The Uncertain Nature of Polar Lunar Regolith

1
The Uncertain Nature of Polar Lunar Regolith

• Jeff Taylor, Josh Neubert, Paul Lucey, and Ed
  McCullough

2
A Tale of Two Moons

• Perhaps the Moon should be divided into two major
  parts
• dry (almost all of it)
• icy (permanently-shadowed areas in polar regions)
• Potential differences in
• Regolith grain size distribution
• Agglutinate physical properties, which might
  affect bulk regolith properties
• Effect of ice and other compounds if present
• We have no direct measurements of any of these
  featureswe need them

3
Regolith Grain Size

• In non-polar regions
• Regolith is fine-grained
• Roughly
• 10 smaller than 10 ?m
• 20 smaller than 20 ?m
• Predictable physical properties (porosity,
  thermal conductivity, shear and bearing strength,
  angle of repose, tribology)

Carrier et al. (1991)
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Regolith Grain Size
Hartmann (2003) Early heavy bombardment may have
produced deep megaregolith as fine-grained as
surface regolith is today.
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Regolith Grain Size

• Could regolith in polar regions be much finer
  grained as a result of this early bombardment,
  with subsequent regolith development atop this
  fine-grained deposit?
• Finer grained deposit would affect permeability,
  but would create larger surface area
• In this case the effect may apply to much of the
  highlands

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Agglutinate Properties Normal Dry Moon
Photos courtesy of Larry Taylor
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Agglutinate Properties Icy Moon
Agglutinates formed in icy regolith might be
extremely frothy, like this sample of reticulite
(basaltic pumice in which cell walls of
bubbles have burstleaves a honeycomb-like
structure.
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Agglutinate Properties Icy Moon

• Extremely porous agglutinates might lead to
  change in physical properties of bulk regolith
• Agglutinates make up at least half the volume of
  a mature regolith
• Would be substantially weaker to both compressive
  and tensional forces
• Might fragment easily, leading to finer-grained
  regolith
• Might provide greater grain-to-grain friction
• Could provide places for H2O to precipitate

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Potential Sources of Polar Hydrogen

• Solar wind hydrogen (indirectly deposited)
• H2O (and possibly CH4 and CO) released from
  non-polar soil grains that contain solar wind
  gases
• Impact of hydrous meteorites
• Impact of comets
• Key point We do not know which of these is most
  important

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Polar Temperatures
Noon

• Models suggest temperatures substantially less
  than 100 K in permanently-shadowed regions
• Model at right (Vasavada, unpublished) is for a
  flat-floored crater like Amudsen

Midnight
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H2O Deposition as Amorphous Ice

• Numerous H2O structures
• At low lt 100 K and P lt 2 x 108 Pa, precipitates
  as amorphous solid

12
Comet Gases

• We do not have solid data on composition of polar
  volatile deposits
• A good guess is that they contain gases from
  comets. For example, in Hale Bopp
• H2O 100 CO2 6
• CO 20 NH3 0.7
• CH3OH 2 CH4 0.6
• These could result in deposits of H2O and other
  gases
• If source is non-polar regolith, there still be
  CO and CH4

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Amorphous Ice Structure Allows Trapping of
Cometary Gases
High-density amorphous ice
Low-density amorphous ice
Jenniskens et al. (1995)
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Gas Release from Amorphous Ice

• Amount of trapped gas depends on T
• Can be up to 3.3 times the amount of ice at 20 K
  (Laufer et al., 1987)
• Trapped Ch4/ice and CO/ice are 0.01 at 70 K
• Gas begins to be released at 120 K and increases
  exponentially as T approaches 135 K
• Transformation to crystalline ice is exothermic,
  so there could be a runaway effect, resulting in
  losses

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Clathrate Hydrates

• Duxbury et al. (2001) propose that compounds like
  CH46H2O and CO2 6H2O could form at depths of a
  few mm to a few meters
• Source of gases could be solar wind interactions
  in non-polar regions or comets
• Their presence might lead to large changes in the
  physical properties of the regolith
• On Earth, their presence is a concern for
  undersea drilling operations because they may
  create unstable slopes

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Other Possible Processes

• Crystallization of amorphous ice and
  micrometeorite bombardment add heat to regolith,
  which could result in
• Loss of H2O
• Formation of frothy agglutinates
• Formation of organic compounds
• Formation of hydrated silicates
• Impact into ice-bearing regolith could produce a
  distinctive morphology surrounding craters a few
  meters (up to 20 m?) in diameter

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Conclusions

• Permanently-shadowed regions might be drastically
  different from the well-characterized and
  reasonably well understood non-polar regions
• Laboratory experiments can help shed light on the
  possibilities and what types of measurements are
  needed
• It is essential to study permanently-shadowed
  regions in great detail to understand
• Nature of hydrogen deposit
• Form of solid water if present
• Stratigraphy of deposits
• Volatility of the regolith, including rate of
  loss during heating and handling
• Regolith physical properties
• Surface morphology