Nature and Evolution of Lunar Soil David S' McKay Chief Scientist for Astrobiology Astromaterials Re

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Nature and Evolution of Lunar Soil  David S.
McKayChief Scientist for AstrobiologyAstromateri
als Research and Exploration Science (ARES)
DirectorateNASA Johnson Space CenterLarry
TaylorProfessor of GeologyUniversity of
Tennessee
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Nine Lunar Sample Sites
Apollo 15
Apollo 17
Luna 24
Luna 20
Apollo 11
Luna 16
Apollo 12
Apollo 16
Apollo 14
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What is lunar soil?

• Lunar soil is defined as the loose fragmental
  material with a grain size smaller than 1 cm on
  and near the surface of the moon. It is a subset
  of the lunar regolith which includes all size
  fragments including boulders.
• Lunar dust is the finest size fraction of lunar
  soil. A working definition of lunar dust is that
  it is all grains smaller than 20mm.

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Lunar Soil Evolution

• Lunar soil is formed by a combination of
• Communition by impact processes
• Agglutination by impact processes
• Addition of volcanic ash
• Space weathering (solar particle sputtering and
  vapor generation and deposition)
• Mixing including regolith gardening

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Our Unhappy Moon
From Larry Taylor
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Lunar Soil Formation
Comminution, Agglutination, Vapor Deposition
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Major Processes

• Impact communition
• Impact melting
• Formation of agglutinates
• Solar wind sputtering
• Impact vaporization
• Impact vapor condensation
• Shock welding of grains
• Thermal welding of grains

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Lunar Mare Soil
Volcanic Glass Bead
Impact-Glass Bead
Agglutinate
Rock Chips
Impact Glass
1 mm
Plagioclase
Regolith broken up rock material
Soil lt1 cm portion of the Regolith
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Mare-Soil Agglutinate
Courtesy Dave McKay
Pieces of minerals, rocklets, and glass cemented
together by shock-melt glass
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Agglutinates
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Concept of soil maturity

• Maturity of lunar soil is the collection of
  properties which have changed over time as the
  soil has been exposed at or near the surface
• Immature soils have had little exposure
• Mature soils have have significant exposure
• The most mature lunar soils have had around 100my
  exposure time

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Grain Size

• Lunar soils are poorly sorted by terrestrial
  standards
• they have a large standard deviation in their
  grain size distribution curves

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What happens as lunar soils mature?

• The mean grain size decreases
• agglutinate abundance increases
• The shape of the grain size distribution changes
• The standard deviation decreases

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How do lunar size distributions compare to
experimental data?
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How does regolith evolution relate to regolith
thickness?
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Mare Soil Maturation
Agglutinates
Particle
Minerals
Basalt Fragments
Taylor McKay, 1992
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Regolith Breccia
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Regolith Breccia
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JSC-1 Simulant Soil
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Bulk AP11 soil on glass slide
20 mm
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Grain Shapes

• Agglutinates have unique shapes not generally
  found in terrestrial soils
• Most mineral grains are equant or subspherical
• Elongated grains or fibers are present but
• rare

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Large silicate grain
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Small grains on microscope stub
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Pyroxene fragment
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Mineral fragments
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Small grains on microscope stub
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Core sample from orange/black glass
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Grain surfaces

• Space weathering produces unusual grain surfaces
• Vapor deposited metals, sulfur compounds, halide
  compounds
• Unsatisfied and reactive bonds from fracturing in
  high vacuum, ion sputtering, ion inplantation
• Highly reduced compared to any terrestrial soils
• Highly desiccated compared to any terrestrial soil

