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Microcrater populations on Apollo 17 rocks
~CARUS22, 459-473 (1974)
Microcrater Populations on Apollo 17 Rocks E.
SCHNEIDER
I
Lunar Science Institute, 2 3303 Nasa Road 1, Houston, Texas 77058
AND
F. HORZ NASA-Johnson Space Center, Houston, Texas 77058
Received December 27, 1974; revised March 4, 1974 Approximately 6000 microcraters were investigated using binocular microscope techniques on Apollo 17 rocks 70215, 72215, 72235, 72395, 72435, 73216, 73218, 73275, 74275, 76135, 76136 and 79155. The crater populations observed have identical characteristics to those obtained from previous missions. Special emphasis was placed on assessing the influence of target properties on the observable crater populations. Although these properties cannot be quantitatively evaluated at present, the empirical results indicate that crater populations on glass, breccia, and crystalline rock surfaces may differ fundamentally. As a consequence, lunar surface exposure ages of individual rocks based on micrometeoroid craters may be subject to criticism. INTRODUCTION Microcrater p o p u l a t i o n s on whole lunar rock surfaces o b t a i n e d b y binocular microscope techniques have been r e p o r t e d from previous Apollo missions (HSrz et al., 1971 ; iY[orrison et al., 1972 ; N e u k u m et al., 1973). This r e p o r t summarizes investigations of t h i r t e e n Apollo 17 rocks. I n a n a l o g y t o large scale lunar surfaces (Gault, 1970; S h o e m a k e r et al., 1969; a n d others), t w o basic t y p e s of crater populations h a v e been distinguished on l u n a r rocks, i.e., " p r o d u c t i o n " a n d "equilibr i u m " populations, with an i n t e r m e d i a t e " t r a n s i t i o n " condition (see Fig. 1). B y definition, p r o d u c t i o n p o p u l a t i o n s are limited to rocks of low absolute crater density, where crater d e s t r u c t i o n b y superposition can be neglected. W i t h time,
more a n d more i m p a c t s will occur on a l r e a d y cratered areas until finally a surface becomes so densely cratered t h a t on t h e statistical average e v e r y new e v e n t will d e s t r o y an existing crater; the surface has reached crater equilibrium. The dev e l o p m e n t from p r o d u c t i o n into equilibrium populations is g r a d u a l a n d characterized b y a transition population. P r o d u c -
TIME
1 Permanent address: Max Planck Institut fiir Kernphysik, 6900 Heidelberg, West GerFIG. 1. Schematic development of cratermany. populations on lunar rock surfaces where craters 2 The Lunar Science Institute is operated by are obliterated only by superposition of subsethe Universities Space Research Association quent events. Note the gradual transition under Contract No. NSR 09-051-001 with the between genuine production and equilibrium National Aeronautics and Space Administration. surfaces. (Schneider and H6rz, 1974.) Copyright ~ 1974 by Academic Press, Inc. 459 All rights o f reproduction in any form reserved. Printed in Great Britain
460
SCHNEIDER AND HORZ
tion, transition, and equilibrium surfaces are frequently observed on lunar rocks and are also obtained in computational models (Neukum et al., 1973 and Hartung et al., 1973). The recognition and interpretation of production populations appears relatively straightforward and their analysis has yielded valuable information about the mass-frequency and absolute flux of micrometeoroids, recently summarized by HSrz et al. (1973). In contrast, considerable ambiguities exist in establishing transition and especially equilibrium populations (Neukum et al., 1973; Morrison et al., 1973; Neukum, 1973; and Hartung et al., 1973). These ambiguities are caused by a variety of effects: 1. The numbers of craters counted may not be sufficient to obtain reliable statistics on finite sized rocks to distinguish unambiguously between transition and equilibrium conditions; the surface areas of individual lunar rocks are simply too small for adequate statistics, especially for pit crater sizes D v > 500t~m diameter (Hartung et al., 1973). 2. Genuine production surfaces have cumulative size frequency distributions that display a changing log-frequency versus log-diameter slope from - 3 to - 1 or even less (HSrz et al., 1973). In contrast to large scale regolith crater counts, the absolute slope 'is, therefore, not a diagnostic criterion for transition or equilibrium conditions. 3. The absolute crater density, t h a t is traditionally taken as an indication for production, may not be diagnostic because lunar rocks differ vastly in their physical properties (Hartung et al., 1973). Effects 1-2 have been the subject of previous investigations (HSrz et al., 1971 ; Morrison et al., 1972; Neukum et al., 1973) and have led the various investigators to different interpretations. The main purpose of this report is to evaluate possible effects of target properties on the observable crater populations and to clarify, hopefully, some of the interpretational difficulties, among which the implications for surface exposure age dating rank foremost (Morrison et al., 1972, 1973;
Neukum, 1973).
1973;
and
Hartung
et
al.,
OBSERVATIONS AND R E S U L T S
Rock Samples A total of 13 different Apollo 17 rocks were investigated at binocular microscope magnifications of 8×, 10f, 16×, 20×, 25×, and 50× in the manner described previously, e.g., Neukum et al. (1973). The rock specimens were selected according to petrographic character as well as lunar surface history, absolute crater density, and a potential correlation of physical target properties and crater density. The rock specimens investigated and some of the above selection criteria are listed in Table I. In addition, scanning electron microscope (SEM) observations of glasssurface 64455 were obtained. Definitions of Observed Features Whereas previous crater counts were predominantly, if not exclusively, concerned with " p i t " craters only, we have attempted to gather additional statistical information on other impact features such as "pitless craters," "stylus-pits," and "concentric cracks". These features are defined as follows : P i t crater. The most common impact phenomenon on lunar rocks. I t is characterized by a cup shaped depression t h a t is glass lined. The pit is surrounded by a concentric "halo zone" of finely crushed material t h a t in turn sits within a still larger "spall zone". The diameters of these zones are defined as Dp (Pit), D h (Halo), and D s (Spall zone) (H5rz et al. 1971). Pitless crater. Depressions t h a t are shallower than glass lined pits and t h a t do not contain any glass. Instead the crater bottom and walls are made up of microfractured material identical to the "halo" zone. These features were first described by McKay, 1970, and HSrz et al., 1971, and attributed to secondary, low velocity impact. Hartung and HSrz, 1972, however, established evidence t h a t pitless craters m a y indeed be due to primary hyper=
IM_ICROCRATERPOPULATIONSON APOLLO 17 ROCKS
461
TABLE I S U ~ Y O F P E R T I N E N T DATAACCORDINGTOLSPET(1973) Rock type a 70215 72215 72235 72235 72395 72435 73216 73218 73275 74275 76135 76136 79155
Mass Pyrex- PlagioIlmenite Graingr ene clase Olivine & others Clasts Matrix size mm
X, B 8110 40-45 ~ 30 Br, e 379 Br,e ) , 62 Br, M~t----~ 62 Br, M 536 Br, M 161 Br, M 162 Br, M 40 Br, M 430 X, B 1493 Br, M 133 45 ~ 40 X, B 87 X, G 319 40-45 ~ 35-40
~5
~ 25 40 30 ~ 10 3 5 15 ~ 15
~5 ~3
~ 10 ~ 15
60 70 90 97 95 85 ~ 85 95
1-2 <0.1 >0.1 >>0.1 <0.1 <0.1 <0.1 >0.1-2 <0.1 ~ 0.2 0.5-2.5 0.3-0.8 1-3
Surface history ~ Complex Simple Simple Simple Simple Simple Complex Simple Complex Simple Simple Simple Complex
a According to LSPET (1973): Br, e----Breccia, light grey, friable; Br, M----Metaclastic rocks; X, B = Crystalline, basalt ; and X, G = Crystalline, gabbro. Simple: At least one side is void of microcraters, i.e., rock has not repeatedly tumbled. Complex : All sides of the rock are cratered, indicating at least a two stage surface exposure history (in part according to R. L. Sutton, pers. comm., 1973). velocity i m p a c t with the glass-lined pit r e m o v e d either during the crater forming e v e n t or subsequently. Completely isolated " p i t s " are occasionally f o u n d in regolith fines, t h u s s u p p o r t i n g the h y p o t h e s i s t h a t t h e y are dislodged in tote from their t a r g e t surface. F u r t h e r m o r e m a n y pitless craters also possess a spall-zone similar to pit craters, t h a t f u r t h e r s u b s t a n t i a t e s their transitional n a t u r e from pit craters. T h o u g h some pitless craters are demons t r a b l y modified h y p e r v e l o c i t y pit craters, one c a n n o t exclude a low velocity origin for some others. A t present a q u a n t i t a t i v e assessment of " p r i m a r y " versus "seconda r y " origin is n o t possible. However, all pitless craters e n c o u n t e r e d in this s t u d y are interpreted to be of p r i m a r y origin. Stylus crater. Glass lined cup, which rests on an elevated pedestal significantly a b o v e the i m m e d i a t e t a r g e t surface inside t h e spallation zone. (HSrz et al., 1971.) The r e q u i r e m e n t s for stylus f o r m a t i o n are u n k n o w n at present; however, t h e y are n o t the result of erosion processes. Vedder (1971), a n d N e u k u m et al. (1972), d e m o n s t r a t e d their f o r m a t i o n due to
p r i m a r y i m p a c t in the l a b o r a t o r y a t velocities > 3 k m / s e c . Concentric cracks. A fracture s y s t e m t h a t concentrically surrounds the central pit. I f present, a few fracture systems, a t most, can be observed a n d the m e a s u r e d radius, De, refers to the largest d i a m e t e r system. I t is m o s t l y confined to the halo zone. The spallation shoulder is n o t considered to be p a r t of the concentric crack system(s). I t is b e l i e v e d - - a s i n d i c a t e d - - t h a t pit craters, pitless craters, a n d stylus pits are transitionally related, t h o u g h the e x a c t conditions of f o r m a t i o n are presently u n k n o w n . The most c o m m o n t y p e s are genuine pit craters which are solidly e m b e d d e d in the s u r r o u n d i n g t a r g e t m a t e r ial; as the height of the stylus increases, the a t t a c h m e n t to the t a r g e t becomes increasingly weaker a n d the n u m b e r o f concentric cracks increases. F i n a l l y t h e b o n d i n g of the glass lined pit becomes so weak t h a t it is dislodged from the target. I n some instances it is also observed t h a t the r e m o v a l of the pit does n o t necessarily involve stylus formation, b u t as indicated
Q
Ds/ D~ ~
Illllllllll Pc/f)pa
illiiSii[[i
% Stylus pits*
I
~o Pitless craters*
% Concentric cracks*
lill[~lLIII
X
tic
c~
T
A
Tdarkdast T breccia totalrock a
72235 72235 73216
1.26 1.26 --
5.81 6.64 4.15 5.61 5.73 6.64 8.22 3.32 4.77 0.83 3.82 1.58 7.39 3.82 1.99 8.96 4.73 3.15 1.66 --
-. --
4.4 4.4 -4.1 --. --. . . . . 4.8 . . . 4.0 4.6
.
