Lunar crater Copernicus: search for debris of impacting body at Apollo 12 site

Acta,1978, VOX.$7, pp. 141 to 164. Pergamon Press. Printed in Eortbern I&and

CJewbimiea et Ernie

Lunar

crater Cope~c~ : search for debris of

impacting body

satApollo 12 site JOHN W. MORUAN, R. GANAPATHY, J. C. LAUL* and EDWARD ANDERS Enrico Fermi Institute and Dep~ment

of Chemistry, University of Chicago, Chicago, Illinois 60637, U.S.A.

(Received

31 May

1971; accepted

in revised form

23 August

1972)

Ah&a&--In an attempt to characterize meteoritic material at the Apollo I2 site, 4 KREEP concentrates from soil 12033 have been analyzed by neutron activation analysis. These contain a meteoritic component in which siderophileIr, Re and Sb are depleted by about a factor of 2, while volatile Se, Zn, Ag and Bi are depleted by a factor of more than 5 relative to Au. This pattern does not closely resembleany major ehondriteor iron meteorite group, but is very similar to that observed in high-alkali samples from Apollo 14. The meteoritic component in KREEP at both sites is therefore predominantly derived from Imbrian ejecta. Kowever, a second, small component of primitive composition seems to be present in Apollo 12 KREEP, judging from the slight, uniform enrichmentsin Ir, Re, Sb, Se and Zn relative to Au. This component does not seem to be due to micrometeorites. If it is attributed to the Copernican projectile, the crater Copernicusmay have been formed by a cometary nucleus, 4 km in diameter, with an impact velocity of 30-40 km/see. These conclusions depend critically on the assumption that the meteoritic component in Apollo 12 KREEP is representativeof the entire impact.

INTRODUCTION APOLLO 12 soil contains a component rich in K, rare earth elements (REE), and P, acronymi~a~y called KREEP (HUBBARD et d., 1971). Many soil ch~ac~ristics

can be explained by simple mixing of two components: KREEP and average Apollo 12 crystalline rock (SCHNETZLERet al., 1970; HUBBARD et al., 1971). KREEP is commonly shocked, even glassy,7 and seems to be derived from an exotic rock type by impact (MEYER and HUBBARD, 1970; HUBBARD and CAST, 1971; MEYER et al., 1971; MARVIN et al., 1972). Certain light grey soils, which were collected from crater rims at the Apollo 12 site, contain particularly large amounts of this material. By virtue of their relatively high albedo, these soils have been associated with the Copernican ray which is telescopically observed to cross the Apollo 12 site (LSPET, 1970). The spectral reflectivity, albedo, and polarization of 12032 and 12033 strongly support a Copernican origin for KREEP, since these optical properties suggest a major contribution of ray material to the crater rim soils (ADAMS and MCCORD, 1971; DOLLFUS et at., 1971). Lead isotope data suggest that the Pb-U-Th system in KREEP-rich materials was disturbed relatively recently (TATSUMOTO, 1970; TATSU~OTO et al., 1971; SILVER, 1971; CLIFF et aZ., 1971). The apparent date of the disturbance, 850 Myr (Silver, 1971; SCHONFELD and MEYER, 1972; EB~RHARDT et al., 1972), is in fair agreement with the age of 2000 Myr for Copernicus estimated from crater counts (HARTMANN, 1968), considering the uncertainties in that method. Though some authors have proposed alternative sources for KREEP * Presentaddress: Radiation Center,Oregon State University,Corvallis,Oregon 97331, U.S.A. t We shallmakeno ~stinetionbetweenglassyand crystallineKXEEP, becauseall samples measuredin this work containedboth kinds. 141

142

JOHN W. MORUAN, R.

