Book of Abstracts- Lunar Regolith Simulant Materials Workshop

EVOLUTION OF THE LUNAR REGOLITH. David S. McKay, Johnson Space Center, Mail Code KA, Houston TX
77058, david.s.mckay@nasa.gov
Introduction: In order to properly design and produce
simulants, it is necessary to have some insight
into the evolution of the lunar regolith. In particular,
the physical and engineering properties of the lunar
regolith result from the complex processes that produce
it and make it unique. In addition, the chemistry
of the lunar regolith depends not only on the chemistry
of the bedrock underlying it, but also on the evolution
paths that produced it. In general, the chemistry of the
regolith does not exactly correspond to the chemistry
of the underlying bedrock. Furthermore, the chemistry
of a given grain size fraction is likely to be different
from that of another fraction. To understand these
complexities, we must consider how lunar regolith has
formed over geologic time.
The lunar regolith. The lunar regolith is the
fragmental layer that overlies nearly all rock formations
on the moon. It varies in thickness from less than
a meter in some areas to 10s of meters elsewhere. The
maximum thickness is not known but is likely to be
less than 100m and certainly less than 200m. Meteorite
bombardment and secondary processes related to
bombardment mainly produce the regolith. However
the regolith is not simply ground up or milled bedrock.
It is a dynamic material, sometimes becoming finer
and other times becoming coarser in grain size. At any
site the regolith may reach a steady state grain size but
this grain size will likely differ from site to site. One
type of regolith, represented by the black and orange
glass at Apollo 17, is not the primary product of meteorite
bombardment, but was produced by volcanic
eruption of pyroclastic ash. In some places it constitutes
the main regolith and is termed dark mantle.
Dark mantle has many qualities that make it an attractive
resource target for lunar propellant production.
Typical lunar regolith contains rock fragments,
mineral fragments, and glass. The primary glass type
is agglutinates, which are constructional particles produced
by small impacts. Because constructional particles
are produced, the lunar regolith is not simply a
product of grinding; its grain size distribution is much
more complex. Figure 1 [1] shows the mean grain size
and graphic standard deviation for 42 Apollo 17 soils.
Finer Soils. These parameters show an inverse correlation;
finer soils are better sorted. The maturity of
lunar soils was first defined by this figure based on
grain size parameters. Maturity is an important parameter
because it determines how much solar wind
components (hydrogen, carbon, nitrogen, etc.) are present.
Independent measurements of agglutinate content
showed that more mature soils have more agglutinates.
This correlation is also shown in Figure 2.
Mature Soils. The most mature soils contain more
than 60% agglutinates in the intermediate size fraction
90-150µm. Extrapolation to 100% agglutinates would
predict a mean grain size of 13µm. This is much finer
grained than any Apollo soil; a soil this fine is unlikely
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