There is significant interest in exploring the polar regions of the Moon with the aim of finding water ice at the bottom of permanently shadowed craters. However, little is known about the regolith properties in these shadowed regions and there are concerns that the regolith could be highly porous and that exploration rovers and landers may sink irretrievably into soft soils. In this study, conducted by graduate interns at the Lunar and Planetary Institute, we use imagery of lunar boulder tracks to investigate the regolith properties in polar craters.

1. How traversable are the Moon’s
polar craters?
H.M. Sargeant,
V.T. Bickel, C.I. Honniball, S.N. Martinez, A. Rogaski, D.A. Kring
S.K. Bell, E.C. Czaplinski, B.E. Farrant, E.M. Harrington, G.D. Tolometti

2. 2

3. About this talk
• Introduction to the Moon
• The challenge: Can we safely access
unexplored regions?
• Measuring soil strength with boulder
tracks
• Image processing & measurements
• Findings
• Implications
3
Detail of M124743115LE
30 m

4. The Moon
4
Highland
Mare
Credit: Gregory H. Revera

5. Previous Lunar Exploration
• Mostly nearside equatorial
• Big influx in missions recently
• We’re going ‘Back to the
Moon!’
• Now we need to explore
elsewhere on the Moon
5
Credit: Cmglee

6. Pyroclastics
• Volcanic deposit
• Glass beads
6
Credit: NASA
Image: AS17-137-20990

7. Permanently Shadowed Regions (PSRs)
• Regions that receive no direct sunlight
• Temperatures reach as low as 40 K
(-233ºC)
• Small PSRs identified at Apollo
landing sites
7
Image: AS16-106-17413
Permanently Shadowed Illuminated
Credit: LPI & CLSE resources
Map view

8. Scientific Significance
• Pyroclastic Deposits
• Understanding lunar volcanism and its evolution (NRC 2007 5c&d)
• Deposition of volatile material (NRC 2007 7c&d)
• Thermal and magmatic evolution of the Moon (NRC 2007 5a)
• Permanently Shadowed Regions
• Determine the state and distribution of volatiles (NRC 2007 4a)
• Understanding the mobility of volatiles in PSRs (NRC 2007 4c)
• Understand the physical properties of the extremely cold polar
regolith (NRC 2007 4d)
8
NASA file: JSC2007e045388

9. In-Situ Resource Utilization Significance
9
• Pyroclastic Deposits
• Glasses enriched in volatiles
• Water production from associated
minerals
• Life support
• Fuel
• Permanently Shadowed Regions
• Possible cold trap for stable
water ice
• 1 to 10% water content inferred
at LCROSS impact site (Paige et. al.,
2010)
Credit: Milliken and Li, 2017

10. PSR Regolith Properties
10
• FUV albedo maps show larger porosities than non-PSR regions
(e.g., 70% compared to 40%) (Gladstone et al., 2012)
• Reflectance data suggest the upper 1 cm of regolith have 83%
porosity in PSR regions (Hapke and Sato, 2015)
• Mini-RF data suggests a low
density layer (70% porosity)
exists to depths of at least
1 - 2 meters (Paul Spudis)
Optical layer:
extremely high
porosity (>70%)
and/or frost

11. Exploration Challenges
• Apollo 15 Lunar Roving Vehicle
(LRV) experienced 100% wheel
slip
• Astronauts manually freed the
vehicle
• Mars Spirit rover became stuck in a
deposit of soft soil
• Incidences like these are of
concern for robotic missions
11
Credit: NASA JPL

12. Determining Soil Strength with Boulder Tracks
• Lunar boulder tracks have been used
to measure soil strength since the
60’s (Filice, 1967; Eggleston et al., 1968;
Moore, 1970)
• Bearing capacity is a measure of
soil strength and is calculated in
this work.
12
Detail of M1234588088LE
Detail of M1234588088LE

13. Finding boulders… LROC Quickmap
13

14. Updated images
• Lunar Reconnaissance Orbiter images are used to search for
boulders with tracks
• Selected images have spatial resolutions of 0.38 to 1.27 m/pix
14
Detail of LRO M135433752LC
Detail of LO 5168_h2
30 m
30 m
1967
2013