. .
. . . .
.
.
.
--
--
---
.
.
. .
2.1 2.1 2.2 ---. --. . . .
.
. .
.
. -1.4
.
1.6 1.8
1.6
1.4
. .
. . . .
.
-------
.
.
3.3
-5.8
25
.
.
6.8 4.8
17 10 11 3.7 3.0 4.6
.
. .
16 45 --
14 9
12 50
31 23 23 20 18 44 17 26 17 53 30 35 24
127 ---
51.5
48 56 34 42 168 87 193 68 145 27 42 33 179 94 14 188 11 88 15 --
206 23 29 17 16 106 70 89 46 83 14 15 21 83 51 5 111 6 64 8 132
309
91 51 --
103 27 34
206
58 3 40 4
12 15 11 7 53 43 30 28 44 7 7 11 44 28
412
20 24 94
257.5
34 1 29 3
7 7 6 6 29 28 15 21 24 4 3 4 24 13
515
14 17
309
17 2
20
2 3 3 2 19 19 8 14 14 3 1 2 11 8
618
4 1
2 4 2
10 16
360.6
9 1
n
9
412
6 1
6
1 4 9 3 8 4 1
11
1
2 12 14 6 11 5 2
824
2
721
5 8
463.3
4 1
3
3 1
2 6 2 7 2
1
927
Stylus pits, % Pitless craters, % Concentric cracks.
50 45
154.5
Magnification 50 × a
76 97 61 82 209 100 351 99 210 43 104 62 331 156 24 332 20 121 26 --
103
Ds/DP, Dc/D~, %
1.3 -53
-17
---
~ 4.4 -----
--3.8 3.7 ---
Surface area scanned. Da/Dp = 8 p a l l d i a m e t e r / P i t d i a m e t e r . Dh/DP = H a l o d i a m e t e r / P i t d i a m e t e r . d Dc/Dp = C o n c e n t r i c c r a c k d i a m e t e r / P i t d i a m e t e r . ' Percentage taken from total number of craters (including pitless craters). f Only pit craters. g P i t d i a m e t e r s (~). h All craters larger than a minimum diameter have been measured to derive
W T B B spallarea B T breccia T dark ciast T B B S N T N B T B T 8 glasscoat totalrock a
7 02 15 7 02 15 70215 7 02 15 72215 72235 72235 T2395 ?2435 7 32 16 73218 73218 73218 74275 74275 74275 7 61 35 79155 79155 79155
Magnification 25 × #
3 8
515
3
2
2 1
1 5 1 5 1
1
1030
1 5
566.6
2
2
1
2 1
4
1133
1 4
618
1
1
1
1 1
4
1236
3
669.5
1
1 1
3
1339
3
721
1
1 1
3
1442
1
1
2
2
--
772.5>772.5
1
1
3
1543 > 1 5 4 5
d~
O ~-~
0
O
O
© h# ~
~J
O C)
464
SCH~IEIDER
AI~D KORZ
b v H a r t u n g and HSrz (1972), it m a y result from a direct u n d e r c u t t i n g of the pit b y concentric cracks w i t h o u t stylus development. Because of this transitional n a t u r e the distinction of stylus and pit craters is somewhat a r b i t r a r y ; only those craters t h a t had a well developed stylus were recorded as stylus pits.
used, however, are considered to constitute a representative sample. The crater counts listed in Table I] are illustrated in Figs. 2-6 in the traditional log-diameter versus log-frequency format. I n some cases, pitless craters are p l o t t e d individually. Stylus craters are included in the pit crater counts because t h e y do possess a glassy pit. The areal density coefficient (A) as defined b y N e u k u m et al. (1973) is also included. A chip of sample 64455 was investigated via SEM techniques according to Schneider et al. (1973). The chip was curved, because it was cut from an egg-shaped p a r e n t rock (Fig. 6). Accordingly, three surface areas were distinguished. The " t o p surface" (II) measured 1.12 cm 2 and the two "side surfaces" (I and I I I ) were 0.16cm 2 and 0.32cm 2, respectively. The crater counts illustrated in Fig. 6 are based on a 100 × photomosaic t a k e n of the entire chip; the crater p o p u l a t i o n of area I I I is n e a r l y identical to t h a t of I and not illustrated. Also illustrated are binocular
Results
The crater populations investigated are s u m m a r i z e d in Table II. As in our previous studies, a v a r i e t y of rock faces of different exposure angle were investigated for some individual rock specimens. The sides investigated are designated T, B, E, W, S, and N in accordance with the orthogonal " m u g s h o t " p h o t o g r a p h y t a k e n at the L u n a r g e c e i v i n g L a b o r a t o r y . Also listed in Table I I are m e a s u r e d ratios of D s / D p, D h / D p and D c / D p t o g e t h e r with relative frequencies of pitless a n d stylus craters as well as craters displaying concentric crack systems. These special investigations were not p e r f o r m e d on all rocks. The rocks
,oo, . °s,op:,.5, j ,0[
I
...o Y: °I
Ds/Dp = 4.5
: 72435, B
1F 7221s '/''x: OgcRATER' :~
.
1
J CRATERS o °'1/C'ATER,So , /x'Ox:''
x20x
: 196
CRATE?
~
T
i
10
100
---•
10
i
Ds/Dp = 4.5 A = .20
o
•
~e o
7
79155, T
e l O x : 93 CRATEI~S
1000
10 OO0 10
100
100
I
.36
5
Ds/Dp = 4.6 A = .34
,
10
~
~
O~o~ 79155, N
x25x : 121 CRATERS
• 2 5 x : 9 9 CRA
100
o~,~=
III ,
10
Ds/Dp = 4.6 I ! o
'elOx
10OO
10 OOO 10
: 1 7 0 CRATERS I
IOO
10()O
10 OO0
DIAMETER ( F' m)
FIO. 2. Cumulative size frequencies of pit diameters (Dp) (solid circles) and corresponding spall diameters (Ds) (open circles). The best fit of a --2 slope through the spall-diameter frequency curve yields a numerical measure for-crater density as described by Neukum et al. (1973). Figures 3-6 axe constructed in identical fashion.
I~ICROCRATER POPULATIONS ON APOLLO 17 ROCKS I00 ~ - -
- ~
.
465
~.
A = 07
I
ROCK 70215 Ds/Dp = 46
• A~= O8~
.25x
~
CRATERS
)
I
25
x
° ~
: 97
'
CRATERS
l .1 k _ _ _ ±
T
~
~ ,~ ,~
~
'
25 x - 82 CRATERS IN LARGE SPAL[ AREA
~\ . ~
. . . .
"A : ' ~
i
E
A = .10
A = 23
ROCK ' 73218 D s / O p = 4.5
I
r 2 5 x : 107 CRATERS
.I~
u
~,
r
\ \
: 25 x : 62 CRATERS
~
~
~
lOO
A = .23
\
I
.
.
25 x : 343 CRATERS
~ •
.
.
•
.
.\
;
.
.
l
.
A = 25
; = o~
ROCK 74275 r Ds/Dp = 4 8
6
10
eo0e~
~ n
10
.\
l
CRAiERS
Z~[~
oN
I 100
I[~ IOOO
n 2 5 x : 167 CRATERS
o 10 000 10
s
J q 2 5 x : 24 | CRATERS
j \ ~ I
;
I
100 1000 DIAMETER (~ m)
10 OOO 10
1OO
1000
10 OO0
Fro. 3. Cumulative frequencies of pit- and spall-diameters for specific faces of individual rocks. l~ASA-S-73-3069. crater count (1973).
data
of N e u k u m
et al.
b e e n r e p e a t e d l y d e m o n s t r a t e d t h a t on genuine p r o d u c t i o n surfaces t h e c u m u l a t i v e slope, a, is changing w i t h values o f a p p r o x i m a t e l y --3 a t p i t d i a m e t e r s > 3 0 0 F m a n d values of ~ - 2 at pit d i a m e t e r s 50-300Fro; a t still smaller sizes, i.e. 1 0 - 5 0 ~ m , t h e slope is e v e n
INTERPRETATION C u m u l a t i v e c r a t e r frequencies as a f u n c t i o n of c r a t e r d i a m e t e r m a y be expressed in t h e f o r m N ~ = K . D -~. I t has 100
i
i
?
Ds/Dp
= 4.5
Dh/Dp
= 2.0
>-
~
A(:] = .O63 A l l = .022
-N lO
O
0
O
I
D~/O. : 45t
eo
.Oo
O
h/Dp
=
= 4.s I = 2.0
Ao=.
~
o
o
0
o=,~
I
Dis/Dp Dh/Dp
D
I
AO= .4 A I = ,OI
0 I i
~
1
f 20 PIT T CRATERS L T o 2 5 x : 41 PIT A N D ~ •
u
25
:
PtTLESS,CRATERS],
10
100
1000
"~ \
o
~32~6
B
\
\ I~ \
\
I
10 0 0 0 10
CRATERS
CRATERS 1 o 16x : 323 PIT AND PITLESS CRATERS
o lOx : 108 PIT AND
I
100 I000 DIAMETER (F.m)
L¥ |
~ ~ ~
P,nESS(RATERS~,--
10 0 0 0 10
I00
1000
10 0 0 0
l~zo, 4. Cumulative frequencies of pit craters and pit and pitless craters for rocks 76135, 73275, and 73216. The corresponding spall frequencies are indicated by squares, l~ote that the size-frequency distribution of pifless craters is similar to pit-craters. NASA-S-73-3068. 17"
466
SCHNEID~IR ]
100
72235,T A.
= .18
o = .15
"RECC,A-MATR,X 10
D s / D p = 3.4 ~ • 5 0 x : 51 CRATERS 1.L ÷ 2 S x : IO0 CRATERS ~ o SPAtL DIAMETER
Z O
"
-
ao
DARK CLAST D s / D p = 3.9 • 5 0 x : 127 CRATERS 425x: 351 CRATERS o SPALL DIAMETER .I
I 1000 1o0 DIAMETER (/4m)
10
10 0 0 0
FIG. 5. Crater counts on rock 72235 (see also Fig. 7). flatter ( z - l ) (HSrz et al., 1973). However also changes because of equilibration effects t h a t result in the preferential destruction of smaller craters (Neukum et al., 1973 and Hartung et al., 1973). In addition observational effects m a y cause a change in ~ because of optical resolution limits for the smallest craters and insufficient statistics for large structures.
100
, I |~+
c~
'r ] [ I
__
..