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Space Weathering of Intermediate-Size Soil Grains
in Immature Apollo 17 Soil 71061
Susan J. Wentworth,1 GeorgAnn Robinson1, and
David S. McKay2 (susan.j.wentworth_at_jsc.nasa.gov) 1
ESCG, Johnson Space Center, Houston, TX, 77058
2NASA-JSC, Houston, TX, 77058
Results and Discussion A few grains of 71061
plagioclase contain trace to moderate amounts of
weathering features. Most grains examined thus
far show little or no evidence of space
weathering, as expected. Two features are known
to be characteristic of materials that have been
exposed directly to space microcraters and glass
pancakes (flat, circular glass accretions). In
general, the features on these plagioclase grains
are very small. The microcraters generally do not
have spall zones the absence of spall zones,
which are characteristic of larger microcraters,
is typical of microcraters lt5 micrometers in
diameter. Microcraters (and nanocraters) are
identifiable at sizes down to the limits of
resolution of the images, as are glass pancakes.
Other accretionary features seen on the 71061
grains include glass splashes, glassy spherules,
and tiny mineral fragments, all of which also
range down to the limits of resolution such
features are not indicators of direct space
weathering but are ubiquitous to most lunar soil
grains. Summary and questions. Space weathering
features on 71061 plagioclase grains appear to be
the same as those found on finest lunar soil
grains and/or those on large lunar rocks. The
range of weathering intensities in the 71061
grains suggest that they represent a broad range
of weathering intensities, and they will provide
a good range of samples for future FIB and TEM
studies. In general, the unanswered questions for
the 71061 feldspars are similar to those for
other space-weathered materials. For example, how
closely can the abundance and types of space
weathering features on individual grains be
correlated with the overall maturity of each
lunar soil? Are most space weathering features
on the 71061 feldspars the result of vapor
deposition? Can vapor deposits eventually be
distinguished from splash glasses in SEM studies?
What are the fundamental differences between
glass pancakes, spherules, and other glass
splashes? Did reduced iron seen in TEM studies
of other space weathered samples 1 form at the
same time(s) as the other features, and what is
the source of the iron? What are the sources of
all of the accretionary materials (including
vapor deposits, glass splashes, and accreted
mineral grains)? Are they the result of
deposition from nearby regolith, or do they
(especially glass spherules) travel long
distances? Why are glass pancakes only found on
surfaces that also contain microcraters? Do
micro- and nano-scale craters contain any record
of the impactors from which they formed? Is any
of the impactor material implanted into the soil
grain? References 1 Keller and McKay (1997)
GCA 61, 2331-2340 2 Wentworth et al. (1999)
MAPS 34, 593-603 3 Wentworth et al. (2004) LPS
XXXV, Abstract 2078 4 Morris et al. (1983)
Lunar Soil Handbook 5 Taylor et al. (2000)
6 Pieters et al. (2000) MAPS 35, 1101-1107.
Abstract Overview Understanding space
weathering, which is caused by micrometeorite
impacts, implantation of solar wind gases,
radiation damage, chemical effects from solar
particles and cosmic rays, interactions with the
lunar atmosphere, and sputter erosion and
deposition, continues to be a primary objective
of lunar sample research. Electron beam studies
of space weathering have focused on space
weathering effects on individual glasses and
minerals from the finest size fractions of lunar
soils 1 and patinas on lunar rocks 2. We are
beginning a new study of space weathering of
intermediate-size individual mineral grains from
lunar soils. For this initial work, we chose an
immature soil (see below) in order to maximize
the probability that some individual grains are
relatively unweathered. The likelihood of
identifying a range of relatively unweathered
grains in a mature soil is low, and we plan to
study grains ranging from pristine to highly
weathered in order to determine the progression
of space weathering. Future studies will include
grains from mature soils. We are currently in the
process of documenting splash glass, glass
pancakes, craters, and accretionary particles
(glass and mineral grains) on plagioclase from
our chosen soil using high-resolution field
emission scanning electron microscopy (FE-SEM).
These studies are being done concurrently with
our studies of patinas on larger lunar rocks
e.g., 3. One of our major goals is to correlate
the evidence for space weathering observed in
studies of the surfaces of samples with the
evidence demonstrated at higher resolution (TEM)
using cross-sections of samples. For example, TEM
