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discovery from the difference map is that iron in fresh craters derived by PLS is often
lower than Lucey's result, represented by Tycho crater (located in south-southwest
of the map, about 43.3
ı
S, 11.2
ı
W). This may be caused by different degrees of
maturity suppressing between the two methods. Statistical result (Fig.
1.10b
)shows
a nearly Gaussian distribution of the difference, with an average value around
0.27 wt%, and root mean square error (RMS) is 1.13 wt%, indicating a relatively
small difference of the global iron abundance between the two maps.
Lawrence et al. (
2002
) derived a global iron map from the gamma-ray counting
rate of the lunar surface detected by Lunar Prospector (LP). The Fe derived by
gamma-ray spectrometer is a direct measurement of Fe gamma-ray line (7.6 MeV).
Moreover, gamma-ray spectrometer usually detects signal from over 10 cm below
lunar regolith, so the result would be less affected by space weathering effect, mak-
ing the FeO abundance more reliable than that derived by experienced algorithms.
The global mode of LP is 6.4 wt%, compared to 5.1 wt% derived from PLS model.
Comparing to Lucey's result, iron abundance derived by PLS modeling is more
consistent with that detected by LP as far as highland regions are considered. In
other words, the higher iron value that is derived from PLS model for the lunar
highland regions when compared to Lucey's algorithm may suggest a more reliable
result. As for iron distribution of Tycho crater, Lawrence et al. (
2002
) also found a
large discrepancy, i.e., Lucey's results show moderate FeO abundances at 7-9 wt%,
while the LP data show very low FeO abundances at 3-4 wt%. FeO content from
PLS modeling is about 3.8 wt% which supports the LP result (Fig.
1.12a, b
).
Considering the different spatial resolutions of the FeO maps from the spectral
and gamma-ray datasets, in order to compare in detail with iron map derived by LP,
we resample the Clementine iron map derived by PLS to the same spatial resolution
as LP iron map (15 km/pixel) and make a global difference map (regions exceed
70
ı
S-70
ı
N are not included) (Fig.
1.11a
). The difference distribution on the global
map isn't very homogenous, but when we take a look at the global iron distribution,
we find the global difference is concentrated within
0.9 to 1.0 which is colored by
green, and iron abundance detected by LP in mare areas and high-latitude regions
is higher than PLS model. The difference in high latitudes tends to be greater; this
effect may be the influence of topographic shading or illumination conditions.
Furthermore, our iron content of PLS model for the South Pole-Aitken (SPA)
basin is higher than LP's but lower than Lucey's (Fig.
1.12c
). As the largest impact
crater on the Moon, the SPA impact event didn't penetrate the materials from lunar
mantle, which are expected to be more mafic and iron rich. The specific noritic
mineralogy may account for this low FeO concentration (Lucey
2004
). From the
statistical result of the difference map, the global average of iron difference is within
1 wt% and the RMS is 2.3 wt%, suggesting a good consistency of PLS model and
LP iron map.
In a word, we find the iron map from PLS model agrees with those from the
Lucey's and LP's, though subtle difference appears for the global maps. This
suggests our PLS model is a robust algorithm for the extraction of lunar iron content.
Furthermore, the application of PLS method on the global lunar mosaic seems to be
more consistent with LP results than those of Lucey 2000. Note that our PLS model
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