~ O.o o e
AREA t
HORZ
All Apollo 17 rocks investigated display a change in slope (Figs. 2-6). The interpretation thus focuses on which of the above processes was responsible. Rocks 70215 (W, T and B) and 74275 (S) are interpreted to be genuine production surfaces because crater-overlap is negligible upon visual inspection and the absolute areal density coefficient is small (A < 0.08) (Fig. 3). All other surfaces display various degrees of crater destruction by superposition and therefore are considered to be in transition or equilibrium condition; their areal density coefficients range from 0.10 to 0.36 and thus extend into the "limiting frequency range" of ~orrison et al. (1972). A variety of independent equilibrium criteria were established by Neukum et al. (1973) on Apollo 16 rocks. The Apollo 17 rocks were subjected to a similar evaluation and the results are indicated in Table III. The only surfaces qualifying unambiguously for equilibrium conditions are 79155, T and N (Fig. 2). With the exception of these and the above genuine production distributions, the interpretation of all other surfaces remains ambiguous. Despite E~ AREA ~W } SEM-SCAN AREA IOOX AREA OPTICAL SCAN, 2OX
,
"° k - Ds/Dp=3.7 I \ o A=0.15 / 1o - PiT 11~I • ~ A--O.12-1 DIAMETER: IT t. o ~ | (SEM, 100X) 1 . + T o\ / " 36 CRATERS, I 1 P +\ I
lI~
~"
?
• 77 CRATERS, I
OPTICAL, 2 0 X / • 173 CRATERS, | ENTIRE ROCK I o
SPALL DIAMETER: ] , "Ds/Dp:4.9 J
AND
1,
+ \
~
"
2 cm
I
+
-J
'~ \ / \l
.1
10
100 1,000 CRATER DIAMETER (/.m)
10,O(~
Fig. 6. Crater populations on rock 64455. Notice the abundance of overlapping craters that essentially destroyed large parts of the original glass surface, indicating that the "optical" counts and "SE]¢[ a.rea II" counts are close to equilibrium conditions for glass surface 64455. NASA-S-73-3065.
467
MICROCI~ATER P O P U L A T I O N S O:N AtJOLLO 1 7 R O C K S TABLE
III
EQUILIBRIUM CI~ITEI~IA FOR VARIOIYS APOLLO 17 ROCKS
Equilibrium criteriaa Rock
(1)
(2)
x x
× x
72235, M b
x
72395
x
72435 73216 73218
x
x
x x
73275
x
74275
76136 79155
72215 72235, C b
(3)
(4)
(5)
(6)
Equilibrium
x
x x
? ?
x
x
x
×
×
x
x
x
x
x
x
x
x
x
x
x
x
x
? ?
x
?
x
x x
? Likely
x
x
?
x
x
x
x
?
? Yes
a According to Neukum et al. (1973). b C --~clast, M--~ breccia matrix. (1) A measured A analytical > 1. (2) Simple exposure history. (3) Surfaces of various exposure angles were measured.
(4) Same A on various surfaces. (5) Extended D -2 plateau. (6) Strongly abraded, i.e., rounded surface.
e x t e n d e d D -2 plateaus t h a t are suggestive for equilibrium on a v a r i e t y of rocks, their corresponding crater density m a y v a r y as m u c h as a f a c t o r of 6. I n contrast, sample 72215, B lacks an e x t e n d e d D -2 region, t h o u g h it possesses the highest crater density (A = 0.36; Fig. 2). H a r t u n g et al. (1973) have emphasized the potential role of physical properties of various rocks for the f o r m a t i o n of microcraters a n d in p a r t i c u l a r for their ease of retaining pit craters which constitute the observable preserved crater p o p u l a t i o n in contrast to the actual craters formed. A striking example illustrating the influence of mechanical t a r g e t properties is rock 72235. As illustrated in Fig. 7, its cratered surface consists of two materials: one is the regular light-grey breccia m a t r i x a n d the o t h e r is a dense, aphanitic, coh e r e n t clast. The entire rock is a chip dislodged f r o m a big boulder; its exposure g e o m e t r y is well k n o w n a n d it therefore m u s t be p o s t u l a t e d t h a t the two materials constituting 72235 had identical surface histories and exposure angles. N e v e r t h e -
less their crater populations differ signific a n t l y b e y o n d statistical error (Fig. 5). N o t e t h a t the "breccia m a t r i x " surface has a f a c t o r of 4 more pit craters for craters with Dp > 300/~m in d i a m e t e r ; at crater sizes Dp > 800/~m, the difference is even larger. These differences are ascribed to physical t a r g e t properties. Figure 8a illustrates some unusually soft soil breccias from Apollo 17. Their well r o u n d e d shapes indicate considerable erosion and it is p r o b a b l y safe to assume t h a t mass-wasting has progressed sufficie n t l y t h a t no "original" rock surface is preserved which would qualify t h e m b y definition (see Fig. 1) to be in crater equilibrium. A n d yet, upon visual inspection t h e y display v e r y few craters, if at all. Identical examples have been observed from other Apollo missions. Thus these materials present additional evidence t h a t physical t a r g e t properties strongly control the preserved crater population. A n o t h e r example is illustrated in Fig. 8b, t h a t contrasts a coherent glass-coating with a crushed, crystalline surface. Again different
468
SCHNEIDER
A7L~D H O R Z
! Cm
I Fio. 7. Sample 72235 consisting of a "hard" clast (left) and "friable" matrix (right). Notice the abundance of large craters (1-3 ram) in the matrix surface and compare them with the structures visible on the clast, l~IASA-S-73-3064. crater densities are observed even with the unaided eye. T h o u g h the crystalline surface was more f a v o r a b l y exposed (H5rz et al., 1972) it displays fewer craters t h a n the glass coating; the thinning of the glass coating a r o u n d the contacts with the exposed crystalline surface allows the reasonable a s s u m p t i o n t h a t the glass
coating once covered the entire specimen a n d t h a t meteorite i m p a c t completely r e m o v e d the glass in the area n o w occupied b y the underlying crystalline material. While the above examples t r u l y represent extremes t h a t serve to illustrate the point, our new, detailed m e a s u r e m e n t s of pitless craters, stylus pits a n d concentric
Fx(~. 8. Sample 79035. Soft soil-clod from Apollo 17. l~otice the absence of microcraters despite the advanced state of rounding and erosion of the rock. Sample 60135. Difference in crater density on a glass surface (dark area) and crystalline surface (light area). According to the reconstructed lunar surface orientation, the crystalline part faced upwards and thus had a more favorable exposure geometry, and yet, the craters are more numerous on the glass coating. NASA-S-73-3066.
M I C R O C R A T E R POPULATIO:~'S O1~ APOLLO
cracks suggest that there are also differences on a more subtle scale (see Table II): (a) Pitless craters. Their abundance varies between 9 and 50% of the total crater population. According to Fig. 9a, they appear to be more abundant on metaclastic breecias than on genuine crystalline rocks and within these two groups they are most abundant on surfaces of low crater density. Thus a mechanism must operate that differs in efficiency on various target-materials and that furthermore changes with increasing crater density. A quantitative explanation is not readily available and the following suggestions are offered, with the assumption that the samples measured are representative for all rocks: (1) Because dislodging of pit craters is due to tensile failure of the pit/target interface, pit removal must be accomplished b y reflection(s) of the shockwave. Such reflection(s) m a y occur at the free target surface and/or within the target along grain-boundaries, cleavage planes, rugs, and macro- and microfractures; these reflections are controlled b y the immediate pit environment. An attempted correlation with physical bulk properties of the rocks failed because these properties do not necessarily reflect the condition of the very surface that is affected b y the mierometeoroid bombardment. As a consequence, a detailed explanation is not possible. We are left with the empirical observation that even similar, genuine basaltic rocks and tough, coherent, highly metamorphosed breccias, that are essentially holocrystalline respond in different ways to the micrometeorite bombardment. This response furthermore changes with increasing crater density, i.e., essentially with time. The decrease in pitless crater frequency on the more densely eratered surfaces m a y be due to an increase in microfractures that cause highly polydirectional reflections which in turn influence significantly the attenuation of the initial shock wave and its associated reflections. (2) The inverse relation of "pitless" craters and absolute crater density (Fig.
17 ROCKS
469
9a) m a y also be taken as a sign that pitless craters are caused b y secondary projectiles. Their frequency on freshly exposed surfaces m a y be particularly high, because of the increased probability to encounter secondary projectiles during the rock's ballistic trajectory to the eventual site of collection. After landing, the surfaces would be essentially exposed to the bombardment of "primaries" and thus the population of secondaries would decrease with time. Because of the demonstrated transitional nature of some pit into pit-less craters and in particular because all pit-less craters have identical relative size-frequencies to pit craters (Figs. 2-6), we prefer to ascribe them to primary cratering events and therefore favor explanation 1. Regardless, however, which explanation is correct, the basic observation remains: different rock materials respond differently and display different numbers of pit craters. (b) Stylus-pits. The frequency of stylus pits also varies significantly from essentially 0-17% of the total crater population (Table 2). Figure 9b illustrates that there is no systematic correlation between pitless and stylus craters for different rock types. (c) Ds/Dp ratio. Figure 9c iUustrates DJDp ratios from all Apollo 12, 14, 15, 16 and 17 rocks measured to date. Though there is considerable data scatter, two general trends are apparent. Firstly basaltic rocks have larger Ds/D p ratios than do breecias, which we again take as indication for different target response. Secondly the ratios appear to be independent of absolute crater density and thus seem not to change with increased microfraeturing of the rock's surface in contrast to the pit-less craters of Fig. 9a. (d) Dc/D p and Dh/D r ratios. The newly measured D J D p and Dc/D p values are listed in Table II. Their absolute values differ from rock to rock although a quantitative correlation with other parameters cannot be established. All that can be concluded is that different Ds/Dp, Dh/D r and D d D p ratios also are indicative of differing material response to micrometeoroid cratering.
470
SCHNI~,IDEI~ AND HOI~Z
60
SO
-~
• 73216, B
•
CRYSTALLINE SAMPLES
•
BRECCIAS
" ~ ' ~ , ~• 73275 T 76135, B ~
40 • 73218, N • 70215, W
3O
&&"'~.~ ~" 2O
70215, T j
(a)
~ • •
~7021~,
72395, T ~ .
~',~.~.,~
73218, T "" ~•
10
"e~..~.