studies verified the existence of vapor deposits
on soil grains 1 we do not yet know if they
can be readily distinguished by surfaces studies
of samples. A wide range of textures of rims on
soil grains is also clear in TEM 1 might it be
possible to correlate them with specific
characteristics of weathering features seen in
SEM? Samples and Techniques For the first
phase of our single grain studies, we chose
Apollo 17 soil 71061, an immature (bulk Is/FeO
14) mare soil 4 for the reasons given above.
This soil was also one of the soils recently
studied in a comprehensive way by the Lunar Soil
Characterization Consortium (LSCC), so a lot of
pertinent data from the fine (lt45 micrometer)
fractions are available e.g., 5, 6 for
comparison to the results we obtain for larger
grains. We are studying individual plagioclase
grains from intermediate size fractions (150-200
µm and 250-500 µm) using both standard and field
emission scanning electron microscopy (JEOL
5910LV and JEOL 6340F for SEM and FE-SEM,
respectively). Some of the individual grains
will be prepared for transmission electron
microscopy (TEM) using focused ion beam (FIB)
sectioning. The locations of the FIB sections
will be precisely known and, therefore,
subsequent TEM results can be exactly correlated
with the surface weathering features documented
during the earlier SEM work.
Range of 71061 Plagioclase Grain Surface Textures
Fresh Smooth, relatively clean surface with
minor amounts of accretionary particles
and slight chipping. Fresh surfaces can also be
highly fractured.
Altered Mineral surface obscured by splash glass
and accretionary particles. Important note this
view shows no evidence of direct space weathering
71061 Microcraters
Craters are present on only a few plagioclase
grains, consistent with immaturity of soil 71061.
Highest abundance is on a grain containing
traces of Mg-rich olivine and pyroxene,
suggesting a nonmare origin for the grain.
This grain may have been space weathered in a
more mature soil before it was added to 71061, a
mare soil. All crater images Shown here are from
that grain (Mount no. 150-250C Grain 09). No
craters identified thus far in the 71061 samples
are large enough (gt5 micrometer diameter) to have
classic features shown in 62255 crater. Craters
range from 1 micrometer pit diameter down to
limits of resolution of the SEM. Many are in the
10-100 nm range, as shown in images below.
Craters are always found on surfaces also
containing glass pancakes, consistent with
original Apollo studies.
Common lunar rock and soil surface features NOT
indicating direct exposure to space at surface
of moon
splash glass
Space Weathering Evidence (SEM)
Impact-generated From small-scale impacts
nearby Size range -small droplets to large
sheets Can completely cover mineral/rock
surface Multiple layers/events possible
Hypervelocity microcraters Craters gt5
micrometers in diameter characterized
by glassy pit liners distinctive spall
zone radial fractures Smaller craters as shown
here do not have those characteristic
features Glass "pancakes" Possible solar wind
etching 1
accretionary particles
Mineral, glass, and rock fragments can obscure
mineral/rock surface (e.g., marked areas at
right) Sintering mechanisms not
completely understood (sintering by individual
heated particles, bonding by small amounts of
glass, electrostatic charge?)
spherules
Common accretionary particle Impact or volcanic
origin Impact glass composition can be highly
fractionated (volatile-poor) Size range down to
nm-scale (see high-mag images)
Glass Pancakes
-Abundant -Largest shown a few micrometers
across -Smallest limits of resolution (not
shown here) -Microcraters
also abundant in this area -Spherules and other
accretionary particles also common
Solar Wind Etching?
TEM studies of specific areas like this will
be essential for determining whether solar wind
etching has occurred, or if the apparent
crystallographically controlled erosion is the
result of some other process(es). Such
features would be ideal for focused- ion beam
(FIB) sectioning and subsequent TEM studies.
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Vapor-Deposited Nanophase Feo on Plagioclase
Courtesy Lindsay Keller
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Distribution of Nanophase Feo
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Diameter nm
5
10
15
20
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Feo Metal spheres
Cumulative Mass Fraction
Diameter (Å)
TEM-measured Size Distribution of Fe Metal
Spheres in Agglutinitic Glass of Apollo 11
Soil 10084
Housley et al, 1974
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What are the differences between Lunar Soils and
the Rocks from which they were derived ??
Major Difference 10 X more native Feo in the
soil
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Comparison to simulated lunar soils

