1. First-Order Relationships Between Lunar Crater Morphology,
Degree of Degradation, and Relative Age:
The Crater Degradation Index
William A. Ambrose
Bureau of Economic Geology
John A. and Katherine G. Jackson
School of Geosciences
Apollo 17
photograph

2. Outline ● Cratering Processes ● Crater Type Versus Morphology
-The USGS Main Sequence
● Crater Degradation Processes
-Fracturing, Lava Flooding, Subsequent Impacts
● Crater Degradation Index
-Statistical Trends for Each Crater Type
● Summary and Conclusions

3. Main Sources and Acknowledgments
NASA & The Lunar and Planetary Institute
-Lunar Orbiter ( ), Apollo ( )
Clementine (1994), Lunar Prospector (1999)
USGS
-G. K. Gilbert, Eugene Shoemaker, and Don Wilhelms
USAF
-Lunar Aeronautical Charts (1965)
Ralph Baldwin, The Face of the Moon (1949)
Peter Schultz, Moon Morphology (1972)
C. A. Wood, The Modern Moon (2003)
Antonín Rükl, Atlas of the Moon (2004)

4. Outline ● Cratering Processes ● Crater Type Versus Morphology
-The USGS Main Sequence
● Crater Degradation Processes
-Fracturing, Lava Flooding, Subsequent Impacts
● Crater Degradation Index
-Statistical Trends for Each Crater Type
● Summary and Conclusions

5. Typical Moon Crater Langrenus: 144 km across Attributes Deep Floor
Apollo 8 photograph
Attributes
Deep Floor
Gentle Profile
Ejecta Blanket
Large Diameter
Dimensional data: Cherrington (1984)
2.9 km
5 km
V.E. >25:1
LPI

6. Outline ● Cratering Processes ● Crater Type Versus Morphology
-The USGS Main Sequence
● Crater Degradation Processes
-Fracturing, Lava Flooding, Subsequent Impacts
● Crater Degradation Index
-Statistical Trends for Each Crater Type
● Summary and Conclusions

7. The Modified USGS Main Sequence
Small
Large
Crater Types (diameter in km)
Simple 1 10
Moltke
Complex
2 & 3 25 93
Euler: Type 2
Copernicus:
Type 3
Walled
Plains 4
227
Schickard
Small
Basins 5
320
Schrödinger
Large
Basins 6
930
Mare Orientale

8. Type 1 (Simple Craters)
Lunar and Planetary Institute
10 km
Moltke

9. Types 2 and 3 (Complex Craters)
Lunar and Planetary
Institute
Type 3 (Aristarchus)
40 km
Type 2 (Euler)
10 km

10. (Walled Plains and Small Basins)
Types 4 and 5
(Walled Plains and Small Basins)
Ptolemaeus (Type 4)
Schrödinger (Type 5)
Apollo 16
Clementine
150 km
75 km
Grimaldi Basin
Type 5
Wood (2003)

11. Type 6 (Large Basins) Mare Orientale Mare Imbrium 900 km 1,300 km
Lunar Orbiter
Lick Observatory
900 km
1,300 km
Mare Orientale
Type 6
Wood (2003)

12. Outline ● Cratering Processes ● Crater Type Versus Morphology
-The USGS Main Sequence
● Crater Degradation Processes
-Fracturing, Lava Flooding, Subsequent Impacts
● Crater Degradation Index
-Statistical Trends for Each Crater Type
● Summary and Conclusions

13. Crater Degradation Index
● Empirical measurement of crater maturity
based on presence or absence of key
degradation factors.
● Data sources:
Observations: >700 primarily nearside craters,
farside craters, small basins and large basins
Maps and photographs:
●Lunar Orbiter, Apollo, Clementine
●Antonín Rükl, Atlas of the Moon (2004)
●Lunar Aeronautical Charts (USAF)
Publications:
●Wood (The Modern Moon, 2003)
●Westfall (Atlas of the Lunar Terminator, 2000)
●Schultz (Moon Morphology, 1972) +many others

14. Crater Degradation Factors
Extant?
● Subsequent impacts
-New craters, landslides, and ejecta
Yes
● Lava flooding
-External and internal No
● Fracturing No
● Degassing/Volcanism
Maybe

15. Crater Superposition and Relative Age
Overlapping
Non-overlapping
Theophilus
Werner
Cyrillus
Aliacensis
80 km
Lunar Orbiter
Photograph
008