~72215
74275, T \~-72435, B
I
I
I
I
I
I
A
.1
I
I
I
I
I
I
.2 AREAL DENSITY
I
i
I
I
I
.3
I
.4
30 25
~2o u
~i5
(b)
10
• 0
0
10
•
•
t
I
15
20
•
e
I
I
i
I
f
40
45
50
60
i
25 30 35 % PITLESS CRATERS
•
A&
•
~
•
4
•
2
I .10
•
|
I I .20 .30 AREAL DENSITY
I .40
I .50
FIG. 9a. Percentage of pit craters versus absolute crater density coefficient (A). Note that the surfaces of low crater-density tend to have relatively more pit less craters. FIG. 9b. Correlation of pitless and stylus craters for different rocks and individual rock faces (see Table II). FIG. 9c. Correlation of absolute crater density coefficient (A) and spall to pit diameter ratio (Ds/Dp) of all rocks (ApoUos 12-17) measured to date. NASA-S-73-3067.
I~IICROCRATERPOPULATIONSON APOLLO 17 ROCKS
(e) Absolute crater density. Figure 10 illustrates the areal crater density coefficient as defined b y Neukum et al. (1973), for all rocks studied to date. Pe r each specimen, only t h a t surface was included which displayed the highest crater density observed. There is again considerable scatter in the data although once more basaltic rocks seem to differ from breccias. Breecias generally possess higher crater densities t han basaltic rocks. (f) Glass-surfaces. Included in Fig. 10 are glass surfaces 64455 and 60135 which are the most densely cratered glass coatings observed to date. Detailed SEI~ cratercounts for the top-surface of 64455 (Fig. 6) revealed t h a t despite its relatively low absolute crater density, its crater population is unambiguously in transition and close to equilibrium conditions as illustrated in Fig. 6. Note the considerable overlap of craters and the almost complete lack of original target surface in Fig. 6. The areal density measured on the surface is A = 0.12. I t is impossible to envision t h a t even with sufficient time the crater density could reach an A of 0.40 or even 0.62 like breccia 14306. Thus we conclude t h a t pits formed in breccias are retained with significantly greater ease t han on glass surfaces and according to Fig. 10,
471
crystalline rocks are somewhat intermediate between glasses and breccias. I)ISCUSSIOI~T Some specific investigations on Apollo 17 rocks together with observations on Apollo 12, 14, 15 and 16 rocks present unambiguous evidence t h a t physical properties of the rocks strongly govern their microcrater populations. These findings are in complete accordance with experimental impact cratering (Gault, 1973). The evidence, however, is only observational and empirical for lunar rocks and is not amenable at present to a more quantitative analysis and understanding, ttowever, it has serious implications for a variety of studies related to micrometeoroids. The flux of micrometeoroids derived from lunar rocks strongly depends on the conversion of a crater diameter (either Dp or Ds) into the corresponding projectile mass. A vari et y of microcratering experiments are available for this calibration (~andeville and Vedder, 1971; Bloch et al., 1971 ; Neukum et al., 1972 ; Schneider et al., 1973; Nagel, 1973; Gault, 1973; Vedder and ~andeville, 1973). None of these experiments were conducted on genuine lunar materials. The target simu-
~
LIMITING FREQUENCY (M O RRIsRANNeGEal 1972)
62235 72395 12038 73275, 72235--- 1 12073 684153.-i"~ i 61156--'1 I II -
-
-
7321671 I II
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• 12051 12047, 62006, 60315 12017, 6445S-'11 I Ill
ill
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b
5%
I
10%
-
-
-
• BRECCIA SURFACES _73218 • CRYSTALLINE SURFACES 69935 • GLASS SURFACES --74275 r--61015 60016 76136 62295 12063
/ Irr6117s ,243s / IIIF / IIII
. 25%
-6oo7s, 791s5, 7221s
Ir',42~1
~14306
.
.~,
.
:~ .~ ~, .:' ,~ 1:o AREAL .
SQ%
DENSITY SATURATION PERCENT (GAULT, 1970)
Fie. 10. Comparison of highest areal density coefficient observed to date on rocks from all missions. Crystalline only refers to genuine basaltic rocks ; metaelastic specimens are included in breeeias . The "limiting frequency range" is an empirical boundary above which no crater populations were observed to date.
472
SCHNEIDER AND HORZ
lants used were predominantly artificial glasses and a few crystalline materials. Thus caution has to be exercised in correlating the same diameter pit crater on a diversity of lunar materials (glass, crystalline rocks and breccias) with the same projectile mass. Because glasses are the best simulated materials, lunar glass coatings are probably the most reliable targets to use in deriving the micrometeoroid flux. Furthermore, the different material response to microcratering may strongly control the preserved crater populations (Figs. 5-10). 1VIorrison et al. (1973) and Neukum (1973) have suggested that the surface exposure time of individual rock specimens may be obtained from such populations. While Morrison et al. essentially equate the total number of craters for rocks that satisfy certain criteria (Morrison et al., 1973) with total exposure time, Neukum equates the extent of the D -~region with the time parameter. Hartung et al. (1973) have already raised serious questions concerning both approaches, because transition and especially equilibrium populations cannot be used for exposure age dating. Both approaches rely on the unambiguous recognition of production populations. Clearly both approaches can not quantitatively account, at present, for the different response of various rock materials. We maintain that the absolute crater density that can be preserved on a rock is both a function of target material and total exposure age. Because the influence of target properties at present escapes quantitative treatment, exposure ages derived from absolute crater-densities and/or extended D -2 regions are subject to serious criticism. ACKNOWLEDGMENTS This work has been made possible with the first author being a visiting scientist at the Lunar Science Institute, which is operated by the Universities Space Research Association under Contract No. N S R 09-051-001 with the National Aeronautics and Space Administration. We are indebted to R. L. Sutton for information about lunar surface orientation of some rocks and to D. A. Morrison for fruitful discus-
sions. This paper constitutes the Lunar Science Institute Contribution No. 178.
I~EFERENCES BLOCH, M. R., FECHTIG, H., GENTNER, W., NElmu~t, G., Am) SCHNEIDER, E. (1971). Meteorite impact craters, crater simulations and the meteoroid flux in the early solar system. I n "Proceedings of the Second Lunar Science Conference," pp. 2639-2652. MI T Press. GAImT, D. E. (1970). Saturation and equilibrium conditions for impact cratering on the lunar surface: Criteria and implications. R a d i o Science 5, 273-291. GATYLT, D. E. (1973). Displaced mass, depth, diameter, and effects of oblique trajectories for impact craters formed in dense crystalline rocks. T h e M o o n 6, 32-44. HA~G, J. B., ~rD HSl~Z, F. (1972). Microcraters on lunar rocks. I n "Proceedings of the 24th International Geological Congress," Montreal, Canada, Section 15, pp. 48-56. HARTUNG, J. B., HSRZ, F., AITKEN, K. F., GAULT, D. E., AND BROWI~LEE, D. E. (1973). The development of microcrater populations on lunar rocks. I n "Proceedings of the F o u r t h Lunar Science Conference," Vol. 3, pp. 32133234. Pergamon Press, Elmsford, N.Y. HSRZ, F., ~IAI~TIYNG, J. B., ~ D GAULT, D. E. (1971). Micrometeorite craters on lunar rock surfaces. J. Geophys. Res. 76, 5770-5798. HSRZ, F., CARRIER, W. D., YOUNG, J. W., DUKE, C. M., NAGLE, J. A., AND FI~¥XELL, R. (1972). Apollo 16 special samples. NASA SP-315, 7-24-7-54. H6RZ, F., BROWNLEE, D. E., FEOHTIG, H., HARTUNG, J. B., MORRISO~, D. A., NEUKUM, G., SCHt~-EIDER, E., A~CD VEDDER, J. F. (1973). Lunar microeraters--implications for the micrometeoroid complex. Planet. Space Sci. in press. MAIqDEVILLE, J.-C., AND VEDDER, J. F. (1971). Microcraters formed in glass by low density projectiles. Earth Planet. Sci. Lett. 11, 297306. McKAY, D. S. (1970). Microcraters in lunar samples. I n "Proceedings of the 28th Annual Meeting of the Electron Microscope Society of America" (C. J. Arceneaux, ed.), pp. 22-23. Claitor's Publishing Division, Baton Rouge. MORRISON, D. A., McKAY, D. S., HEIKEN, G. H., AND MOORE, H. J. (1972). Microcraters on lunar rocks. I n "Proceedings of the Third Lunar Science Conference," Vol. 3, pp. 27672791. MIT Press.
MICROCRATER POPULATIONS ON APOLLO 17 ROCKS MORRISON, D. A., McKAY, D. S., FRULAND, 1~. M., AND MOORE, H. J. (1973). Microcraters on Apollo 15 and 16 rocks. I n "Proceedings of the F o u r t h Lunar Science Conference," Vol, 3, pp. 3235-3253. Pergamon Press, Elmsford, N.Y. NAGEL, K. (1973). Experimente zur kratersimulation. Master's Thesis, University of Heidelberg, Germany, unpublished. NEUXUM, G. (1973). Micrometeoroid flux, microcrater population development and erosion rates on lunar rocks, and exposure ages of Apollo 16 rocks derived from crater statistics. I n " L u n a r Sci. I V " (J. W. ChamberlainandC. Watkins, Eds.), pp. 558-560. Lunar Science Institute, Houston. NEUKUlVI, G., SCHNEIDER, E., MEHL, A., STORZER, D., W A G N E R , G. A., FECHTIG, H., AND BLOCH, M. R. (1972). Lunar craters and exposure ages derived from crater statistics and solar flare tracks. In "Proceedings of the Third Lunar Science Conference," Vol. 3, pp. 2793-2810. MIT Press. NEU~:O-I~, G., H6RZ, F., HARTUNG, J. B., AN2) MORRISON, D. A. (1973). Crater populations
473
on lunar rocks. I n "Proceedings of the Fourth Lunar Science Conference," Vol. 3, pp. 3255 3276. Pergamon Press, Elmsford, N.Y. SCHNEIDER, E., STORZER, D., HA~T~nVG, J. B., FECHTIG, H., A_WD GENTNER, W. (1973). t~crocratcrs on Apollo 15 and 16 samples and corresponding cosmic dust fluxes. I n "Proceedings of the Fourth Lunar Science Conference," Vol. 3, pp. 3277-3290. Pergamon Press, Elmsford, N.Y. SCHNEIDER, E., AND HSRZ, F. (1974). Lunar rock erosion. I n " L u n a r Science V," p. 666. Lunar Science Institute, Houston. SHOEMAKER, G. M., BATSON, R. ~¢[., HOLT, H. E., MOI~RIS, E. C., RENm-LSON, J. J., AND WHITAKER, E. A. (1969). Observations of the lunar regolith and the earth from the television camera on Surveyor 7. J . Geophys. Res. 74, 6081-6119. VEDDER, J. F. (1971). Microcraters in glass and minerals. E a r t h Planet. Svi. Lett. 11, 291296. VEDDER, J. F., AND MANDEVILLE, J.-C. (1973). Microeraters formed in glass by projectiles of various densities (in preparation).
