IM P A C T C R A T E R ｝N G A G eological Process on the Planets by

R ichard A .F . G rieve and Jam es W 。H ead, III

Exploration of space in recent years has highlighted the im portance of im pact cratering as a geological process operating on terrestrial planetary bodies. T his review outlines som e aspects of the process and sum m arizes the key observational data available on im pact cratering. It illustrates how the preserved cratering histories of the planets can provide im portant inform ation on the nature and evolution of planetary surfaces. In tro d uc tio n of several m egabars. W ith the recent vigorous exploration of space by the U nited R arefaction w aves, generated at free surfaces, follow the States and the Soviet U nion, the am ount of inform ation com pressional w ave. These release w aves travel faster currently available on the planets is considerably greater through the now com pressed target rocks than the initial than som e fi ft een years ago. Param ount in this data base are sh o ck w a ve an d ov e rta ke it. T h e rarefa ction w a v e fro nts a re high quality im ages of planetary surfaces. A planet's surface not parallel to the com pressional w ave, except in the volum e reflects the effects of internal and external geological pro- directly below the im pacting body, and the resultant stress cesses, and thus these im ages provide a fram ew ork for vectors deflect the original radial particle m otions upw ard understanding the geological evolution of the planets. and outw ard leading to the excavation and ejection of target A lthough terrestrial or silicate planets m ay have had sim ilar m aterial. Thus a cavity know n as the ．t ransient cavity is early histories, they vary considerably in their present state. form ed, partly by excavation and partly by displacem ents A t least part of the variation is related to planetary size, a induced by those particle vectors w hich are insufficiently de fle c te d to resu lt in e xc a va tio n . m ajor factor affecting therm al evolution, w hich is prim arily controlled by the ratio of heat radiated from the surface to heat generated w ithin the planetary volum e (H ead and Solo- W hen rocks are subjected to shock pressures above their m on, 1981a). H ugoniot elastic lim it, P 40-50 kb for crystalline rocks, they deform plastically and there is an increase in their internal In spite of the varied nature of the different planets, there is energy. The locus of pressure-volum es states defines the a geological process w hich is ubiquitous to them all 一the H ugoniot equation of state of the rock and inflections in the exogenic process of im pact cratering. Its effects are m ost H ugoniot correspond to phase changes associated w ith com - obvious and im portant on planets w hich have preserved a high pression. T he pressure release path is controlled by -the percentage of their prim itive initial crusts. For exam ple, adiabat so that som e internal energy is trapped irreversibly, num erous photogeological, geophysical, geochernical and m anifesting itself as w aste heat. petrographic observations attest to the fact that the M oon underw ent a period of intense im pact cratering in the first T his behaviour of rocks subjected to shock leads to the few hundred m illion years of its history. This early bom bard- production of characteristic shock m etam orphic effects m ent w as responsible for features ranging from the second- w hich have been recognized in m any lunar sam ples and order topography of the M oon to the fact that som e 90% of m eteorites and are the principal criteria for identifying the sam ples returned from lunar highlands are im pact pro- terrestrial im pact structures. Their duplication in experi- d u c ts . m ents enables specific pressures to be assigned to particular features. In tectosilicates, for exam ple, planar deform ation G iven the im portance of im pact in the early evolution of features w ith various orientations form at ＞100-150 kb, solid planetary crusts, considerable effort has been devoted in state glasses resulting from lattice disordering form at 300- recent vears to studviniz the craterinQ Drocess (R oddv et al二 400 kb, incipient m elting at 500-600 kb, and finally, vapori- IW ，）· in is p ap e r su m m arize s ou r cu rre nt u na e rstan a in g o t zation at pressures of P 1000 kb and greater. the im pact process and outlines som e of its im plications for the geological evolution of the planets. T ransient cavity form ation is an extrem ely rapid process, generally only a Jew m inutes long even fo r a c ra ter as la rge T he C ratering Process as 20 km . M aterial nearest the point of im pact iE ejected A m ajor problem in deciphering the results of im pact cra- first. S in c e this m ate ria l w a s subjected to the highest tering lies in the fact that it is a highly destructive event, lasts for a very short tim e and results in m etam orphism of rocks and other m aterial w ithin the im pact area. M uch of the current know ledge on im pact processes com es from results of experim ents using high-explosive and nuclear ex- plosions, theoretical com putational studies and geological and 维 geophysical studies at terrestrial im pact craters. A lthough 粼孤书鬓鑫馨 the earth is the m ost endogenically active of the terrestrial planets and thus has relatively few preserved im pact craters (,r 100 with diam eters greater than a km are known; Grieve and R obertson, 1979), it is the source of virtually all w ell- constrained ground truth data. B efore considering the effects of cratering, it is useful to briefl y review the physics of ‘叨pac七IFor typical terrestrial im pact velocities of 15-25 K M 5 ‘ the im pacting body penetrates to 2-3 tim es its radius, and transfers the bulk of its kinetic energy to the target rocks as a com bination of kinetic and internal energy. T he im pact generates an ex- ponentially decaying com pressional shock w ave w hich pro- pagates radially into the target body, w ith initial particle v eloc itie s m e asu re d in km s-1． 