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Table 1 Om' *) O Lunar and Planetary Institute L Provided by The TARGET-DEPENDENCE OF CRATER DEPTH ON THE MOON. Richard J. Pike, U.S. Geological Survey, Menlo Park, CA 94025. Head (1976) has developed and amplified Wegener's (1921) concept that layering of the target can affect the morphology of large impact craters. Head's model attributes the differences between simple (small) and complex (large) craters to presence of a thick and "soft" megaregolith over a "hard" substrate. The idea has met with some resistance (e.g., Floran and Dence, 1976) because the megaregolith must be absent over much of the maria, and the expected depth/diameter (d/~)differences between craters on the maria and those on the uplands were not observed (Quaide et al. , 1965; Pike, 1974). Although Marcus (1967) and Pike (1977) had found complex upland craters to be slightly deeper than complex mare craters, the data were meager, and both authors dismissed the differences as insignificant . Table 1 The most recent measurements, improved over those available for the latter four studies, have demonstra- ted that fresh complex craters on the lunar uplands clu. D d are indeed deeper than those on mare surfaces (Wood om' *) and Andersson, 1978; Croft, 1978; Pike, 1980a; DeHon, bp.4~- 93.0 3.80 1980). The new d/D data indicate that the morphologic briarorah. m 87.3 3.w rat^^. M 58.3 3.43 transition, whatever its origin, is markedly influenced Arbtill- M 55.3 3.30 p~inir~ 42.1 3.07 by the nature of the target, but do not necessarily m 40.0 2-75 confirm the specific mechanism proposed by Head (1976). Warin Pn 33.50 3.00 Mert m 29.9 2.60 The data (revised from Pike, 1980a) are summarized in Ilmc1.u~ R1 27.40 2.6 Uldler 27.13 2.m Fig. 1 and plotted individually in Fig. 2. Simple bler 26.63 2.24 craters less than km across are not shown. Table Poss 26.3 2.1 5 1 &.go RI 26.0 2.51 lists all 61 complex craters; data on the simple cra- Dclisle m 25.38 2.42 ~(u~i~~m 23.0 2.46 ters (Pike, 1980a) are available from the author upon Pytheas Peirce :::i6 :::: request. Fitted least-squares equations for complex 0aes m 17.40 2.31 craters,which are given on Fig. 2, differ slightly J-en B FM 15.9 2.19 Beesel m 15.5 1.77 from those in Pike (1980a) because some very old Ldande FW 23.25 2.65 ~~i.~.~~~40.2 3.15 (Nectarian) upland craters inadvertently included in Ib*O~hilU6 mC 102.0 4.1 the earlier work have been purged from the revised Lrrsrmus R(c 136.0 4.5 Airken c 137.10 4.90 data set. ~lodovska C 128.65 4.28 &plysin c 127.90 5.10 Simple craters on the uplands (Class C) and mare L.naemkTYC~O c 85.0 beso4.6 surfaces (Class PM) are alike in that they follow the Perowe C 81.0 4.18 same d/D trend (Pike, 1980a), but differ in that simple bins c 78.2 4.15 &=obius c 64.5 4.05 craters attain larger sizes on the uplands (D=21 km) Ritz C 59.0 3.75 ~bulwafa c 55.9 3.7 than they do on the maria (D=15 km). Complex craters SchorrSchubert 53.9 3.35 differ in both respects. Those on the uplands average C 53.5 3.7 klaurin c 52.5 3.38 12% (@D=100 km) to 18% (@D=15 km) deeper than those Bnmner C 51.63 3.63 S~-N c 50.5 3.64 on mare surfaces. Moreover, complex mare craters are hpella C 47.0 3.5 Dclporte 43.38 3.19 as small as 15 km across, whereas the smallest upland Cmskiy C 43.38 3.37 crater is 21 km across. Thus the simple-to-complex w. Bcrmchel C 40.5 3.65 Taylor c 36.25 3.1 transition occurs at a crater diameter of 15 km on the Schubert B C 35.8 2.9 Tisserand c 35.0 2.93 maria but at 21 km on the uplands. GilkrtU C 34.5 3.03 Iatcholsky C 34.12 3.06 The d/D distinctions between mare and upland cra- kch0 c 33.75 3.3 ters are less conspicuous for craters in mixed targets. Ctf~ibius C 33.25 3.06 Izsak c 33.2 3.4 Simple craters that formed at a mare-upland contact Eorrocka c 31.0 2.92 P~EC~~I c 30.38 3.1 or penetrated only a thin mare veneer (Head, 1976; Cellin1 c 29.63 2.99 Croft, 1978) plot primarily with "pure" upland craters Ibuteequieu C 28.38 3.06 &Intin c 27.75 3.05 (Figs. 1, 2). There is essentially no difference in Bingh-R C 26.88 2.71 23.5 Z65 d/D for these simple Class PMC craters. However, the C 22.3 2.7 Woismm C 21.3 2.9 four complex Class PMC craters (Lalande, Aristarchus, conon c 20.75 2-93 Theophilus, and Langrenus) are slightly shallower than O Lunar and Planetary Institute