Microcrater Populations on Apollo 17 Rocks E.
SCHNEIDER
I
Lunar Science Institute, 2 3303 Nasa Road 1, Houston, Texas 77058
AND
F. HORZ NASA-Johnson Space Center, Houston, Texas 77058
Received December 27, 1974; revised March 4, 1974 Approximately 6000 microcraters were investigated using binocular microscope techniques on Apollo 17 rocks 70215, 72215, 72235, 72395, 72435, 73216, 73218, 73275, 74275, 76135, 76136 and 79155. The crater populations observed have identical characteristics to those obtained from previous missions. Special emphasis was placed on assessing the influence of target properties on the observable crater populations. Although these properties cannot be quantitatively evaluated at present, the empirical results indicate that crater populations on glass, breccia, and crystalline rock surfaces may differ fundamentally. As a consequence, lunar surface exposure ages of individual rocks based on micrometeoroid craters may be subject to criticism. INTRODUCTION Microcrater p o p u l a t i o n s on whole lunar rock surfaces o b t a i n e d b y binocular microscope techniques have been r e p o r t e d from previous Apollo missions (HSrz et al., 1971 ; iY[orrison et al., 1972 ; N e u k u m et al., 1973). This r e p o r t summarizes investigations of t h i r t e e n Apollo 17 rocks. I n a n a l o g y t o large scale lunar surfaces (Gault, 1970; S h o e m a k e r et al., 1969; a n d others), t w o basic t y p e s of crater populations h a v e been distinguished on l u n a r rocks, i.e., " p r o d u c t i o n " a n d "equilibr i u m " populations, with an i n t e r m e d i a t e " t r a n s i t i o n " condition (see Fig. 1). B y definition, p r o d u c t i o n p o p u l a t i o n s are limited to rocks of low absolute crater density, where crater d e s t r u c t i o n b y superposition can be neglected. W i t h time,
more a n d more i m p a c t s will occur on a l r e a d y cratered areas until finally a surface becomes so densely cratered t h a t on t h e statistical average e v e r y new e v e n t will d e s t r o y an existing crater; the surface has reached crater equilibrium. The dev e l o p m e n t from p r o d u c t i o n into equilibrium populations is g r a d u a l a n d characterized b y a transition population. P r o d u c -
TIME
1 Permanent address: Max Planck Institut fiir Kernphysik, 6900 Heidelberg, West GerFIG. 1. Schematic development of cratermany. populations on lunar rock surfaces where craters 2 The Lunar Science Institute is operated by are obliterated only by superposition of subsethe Universities Space Research Association quent events. Note the gradual transition under Contract No. NSR 09-051-001 with the between genuine production and equilibrium National Aeronautics and Space Administration. surfaces. (Schneider and H6rz, 1974.) Copyright ~ 1974 by Academic Press, Inc. 459 All rights o f reproduction in any form reserved. Printed in Great Britain
460
SCHNEIDER AND HORZ
tion, transition, and equilibrium surfaces are frequently observed on lunar rocks and are also obtained in computational models (Neukum et al., 1973 and Hartung et al., 1973). The recognition and interpretation of production populations appears relatively straightforward and their analysis has yielded valuable information about the mass-frequency and absolute flux of micrometeoroids, recently summarized by HSrz et al. (1973). In contrast, considerable ambiguities exist in establishing transition and especially equilibrium populations (Neukum et al., 1973; Morrison et al., 1973; Neukum, 1973; and Hartung et al., 1973). These ambiguities are caused by a variety of effects: 1. The numbers of craters counted may not be sufficient to obtain reliable statistics on finite sized rocks to distinguish unambiguously between transition and equilibrium conditions; the surface areas of individual lunar rocks are simply too small for adequate statistics, especially for pit crater sizes D v > 500t~m diameter (Hartung et al., 1973). 2. Genuine production surfaces have cumulative size frequency distributions that display a changing log-frequency versus log-diameter slope from - 3 to - 1 or even less (HSrz et al., 1973). In contrast to large scale regolith crater counts, the absolute slope 'is, therefore, not a diagnostic criterion for transition or equilibrium conditions. 3. The absolute crater density, t h a t is traditionally taken as an indication for production, may not be diagnostic because lunar rocks differ vastly in their physical properties (Hartung et al., 1973). Effects 1-2 have been the subject of previous investigations (HSrz et al., 1971 ; Morrison et al., 1972; Neukum et al., 1973) and have led the various investigators to different interpretations. The main purpose of this report is to evaluate possible effects of target properties on the observable crater populations and to clarify, hopefully, some of the interpretational difficulties, among which the implications for surface exposure age dating rank foremost (Morrison et al., 1972, 1973;
Neukum, 1973).
1973;
and
Hartung
et
al.,
OBSERVATIONS AND R E S U L T S
Rock Samples A total of 13 different Apollo 17 rocks were investigated at binocular microscope magnifications of 8×, 10f, 16×, 20×, 25×, and 50× in the manner described previously, e.g., Neukum et al. (1973). The rock specimens were selected according to petrographic character as well as lunar surface history, absolute crater density, and a potential correlation of physical target properties and crater density. The rock specimens investigated and some of the above selection criteria are listed in Table I. In addition, scanning electron microscope (SEM) observations of glasssurface 64455 were obtained. Definitions of Observed Features Whereas previous crater counts were predominantly, if not exclusively, concerned with " p i t " craters only, we have attempted to gather additional statistical information on other impact features such as "pitless craters," "stylus-pits," and "concentric cracks". These features are defined as follows : P i t crater. The most common impact phenomenon on lunar rocks. I t is characterized by a cup shaped depression t h a t is glass lined. The pit is surrounded by a concentric "halo zone" of finely crushed material t h a t in turn sits within a still larger "spall zone". The diameters of these zones are defined as Dp (Pit), D h (Halo), and D s (Spall zone) (H5rz et al. 1971). Pitless crater. Depressions t h a t are shallower than glass lined pits and t h a t do not contain any glass. Instead the crater bottom and walls are made up of microfractured material identical to the "halo" zone. These features were first described by McKay, 1970, and HSrz et al., 1971, and attributed to secondary, low velocity impact. Hartung and HSrz, 1972, however, established evidence t h a t pitless craters m a y indeed be due to primary hyper=
IM_ICROCRATERPOPULATIONSON APOLLO 17 ROCKS
461
TABLE I S U ~ Y O F P E R T I N E N T DATAACCORDINGTOLSPET(1973) Rock type a 70215 72215 72235 72235 72395 72435 73216 73218 73275 74275 76135 76136 79155
Mass Pyrex- PlagioIlmenite Graingr ene clase Olivine & others Clasts Matrix size mm
X, B 8110 40-45 ~ 30 Br, e 379 Br,e ) , 62 Br, M~t----~ 62 Br, M 536 Br, M 161 Br, M 162 Br, M 40 Br, M 430 X, B 1493 Br, M 133 45 ~ 40 X, B 87 X, G 319 40-45 ~ 35-40
~5
~ 25 40 30 ~ 10 3 5 15 ~ 15
~5 ~3
~ 10 ~ 15
60 70 90 97 95 85 ~ 85 95
1-2 <0.1 >0.1 >>0.1 <0.1 <0.1 <0.1 >0.1-2 <0.1 ~ 0.2 0.5-2.5 0.3-0.8 1-3
Surface history ~ Complex Simple Simple Simple Simple Simple Complex Simple Complex Simple Simple Simple Complex
a According to LSPET (1973): Br, e----Breccia, light grey, friable; Br, M----Metaclastic rocks; X, B = Crystalline, basalt ; and X, G = Crystalline, gabbro. Simple: At least one side is void of microcraters, i.e., rock has not repeatedly tumbled. Complex : All sides of the rock are cratered, indicating at least a two stage surface exposure history (in part according to R. L. Sutton, pers. comm., 1973). velocity i m p a c t with the glass-lined pit r e m o v e d either during the crater forming e v e n t or subsequently. Completely isolated " p i t s " are occasionally f o u n d in regolith fines, t h u s s u p p o r t i n g the h y p o t h e s i s t h a t t h e y are dislodged in tote from their t a r g e t surface. F u r t h e r m o r e m a n y pitless craters also possess a spall-zone similar to pit craters, t h a t f u r t h e r s u b s t a n t i a t e s their transitional n a t u r e from pit craters. T h o u g h some pitless craters are demons t r a b l y modified h y p e r v e l o c i t y pit craters, one c a n n o t exclude a low velocity origin for some others. A t present a q u a n t i t a t i v e assessment of " p r i m a r y " versus "seconda r y " origin is n o t possible. However, all pitless craters e n c o u n t e r e d in this s t u d y are interpreted to be of p r i m a r y origin. Stylus crater. Glass lined cup, which rests on an elevated pedestal significantly a b o v e the i m m e d i a t e t a r g e t surface inside t h e spallation zone. (HSrz et al., 1971.) The r e q u i r e m e n t s for stylus f o r m a t i o n are u n k n o w n at present; however, t h e y are n o t the result of erosion processes. Vedder (1971), a n d N e u k u m et al. (1972), d e m o n s t r a t e d their f o r m a t i o n due to
p r i m a r y i m p a c t in the l a b o r a t o r y a t velocities > 3 k m / s e c . Concentric cracks. A fracture s y s t e m t h a t concentrically surrounds the central pit. I f present, a few fracture systems, a t most, can be observed a n d the m e a s u r e d radius, De, refers to the largest d i a m e t e r system. I t is m o s t l y confined to the halo zone. The spallation shoulder is n o t considered to be p a r t of the concentric crack system(s). I t is b e l i e v e d - - a s i n d i c a t e d - - t h a t pit craters, pitless craters, a n d stylus pits are transitionally related, t h o u g h the e x a c t conditions of f o r m a t i o n are presently u n k n o w n . The most c o m m o n t y p e s are genuine pit craters which are solidly e m b e d d e d in the s u r r o u n d i n g t a r g e t m a t e r ial; as the height of the stylus increases, the a t t a c h m e n t to the t a r g e t becomes increasingly weaker a n d the n u m b e r o f concentric cracks increases. F i n a l l y t h e b o n d i n g of the glass lined pit becomes so weak t h a t it is dislodged from the target. I n some instances it is also observed t h a t the r e m o v a l of the pit does n o t necessarily involve stylus formation, b u t as indicated
Q
Ds/ D~ ~
Illllllllll Pc/f)pa
illiiSii[[i
% Stylus pits*
I
~o Pitless craters*
% Concentric cracks*
lill[~lLIII
X
tic
c~
T
A
Tdarkdast T breccia totalrock a
72235 72235 73216
1.26 1.26 --
5.81 6.64 4.15 5.61 5.73 6.64 8.22 3.32 4.77 0.83 3.82 1.58 7.39 3.82 1.99 8.96 4.73 3.15 1.66 --
-. --
4.4 4.4 -4.1 --. --. . . . . 4.8 . . . 4.0 4.6
.
