Partial Least Squares Modeling of Lunar Surface FeO Content with Clementine Ultraviolet-Visible Images - Planetary Exploration and Science: Recent Results and Advances - page 2

as the crustal evolution. Iron is often expressed as FeO in astrochemistry. The
absorption properties in the ultraviolet-visible (UVVIS) and near-infrared (NIR)
spectral regions of iron-bearing minerals (e.g., pyroxene, olivine, and ilmenite)
are dominated by Fe 2C or Ti 4C /Ti 3C (Lucey et al. 1998 ). The absorption features
from lunar sample or remotely sensed spectra would mix up influences from
the exposures of lunar soils to the space environment, i.e., the Moon has been
suffering from bombardments by micrometeorites, solar wind ions, cosmic rays,
and solar flare particles (Fischer and Pieters 1994 , 1996 ). Sustained bombard-
ments will cause the lunar surface material change in petrography and chemistry.
These changes include reduction of mean grain size, the production of nanophase
iron (npFe 0 ) and complex glass-welded aggregates of lithic and mineral frag-
ments (agglutinates), and so on (Fischer and Pieters 1994 , 1996 ; Mckay et al.
1974 ). This so-called process “space weathering” will bring about the maturation
of lunar regolith, i.e., the mature regolith usually has suffered from a longer
time of space weathering compared to immature regolith. Space weathering will
cause an overall reduction in the reflectance, and reduce the absorption band
strengths, creating and steepening a red-sloped continuum (Fischer and Pieters
1994 , 1996 ).
Many authors have obtained the empirical relationships between spectral prop-
erties and iron abundance of lunar soils with intent to get a more accurate lunar
iron model (Lucey et al. 1995 , 2000 ; Blewett et al. 1997 ; Gillis et al. 2004 ; Wilcox
et al. 2005 ). Lucey et al. ( 1995 ) firstly provided a method for the derivation of
iron from Clementine multispectral images, utilizing the laboratory spectra and iron
abundance of lunar soils. A Fe parameter was defined based on compositional and
maturity-related trends on a plot of 950 nm/750 nm versus 750 nm reflectance,
which was found to have a strong linear relationship with iron abundance (Fig. 1.1 )
(Lucey et al. 1995 ). It can be seen from Fig. 1.1 that iron content has an orthogonal
effect, where low iron abundance has high reflectance and high ratio whereas high
iron abundance has low reflectance and low ratio. This trend has a hypothetical
Fig. 1.1 A schematic
diagram of NIR/VIS ratio
versus VIS for lunar samples.
Samples with high iron
abundance toward lower left
on the plot. Samples with
same iron abundance but
different maturities locate
along a line radial to a dark
red mature end-member at the
upper left . The Fe parameter
() could decouple iron
content from maturity