designed to understand the relationship between CPR
and ice on the moon, and why the LCROSS mission is being sent to directly probe
the surface and test these hypotheses through observation and experiment.
And that is how the scientific method deals with
controversy.
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