丁he highest p re ss u re s g e n - FiW e 1. A erial photo少aph 可 the 3 ．2 k m d ia m e t e r N e w erated by the im pact of iron and stony bodies a re in t h e o r d e r Q uebec sim ple crater, Q uebec, C anada. E PISO D ES, Vol. 198 1, N o. 2. 3 pressures, it is in the form of im pact m elt/vapour. It also has Earth, the transition from sim ple to com plex form occurs at the highest particle velocities and travels farthest from the ,r 4 krn in crystalline rocks and r 2 km in sedim entary rocks. im pact point. A s cavity grow th continues, successive H ow com plex structures achieve their present shallow form is volum es of progressively less "shocked" m aterial are exca- a m ajor problem yet to be resolved in cratering m echanics. vated and land closer to the cavity rim . O ne suggestion is that they are produced by shallow excav- N ot a ll th e m ate ria l se t in m otion is e xc av a te d . P a rtic le ation due to low density, com etary bodies. This, how ever, m otions beneath the projectile are such that som e of the requires a distinct size distribution betw een high density im pact m elt and vapour and som e com m inuted crystalline bodies (sim ple crater producers) and low density ones (com - debris rem ain w ithin the cavity. They form a lining to the plex structure producers), and a system atic variation in this cavity and a pool or sheet of im pact m elt rock and breccia in size distribution betw een planets as the sim ple-com plex tran- the final cavity. These m elt rocks are distinguished from sition diam eter is not constant betw een planets. It also is at norm al igneous rocks by variance w ith the data on siderophile abundances in terres- the presence of shocke d inclusions, trial im pact m elts, w hich ind icate that the various crater petrographic evidence for super-hea t , form s can be produced by a variety of projectile types (W olf rem arkably hom ogeneous and often u nusual com position et al., 1980). corresponding to a m ixture of the target rocks (G rieve 丝 aL , 1977), and M ost w orkers consider that com plex form s are the result of in som e cases, excess siderophile elem ents w ith relative m odification processes during and shortly follow ing im pact. abundances m atching those of know n m eteorite types (W olf Subsurface structural data and the pattern of shock m eta- et al., 1980). m orphism in autochthonous target rocks at terrestrial struc- tures indicate that m odification involves m assive uplift of T h e F in a l C ra te r F o r m the transient cavity floor. M odification m ay be achieved by The classes of crater form s noted in relatively inactive a com bination of post-shock rebound of the transient cavity planetary bodies such as the M oon can be recognized an floor and g ravitational collapse of the unstable rim area. Earth, although the m ore active nature of the Earth's crust T his process can be described by a phenom enological m odel has com m only resulted in severe m odification of im pact w hich, as indicated by observational data, has am ong its craters. D espite this, terrestrial craters reveal im portant param eters rock strength and gravity (M elosh, 1981). su b su rfa ce stru ctu ra l in fo rm a tio n . The problem of the nature of m odification processes in large R elatively sm all im pact craters are designated as sim ple com plex structures is not trivial, as reconstruction of the craters and have a bow l-shaped form (Fig. 1), an uplifted rim original transient cavity form establishes lim its for the depth capped by an overturned flap of ejecta, and depth/diam eter of sam pling by ejecta. P resent lim its are not precise, but it dim ensions of r 115 (P ike, 1977). B eneath the floor of sim ple is suggested that sam ples from perhaps as deep as 30-60 km craters is a bow l-shaped breccia lens. D rilling results indi- in the lunar crust (G rieve, 1980a; Head 丝旦．，1975) m ay be ca te tha t this is a llo ch tho n o us m a te rial an d tha t the excavated from im pact structures the size of the Im brium basin (diam eter ，1300 km ). Sam ples from 20-30 km m ay, in depth/diam eter ratio of the crater defi ned by the autochthonous target rocks is closer to 1/3-1/4. T his breccia fact, be represented in the "returned" highlands rocks (H erz- lens, w hich m ay contain a basal pool of im pact m elt, is berg and B aker, 1980). generally considered to result from the inw ard collapse and C ratering on the P lanets slum ping of the w alls and rim of the transient cavity. M o o n A feature com m on to all the terrestrial planets is that T he M oon is the principal data source for assessing the im pact craters above a certain critical diam eter change in form from sim ple craters to com plex structures. The latter effects of sustained cratering on crustal evolution. The lunar have a variety of form s characterized by the presence of surface is characterized by im pact craters of all sizes, central peaks and/or interior rings (W ood and H ead, 1976) and ranging up to ＞ 1300 km -sized m ulti-ring basins. L ow are considerably shallow er than sim ple craters (F ig. 2). O n degradation rates, high resolution im ages and the large 录 num ber of craters com bine to provide an excellent data base f 抓一尹 浏 一 for determ ining the m orphology of fresh crate rs. 、 》· 、飞筋 巍 沪 龚改 It is apparent that the transition from sim ple, bow l-shaped 、狱 craters to shallow er, com plex form s occurs over a diam eter 潇蒙 臀 粼舞 range of several km . This, and m inor variations in the .:一I } IV - diam eters at w hich specific m orphological features occur, is 尹甲石扩“ 蒙锹 熟 瑟

疾归 the result of variations in im pact conditions, particularly variations in the physical nature of the target. Subsequent 哪 摹 m odification of the bow l-shaped form initially appears as a flattening of the crater floor due to m ass w asting of the cra te r w a lls.

