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The Uncertain Nature of Polar Lunar Regolith

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Title: 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)
4
Regolith Grain Size
Hartmann (2003) Early heavy bombardment may have
produced deep megaregolith as fine-grained as
surface regolith is today.
5
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

6
Agglutinate Properties Normal Dry Moon
Photos courtesy of Larry Taylor
7
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.
8
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

9
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

10
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
11
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

13
Amorphous Ice Structure Allows Trapping of
Cometary Gases
High-density amorphous ice
Low-density amorphous ice
Jenniskens et al. (1995)
14
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

15
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

16
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

17
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
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