15. A Little Perspective…
15

16. A Little Perspective…
16

17. PSRs – North and South Pole
17
Credit: Shaui Li et al. / PNAS

18. Boulders and Tracks on the Edge of a PSR
18
M141032202RE/LE

19. 19
Boulders and Tracks on the Edge of a PSR
M141032202RE/LE

20. 20
Boulders and Tracks on the Edge of a PSR
Detail of M141032202RE
M141032202RE/LE

21. 21
Boulders and Tracks on the Edge of a PSR
Detail of M141032202RE

22. 22
Boulders and Tracks on the Edge of a PSR
Detail of M141032202RE

23. Illumination of PSRs
Light reflected off crater walls provides illumination inside PSRs
23

24. PSR Image Filtering
24
M1117841678R&L
Original

25. PSR Image Filtering
25
M1117841678R&L
Original
Contrast & brightness
enhanced

26. PSR Image Filtering
26
M1117841678R&L
Original Filtered
Contrast & brightness
enhanced

27. PSR Image Filtering
27
Original contrast enhanced
Filtered
Detail of M1117841678R&L
400 m

28. Boulder and Track Inside and Outside a PSR
28
Detail of M117841678LE/RE

29. Boulder and Track Inside and Outside a PSR
29
Detail of M117841678LE/RE

30. PSR Image Filtering
30
Filtered
Detail of M1117841678R&L

31. Map of PSR Locations
31
Credit: NASA Goddard
PSR
Boulder tracks
70º S

32. Map of Comparison Locations
32

33. Boulder Tracks in all Regions
33
Detail of M168007359RE Detail of M175375107LE
Highland Mare
Pyroclastic Deposit
Detail of M135215829RE
PSR
Detail of M117841678LE/RE

34. Bearing Capacity
• The maximum load the soil can
sustain before failure
• The resisting pressure of a soil
against a load
• kN/m2
• Consists of:
− Soil cohesion
− Surcharge/normal force
− Soil friction angle (Angle of
repose)
34
Credit: NASA C-89-6201

35. Hansen (1970) Equation
35
Symbol Parameter
c Cohesion of the soil
q0 Vertical stress of the soil
γs Soil unit weight
Bav Diameter of footing
N(c,q,γ) Bearing capacity factors
s Shape factor
d Depth factor
i Load Inclination factor
b Footing Inclination factor
g Local slope inclination factor
qf=cNcscdcicbcgc+qoNqsqdqiqbqgq+0.5γBHNγsγdγiγbγgγ
Cohesion Surcharge Friction

36. Values Taken from Literature
• A cohesion value of 1 kPa is used (Mitchell et al., 1972c)
• Soil density values are selected from Apollo core tube analysis
that best represent pyroclastic, highland, and mare material
36
Terrain Sample # 𝝆 (g/cm3)
Literature values*
𝝆 (g/cm3)
Value used
Pyroclastic 15010 1.79-1.91a 1.85
Highland 64001 1.66b 1.66
Mare 12025 1.96c 1.96
a- Mitchell et al., 1972a (Apollo 15 preliminary report)
b- Mitchell et al., 1972b (Apollo 16 preliminary report)
c- Scott, 1971
* Density values for 30 - 60 cm

37. Internal Friction Angle
Landslides are used to constrain internal friction angles
37
Terrain Internal
Friction angle
n*
Pyroclastic 31 21
Highland 30 20
Mare 28 30
PSR 29 22
*2 DEMs per-location
*Number of measurements made
Detail of M1162383484RC
300 m

38. Assumptions
• Rectangular surface
• Boulder is static
• The load is entirely vertical
38
Cross Section
Map view
Cross Section

39. Boulder and Track Measurements
• Boulder long and short diameter
• Track width
39
Map view
Map view
• Shadow length
• Slope angle
Detail of M175375107LE

40. Boulder and Track Measurements
• Boulder long and short diameter
• Track width
40
Map view
Map view
• Shadow length
• Slope angle
Cross Section
of track
𝐷 =
𝐿
tan 𝑖
Detail of M175375107LE

41. Effective Incidence Angle
• Effective incidence angle is derived from the illuminated slope
41
Cross Section
𝐷 =
𝐿
tan 𝑒
Crater
Track
Depth

42. Effective Incidence Angle
• Effective incidence angle is derived from the illuminated slope
42
Cross Section
𝐷 =
𝐿
tan 𝑒
Crater
Track
Depth
Detail of M1117841678R&L