?i C o m m o n ch a ra c te ristic s o f lu n ar c ra ters 15-2 0 k rn in d ia - m eter are the developm ent of cuspate rim s and scallop deposits at the base of the walls (Fig. 3a). These indicate failure and inw ards slum ping of the cavity w alls. W ith inc reasing diam eter, w ell-developed rim terraces and a dis- tinct central peak appear (Fig. 3b), thus characterizing the 呢甲澎 群 布 so-called central peak craters. R im restoration m odels can describe the change in m orphology by w all failure of an initial cavity w ith a form equivalent to bow l-shaped craters (Settle and H ead, 1979). At diam eters larger than ，70 km , 门 必 the observed geom etry cannot be derived from a presum ed 即 粕 original bow l-shaped cavity by terracing alone. U plift and rebound of the cavity floor, as indicated by subsurface data from terrestrial craters, m ay becom e increasingly im portant F igure 2. LA N D SA T im age 可 the M anicouagan com plex at these size ranges. im pact structure, Q uebec, C anada. T he annular lake is 65 km A t diam eters g reater than 140 km , lunar structures develop a in diam eter and approxim ates the present diam eter of the s tr u c tu r e . fragm entary interior ring surrounding the central peak (F ig . E PISO D E S, Vol. 1981, N o. 2. 4 黔

蒸

a b

C d

F igure 3. (a) The lunar crater D aw es, diam eter 18.4 km , show ing cuspate rim outline and scallop deposits due to slum ping at the base of th e w alls; (b) The lunar crater Tycho, diam eter 84 k7n, w ith w ell-deveZoped rim terraces and a prom inent central peak, (c) C om pton, a 175 kin diam eter lunar im pact structure w ith a central peak and 厂ragm entary inner ring, (d) SchrU dinger, a lunar peak ring basin , diam eter 320 krn, show ing a w ell- developed inner ring and no central peak.

3c). A bove 175 km , this ring becom es the dom inant m orpho- logical elem ent w ithin the im pact structure (W ood and H ead, 1976), a central peak is no longer evident and the structure is classed as a-peak ring basin (Fig. 3d). A dditional interior rings develop at diam eters of 350 km or m ore, and im pact structures take the form of a m ulti-ring basin. The later filling of m any of these large m ulti-ring basins by basaltic volcanics defines the dichotom y betw een the cratered high- la n d s a n d m a r ia . It is generally accepted that the lunar highlands (the initial anorthositic gabbro crust form ed 4.6妙-4.4 G a ago) preserve a re co rd o f inten se b o m b a rd m e n t in those inter-planetary bodies w hich w ere not incorporated t h e m ajor accretionary processes of planetary form atior (F ig 4 ). C rater size- frequency distributions, com bined w ith absolute ages deter- m ined by radiom etric dating of lu n a r sam ples, indicate an extrem ely high early flux (num ber of im pacting bodies) w hich decayed exponentially w ith tim e. F or exam ple, the preserved ,r 4.2 G a) 1s crater density of the average highlands (age 沪 F igure 4. O verview of the heavily cratered lunar highlands. som e 30 tim es greater than that of the average m aria (age The structure w ith m are-fill (dark m aterial) and a prom inent 3.4 G a; H artm ann et al., 1981). W he n th e te rre strial d a ta fo r central peak in the center of the im age is Tsiolkovsky, 190 cratering during P hanerozoic tim e is included, the resultant k-m in d ia m e ter. EPISO D ES, V ol. 1981, N o. 2. 5 ︵F cratering flux-tim e curve foig. 5) provides a m e tho d fo r T A B L E 1. E F F E C T S O F IM P A C T C R A T E R IN G O N L U N A R establishing a tim e fram e r t h e o c c u r r e n c e of processes H IG H L 八 N D S C R U S T s u c h a s v o lc a n is m an d te cton ism o n planets w here sam ples G eophysical are u na va ilab le for absolute dating. (TYe technique in v o lv e s counting craters of a given size per unit a r e a a n d co m p aring T opography 一C reation of to内graphic d iffe re n ce s up to 8 this crater density w ith the derived E arth-M oon flux ra te , km ; resultant surface d ic h o t o m y b e t w e e n adjusted for differences in the m odal iM Dact veloc highlands and m are. ary gravity and the like betw een planets, to derive an age for G ravity 一H ighs up to 200 m gal associated w ith the planetary surface under investigation.) m ascons w ithin m ulti-ring basins; local low s 10 0 associated w ith ejecta and brecciation. lest Crul M agnetics Local high-intensity anom alies m ay be 山 associated w ith high tem perature im pact 止 －Av rhig V M O O N 芝 deposits. 旧 UJ C ra te r re ten tio n rate S e ism ic g Low seism ic velocities in upper 20-25 km of 哎 止 粉A16 crust due to fracturing and m ega-regolith 山 乡 developm ent. 