l Provided by the NASA Astrophysics Data System Target-Dependence of Crater Depth Pike, R. J. the mean upland-crater curve (although still within the d/D envelope for up- land craters), probably in response to the mare materials. Within the diameter 1 I1111 interval 15 km to 21 km usually all mare craters are complex and almost all upland craters are simple, - but at least two craters, Diophantus (mare) and Dionysius (upland) have - ambiguous morphologies. Neither is a fully fledged 4 r, 3 -Upland (Class C) complex crater (Pike, 1977) 6 - Ham (Class PM) or has a mixed target -wand with &re ven- (PMC). Three larger "tran- - sitional" craters on the 1 I I ,,At ' uplands, Proclus, Auzout, 10 20 5 40 70 loo 150 and Kant, also lack clear- CRATER DIAHETEFL, lan cut simple or complex forms Figlm 1 (Pike, 1977). These five craters do not lie fully within either simple or complex d/D 'fields (Figs. 1, 2). The correspondence of immature complex-crater morphology to ambiguous d/D values suggests a close relation between the visual appearance and the topographic geometry of fresh impact craters on the Moon. The five transi- tional craters are statistically rare occurrences that may exemplify incom- pleteness in the processes by which complex craters form. The marelupland differences in crater form shown here are ascribed to preqence or absence of significant layering within the target, rather than to a thick unconsolidated layer preferentially blanketing the uplands (Head, 1976; DeHon, 1980). This alternative interpretation follows from terrain-related dif- I I I III ferences in crater sizes for the simple-to-complex transition in terrestrial b meteorite craters (Pike, 1980a; Pike, 1981). On - both Earth and the Moon the smaller onset diameter - occurs in demonstrably layered rocks: sedimentary - strata on Earth and basalt lava flows on the Moon. The larger onset diameter w 1 --. is observed on both plan- ets in what are known or inferred to be less sharply stratified mate- (Fquations for simple craters rials: igneous and meta- 4 are given in Pike (1980a) ) morphic rocks on Earth and upland breccias on (Amow marks - Simple Fow -1 the Moon. These intra- - transition) X - Complex Form - planetary contrasts in - . - 1 I 1 I I1Ill the transition diameter 0.7 j I I 10I 20 40 70 lo0 150 thus do not seem to re- CRATER DIAMETER,^ flect simply hard versus Figure 2 O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Target-Dependence of Crater Depth Pike, R. J. soft targets. Were that the case, then the transition on the Moon should occur in smaller craters on the softer highland breccias than in the harder mare basalts. This is not what is observed in Figures 1 and 2. I propose here a modification of the hypothesis offered by Head (1976): Layering scatters the impact's rarefaction wave along contacts between strata such that a significant portion of the wave is propagated laterally rather than downward as it is in a homogeneous medium. Thus complex (shallow) cra- ters form at smaller sizes in layered targets. Because similarly-shaped simple craters are observed on both layered and nonlayered targets, the layering cannot itself trigger the formation of complex craters. What, then, is the trigger mechanism? The scattering mechanism proposed here may occur only above a critical level of impact energy at which physical processes -- perhaps certain aspects of the flow regime -- change and complex craters will form, below which only simple craters form (Pike, 1980b). Alternatively, small dense meteoroids may form simple craters and large diffuse meteoroids form complex craters (Quaide et al., 1965; Milton and Roddy, 1972). The problem is complicated and remains unsolved. References Head J. W., 1976, Proc. Lunar Planet. Sci. Conf. 7th, 2913-2929. Wegener A., 1921, Natur. u. Tech. -55, 48 pp. Floran R. J., and Dence M. R., 1976, Proc. Lunar Planet. Sci. Conf. 7th, 2845-2865. Quaide W. L., et al., 1965, Annals N.Y. Acad. Sci., -123, Art. 2, 563-572. Pike R. J., 1974, Geophys. Res. Letts., &, 291-294. Marcus A. H., 1967, Icarus, -5, 56-74. Pike R. J., 1977, Proc. Lunar Sci. Conf. 8th, 3427-3436. Wood C. A., and Andersson L., 1978, Proc. Lunar Planet. Sci. Conf. 9th, 3669-3689. Croft S. K., 1978, Proc. Lunar Planet. Sci. Conf. 9th, 3711-3733.
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