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46th Lunar and Planetary Science Conference (2015)
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RULES FOR REGOLITH THICKNESS ESTIMATION USING CRATER MORPHOLOGY AND ITS
APPLICATION TO OCEANUS PROCELLARUM. Tiantian Liu1, Wenzhe Fa1, and Meng-Hua Zhu2, 1Institute
of Remote Sensing and Geographical Information System, Peking University, Beijing 100871, China (ttliu@pku.
edu.cn, [email protected]), 2Space Science Institute, Macau University of Science and Technology, Macau, China.
Introduction: Laboratory impact experiments
showed that morphologies of small fresh craters (normal, flat-bottomed, and concentric) depend primarily
on regolith thickness [1], and this relation was previously used to estimate lunar regolith thickness over
local regions using Lunar Orbiter (LO) photographs [1,
2]. With the release of high-resolution optical images
from recent lunar missions (e.g., Lunar Reconnaissance Orbiter, Chang’E-2, Kaguya) [3-5], this method was
revived and applied for regolith thickness estimation
over larger regions [6, 7]. Nevertheless, previous rules
for crater morphology identification are mainly based
on LO photographs, and it is not sure whether these
rules can be applied directly to recent images that were
obtained at different imaging conditions (e.g., illumination angle and spatial resolution). In addition, morphologies of small craters over the lunar surface are
very complicated because of target property and degradation [8]. For example, a degraded concentric crater
may exhibit a shadow pattern that looks like that of a
flat-bottomed one. All these issues might result in serious problems in the estimated regolith thickness. So far
as we know, there is no systematical summarization on
rules about optical images selection and crater morphology identification.
In this study, rules for regolith thickness estimation
using crater morphology are summarized by comparing the results from LO photographs and Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle
Cameras (NACs) images. Based on these rules, regolith thickness over Oceanus Procellarum is estimated
using LROC NAC images. The estimated regolith
thickness is helpful for landing site selection for China’s future lunar sample return missions.
Rules: When crater morphology method was first
proposed, regolith thicknesses over 12 sites were estimated using LO photographs, and it was found that
each site has one of four thickness types with median
values of 3.3, 4.6, 7.5 and 16 m [2]. These four regolith
thickness distribution types were consistent with a later
Monte Carlo simulation of regolith thickness distribution [9], indicating that crater morphology method and
the estimated thickness at these sites are reliable. In
this study, these 12 sites were taken as the calibration
sites for regolith thickness estimation.
Crater selection: By replicating regolith thicknesses over these regions using both LO photographs
and LROC NAC images [2], two basic rules were
found for crater morphology identification. Firstly,
only small craters with diameters less than ~250 m can
be used for regolith thickness estimation [1]. Small
craters display various but distinct morphologies with
respect to their sizes [1]. With increasing crater diameter, normal, flat-bottomed, and concentric morphologies appear in sequence. As crater diameter continues
to increase, a larger crater will turn into normal again,
and the formation mechanism will be different. Secondly, to identify the morphology of a small crater
unambiguously, the selected craters should be fresh. A
fresh crater suffers less later surface modification since
its formation, and as a result, its original morphology
can be well preserved. A crater with sharp feature,
uplifted rim, and a small number of superposed craters
can be regarded as fresh.
Crater morphology identification: Identification of
crater morphology is the key point of this method. Crater morphology is identified visually according to its
shape and shadow pattern. A normal crater has a bowl
or conical shape with a single arcuate shadow pattern.
A flat-bottomed crater has a pronounced flattened floor
as deduced from a single annular shadow pattern. A
concentric crater, characterized by a smaller central
crater, can be easily evidenced by a double annular or
ring-shaped arcuate shadow pattern. In addition, relative size, rim characteristic, and block distribution of
small craters can be helpful in distinguishing the type
of crater morphology. Normal craters with welldeveloped rims are usually smaller than a few tens of
meters [7], and there are no visible blocks on the rims.
Craters with flat-bottomed geometry have relatively
larger diameters and prominent rims, and there are few
or no blocks on the rims. It should be noted that flat
floors of smaller craters are usually smooth, nevertheless, floors of larger craters may be hummocky and
cracks might occur. The diameter of concentric craters
ranges from ten to more than two hundred meters, and
there are obvious blocks on their low rims that appear
as the ejecta apron.
Optical image selection: When high-resolution optical images are used, illumination angle and spatial
resolution of the selected images are two important
factors that can affect crater morphology identification.
The higher the spatial resolution of the image, the
smaller the crater whose morphology can be identified.
Therefore, if other conditions are the same, an image
with the highest spatial resolution is preferred. More
46th Lunar and Planetary Science Conference (2015)
craters can be identified in an image with a lower illumination angle. Nevertheless, when illumination angle
is smaller than the repose angle of regolith, shadow
effect will become a serious problem and might result
in misrecognition of the crater morphology [1, 7]. On
the other hand, if illumination angle is too large
(e.g., >55°), an optical image could become blurry and
less craters will be identified. Therefore, images with
illumination angles just slightly larger than regolith
repose angle are the best choice. In case of illumination angle smaller than the repose angle of regolith, the
recognition boundary of crater morphology in regolith
thickness calculation should be revised [1, 7].
It should be noted that regolith thickness estimated
from concentric craters is usually concordant with that
from normal craters. Therefore, we suggest that regolith thickness estimated solely from normal craters is
presented [1]. To summarize, Fig. 1 shows the key
points for rules in regolith thickness estimation using
crater morphology.