咤 入A14 0 1 社、 G eological 山 A17 A I l M il 比 D istrib u tio n V A 书 一G eologic units related to m ajor ejecta J 山 、1 L 24－ 泛； deposits; m any recovered sam ples tran- 比 ～ 卜 sported from original location. 1 的1 侧物恢 P hysical nature 一90% of sam ples are im pact products: 之 山 brecclas or m elt rocks; m any show the 0 J 、 ＿ IE UR 比 e ffe c ts of sh o ck . 山 卜 咤 C h e m ic a l n a t u r e 一B ulk chem istry of sam ples often a m ix- 匕 0 ture of pre-existing rock types, NA co m m only enriched in m e teoritic sidero- phile elem ents and w ith reset ages. 3 2 ｝ 0 A G E O F S U RFA C E (b .y.) F igure s． V a r ia t io n in preserved crater density w ith Sur厂ace The large m ulti-ring basins provide the fundam ental tectonic age on the M o o n . E U R and N A are crater densities of Europe and stratigraphic fram ew ork of the lunar surface and are the a n d N orth A m erica cratons respectively, adjusted to lunar cause of m ajor and long-lasting effects on crustal evolution. fm pact conditions. The m ajor fraction of m ass and energy delivered to -the lunar surface by im pact bom bardm ent is expressed in these basins. T he use of c ra ter de n sities a s a crude dating technique A b rie f d iscu ss io n of th e sc ale of su ch e ve nts is w a rra n te d . assum es that the preserved record on the M oon represents an F or exam ple, the kinetic energy of im pact associated w ith accurate m easure of the num ber of craters produced 一that it th e form a tio n o f th e O rie nta le b asin 二6b)）is estim ated to is a "production population". Such an interpretation is (Fig be approxim ately 1031 一1032 ergs. T h is m eans that 103 一104 favoured by detailed analyses of size-frequency distributions t im e s the present annual output of be tw ee n t e rr a in s w it h in t h e internal energy of the highlands. A m odel cratering E a r t h w as deposited at a specific lo ca tio n on th e lun a r histo ry c a n be established for the highlands and, w ith the surface in a tim e span m easured in m inutes. c o n s t ra in t s available from terrestrial and experim ental craters, it is possible to estim ate such param eters as the a ve rag e depth of sam pling of the lunar crust, the average n u m b e r of im pacts recorded in the "returned" sam ples, the total volum e of im pact m elt produced in the lunar highlands, and the relationship betw een reset A r ages and the cratering h isto ry . There is a clustering of A r ages at 4.1-3.9 G a. It is w ell e sta blished fro m te rrestria l c ra te rs tha t a sh o ck e ve n t re sults ls in radiogenic A r loss from the target. 丁h e A r c lo c k a com pletely reset on im pact m elting (P 孚 6 0 0 kb) and sizeable fraction of the radiogenic A r m ay be lost at pres- sures as low as 300 kb. T his clustering of ages ha s thu s be e n interpreted in term s of the bo m bardm ent of the highlands. It has been called the "term inal cataclysm " (Tera et al., 1974) and linked to the form ation of I m b r iu m a n d s e v e ra l other large m ulti-ring basins at 3.9 1 0.1 G a. A lthough m uch quoted and suggested as a tim e m arker for other planets, the term inal cataclysm is only a w orking hypothesis. It is not clear either from the geochronological data or the cratering record w hether the clustering of highland ages reDresents a aisc re te p erioa w ith fo rm a tio n of a n um be r of la rge IM D ac t Dasins w ithin r U-Z Cia, or a longer period of basin form ation term inating at r 3.9 G a. The im pact of planetism al-sized bodies during the term inal F igure 6. T he O rientale m ulti-ring basin on the M oon. It has stages of lunar accretion undoubtedly contributed to the three distinct rings; the outerm ost, the C ordillera, defin es a m elting of the outer several hundred km of the M oon and the ba sin 9 00 k m in dia m e te r. form ation of the initial lunar crust. T he effects of post- accretional im pacts on the subsequent evolution of the lunar 少‘叨tale，，h as 功ree conspicuous rings.＿The outerm ost one, crust cannot be adequately sum m arized w ithin the lim its of 只le 卜ord“‘era，工？rm s a c人rcular scarp de士‘ning a basin 900 km th is c on trib ution . S o m e direc t an d in direc t e ffe c ts h av e b ee n 坦 aia贝cie仁· AS tne ．”youn尽est”＿o 土 the m ajor m ulti-ring uasins, ics iorm 1s tne least degraded and m odified by such re fe rre d to an d othe rs a re listed in T a ble I． D e ta iled processes as viscous relaxation; the preserved im pact-induced discussions can be found in P apike and M errill (1980). r e lie f f ro m t h e C o r d ille ra t o th e b a s in c e n t e r is a r o u n d 8 k m EP ISO D E S, Vol. 198 1, N o. 2. 