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Central Peaks in Lunar Craters : Morphology and Morphometry

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CENTRAL PEAKS IN LUNAR CRATERS : MORPHOLOGY AND MORPHOMETRY. Wendy Hale and James W. Head, Dept. of Geological Sciences, Erown University, Provi- dence, R I 02912. Central peaks are an important structural and morphological feature of fresb craters on the Moon. Previous work has established their occurrence as a function of fresh crater size (100% in craters with diameters >35 km), 1 their relationship to substrate type (more frequent occurrence in mare craters than in highland craters below%35 ki lometers),' thei r abundance as a function of crater type,' that peak height is directly proportional to crater diameter (h = .006 D~,~.'~),~"and that a transition from central peak craters to cen- tral peak basins to peak ring basins occurs at d iameters between 150-250 km.4'5 A1 though virtua l l y a l l workers agree that central peaks are related to ~hcsurrounding impact crater in origin, there is uncertainty about the rode of formation, with some associating peak formation with the gravity-induced fa; ure of the transient cavity during the modification stage of the cratering event, 6 a7d others proposing that central uplift begins during the excavation stag', initiated by high stresses associated with shock and rarefaction waves ccnc~ntratedin the sub-impact p~int.~'~Relati~nships between central peak hsicjht and crater diameter,3 and the diameters of transition to central peak zna peak-ring basins on different planets4 have been cited as evidence favor- ing the latter view. The purpose of the present study is to examine central peak regions in fresh lunar craters in order to I ) define a relationship be- tlween diameter of central peaks, and crater rim crest and floor diameter, 2) characterize their morphology, 3) provide a stronger basis for comparison of central peaks to peak rings in lunar basins, and 4) assess the effects of degradation. In order to minimize effects from erosion and degradation, only the mor- pholog;cally freshest examples of craters with central peaks were chosen for study (n = 175; Copernican to lmbrian in age). Measurements were made from [Lunar Orbiter and Apol lo images and topographic Taps (LTO series). Craters in the diameter range between central peak onset and 17 km were not included because of the morphologic comp levity of the crater interior.'" The sample includes 90 nearside craters, which comprise 98% of all fresh nearside craters with central peaks as l isted in the LPL catalogue, as we1 l as 85 craters from polar and farside regions. In addition, 29 craters of pre- lmbrian age with prominent central peaks were measured to test for evidence of degradation in periods of more intense erosion. For each crater, meas- urements were made of rim crest diameter (Drc), f loor diameter (Df 1, crater age, central peak morphology, and central peak diameter (Dcp). Since central peaks commonly occur in i rregu lar clusters, the latter val ue is not as easi ly defined as Drc or Df . We def ine Dcp here as the d iameter of a best-f i t c i rc l e enclosing all of the central peak mass. I) Relationship between Drc and Dcp - A plot of Drc versus central peak region diameter (Dcp) for the fresh crater population (Figure I) shows a linear relationship defined by Dcp = 0.259 Drc - 2.57 (r = 0.88) for the popu- lation diameter range of 17-180 km. A previous study of central peak areas for central peak craters of all states of degradation in the central nearside region showed only a poor correlation between increasing peak area and crater diameter.g Apparently all craters with central peaks were included in this studyg regardless of degradation state, a factcr that may have added to the scatter of the data. 2) Relationship between Df and Dcp - Measurements of crater floor diam- eter for the same population of craters yields a linear relationship between Df and D (Dcp = 0.398 Df - 0.51; r = 87) (Figure 2). cP 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System CENTRAL PEAKS IN LUNAR CRATERS Hale, W. and Head, J.W. 3) Morphological characteristics of central peaks - Central peaks were classified according to