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Lunar Crater Morphometry: New Data

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Lunar Crater Morphometry: New Data LUNAR CRATER MORPHOMETRY: NEW DATA. C. A. Wood, Dept. of Geological Sciences, Brown Univ., Providence, RI 02912 and L. Andersson, Lunar and Plan- etary Lab. , Uni v. of Arizona, Tucson, AZ 85721. Investigators have long recognized that systematic morphologic changes occur with increasing diameters in lunar craters; simple bowl-like pits give way to broad craters characterized by central peaks and terraced walls. ~ence' called the smaller landforms "simple craters" and the larger ones with peaks and terraces "complex craters." Previous studies of the transition from sim- ple to complex complex craters were based on restricted samples of the lunar crater population. We now document changes in crater morphology for all fresh nearside craters >7 km, and for ~1500smaller ones. The large data base per- mits a detailed description of the onset diameters and ranges of significant occurrences for flat floors, central peaks, and wall slump features. Data source: Our data come from a preliminary version of the new "LPL Cata- log of Lunar which includes morphometric information for ~11500 craters of all degradation classes on the lunar nearside. To isolate pristine morphologies from effects due to degradation only the 3001 freshest (LPL class 1) craters are used. Unlike previous crater listings each crater in the new catalog is classified on the basis of overall morphology. A1 though 18 differ- ent morphological types are now recognized, only about a third are numerically significant; the others represent uncomnon but distinctive morphological vari- ations (e-g. , concentric craters). The principle types and their prototype craters are: ALC - (A1 bategnius C) bowl-shaped craters with smooth rims, di- ameters up to ~20km; BIO - (Biot) similar to ALC but with a flat floor having a clear break in slope at the contact with the crater wall; TRI - (Triesnecker) craters typically between 15-40 km diameter with wall slumps or scallops; TYC - (Tycho) multiple tiers of terraces, large flat floors, diameters from ~25- 175 km; SOS - (Sosigenes) terrace-free rims varying from 5-40 km diameter, shallow craters with flat floors. ALC and BIO correspond to the Class I11 craters of Pohn and Offield4, TRI are approximately their Class 11, and TYC are Class 8. Pohn and Offield did not recognize SOS (or the other less common LPL types) as fresh crater morphologies. The remaining LPL morphology types represent only 1%of class 1 craters, and many may be volcanic or volcanically modified craters. Depth-diameter relations: In Fig. 1 the depths (d) and diameters (D) of each of the main morphological types are compared. ALC-BIO have nearly identical d-D relations, TRI-TYC follow a second trend, and SOS defines a third5. Least square fits for each trend are given in Table I. The transition between simple (ALC-BIO) and complex (TRI-TYC) craters occurs over a diameter range (15-20 km) in which both types are found. This agrees with Pike's6 value of 17.5 km, but not with the 6.2 km estimate by Smith and sanchez7. D-d rela- tions for SOS craters do not inflect at the transition zone, suggesting that these craters are not part of the main sequence of impact craters. The parel- lelism of the d-D plots for SOS and ALC-BIO suggests that SOS are ALC-BIO types shallowed by lava flooding. Two observations argue against that inter- pretation: SOS craters tend to be more polygonal and occur at larger diameters than ALC-BIO craters. There is, however, no compelling evidence that SOS are entirely of volcanic origin. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LUNAR CRATER MORPHOMETRY: NEW DATA Wood, C. A. Flat floors: Nearly all class 1 craters, except ALC, have flat floors (Fig. 2). Table 1 includes slope-intercept values for exponential least squares fits to floor diameter vs. D for each morphological type. Central peaks: The occurrence of central peaks in class 1 craters varies ac- cording to crater diameter8 and morphological type (Fig. 3). The substrate in which craters form also effects peak occurrenceg, but the differences due to types are significant enough to ignore substrate effects in this preliminary analysis. Fig. 3 sumnarizes the new results, and makes apparent a major dif- ference between 2 similar crater types, ALC and BIO. Whereas no peaks have been detected in the 1670 class 1 ALC craters, peaks are comnonly found in BIO craters, increasing from 4% (D=O-5 km) to 35% (D=15-20 km). Wall failure: Onset of wall slumps (TRI) occurs at D=15 km, by D=20-25 km 50% of class 1 craters have slumped rims, and beyond D=50 km wall failure occurs excl usi vely by terracing (TYC) . Discussion: The relationship between ALC and BIO is unclear. As well as the absence of flat floors and central peaks discussed above, interior wall slopes of ALC craters are systematically 50-60 less steep than for BIO. The major process of lunar crater degradation - mass wasting - tends to decrease slopes1O, suggesting that bowl -shaped craters (ALC) are erosional modi fica- tions of flat-floored craters (BIO). A small amount of mass wasting of rim materials buries tiny peaks, reduces wall slopes, and smooths out the wall- floor inflection without significantly affecting d-D relations. The transition from simple to complex craters is commonly said to re- flect the onset of flat fl oors, central peaks, and wall sl umps/terracesl '7'11. The new data presented here illustrate (a) that flat floors are characteristic of a1 1 unmodified craters (i .e., a1 1 types but ALC) from a few kilometers di- ameter to hundreds of kilometers; (b) that central peaks become progressively more abundant with diameter (4% at D=0-5 km to 100% for D235 km); and (c) that wall failure appears at D=15 km. Thus, there is no onset diameter for flat floors, and central peaks exist in craters as small as D=2.6 km. The abrupt transition from simple to complex craters in due only to the onset of wall failure and associated shallowing. The independent development of central peaks and wall terraces is strong evidence against any theory in which peak formation is related to wall failure (e.g., {I ,121). Additionally, the occur- rence of stratigraphic central uplifts in terrestrial craters is insufficient evidence to consider craters to be "complex," and calls into question argu- ments for gravity scaling based on comparison of uplifts in terrestrial craters and onset of terraces in lunar craters. References: lDence, M. R. (1968) Shock Metamorphosis -of Natural Materials, 169. 2Wood, C. A. and Andersson, L. (IF(1978)~P~ Catalog ofTunar Cratmrep.). 3Wood, C. A. (1971) --Moon 3, 408. sPohn, H. A. and Offield, T. W. (1970) USGS Prof. Pa . 700-C, C153. ~WOO~,C. A. (1973) Bull. Am. Astron. Soc. 5, 36- we,+ J.7574) GRL 1, 291. 7Smith, E. Ixdxnc-. c(1g75) Mod. Geol . 5, 175. 8~ood,C.K. (1973) Icarus 20, 503. 'Cintala, M. J., Wood, C. A. and Hezd, J. W. (1977) Proc. LunarLunar? Sci. CEf. 8th, 3409. locannon, P. J. (1970)- Sky and Telescope 40, 215"Howard,215"Howard,xA(1974)~roc. K. ~(1974)~roc. Lunar Sci . -~onf.Conf. 5th, 61. Tuaide, W. L., Gault, D. E., and Schmidt, Rx(1965JWAcad.sci.m -- - -123, 563. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LUNAR CRATER MORPHOMETRY: NEW DATA Wood, C. A. Fig. 1 : Depths vs. Diameters - 10 - Fig. 3:Central Peaks - Table 1: Least Squares Relations Between Fresh Lunar Crater Depths, Diameters, and Floor Diameters. Morph Dia (km) d=aD b F=~D' ; Type Range a b *SE N r s kSE N ALC 1-20 .20 .92 .21 902 - - - - BIO 1-20 .18 1.00 .28 681 .131.25 .71 769 SOS 5-40 .I4 -91 .56 133 .32 1.15 1.82 173 lRI 15-40 1.51 .18 .60 75 .30 1.14 3.24 92 TY C 25-1 75 .56 .45 .48 13 .20 1.25 3.28 41 d=crater depth, D=crater diameter, F=floor diameter, SE=std. error of estimate, all in km; preliminary data. ALC BIO SOS TR I TYC 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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