6 T h e re is only a sm a ll v olu m e of vo lca nic m a r e m a t e ria l crater density of the heavily cratered terrain is only 70% of w it h in O rie n ta le and im pact m elt-bearing f a c ie s a re w e ll that of the lunar highlands. T he orbits of various classes of inter-planetary bodies indicate a higher theoretical crater 谬戳 戮mvoa胃ea otf ．.2 m5e lxt- b1e0a6r糕架粼‘留脚吮郭 production rate for M ercury than for the M oon. T his suggests parable to that of the C olum bia R iver plateau basalts on that the m ercurian "highlands" m ay be slightly younger than Earth. M uch of the im pact m elt form ed initially is not w ithin the lunar highlands and that they w ere subjected to a reduced the O rientale basin ; it w as redistributed in ejecta, w ith the flux rate m ore consistent w ith a form ational age of r 4.0 G a. continuous ejecta deposits of m elt and breccias covering som e 60-70' of arc of the lunar surface. T he m ost spectacular im pact feature on the m ercurian sur- face is the C aloris basin, w hich is defined by a ring of M ercu ry m ountains 1300 km in diam eter, rising som e 2 km above the surrounding terrain. A lthough com parable in size to the T he approach encounter of M ariner 10 w ith M ercury revealed Im brium basin on the M oon, C aloris lacks a w ell-defined a heavily cratered surface not unlike that of the lunar highlands (Fig. 7). Following flyby, the outgoing view s m ulti-ring form . T here is, how ever, a w eakly developed revealed both heavily cratered terrain and a younger, re- annular scarp at 1450 km , and the apparent presence of tw o latively lightly cratered, sm ooth terrain of probable volcanic interior rings, appearing as com plex circum ferential ridge origin. A lthough sim ilar in general m orphology to those on system s w ithin the basin, has been suggested. the M oon, the im pact craters of M ercury show som e signif- M a r s ica nt d iffe re n ce s. M ars has a varied and dynam ic surface. A lthough craters are com m on, heavily cratered terrain is preserved only in the southern hem isphere. T he rem ainder of the surface is dom inated by volcanics and includes im m ense, relatively young shield volcanoes such as O lym pus M ons, w hich rise up to 25 km above the local datum . P olar deposits and w ind- blow n and fluvial sedim ents are present, as is a com plex array of broad sinuous fluvial channels. Tectonism is evident by a spectacular series of fractures and grabens associated w ith the T harsis region, w here there is a m assive bulge 8000 km in diam eter and 11 km high. O n the east side of Tharsis lies the equatorial canyon system of Valles M arineris, w hich ex te nd s fo r a lm ost 50 00 k m and rea ch e s a 10 0-km w id th a n d a 6-km depth in places. W ith such evidence for extensive and varied endogenic activ- ity, it is not surprising that m artian craters are different from their lunar and m ercurian counterparts. T here is a paucity of sm all craters; readily identifiable secondary craters form ed by ejecta are rare, and the topography of the preserved craters appears soft or subdued (Fig. 8). The effective rem oval of sm all craters, either by degradation or aggradation, is particularly evident in the heavily cratered terrain, w here the density of large craters,＞64 km diam eter, is 13 tim es that of the average lunar m are and gives an F igure 7. M ariner 10 im age of heavily cratered terrain on M ercu ry . e s t im a t e cl a g e f o r t h e su r fa c e o f 4 .U 士 U .Z G a (H a rt m a n n e t 里·，i} ,6 i). i ne aensity of craters ＜ 4 Rm , how ever, is oniy M ost obvious is the effect 可 the higher planetary gravity 1.4 tim es that of the average m are and suggests a consider- (370 cm s-2 versus 162 cm s-2 for the M oon), with the radial ably younger age. 六 臀 extent of ejecta from m ercurian craters being r 0.65 that of 撰物魏尸殡别颧 瀚蔑 书I阅 翼 塔 蘸 COM Darable lunar craters (G ault et al二1975). In addition. the 全 羹 睽 ala m eTe rs at w n icn trie v ariou s m o rp no iog ica i fo rm s o c cu r are different. For exam ple, the transition from the sim ple to 黝谈 沧妥 com plex form occurs at approxim ately 10 km , interior rings 蕃 appear at approxim ately 90 km and peak ring basins at 120 鬃 馨 km (W ood and H ead, 1976). These diam eters scale only approxim ately w ith gravity w hen com pared to lunar data, and 戴旦 suggest an effect due to differences in m odal im pact velocity 黔 and target characteristics. A detailed com parison of the 蒸 relationships betw een various m orphological param eters in- dicates that craters on the so-called cratered plains of M ercury have m ore in com m on w ith craters on the lunar 巍 m aria than the lunar highlands (Cintala 丝旦．