their morphological complexity and geometry. Simple peaks are either single peaks or single coherent ridges while complex peaks are clusters of peaks, of either symmetrica I, I inear, or arcuate geometry. The distribution of these types as a function of diameter is shown in Figure 3. The same population was subdivided according to peak geometry into I inear peaks (elongate sing l e ridges or c l usters), arcuate peaks (arcuate single ridges or clusters), and symmetrica I peaks (consisting of a single central ly located peak or centrally oriented clusters) (Figure 3). There is no clear trend in increasing peak complexity with increasing crater size. There is a sl ight tendency for peaks to be less symmetrical with increasing crater size. 4) Substrate influence on peak complexity and morphology - Table I shows the distribution of subdivisions of the complexity and geometry classifica- tions as a function of substrate type. Central peak complexity appears in- sensitive to substrate type. However, development of l inear central peaks appears to be favored in hiqhland- terrain. 5) Pre-lmbrian central peak craters - Twenty-nine central peak craters of pre-lmbrian aqe (20-186 km in diameter) were analyzed. Central peak diam- eter for these ckters increases at a lower rate, rejative to rim crest diam- eter, then for fresh craters (Drc = 3.99 Dcp + 24.6; r = .76). They also increase at a lower rate relative to the floor (Df = 2.14 Dcp t 21.651, but at the same time the floor itself is increasing at a faster rate relative to the rim crest (Drc = 1.29 Df f 14.37). If these pre-lmbrian craters formed with the same initial geometry as later craters, these trends are easily explained by small additions of material to the crater floor (probably from degradation mechanisms8), causing widening of the floor and partial covering of the central peaks. Discussion and Conclusions - For fresh lunar craters, a I inear relation- ship has been shown for the morphometric parameters central peak diameter (Dcp) and rim crest (Drc) and floor diameter (Df). Linear relationships have previously been demonstrated for Drc and peak height3 and peak ring 4 diameter, and cited as evidence for these elements being related to the kinetic energy of impact. 3'4 These relationships differ for pre- lmbrian craters with central peaks in a manner consistent with the addition of small amounts of floor material probably resulting from the more intense degrada- tion of this period. There is no evidence for size or substrate control on peak complexity. Peak geometry is apparently not influenced by crater size, but linear peaks are more prominent in the highlands than the maria, perhaps due to highland crustal fractures. lo Several l ines of evidence suggest that the transition from craters to basins (peaks to peak rings) is more com- plicated than a sim le expansion of central peak complexity and size, as pre- vious ly proposed. "" First, there is no we1 I developed trend in simple to complex peak morphology as crater diameter increases. Secondly, the slope of the Drc/Dcp, l ine differs from that of the Drc/peak ring diameter establ ished previously. Analysis of the characteristics of craters and basins with transitions l morphologies is presently underway. References: 'wood C.A. and L. Andersson (1 978) PLSC9, 1267; inta tala M., C.A. Wood and J .W. Head ( 1977) PLSC8, 3409; 3~oodc.A.973) Icarus, 503; 4~ead J.W. (1978) Lunar Science IX, 485; 'wood, C.A. and J.W. Head (1976) PLSC7, 3629; 6~enceM. --et a1 . (19m) Shock ---Met. Nat. Mat., p. 368 and Gault D. --et al. (1975) J. Geophys. Res., 2444; 'Milton D. and D.J. Roddy (1972) IGC, Sess. 24, p. I19;-'~ead J .W. (1975) --Moon 12, 299; ien C. (1975) --Moon 12,463;'UStr-om R.G. (1964) -Corn. --Lunar Plan. -Lab., 205; ll~odgesC.(1978) PLSC9, 521. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System urajsLs lejaa s~!silydo~jsvVSVN ayj Lq pap!aoq . ajnj!jsuI L~sjauleldpun: mun? 0 CENTRAL PEAK REGION DIAMETER (Dcp) (km) 0 10 20 30 40 50 a CENTRAL PEAK REGION DIAMETER (Dcp) (km) 0 10203040~ - - ,.-. --K! - - PPP m u' E 4 --- - - P m-- -4 L1 Lx rN'J 0 0 IF m . -3s. *¶ xi in yi - 3-4 3% 'M'T pea^ pue .M 'aj~~ S831VH3 HVNfll Nt SlV3d lVHlN33 .
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