• Simulants such as JSC-1 reproduce
• Bulk chemistry
• Grain size distribution
• Approximate correct mixture of mineral types and
  glasses
• Some jagged shapes similar to lunar soils

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Comparison to simulated lunar soils

• Simulants such as JSC-1 do not reproduce
• Agglutinate shapes
• Fine-grained reduced iron
• Reflectance spectral properties
• Implanted species from solar wind, solar flares,
  and cosmic rays
• Reactive radiation-damaged rims
• Reactive vapor deposits on grain surfaces

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MAGNETIC PROPERTIESOF LUNAR SOILS

• Magnetic Susceptibility of Soil Particles
  Increases as Grain Size Decreases

• Effects of Vapor-Deposited Nanophase Feo are a
  Direct Function of Surface Area and Most
• Pronounced in the Finest Grain Sizes

• Virtually All lt10 ?m Particles are Easily
  Attracted by a Simple Hand-held Magnet,
  Plg, Pyx, Ol, and Agglutinitic Glass alike.

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Lunar Soil Cycle
MicrometeoritesS-W Ions
MicrometeoritesS-W Ions
Agglutination
Melting
Vaporization
Fe0 SiO2 Si0
Vapor-Deposited
SiO2 npFe0
Selective Comminution
(especially glass)
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Key questions for toxicity studies

• Can the human respiratory system clear out
  unusual jagged shapes of agglutinates? At 1/6th
  G?
• What is the effect of highly reduced submicron
  iron found on most grain surfaces?
• What is the effect on humans of highly reactive
  bonds from radiation damage, vapor deposition,
  sputtering erosion and deposition?
• When can lunar simulants be used and when must
  actual lunar soil be used in toxicity studies?

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Regolith Processing on the Moon
Buried Habitat
LUNOX / LLH Storage
Mirror
ISR
ISR (InSect Robot)
Mirror
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Gas Release from Lunar Mare Soil
Cumulative
400
1200
800
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Concentrations of Solar-Wind Volatile Species
in Lunar Regolith Samples, in ppm
Haskin and Warren, 1991
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Production of LLOX by H2 Reduction of Ilmenite
recycle
FeTiO3 H2 Fe TiO2 H2O electrolysis
H2 ½ O2
Ilmenite feed
Solid Product
Oxygen Product
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Mining Water on the Moon Living-Off-the-Land
Concept

• H2 is abundant in the Lunar Soil !
• O2 is easily produced from Lunar Resources !
• Therefore, we can readily obtain rocket fuels
• and make H2O !!!

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Premise Need 20 tonnes of LLH hydrogen per year
Hydrogen in lunar soil 200 ppm 50 recovery
0.01 wt H in 1 m3 2.0 g/cc x 106 cc/m3 X
1 x 10-4 2.0 x 102 g/m3 20 tonnes 20t x
103 kg / t x 103 g/kg 20 x 106 g 20 tonnes
20 x 106g / 2.0 x 102 g/m3 10 x 104 m3 105
m3 1 Football Field (Depth of 3m) 5 x 103 m2
x 3 15 x 103 m3 20 t LH 6.2 Football
Fields to 3 m depth
20 tonnes of LLH 6 Football Fields 0.03
km2 (1 / 30 th km2)
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FERROMAGNETIC RESONANCE (FMR)

• Measurement of Single-Domain, Nanophase Fe0
  (IS)
• Normalized IS for Iron Content IS / FeO
• IS / FeO amount of total iron that is
  present as Feo
• IS / FeO is a Function of Agglutinate
  Abundance
• Agglutinate Abundance is a Function of
  Maturity
• Is / FeO Soil Maturity