，1977). If this 暴 葬黔 is correct, it carries the im portant im plication that the substrate for the m ercurian cratered plains m ay be relatively 鬃 coherent volcanic units and not a heavily brecciated m egare- 黝 举 golith such as in the lunar highlands. 馨弊鑫 A nalyses of the size-frequency distribution of m ercurian 蒸 craters can also be used to understand surface history. 鬓 A lthough the size-distribution of large m ercurian craters is 麟 sim ilar to that of large lunar and terrestrial craters, there is an apparent lack of craters ＜ 30 km w ithin the heavily 黔 黔纂下 cratered terrain. Such a departure from the expected F igure 8. Viking im age of heavily cratered terrain on M ars. distribution is generally interpreted as indicating resurfacing, N ote the relatively "soft" outlin es of m artian craters relative leading to the loss or obliteration of sm aller craters (Strom , to lunar and m ercurian craters (Figs. 4 and 7) and lack of 1977). O n a planet such as M ercury, the only likely resu r- sm a ll cra te rs. S om e sin u o us ch an n els are also illustra ted . facing process is volcanism . E ven for large structures, the W idth of im age is r 270 km . EPISO D E S, Vol. 1981, N o. 2. 7 A nalyses of crater densities in specific areas has yielded A s expected, ring basins are confined to the older southern size-frequency distributions w hich deviate from the expected hem isphere and are significantly m odified by eolian and other sim ple pow er law distribution . Som e show considerable surface processes, so that in m any cases the interior rings are structure indicative of periods of local erosion and deposition indistinct. M any m artian basins also show evidence of (M asursky 兰 丝·，1977). Although the high level of surface infilling by lunar m are-like volcanism . A lthough M ars has the activity has com plicated the analysis of size-frequency dis- the largest identifiable im pact basin on the terrestrial tribution of m artian craters, it is possible to place the planets (Hellas, 2000 km in diam eter), it possesses signif- activity in a tim e fram ew ork . It appears that, unlike the icantly few er basins per unit area than the M oon or M ercury. M oon and M ercury, M ars has been active w ell in to the second T his deficiency could be due partly to the destruction of pre- half of solar system history. In spite of considerable uncer- tainty, crater densities indicate that channel form ation m ay units of the northern hem isphere. ha ve o c cu rre d b etw e e n r 3 .0 G a to r 2 .0 G a a nd so m e V e n u s volcanic features such as O lym pus M ons m ay be as young as 0.5 G a (H artm ann et al., 198 1). T he dense cloud cover of Venus prevents direct study of its The transition from sim ple to com plex form occurs at r 5 km surface. E arth-based radar im ages and those of the Pioneer in m artian craters and, as expected, there are variations in m ission indicate a num ber of circular features (M asursky 丝 the m orphology of m artian craters of a given size depending 兰·，1980). They range in size from r 20 krn to over 1000 krn on in situ characteristics. M artian craters show the diversity and have the gross m orphology of im pact craters. T he larger diam eter features have a size-frequency distribution and in m orphology observed on the M oon and M ercury, but also display crater form s unique to M ars. In particular, som e areal density sim ilar to craters on the lunar highlands and, if craters contain a central pit in addition to (or even replacing) interpreted as craters, they indicate that Venus has preserved a central peak. Such craters range in size from 2 to 200 km som e early cratered crust. and the origin of the central pit has b ee n a scribe d to the The venusian "craters" are shallow com pared to their lunar explosive decom pression of subsurface volatiles (W ood et al., counterparts, w ith m axim um recorded depth of r I km , and 1978). it has been suggested that they m ay be partially filled by The presence of volatiles has also been related to the volcanics or sedim ents, or have undergone viscous relaxation . form ation of so-called ram part craters (Fig. 9). These In addition, there m ust be effects on crater topography and structures are characterized by several layers of surrounding ejecta distribution due to the high surface tem perature , r ejecta, typically show ing a lobate form and term inating in an 400'C , and atm ospheric pressure, r 100 bars. A t present, outer low ridge or escarpm ent (Carr 丝 丝·，1977); they