GANAPATHY, J. C. LAULand EDWARDANDERS

(QTJAIDE et al., 1971; BAEDECKER et al., 1971a, b; WASSONand BAEDECKER,1972), the evidence presently available seems to favor a Copernican source. The gross meteoritic content of Apollo 12 soils has been estimated from their trace element composition (LAULet al., 1971). Several soils resemble that of Apollo 11 in that they contain ~1.7 per cent primitive (Cl-chondrite-like) material (GANAPATHYet al., 1970). The crater rim soils contained considerably less meteoritic

4

2 %KREEP

16

loo

1.2

0

loo 0 % KPEEP

100

Fig. 1. Correlation of trace elements with KREEP content in Apollo 12 crater rim soils. Trace elementsmainly from LMJLel al. (1971), Re and OSfrom LOVERING and HUQEES(1971), CREEP contents from W~NKE et al. (1971). Solid circlesrepresent the experimental points, open circles a.re the oalculcttedintercepts. Error bars represent ste;ndard deviations; the unclosed error bar in the Bi diagram representsa single determination only on 12037.

material, of a fractionated type, the average of 12032,12033, and 12037 being equivalent to O-8 per cent L-chondrite, 0.8 per cent H-chondrite, or O-2per cent Group I iron meteorite (LAULet aZ., 1971). The low abundance of meteoritic material in one of these soils (12033) was independently noted by BAEDECKERet al. (1971a), though they had not measured the appropriate elements to notice the fractionated meteoritic pattern. Since Apollo 12 KREEP glass seems to be an impact melt, it is reasonable to suppose that it contains its own meteoritic component. We have, therefore, attempted to estimate its content of meteoritic and other trace elements by a linear regression, on the assumption that Apollo soils 12032, 12033, and 12037 are binary mixtures of ‘100 per cent KREEP’ and ‘0 per cent KREEP’ (Fig. 1). Our mixing model differs from that of HUBBARDet al. and others in that 0 per cent KREEP is not simply average Apollo 12 crystalline rock, but is all the components in the soil except KREEP. The KREEP contents of the soils were taken from WXNKEet al. (1971), whose estimates were based on a regression analysis of 21

Lunar crater Copernicus: search for

debrisof impactingbody at Apollo 12 site

143

elements in 8 soils. Each solid point in Fig. 1 represents a triplicate snalysis. Endmember compositions are represented by open circles. Compared to the KREEPfree end member, 100 per cent KREEP is high in the siderophile elements Re, OS, Ir and Au, but low in the volatile elements Se, Zn, Cd and Bi. This regression, presented in the original version of this paper (MORGANet al., 1971), was deemed unreliable by some experts. We therefore measured 4 hand-picked KREEP concentrates, as well as another sample of 12037 soil. EXPERIMENTAL 8am$es. Two CREEP fragments (one lithic, one glassy), 10 mg total weight, were handpicked from the 2-4 mm fraction of 12033,105 soil by N. J. Hubbard, NASA Manned Spacecraft Center, and were combined for analysis. Two more KREEP samples (possibly less pure) were separated from I.27 g of 12033,20 < 1 mm soil. The +30 mesh fraction (108 mg total) of this sample was cleaned ultrasonically in double-distilled acetone, and 21 mg of compact glass resembling the Hubbard sample was hand-picked. From the remaining +30 mesh fraction we selected a second sample consisting of ‘ropy and cindery’ glass (52 mg), which still retained a light-colored coating. Neither of these fractions appeared to be appreciably magnetic when tested with a small hand magnet. We separated and cleaned 233 mg of 30-100 mesh material from the remainder of the 12033,20 sample, but balked at the task of selecting a well-defined KREEP sample from it. Some of the material (41 mg) in this size range was appreciably magnetic and was taken for analysis. Samples of 12037 soil and BCR-1 were also analyzed as controls. Analytical method. Details of the radiochemicalneutron activation method used in this work are unpublished, but may be obtained from sources given by ANDERS et al. (1971). Two new elements have been added: Sb and Re. Because of the small size of the KREEP concentrates, the uncertainty in their Bi values is quite large ( - &30 per cent). In addition, the Bi fraction of sample 12033,105 was cross-contaminated when a precipitate was accidentally transferredto a waste beaker containing rejected supernates from other samples of higher Bi content (notably BCR-1).