Regolith Thickness over Oceanus Procellarum:
Oceanus Procellarum (20.67°N, 56.68°W, 2592.24 km
diameter) is the largest basaltic lava located on the
western edge of the lunar nearside [10]. It is divided
into 60 spectrally homogeneous units according to the
Clementine UVVIS data, and surface ages of these
units were estimated using cumulative size-frequency
distribution of craters [11]. Based on the rules above,
regolith thickness of each unit is estimated using
LROC NAC images. In total, 69 images with spatial
resolutions from 0.4 to 1.7 m and illumination angles
from 21.4° to 52.6° were selected. 51870 small fresh
craters with diameter from 3.4 to 154.2 m are counted,
of which 32946 are normal and 3204 are concentric.
Fig. 2 shows the spatial distribution of the estimated regolith thickness (median value, in italic) and surface age (in roman) for the 60 geologic units. As can
be seen, median regolith thickness over Oceanus Procellarum varies from 2.1 to 8.4 m. There are substan-
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tial variations in regolith thickness among the 60 geologic units. Several regions (e.g., P57, P60) in Oceanus
Procellarum are considered as the youngest surfaces
across the Moon, and these regions have a median regolith thickness as small as 2.2 m. Regolith thickness
over the northeastern region is larger than that over the
southwestern region. When comparing regolith thickness with surface age, it is found that the median regolith thicknesses of the geologic units older than 2.7 Ga
are obviously greater than those of the remaining
younger units. Nevertheless, the correlation between
regolith thickness and surface age is not as strong as
expected [7], indicating that regolith evolution over the
Procellarum region might be very complex.
Conclusions: Rules for regolith thickness estimation using crater morphology are summarized. These
rules can be used to estimate regolith thickness over
larger region across the Moon using the newly acquired high-resolution optical images from recent missions. As an example, regolith thickness over Oceanus
Procellarum and its correlation with surface age are
estimated using LROC images. Our results show that
median regolith over Oceanus Procellarum varies from
2.1 to 8.4 m, and 45 geologic units (75% of the units)
have a regolith thickness smaller than 5.5 m. The estimated results can help for site selection for China’s
future lunar sample return missions.
References: [1] Quaide W. L. and Oberbeck V. R. (1968)
JGR, 73(16), 5247–5270. [2] Oberbeck V. R. and Quaide W.
L. (1968) Icarus, 9, 446–465. [3] Haruyama J. et al (2008)
EPS, 60(4), 243–255. [4] Zhao B. C. et al. (2011) Sci China
Tech Sci, 54(9), 2237–2242. [5] Robinson M. S. et al. (2010)
Space Sci. Rev., 150, 81–124. [6] Bart G. D. et al. (2011)
Icarus, 215, 485–490. [7] Fa W. et al. (2014) JGR, 119,
1914–1935. [8] Bart G. D. (2014) Icarus, 235, 130–135. [9]
Oberbeck V. R. et al. (1973) Icarus, 19, 87–107. [10] Andrew-Hanna J. C. et al. (2014) Nature, 514, 68–71. [11] Hiesinger H. et al. (2003) JGR, 108(E7), 1–27.
P1 3.59 7.5
P28: 2.94/3.72 5.7
P28
P55 1.67
3.4
P56 1.49
2.5
P10 3.44
7.0
Crater
selection
Size: <~250 m
6.6
P58 1.33
P21
3.12
2.93 P29
5.8
5.1
5.7
Freshness: sharp feature, uplifted
P9 3.47
2.4
rim, superposed craters
P39 2.19
4.5
P37
P53
1.68/3.18
P40
2.14/3.40
7.2
P26
2.96/
3.49
P31
5.5
P22:3.08 4.2
P30: 2.90 6.1
P37: 2.38/3.56 2.3
2.3
P48
6.1
Morphology
identification
Rules
single arcuate; (2) flat-bottomed:
single annular; (3) concentric: ringshaped arcuate or double annular
P60 1.20
3.7
3.8
P52
1.73/3.72
3.7
P16 3.33
3.4
P20
3.12/3.93
3.4
P23
P23: 3.07 3.7
P34: 2.592.3
P38: 2.31 2.5
3.4
2.7
P25
2.96/
3.62
2.41/3.86 P36
Illumination
angle
repose angle & <~55°
2.7
P24
3.00/3.74
4.4
P44 2.11
P34
3.6
5.1
2.3
P15
3.34/3.87
8.0
P2
3.57
P6
3.48
4.8
P12
3.42
P18 3.32
Image
4.5
P5 3.48
2.5
2.5
rim characteristic, block distribution
High is preferred
P43
2.12
P51 1.85
P19 3.31
Other information: relative size,
Spatial
resolution
2.2
P49 2.01
Regolith
thickness
8.4
P32 2.76
P14
3.36/3.62
P22
Shadow pattern: (1) normal:
P7
3.48
P4
P30
conical; (2) flat-bottomed: flattened
floor; (3) concentric: crater-in-crater
P4: 3.48/3.74 6.3
P31: 2.88/3.72 5.4
P48: 2.04 6.2
P59: 1.21 4.8
P59
5.3
Geometry: (1) normal: bowl or
Crater
P13 3.40
P57
1.33
2.2
P33
2.59/
3.63
P41
2.13
P47
2.08
P50
1.87
5.3
4.0
2.1
P35
2.54
P45
2.09/3.70
3.1
2.3
2.8
P11 3.43
4.0
P3
3.53
P8 3.47
7.0
P46
2.08
4.6
P27 2.96
4.3
P38
3.0
< repose angle
Revise recognition
boundary
Figure 1. Flow chart showing the rules for regolith thickness
estimation using crater morphology.
P17 3.32
P54 1.67 2.3
3.6
P42 2.12 2.8
Figure 2. Estimated regolith thickness (m) based
on normal craters over Oceanus Procellarum.