are how ever, the resolution of P ioneer im agery, r 30 km spatial interpreted as evidence of ground flow . T he peculiar flow resolution, reduces m uch of the discussion to speculation. characteristics of the ejecta is attributed to entrained liquid F urther analysis of the cratering history of Venus m ust aw ait or vapor, released by im pact into a volatile-rich target. the Venus O rbiting Im aging R adar (VO IR) m ission. R am part craters a re distributed over the entire planet. The C oncluding R em arks radial extent of the fluidized ejecta, how ever, varies w ith T his brief outline has attem pted to indicate the relative latitude and altitude (M ouginis-M ark, 1979). The most exten- im portance of cratering on the terrestrial planets. C onsider- sive ejecta units occur at low altitudes and high latitudes, able inform ation on the form ation and evolutionary history of suggesting that the fluidizing m edium w as concentrated in a planet's crust can be gained from detailed analyses of its topographic low s and near polar regions. cratering record. T he E arth appears unique am ong the terrestrial planets in that it does not have any preserved heavily cratered terrain. T his is not unexpected , considering its highly active nature. A lthough no evidence is preserved, the Earth m ust have been subjected to the high early flux of large bodies w hich produced the 1000 km -sized im pact basins on the other planets. H ypotheses concerning the effects of such bom bardm ent on the early (pre ‘ 4.0 G a) history of the Earth generally suggest that it could be responsible for or trigger the initial dichotom y betw een oceanic and continental proto-crust. The lack of constraints on the initial cond itions on the early E arth has led, how ever, to opposing view s as to how basin-sized im pacts m ight achieve this dichotom y (F rey, 1980; G rieve, 1980b). W hatever the case, it is am )arent from lunar basins inaT m ajor im pacts result in long term klu。一 IU' years) therm al and tectonic anom alies w ithin a planet's crust (Head and Solom on, 198 1b). T he consequences of such ano m alies, w hich m ay persist long after the evidence for the original basin has been destroyed, m ust be considered w hen m odelling the crustal e volution of the E arth. The effect of im pact cratering on the Earth m ay extend w ell beyond the first few hundred m illion years of its history. R ecent studies suggest that im pact events m ay be F igure 9. The Y uty crater on M ars (18 kin diam eter) is a so- responsible for m ajor faunal extinctions, such as that at the called ram part crater w ith several layers of Zobate ejecta. C retaceous-T ertiarv boundarv (A lvarez et al二 1980: K vte et A n earlier crater, close to rim of Y uty, is partially buried by ejecta. 里·，1}16U, ano otners). it is apparent n at trie exploration ot space has highlighted im pact cratering as a geologica l A third unusual type of m artian crater has a m arked radial process of considerable im portance to the history of the pattern to its ejecta. T hese radially textured c raters are planets and perhaps for the Earth.* localized, being found prim arily on Tharsis or Elysium lavas. In general, craters w ith typical lunar or m ercurian m orpho- R e fe r e n c e s logies are also m ore abundant on volcanic units, as opposed to A lvarez, L .W ., A lvarez, W ., A saro , F ., and M ichel, H .V . le ss c oh e re nt ta r2e t m a te ria ls. 1980, E x traterrestrial cause for the C retaceous-Tertiary

* C on trib u tion N um b e r 9 3 5 o f E arth P hysics Branch, D epartm ent of Energy, M ines and R esources, C anada. E PISO D ES, V ol. 1981, N o. 2. 8 extinction: Science, v. 208, no. 4448, p. 1095-1108. P roc. C onf. Lunar H ighlands C rusts: G eo chim . C os- m ochim . A cta, Suppl. 12, p. 113-132. C arr, M .H ., C rum pler, L .S., C utts, J.A ., G reeley, R ., G uest, J.E . and M asursky, H ., 1977, M artian im pact craters and K yte, F .T ., Z hou, Z., and W asson, J.T., 1980, Siderophile- em placem ent of ejecta by surface flow : J. G eophys. R es., enriched sedim ents from the C retaceous-T ertiary bound- v. 82, no. 28, p. 4055-4065. ary: N ature, v. 288, no. 5792, p. 651一656. C intala , M .J., W ood , C .A ., and H ead, J.W ., 1977, The effects M asursky, H -, Boyce, J.M ., D ial, A .L., Schaber, G .G ., and of target characteristics on fresh crater m orphology: Strobell, M .E ., 1977, C lassification and tim e of form ation Proc. L unar Sci. C cnf. 8th, p. 3409-3425. of m artian channels based on V iking data: J. G eophys. R es., v. 82, no. 28, p. 40 16-4038. F rey, H ., 1980, C rustal evolution of the early Earth: T he role of m ajor im pacts: Precam b. R es., v. 10, no. 3-4, p. 195- M asursky, H ., Eh ason, E ., Ford, P .G ., M cG ill, G .E ., Petten- 2 1 6 . gill, G .H ., Schaber, G .G ., and Schubert, G ., 1980, Pioneer Venus radar results: G eology from im ages and altim etry: G ault, D .E., G uest, J.E ., M urray, J.B., D zurisin, D ., and J. G eophys. R es., v. 85, A . 