Title: The Uncertain Nature of Polar Lunar Regolith
1The Uncertain Nature of Polar Lunar Regolith
- Jeff Taylor, Josh Neubert, Paul Lucey, and Ed
McCullough
2A 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
3Regolith 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)
4Regolith Grain Size
Hartmann (2003) Early heavy bombardment may have
produced deep megaregolith as fine-grained as
surface regolith is today.
5Regolith 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
6Agglutinate Properties Normal Dry Moon
Photos courtesy of Larry Taylor
7Agglutinate 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.
8Agglutinate 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
9Potential 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
10Polar 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than 100 K in permanently-shadowed regions - Model at right (Vasavada, unpublished) is for a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
11H2O Deposition as Amorphous Ice
- Numerous H2O structures
- At low lt 100 K and P lt 2 x 108 Pa, precipitates
as amorphous solid
12Comet 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
13Amorphous Ice Structure Allows Trapping of
Cometary Gases
High-density amorphous ice
Low-density amorphous ice
Jenniskens et al. (1995)
14Gas 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
15Clathrate 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
16Other 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
17Conclusions
- 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