43. Bearing capacity vs slope angle
43

44. Track depth vs bearing capacity
44

45. 45
• Mean and range of bearing
capacity values are
consistent across locations
• Bearing capacity is
comparable to terrestrial
medium dense sand
Bearing Capacity Analogs
• PSR data is skewed as a
result of only shallow
tracks
• Bearing capacities are
equivalent to medium
dense sand
• Bearing Capacity at LPDs
and PSRs are statistically
higher at shallow depths
Terrestrial
Equivalents

46. Final Remarks
• Track characteristics don’t appear to change across locations or
from outside to inside PSRs
• Bearing capacity increases with depth
• Results can be used to estimate sinkage of current lunar rover
designs
46
Detail of M168007359RE Detail of M175375107LE
Highland Mare
LPD PSR
Detail of M135215829RE Detail of M117841678LE/RE

47. Final Remarks
• Track characteristics don’t appear to change across locations or
from outside to inside PSRs
• Bearing capacity increases with depth
• Results can be used to estimate sinkage of current lunar rover
designs
47
Detail of M168007359RE Detail of M175375107LE
Highland Mare
LPD PSR
Detail of M135215829RE Detail of M117841678LE/RE

48. Acknowledgments
• Lunar and Planetary Institute in Houston, TX
• Universities Space Research Association
• NASA Solar System Exploration Research Virtual Institute at NASA Ames
Research Center in Mountain View, CA
• LPI Exploration Science Summer Internship in Houston, TX
• Dr. Julie Stopar of the LROC team for her support
• Christian Venturino for previous boulder track work conducted at the LPI
• LROC team for the stunning NAC images returned from the Moon that enable
research for the new era of lunar exploration
• USGS Integrated Software for Imagers and Spectrometers (ISIS)
• Ames Stereo Pipeline (ASP)
48

49. References
• Carrier, W. (2006). Lunar soil simulation and trafficability parameters. Lunar Geological Institute (< http://www.lpi. usra.
edu/lunar/surface/carrier_lunar_trafficability_param. pdf>).
• Filice, A. L. (1967). Lunar surface strength estimate from Orbiter II photograph. Science, 156(3781), 1486-1487.
• Gaddis, L. R., Staid, M. I., Tyburczy, J. A., Hawke, B. R., & Petro, N. E. (2003). Compositional analyses of lunar pyroclastic deposits.
Icarus, 161(2), 262-280.
• Hansen, J. B. (1970). A revised and extended formula for bearing capacity.
• Hapke, B., & Sato, H. (2016). The porosity of the upper lunar regolith. Icarus, 273, 75-83.
• Heiken, G. H., McKay, D. S., & Brown, R. W. (1974). Lunar deposits of possible pyroclastic origin. Geochimica et Cosmochimica
Acta, 38(11), 1703,IN1703,1705-1704,IN1709,1718. doi:10.1016/0016-7037(74)90187-2
• Kring, D. A., & Durda, D. D. (2012). A global lunar landing site study to provide the scientific context for exploration of the Moon. LPI
Contribution(1694).
• Meyerhof, G. (1957). The ultimate bearing capacity of foundations on slopes. Paper presented at the Proc., 4th Int. Conf. on Soil
Mechanics and Foundation Engineering.
• Mitchell, J., Bromwell, L., Carrier III, W., Costes, N., Houston, W., & Scott, R. (1972a). Soil mechanics experiment. In Apollo 15
Preliminary Science Report (Vol. 289, pp. 7.1-7.28).
• Mitchell, J., Houston, W., Scott, R., Costes, N., Carrier III, W., & Bromwell, L. (1972b). Mechanical properties of lunar soil: Density,
porosity, cohesion and angle of internal friction. Paper presented at the Lunar and Planetary Science Conference Proceedings.
• Mitchell, J. K., Carrier III, W. D., Houston, W. N., Scott, R. F., Bromwell, L. G., Durgunoglu, H., Costes, N. C. (1972c). Soil mechanics.
In Apollo 16 Preliminary Science report (Vol. 315, pp. 8.1 - 8.29).
• Paige, D. A., Foote, M. C., Greenhagen, B. T., Schofield, J. T., Calcutt, S., Vasavada, A. R., . . . McCleese, D. J. (2010). The Lunar
Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Space Science Reviews, 150(1-4), 125-160. doi:10.1007/s11214-
009-9529-2 49

50. Determination of Diameter of Footing
50

51. Effective Incidence Angle
• Effective incidence angle is derived from the illuminated slope
51