13, p. 8232-8261. M alin, M .C ., 1975, Som e com parisons of im pact craters on M ercury and the M oon: J. G eophys. R es., v. 80, no. 17, p. M elosh, H .J., 1981，A B ingham plastic m odel of crater 24 44- 24 60 . collapse: L unar and Planet. Sci. X II, L unar and P lanetary Institute, H ouston, T exas, p. 702-704. G rieve, R .A .F ., 1980a, C ratering in the lunar highlands: Som e problem s w ith the process, record and effects, in M ouginis-M ark, P ., 1979, M artian fluidized crater m orpho- Papike, J.J., and M errill, R .B . (eds.), P roc. C onf. Lunar logy: V ariations w ith crater size, latitude, altitude and H ighlands C rust: G eochim . C osm ochim . A cta, Suppl. 12, target m aterial: J. G eophys. R es., v. 84, no . B 14, p. 8011- 8 0 2 2 . p. 173-196. Papike, J.J., and M errill, R .B. (eds.), 1980, Proceedings of G rieve, R .A .F ., 1980b, Im pact bom bardm ent and its role in proto-continental grow th on the early Earth: P recam b. C onference on L unar H ighlands C rust: G eochim . C osm o- R es., v. 10, p. 217-247. chim . A cta , suppl. 12. G rieve, R .A .F ., and R obertson, P .B ., 1979, T he terrestrial P ike, R .J., 1977, Size-dependence in the shape of fresh im pact craters on the M oon, in R oddy, D .J., Pepin, R .O ., cratering record. 1. C urrent status of observations: and M errill, R .B . (eds.), Im pact and Explosion C ratering: Icarus, v. 38, no . 2, p. 212-229. P ergam on, N .Y ., p. 489-510. G rieve, R .A .F ., D ence, M .R ., and R obertson, P.B ., 1977, C ratering processes: A s interpreted from the occurrence R oddy, D .J., P epin, R .O ., and M errill, R .B . (eds.), 1977, Im pact and E xplosion C ratering: P ergam on, N .Y . of im pact m elts, in R oddy, D .J., P epin, R .O ., and M errill, R.B. (eds.), Im pact and Explosion C ratering: Pergam on, Settle, M ., and H ead, J.W ., 1979, T he role of rim slum ping in N .Y ., p. 79 1-8 14. the m odification of lunar im pact craters: J. G eophys. H artm ann, W .K ., Strom , R .G ., W eidenschilling, S.J., Blasius, R es., v. 84, no. B 6, p. 3081-3096. K .R ., W oronow , A ., D ence M .R ., G rieve, R .A .F ., D iaz, J., Strom , R .G ., 1977, O rigin and relative age of lunar and C hapm an, C .R ., Shoe m aker, E.M ., and Jones, K .L ., 198 1, M ercurian intercrater plains: Physics of the E arth and C hronology of planetary volcanism by com parative studies Planet. Interiors, v. 15, no. 2/3, p. 156-172. of planetary cratering. C hap. 8, Basaltic V olcanism on the T errestrial Planets: Pergam on, N .Y ., in press. Tera, F., P apanastassiou, D .A ., and W asserburg, G .J., 1974, Isotopic evidence for a term inal lunar cataclysm : Earth H ead, J.W ., 1974, O rientate m ulti-ring basin interior and Planet. Sci. L ett., v. 22, no. 1, p. 1-21. im plications for the petrogenesis of lunar highland sam - W olf, R ., W oodrow , A .B ., and G rieve, R .A .F ., 1980, M eteor- ples: M oon, v. 11, no . 3-4, p. 327-356. itic m a te rial a t fou r C an a dia n im i)ac t c ra ters: G eo ch im . 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H erzberg, C .T ., and Baker, M .B., 1980, The cordierite-to spinel-cataclasite transition: Structure and form ation of the lunar crust, in Papike, J.J. and M errill, R .B . (eds.), A B O U T T H E A U T H O R S :

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D r. R ichard A .F . G rieve, a research scientist w ith the E arth P hysics Branch of the C anadian D epartm ent of Energy, M ines and R esources, is currently Visiting Professor at B row n D r. Jam es W . H ead, 111, is Professor of G eology at B r o w n U niversity, U .S.A . H e has w orked on the analysis of lunar U niversity, P rovidence, R hode Island, U .S.A ., and is C h a i「一 sam ples returned by the A pollo m issions and his current m an of the IU G S A dvisory C om m ittee on C om parative research is concentrated in im pact cratering processes and Planetology. H is research centers c i planetary geological their effect on early crustal evolution. D r. G rieve is processes and history. D r. H ead w as involved in the A pollo Secretary of the IU G S A dvisory C om m ittee on C om parative L unar Exploration Program from 1968 to 1972. Planetology. EP ISO D ES, V ol. 198 1, N o. 2.