16. Subsequent Impacts
None
Many
Tycho
Janssen
80 km
100 km

17. Mare Humorum are degraded Degradation is principally
Lava Flooding:
Mare Humorum
European Southern Obs.
Many large craters in
Mare Humorum are degraded
Gassendi
(floor-fractured)
Degradation is principally
due to mare lavas
099
Doppelmayer
Puiseux
Doppelmayer

18. Gassendi: Floor-fractured crater
European Southern Obs.
Apollo photograph
South
100 km
Gassendi

19. Dark-Halo Craters Pu’u O’o Firefountain Alphonsus Dark-halo craters
USGS (1985)
Alphonsus
Dark-halo
craters
Nikolai Kozyrev (1958)
Apollo photograph
50 km

20. Outline ● Cratering Processes ● Crater Type Versus Morphology
-The USGS Main Sequence
● Crater Degradation Processes
-Fracturing, Lava Flooding, Subsequent Impacts
● Crater Degradation Index
-Statistical Trends for Each Crater Type
● Summary and Conclusions

21. Crater Degradation Index
Low
High 1 2
Rays or bright ejecta
Present
Not present
Rim
Pristine or Sharp
Smooth or Rounded
Subdued
Floor fractured or flooded by internal lava No
Yes
Interior or rim flooded by mare lava
Outer Rim Flooded or Overlapped
Breached
Major post-impact craters
1-3
>3
Ejecta from other craters
Albedo in full phase
Bright
Dim

22. Copernicus and Stadius
100 km 1 2
Rays or bright ejecta
Present
Not present
Rim
Pristine or Sharp
Smooth or Rounded
Subdued
Floor fractured or flooded by internal lava No
Yes
Interior or rim flooded by mare lava
Outer Rim Flooded or Overlapped
Breached
Major post-impact craters
1-3
>3
Ejecta from other craters
Albedo
Bright
Dim
Copernicus
(0)
Stadius
(9)

23. Degradation From Base Surge Deposits
(Trask and McCauley, 1972)
Riccioli
70 km 1 2
Rays or bright ejecta
Present
Not present
Rim
Pristine or Sharp
Smooth or Rounded
Subdued
Floor fractured or flooded by internal lava No
Yes
Interior or rim flooded by mare lava
Outer Rim Flooded or Overlapped
Breached
Major post-impact craters
1-3
>3
Ejecta from other craters
Albedo
Bright
Dim
Lunar Orbiter photograph
(6)
Mare Orientale

24. Crater type versus C.D.I. C.D.I. Type 10 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6
n=704
y=1.4x+0.6 9 8 7
mean 6 5
C.D.I.
>100 4
50-100 3
20-50 2
1-20 1 1 2 3 4 5 6
Type

25. C.D.I. distribution per crater type40
300
n=403
n=153 30
200
number of craters
number of craters 20
100 10 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
C.D.I.
C.D.I.
non-mare
mare

26. C.D.I. distribution per crater type20 10
n=103
n=23
number of craters 10
number of craters 5 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
C.D.I.
C.D.I.
non-mare
mare

27. Crater type vs. avg. # subsequent impacts20
Wilhelm
n=596 18
Langrenus 16 14 12 10
# impacts 8
non-mare 6
mare 4 2 1 2 3 4 5 6
Type

28. Normalized subsequent impact data10
n=690 9
Prevalent flooding 8 7 6 5
# impacts/ km2 (x 10-3)
Sinus Iridum
Type 5 4 3 2 1 1 2 3 4 5 6
Type

29. Lunar Crater Densities vs. Time
Number of Craters
per 106 km2
Age (Billions of years)
Modified from Heiken, Vaniman, and French (1991)

30. Arthur Scale ●Modified from Baldwin (1949, 1963) Class C.D.I. 0-1 2
3-4
5-7
>7
Description
Age (109 BY) 1
Fresh rims, rays
0-2.9 2
Freshest post-Mare 3
Softened rims 4
Heavily degraded 5
Faint outline

31. Summary ●Crater morphology is systematically related to crater size.
●Crater degradation tends to increase
with larger crater type.
●Normalized crater-density values
asymptotically increase for
crater types 1-4; lower for type 5.
●Crossplots of degree of degradation
versus crater type, plus crater-
density data provide a useful
framework for estimating crater
maturity.
Clementine
photograph