. .
. . . .
.
.
.
--
--
---
.
.
. .
2.1 2.1 2.2 ---. --. . . .
.
. .
.
. -1.4
.
1.6 1.8
1.6
1.4
. .
. . . .
.
-------
.
.
3.3
-5.8
25
.
.
6.8 4.8
17 10 11 3.7 3.0 4.6
.
. .
16 45 --
14 9
12 50
31 23 23 20 18 44 17 26 17 53 30 35 24
127 ---
51.5
48 56 34 42 168 87 193 68 145 27 42 33 179 94 14 188 11 88 15 --
206 23 29 17 16 106 70 89 46 83 14 15 21 83 51 5 111 6 64 8 132
309
91 51 --
103 27 34
206
58 3 40 4
12 15 11 7 53 43 30 28 44 7 7 11 44 28
412
20 24 94
257.5
34 1 29 3
7 7 6 6 29 28 15 21 24 4 3 4 24 13
515
14 17
309
17 2
20
2 3 3 2 19 19 8 14 14 3 1 2 11 8
618
4 1
2 4 2
10 16
360.6
9 1
n
9
412
6 1
6
1 4 9 3 8 4 1
11
1
2 12 14 6 11 5 2
824
2
721
5 8
463.3
4 1
3
3 1
2 6 2 7 2
1
927
Stylus pits, % Pitless craters, % Concentric cracks.
50 45
154.5
Magnification 50 × a
76 97 61 82 209 100 351 99 210 43 104 62 331 156 24 332 20 121 26 --
103
Ds/DP, Dc/D~, %
1.3 -53
-17
---
~ 4.4 -----
--3.8 3.7 ---
Surface area scanned. Da/Dp = 8 p a l l d i a m e t e r / P i t d i a m e t e r . Dh/DP = H a l o d i a m e t e r / P i t d i a m e t e r . d Dc/Dp = C o n c e n t r i c c r a c k d i a m e t e r / P i t d i a m e t e r . ' Percentage taken from total number of craters (including pitless craters). f Only pit craters. g P i t d i a m e t e r s (~). h All craters larger than a minimum diameter have been measured to derive
W T B B spallarea B T breccia T dark ciast T B B S N T N B T B T 8 glasscoat totalrock a
7 02 15 7 02 15 70215 7 02 15 72215 72235 72235 T2395 ?2435 7 32 16 73218 73218 73218 74275 74275 74275 7 61 35 79155 79155 79155
Magnification 25 × #
3 8
515
3
2
2 1
1 5 1 5 1
1
1030
1 5
566.6
2
2
1
2 1
4
1133
1 4
618
1
1
1
1 1
4
1236
3
669.5
1
1 1
3
1339
3
721
1
1 1
3
1442
1
1
2
2
--
772.5>772.5
1
1
3
1543 > 1 5 4 5
d~
O ~-~
0
O
O
© h# ~
~J
O C)
464
SCH~IEIDER
AI~D KORZ
b v H a r t u n g and HSrz (1972), it m a y result from a direct u n d e r c u t t i n g of the pit b y concentric cracks w i t h o u t stylus development. Because of this transitional n a t u r e the distinction of stylus and pit craters is somewhat a r b i t r a r y ; only those craters t h a t had a well developed stylus were recorded as stylus pits.
used, however, are considered to constitute a representative sample. The crater counts listed in Table I] are illustrated in Figs. 2-6 in the traditional log-diameter versus log-frequency format. I n some cases, pitless craters are p l o t t e d individually. Stylus craters are included in the pit crater counts because t h e y do possess a glassy pit. The areal density coefficient (A) as defined b y N e u k u m et al. (1973) is also included. A chip of sample 64455 was investigated via SEM techniques according to Schneider et al. (1973). The chip was curved, because it was cut from an egg-shaped p a r e n t rock (Fig. 6). Accordingly, three surface areas were distinguished. The " t o p surface" (II) measured 1.12 cm 2 and the two "side surfaces" (I and I I I ) were 0.16cm 2 and 0.32cm 2, respectively. The crater counts illustrated in Fig. 6 are based on a 100 × photomosaic t a k e n of the entire chip; the crater p o p u l a t i o n of area I I I is n e a r l y identical to t h a t of I and not illustrated. Also illustrated are binocular
Results
The crater populations investigated are s u m m a r i z e d in Table II. As in our previous studies, a v a r i e t y of rock faces of different exposure angle were investigated for some individual rock specimens. The sides investigated are designated T, B, E, W, S, and N in accordance with the orthogonal " m u g s h o t " p h o t o g r a p h y t a k e n at the L u n a r g e c e i v i n g L a b o r a t o r y . Also listed in Table I I are m e a s u r e d ratios of D s / D p, D h / D p and D c / D p t o g e t h e r with relative frequencies of pitless a n d stylus craters as well as craters displaying concentric crack systems. These special investigations were not p e r f o r m e d on all rocks. The rocks
,oo, . °s,op:,.5, j ,0[
I
...o Y: °I
Ds/Dp = 4.5
: 72435, B
1F 7221s '/''x: OgcRATER' :~
.
1
J CRATERS o °'1/C'ATER,So , /x'Ox:''
x20x
: 196
CRATE?
~
T
i
10
100
---•
10
i
Ds/Dp = 4.5 A = .20
o
•
~e o
7
79155, T
e l O x : 93 CRATEI~S
1000
10 OO0 10
100
100
I
.36
5
Ds/Dp = 4.6 A = .34
,
10
~
~
O~o~ 79155, N
x25x : 121 CRATERS
• 2 5 x : 9 9 CRA
100
o~,~=
III ,
10
Ds/Dp = 4.6 I ! o
'elOx
10OO
10 OOO 10
: 1 7 0 CRATERS I
IOO
10()O
10 OO0
DIAMETER ( F' m)
FIO. 2. Cumulative size frequencies of pit diameters (Dp) (solid circles) and corresponding spall diameters (Ds) (open circles). The best fit of a --2 slope through the spall-diameter frequency curve yields a numerical measure for-crater density as described by Neukum et al. (1973). Figures 3-6 axe constructed in identical fashion.
I~ICROCRATER POPULATIONS ON APOLLO 17 ROCKS I00 ~ - -
- ~
.
465
~.
A = 07
I
ROCK 70215 Ds/Dp = 46
• A~= O8~
.25x
~
CRATERS
)
I
25
x
° ~
: 97
'
CRATERS
l .1 k _ _ _ ±
T
~
~ ,~ ,~
~
'
25 x - 82 CRATERS IN LARGE SPAL[ AREA
~\ . ~
. . . .
"A : ' ~
i
E
A = .10
A = 23
ROCK ' 73218 D s / O p = 4.5
I
r 2 5 x : 107 CRATERS
.I~
u
~,
r
\ \
: 25 x : 62 CRATERS
~
~
~
lOO
A = .23
\
I
.
.
25 x : 343 CRATERS
~ •
.
.
•
.
.\
;
.
.
l
.
A = 25
; = o~
ROCK 74275 r Ds/Dp = 4 8
6
10
eo0e~
~ n
10
.\
l
CRAiERS
Z~[~
oN
I 100
I[~ IOOO
n 2 5 x : 167 CRATERS
o 10 000 10
s
J q 2 5 x : 24 | CRATERS
j \ ~ I
;
I
100 1000 DIAMETER (~ m)
10 OOO 10
1OO
1000
10 OO0
Fro. 3. Cumulative frequencies of pit- and spall-diameters for specific faces of individual rocks. l~ASA-S-73-3069. crater count (1973).
data
of N e u k u m
et al.
b e e n r e p e a t e d l y d e m o n s t r a t e d t h a t on genuine p r o d u c t i o n surfaces t h e c u m u l a t i v e slope, a, is changing w i t h values o f a p p r o x i m a t e l y --3 a t p i t d i a m e t e r s > 3 0 0 F m a n d values of ~ - 2 at pit d i a m e t e r s 50-300Fro; a t still smaller sizes, i.e. 1 0 - 5 0 ~ m , t h e slope is e v e n
INTERPRETATION C u m u l a t i v e c r a t e r frequencies as a f u n c t i o n of c r a t e r d i a m e t e r m a y be expressed in t h e f o r m N ~ = K . D -~. I t has 100
i
i
?
Ds/Dp
= 4.5
Dh/Dp
= 2.0
>-
~
A(:] = .O63 A l l = .022
-N lO
O
0
O
I
D~/O. : 45t
eo
.Oo
O
h/Dp
=
= 4.s I = 2.0
Ao=.
~
o
o
0
o=,~
I
Dis/Dp Dh/Dp
D
I
AO= .4 A I = ,OI
0 I i
~
1
f 20 PIT T CRATERS L T o 2 5 x : 41 PIT A N D ~ •
u
25
:
PtTLESS,CRATERS],
10
100
1000
"~ \
o
~32~6
B
\
\ I~ \
\
I
10 0 0 0 10
CRATERS
CRATERS 1 o 16x : 323 PIT AND PITLESS CRATERS
o lOx : 108 PIT AND
I
100 I000 DIAMETER (F.m)
L¥ |
~ ~ ~
P,nESS(RATERS~,--
10 0 0 0 10
I00
1000
10 0 0 0
l~zo, 4. Cumulative frequencies of pit craters and pit and pitless craters for rocks 76135, 73275, and 73216. The corresponding spall frequencies are indicated by squares, l~ote that the size-frequency distribution of pifless craters is similar to pit-craters. NASA-S-73-3068. 17"
466
SCHNEID~IR ]
100
72235,T A.
= .18
o = .15
"RECC,A-MATR,X 10
D s / D p = 3.4 ~ • 5 0 x : 51 CRATERS 1.L ÷ 2 S x : IO0 CRATERS ~ o SPAtL DIAMETER
Z O
"
-
ao
DARK CLAST D s / D p = 3.9 • 5 0 x : 127 CRATERS 425x: 351 CRATERS o SPALL DIAMETER .I
I 1000 1o0 DIAMETER (/4m)
10
10 0 0 0
FIG. 5. Crater counts on rock 72235 (see also Fig. 7). flatter ( z - l ) (HSrz et al., 1973). However also changes because of equilibration effects t h a t result in the preferential destruction of smaller craters (Neukum et al., 1973 and Hartung et al., 1973). In addition observational effects m a y cause a change in ~ because of optical resolution limits for the smallest craters and insufficient statistics for large structures.
100
, I |~+
c~
'r ] [ I
__
..
~ O.o o e
AREA t
HORZ
All Apollo 17 rocks investigated display a change in slope (Figs. 2-6). The interpretation thus focuses on which of the above processes was responsible. Rocks 70215 (W, T and B) and 74275 (S) are interpreted to be genuine production surfaces because crater-overlap is negligible upon visual inspection and the absolute areal density coefficient is small (A < 0.08) (Fig. 3). All other surfaces display various degrees of crater destruction by superposition and therefore are considered to be in transition or equilibrium condition; their areal density coefficients range from 0.10 to 0.36 and thus extend into the "limiting frequency range" of ~orrison et al. (1972). A variety of independent equilibrium criteria were established by Neukum et al. (1973) on Apollo 16 rocks. The Apollo 17 rocks were subjected to a similar evaluation and the results are indicated in Table III. The only surfaces qualifying unambiguously for equilibrium conditions are 79155, T and N (Fig. 2). With the exception of these and the above genuine production distributions, the interpretation of all other surfaces remains ambiguous. Despite E~ AREA ~W } SEM-SCAN AREA IOOX AREA OPTICAL SCAN, 2OX
,
"° k - Ds/Dp=3.7 I \ o A=0.15 / 1o - PiT 11~I • ~ A--O.12-1 DIAMETER: IT t. o ~ | (SEM, 100X) 1 . + T o\ / " 36 CRATERS, I 1 P +\ I
lI~
~"
?
• 77 CRATERS, I
OPTICAL, 2 0 X / • 173 CRATERS, | ENTIRE ROCK I o
SPALL DIAMETER: ] , "Ds/Dp:4.9 J
AND
1,
+ \
~
"
2 cm
I
+
-J
'~ \ / \l
.1
10
100 1,000 CRATER DIAMETER (/.m)
10,O(~
Fig. 6. Crater populations on rock 64455. Notice the abundance of overlapping craters that essentially destroyed large parts of the original glass surface, indicating that the "optical" counts and "SE]¢[ a.rea II" counts are close to equilibrium conditions for glass surface 64455. NASA-S-73-3065.
467
MICROCI~ATER P O P U L A T I O N S O:N AtJOLLO 1 7 R O C K S TABLE
III
EQUILIBRIUM CI~ITEI~IA FOR VARIOIYS APOLLO 17 ROCKS
Equilibrium criteriaa Rock
(1)
(2)
x x
× x
72235, M b
x
72395
x
72435 73216 73218
x
x
x x
73275
x
74275
76136 79155
72215 72235, C b
(3)
(4)
(5)
(6)
Equilibrium
x
x x
? ?
x
x
x
×
×
x
x
x
x
x
x
x
x
x
x
x
x
x
? ?
x
?
x
x x
? Likely
x
x
?
x
x
x
x
?
? Yes
a According to Neukum et al. (1973). b C --~clast, M--~ breccia matrix. (1) A measured A analytical > 1. (2) Simple exposure history. (3) Surfaces of various exposure angles were measured.
(4) Same A on various surfaces. (5) Extended D -2 plateau. (6) Strongly abraded, i.e., rounded surface.
e x t e n d e d D -2 plateaus t h a t are suggestive for equilibrium on a v a r i e t y of rocks, their corresponding crater density m a y v a r y as m u c h as a f a c t o r of 6. I n contrast, sample 72215, B lacks an e x t e n d e d D -2 region, t h o u g h it possesses the highest crater density (A = 0.36; Fig. 2). H a r t u n g et al. (1973) have emphasized the potential role of physical properties of various rocks for the f o r m a t i o n of microcraters a n d in p a r t i c u l a r for their ease of retaining pit craters which constitute the observable preserved crater p o p u l a t i o n in contrast to the actual craters formed. A striking example illustrating the influence of mechanical t a r g e t properties is rock 72235. As illustrated in Fig. 7, its cratered surface consists of two materials: one is the regular light-grey breccia m a t r i x a n d the o t h e r is a dense, aphanitic, coh e r e n t clast. The entire rock is a chip dislodged f r o m a big boulder; its exposure g e o m e t r y is well k n o w n a n d it therefore m u s t be p o s t u l a t e d t h a t the two materials constituting 72235 had identical surface histories and exposure angles. N e v e r t h e -
less their crater populations differ signific a n t l y b e y o n d statistical error (Fig. 5). N o t e t h a t the "breccia m a t r i x " surface has a f a c t o r of 4 more pit craters for craters with Dp > 300/~m in d i a m e t e r ; at crater sizes Dp > 800/~m, the difference is even larger. These differences are ascribed to physical t a r g e t properties. Figure 8a illustrates some unusually soft soil breccias from Apollo 17. Their well r o u n d e d shapes indicate considerable erosion and it is p r o b a b l y safe to assume t h a t mass-wasting has progressed sufficie n t l y t h a t no "original" rock surface is preserved which would qualify t h e m b y definition (see Fig. 1) to be in crater equilibrium. A n d yet, upon visual inspection t h e y display v e r y few craters, if at all. Identical examples have been observed from other Apollo missions. Thus these materials present additional evidence t h a t physical t a r g e t properties strongly control the preserved crater population. A n o t h e r example is illustrated in Fig. 8b, t h a t contrasts a coherent glass-coating with a crushed, crystalline surface. Again different
468
SCHNEIDER
A7L~D H O R Z
! Cm
I Fio. 7. Sample 72235 consisting of a "hard" clast (left) and "friable" matrix (right). Notice the abundance of large craters (1-3 ram) in the matrix surface and compare them with the structures visible on the clast, l~IASA-S-73-3064. crater densities are observed even with the unaided eye. T h o u g h the crystalline surface was more f a v o r a b l y exposed (H5rz et al., 1972) it displays fewer craters t h a n the glass coating; the thinning of the glass coating a r o u n d the contacts with the exposed crystalline surface allows the reasonable a s s u m p t i o n t h a t the glass
coating once covered the entire specimen a n d t h a t meteorite i m p a c t completely r e m o v e d the glass in the area n o w occupied b y the underlying crystalline material. While the above examples t r u l y represent extremes t h a t serve to illustrate the point, our new, detailed m e a s u r e m e n t s of pitless craters, stylus pits a n d concentric
Fx(~. 8. Sample 79035. Soft soil-clod from Apollo 17. l~otice the absence of microcraters despite the advanced state of rounding and erosion of the rock. Sample 60135. Difference in crater density on a glass surface (dark area) and crystalline surface (light area). According to the reconstructed lunar surface orientation, the crystalline part faced upwards and thus had a more favorable exposure geometry, and yet, the craters are more numerous on the glass coating. NASA-S-73-3066.
M I C R O C R A T E R POPULATIO:~'S O1~ APOLLO
cracks suggest that there are also differences on a more subtle scale (see Table II): (a) Pitless craters. Their abundance varies between 9 and 50% of the total crater population. According to Fig. 9a, they appear to be more abundant on metaclastic breecias than on genuine crystalline rocks and within these two groups they are most abundant on surfaces of low crater density. Thus a mechanism must operate that differs in efficiency on various target-materials and that furthermore changes with increasing crater density. A quantitative explanation is not readily available and the following suggestions are offered, with the assumption that the samples measured are representative for all rocks: (1) Because dislodging of pit craters is due to tensile failure of the pit/target interface, pit removal must be accomplished b y reflection(s) of the shockwave. Such reflection(s) m a y occur at the free target surface and/or within the target along grain-boundaries, cleavage planes, rugs, and macro- and microfractures; these reflections are controlled b y the immediate pit environment. An attempted correlation with physical bulk properties of the rocks failed because these properties do not necessarily reflect the condition of the very surface that is affected b y the mierometeoroid bombardment. As a consequence, a detailed explanation is not possible. We are left with the empirical observation that even similar, genuine basaltic rocks and tough, coherent, highly metamorphosed breccias, that are essentially holocrystalline respond in different ways to the micrometeorite bombardment. This response furthermore changes with increasing crater density, i.e., essentially with time. The decrease in pitless crater frequency on the more densely eratered surfaces m a y be due to an increase in microfractures that cause highly polydirectional reflections which in turn influence significantly the attenuation of the initial shock wave and its associated reflections. (2) The inverse relation of "pitless" craters and absolute crater density (Fig.
17 ROCKS
469
9a) m a y also be taken as a sign that pitless craters are caused b y secondary projectiles. Their frequency on freshly exposed surfaces m a y be particularly high, because of the increased probability to encounter secondary projectiles during the rock's ballistic trajectory to the eventual site of collection. After landing, the surfaces would be essentially exposed to the bombardment of "primaries" and thus the population of secondaries would decrease with time. Because of the demonstrated transitional nature of some pit into pit-less craters and in particular because all pit-less craters have identical relative size-frequencies to pit craters (Figs. 2-6), we prefer to ascribe them to primary cratering events and therefore favor explanation 1. Regardless, however, which explanation is correct, the basic observation remains: different rock materials respond differently and display different numbers of pit craters. (b) Stylus-pits. The frequency of stylus pits also varies significantly from essentially 0-17% of the total crater population (Table 2). Figure 9b illustrates that there is no systematic correlation between pitless and stylus craters for different rock types. (c) Ds/Dp ratio. Figure 9c iUustrates DJDp ratios from all Apollo 12, 14, 15, 16 and 17 rocks measured to date. Though there is considerable data scatter, two general trends are apparent. Firstly basaltic rocks have larger Ds/D p ratios than do breecias, which we again take as indication for different target response. Secondly the ratios appear to be independent of absolute crater density and thus seem not to change with increased microfraeturing of the rock's surface in contrast to the pit-less craters of Fig. 9a. (d) Dc/D p and Dh/D r ratios. The newly measured D J D p and Dc/D p values are listed in Table II. Their absolute values differ from rock to rock although a quantitative correlation with other parameters cannot be established. All that can be concluded is that different Ds/Dp, Dh/D r and D d D p ratios also are indicative of differing material response to micrometeoroid cratering.
470
SCHNI~,IDEI~ AND HOI~Z
60
SO
-~
• 73216, B
•
CRYSTALLINE SAMPLES
•
BRECCIAS
" ~ ' ~ , ~• 73275 T 76135, B ~
40 • 73218, N • 70215, W
3O
&&"'~.~ ~" 2O
70215, T j
(a)
~ • •
~7021~,
72395, T ~ .
~',~.~.,~
73218, T "" ~•
10
"e~..~.
~72215
74275, T \~-72435, B
I
I
I
I
I
I
A
.1
I
I
I
I
I
I
.2 AREAL DENSITY
I
i
I
I
I
.3
I
.4
30 25
~2o u
~i5
(b)
10
• 0
0
10
•
•
t
I
15
20
•
e
I
I
i
I
f
40
45
50
60
i
25 30 35 % PITLESS CRATERS
•
A&
•
~
•
4
•
2
I .10
•
|
I I .20 .30 AREAL DENSITY
I .40
I .50
FIG. 9a. Percentage of pit craters versus absolute crater density coefficient (A). Note that the surfaces of low crater-density tend to have relatively more pit less craters. FIG. 9b. Correlation of pitless and stylus craters for different rocks and individual rock faces (see Table II). FIG. 9c. Correlation of absolute crater density coefficient (A) and spall to pit diameter ratio (Ds/Dp) of all rocks (ApoUos 12-17) measured to date. NASA-S-73-3067.
I~IICROCRATERPOPULATIONSON APOLLO 17 ROCKS
(e) Absolute crater density. Figure 10 illustrates the areal crater density coefficient as defined b y Neukum et al. (1973), for all rocks studied to date. Pe r each specimen, only t h a t surface was included which displayed the highest crater density observed. There is again considerable scatter in the data although once more basaltic rocks seem to differ from breccias. Breecias generally possess higher crater densities t han basaltic rocks. (f) Glass-surfaces. Included in Fig. 10 are glass surfaces 64455 and 60135 which are the most densely cratered glass coatings observed to date. Detailed SEI~ cratercounts for the top-surface of 64455 (Fig. 6) revealed t h a t despite its relatively low absolute crater density, its crater population is unambiguously in transition and close to equilibrium conditions as illustrated in Fig. 6. Note the considerable overlap of craters and the almost complete lack of original target surface in Fig. 6. The areal density measured on the surface is A = 0.12. I t is impossible to envision t h a t even with sufficient time the crater density could reach an A of 0.40 or even 0.62 like breccia 14306. Thus we conclude t h a t pits formed in breccias are retained with significantly greater ease t han on glass surfaces and according to Fig. 10,
471
crystalline rocks are somewhat intermediate between glasses and breccias. I)ISCUSSIOI~T Some specific investigations on Apollo 17 rocks together with observations on Apollo 12, 14, 15 and 16 rocks present unambiguous evidence t h a t physical properties of the rocks strongly govern their microcrater populations. These findings are in complete accordance with experimental impact cratering (Gault, 1973). The evidence, however, is only observational and empirical for lunar rocks and is not amenable at present to a more quantitative analysis and understanding, ttowever, it has serious implications for a variety of studies related to micrometeoroids. The flux of micrometeoroids derived from lunar rocks strongly depends on the conversion of a crater diameter (either Dp or Ds) into the corresponding projectile mass. A vari et y of microcratering experiments are available for this calibration (~andeville and Vedder, 1971; Bloch et al., 1971 ; Neukum et al., 1972 ; Schneider et al., 1973; Nagel, 1973; Gault, 1973; Vedder and ~andeville, 1973). None of these experiments were conducted on genuine lunar materials. The target simu-
~
LIMITING FREQUENCY (M O RRIsRANNeGEal 1972)
62235 72395 12038 73275, 72235--- 1 12073 684153.-i"~ i 61156--'1 I II -
-
-
7321671 I II
'4-Elll III
• 12051 12047, 62006, 60315 12017, 6445S-'11 I Ill
ill
12o21-7 Itllll I l l 6035~ ~~ ~
b
5%
I
10%
-
-
-
• BRECCIA SURFACES _73218 • CRYSTALLINE SURFACES 69935 • GLASS SURFACES --74275 r--61015 60016 76136 62295 12063
/ Irr6117s ,243s / IIIF / IIII
. 25%
-6oo7s, 791s5, 7221s
Ir',42~1
~14306
.
.~,
.
:~ .~ ~, .:' ,~ 1:o AREAL .
SQ%
DENSITY SATURATION PERCENT (GAULT, 1970)
Fie. 10. Comparison of highest areal density coefficient observed to date on rocks from all missions. Crystalline only refers to genuine basaltic rocks ; metaelastic specimens are included in breeeias . The "limiting frequency range" is an empirical boundary above which no crater populations were observed to date.
472
SCHNEIDER AND HORZ
lants used were predominantly artificial glasses and a few crystalline materials. Thus caution has to be exercised in correlating the same diameter pit crater on a diversity of lunar materials (glass, crystalline rocks and breccias) with the same projectile mass. Because glasses are the best simulated materials, lunar glass coatings are probably the most reliable targets to use in deriving the micrometeoroid flux. Furthermore, the different material response to microcratering may strongly control the preserved crater populations (Figs. 5-10). 1VIorrison et al. (1973) and Neukum (1973) have suggested that the surface exposure time of individual rock specimens may be obtained from such populations. While Morrison et al. essentially equate the total number of craters for rocks that satisfy certain criteria (Morrison et al., 1973) with total exposure time, Neukum equates the extent of the D -~region with the time parameter. Hartung et al. (1973) have already raised serious questions concerning both approaches, because transition and especially equilibrium populations cannot be used for exposure age dating. Both approaches rely on the unambiguous recognition of production populations. Clearly both approaches can not quantitatively account, at present, for the different response of various rock materials. We maintain that the absolute crater density that can be preserved on a rock is both a function of target material and total exposure age. Because the influence of target properties at present escapes quantitative treatment, exposure ages derived from absolute crater-densities and/or extended D -2 regions are subject to serious criticism. ACKNOWLEDGMENTS This work has been made possible with the first author being a visiting scientist at the Lunar Science Institute, which is operated by the Universities Space Research Association under Contract No. N S R 09-051-001 with the National Aeronautics and Space Administration. We are indebted to R. L. Sutton for information about lunar surface orientation of some rocks and to D. A. Morrison for fruitful discus-
sions. This paper constitutes the Lunar Science Institute Contribution No. 178.
I~EFERENCES BLOCH, M. R., FECHTIG, H., GENTNER, W., NElmu~t, G., Am) SCHNEIDER, E. (1971). Meteorite impact craters, crater simulations and the meteoroid flux in the early solar system. I n "Proceedings of the Second Lunar Science Conference," pp. 2639-2652. MI T Press. GAImT, D. E. (1970). Saturation and equilibrium conditions for impact cratering on the lunar surface: Criteria and implications. R a d i o Science 5, 273-291. GATYLT, D. E. (1973). Displaced mass, depth, diameter, and effects of oblique trajectories for impact craters formed in dense crystalline rocks. T h e M o o n 6, 32-44. HA~G, J. B., ~rD HSl~Z, F. (1972). Microcraters on lunar rocks. I n "Proceedings of the 24th International Geological Congress," Montreal, Canada, Section 15, pp. 48-56. HARTUNG, J. B., HSRZ, F., AITKEN, K. F., GAULT, D. E., AND BROWI~LEE, D. E. (1973). The development of microcrater populations on lunar rocks. I n "Proceedings of the F o u r t h Lunar Science Conference," Vol. 3, pp. 32133234. Pergamon Press, Elmsford, N.Y. HSRZ, F., ~IAI~TIYNG, J. B., ~ D GAULT, D. E. (1971). Micrometeorite craters on lunar rock surfaces. J. Geophys. Res. 76, 5770-5798. HSRZ, F., CARRIER, W. D., YOUNG, J. W., DUKE, C. M., NAGLE, J. A., AND FI~¥XELL, R. (1972). Apollo 16 special samples. NASA SP-315, 7-24-7-54. H6RZ, F., BROWNLEE, D. E., FEOHTIG, H., HARTUNG, J. B., MORRISO~, D. A., NEUKUM, G., SCHt~-EIDER, E., A~CD VEDDER, J. F. (1973). Lunar microeraters--implications for the micrometeoroid complex. Planet. Space Sci. in press. MAIqDEVILLE, J.-C., AND VEDDER, J. F. (1971). Microcraters formed in glass by low density projectiles. Earth Planet. Sci. Lett. 11, 297306. McKAY, D. S. (1970). Microcraters in lunar samples. I n "Proceedings of the 28th Annual Meeting of the Electron Microscope Society of America" (C. J. Arceneaux, ed.), pp. 22-23. Claitor's Publishing Division, Baton Rouge. MORRISON, D. A., McKAY, D. S., HEIKEN, G. H., AND MOORE, H. J. (1972). Microcraters on lunar rocks. I n "Proceedings of the Third Lunar Science Conference," Vol. 3, pp. 27672791. MIT Press.
MICROCRATER POPULATIONS ON APOLLO 17 ROCKS MORRISON, D. A., McKAY, D. S., FRULAND, 1~. M., AND MOORE, H. J. (1973). Microcraters on Apollo 15 and 16 rocks. I n "Proceedings of the F o u r t h Lunar Science Conference," Vol, 3, pp. 3235-3253. Pergamon Press, Elmsford, N.Y. NAGEL, K. (1973). Experimente zur kratersimulation. Master's Thesis, University of Heidelberg, Germany, unpublished. NEUXUM, G. (1973). Micrometeoroid flux, microcrater population development and erosion rates on lunar rocks, and exposure ages of Apollo 16 rocks derived from crater statistics. I n " L u n a r Sci. I V " (J. W. ChamberlainandC. Watkins, Eds.), pp. 558-560. Lunar Science Institute, Houston. NEUKUlVI, G., SCHNEIDER, E., MEHL, A., STORZER, D., W A G N E R , G. A., FECHTIG, H., AND BLOCH, M. R. (1972). Lunar craters and exposure ages derived from crater statistics and solar flare tracks. In "Proceedings of the Third Lunar Science Conference," Vol. 3, pp. 2793-2810. MIT Press. NEU~:O-I~, G., H6RZ, F., HARTUNG, J. B., AN2) MORRISON, D. A. (1973). Crater populations
473
on lunar rocks. I n "Proceedings of the Fourth Lunar Science Conference," Vol. 3, pp. 3255 3276. Pergamon Press, Elmsford, N.Y. SCHNEIDER, E., STORZER, D., HA~T~nVG, J. B., FECHTIG, H., A_WD GENTNER, W. (1973). t~crocratcrs on Apollo 15 and 16 samples and corresponding cosmic dust fluxes. I n "Proceedings of the Fourth Lunar Science Conference," Vol. 3, pp. 3277-3290. Pergamon Press, Elmsford, N.Y. SCHNEIDER, E., AND HSRZ, F. (1974). Lunar rock erosion. I n " L u n a r Science V," p. 666. Lunar Science Institute, Houston. SHOEMAKER, G. M., BATSON, R. ~¢[., HOLT, H. E., MOI~RIS, E. C., RENm-LSON, J. J., AND WHITAKER, E. A. (1969). Observations of the lunar regolith and the earth from the television camera on Surveyor 7. J . Geophys. Res. 74, 6081-6119. VEDDER, J. F. (1971). Microcraters in glass and minerals. E a r t h Planet. Svi. Lett. 11, 291296. VEDDER, J. F., AND MANDEVILLE, J.-C. (1973). Microeraters formed in glass by projectiles of various densities (in preparation).