RESULTS The analytical data are summarized in Table 1. The analyses for 12037 soil and BCR-1 agree well with previous determinations (LAUL et a,!., 1971). A genersl idea of our precision and accuracy may be obtained from Fig. 1 of that paper. A few doubtful values are given in italics. It is interesting to compare the average composition of KREEP fragments with that of ‘100 per cent KREEP’ extrapolated from bulk soils (MORGANet al., 1971). Considering that the hand-picked fractions are a biased sample, the agreement is not bad (Table 1, last two columns). DISCUSSION Meteoritic component in 12033 KREEP The trace element composition of 12033 KREEP can be regarded as a simple combination of indigenous lunar debris and a meteoritic component. In principle, it would be possible to derive the meteoritic component directly by subtracting from the KREEP glass abundances the measured trace element content of uncontaminated rock of KREEP composition. In fact, no such rock has yet been found among those analyzed and so the direct method is not feasible. Alternatively, we can take uncorrected abundances and calculate an apparent meteoritic component. These values are then upper limits, but may have significance if they are unusually low. The noble metals Au, Ir end Re are present in Indigenous

contribzction.

143 0.097 0.99 0.43 56 3.3 70 1.25 4.6 I.1 8.0 6.0 19.4 710

Ir Re AU Sb Se Ag Bl? In Bi Zn Cd Tl Rb cs

1.5 0.16 1.5 0.79 84 2.2 160 1.83 0.70 1.6 3.8 4.1 17.0 655

12033,20-4 Glassy KREEP ( + 30 mesh) 2.2 O-19 2-l 1.5 99 2.0 140 1.21 0.4 2.3 7.7 3.3 17.0 625

12033,20-7 Cindery KREEP ( +30 mesh) 6.7 0.67 5.4 3.1 172 2.5 130 1.31 0.4 3.2 13 3.3 14.9 535

12033,20-l Magnetic (30-100 mesh)

* Italicized values are too high, owing to contamination.

12033,105 2 KREEP fragments (2-4 mm)

Element

4.2 0.25 2.2 0.77 166 25 90 222 0.70 6.1 56 2.6 4.9 210

12037,25 Bulk soil (
BCR-I

0.20 4.5 X7 50.5 10-o 500

& o-2 f 40

& 0.12 & 0.6 & 8

170 & 20 100

3.7 & 0.5 0.44 f 0.06 14 f 0.1

f f f jl &

2.4 0.22 2‘0 1.2 50 125 + 40 1.4 f 0.3 0.5 & 0.2 2.1 f O-9 8&4 4.2 f 1.3 17 jc2 630 & 70

3.0 0.25 2.5 1.5 100

100% KREEP This work MORGANet al., (Av. of KREEP 1971 separ&b) (Bulk soils)

Table 1. Abundances in KREEP fragments from crater rim soil 12033 (ppb; Zn, Rb ppm)*

Lunar crater Copernicus: search for debris of impacting body at Apollo 12 site

145

small amounts in all lunar igneous rocks examined to date: mare basalts, highland basalts, and anorthosites (GANAPATHY et al., 1970; ANDERS et al., 1971; LOVERING and HUQHES, 197 1; MORGAN et al., 1972a). They are strongly siderophile and are most unlikely to have remained in the silicate phase in a differentiation process that fractionated lithophile elements by factors of more than 100. There can be little doubt that these elements in 12033 KREEP are predominantly meteoritic. For other elements this assumption cannot be made with equal confidence. It is reasonable to assume that all four 12033 fractions have a similar indigenous trace element composition. The Rb and Cs contents, which are predominantly indigenous, are almost constant in the four samples, having a standard deviation of only 11 per cent. On the other hand, Au varies by a factor of more than 5. If we plot the abundance of an element against Au, the intercept at Au = 0 will approximate the indigenous contribution, and the slope of the line will represent the ratio of the element to Au in the meteoritic component. The meteoritic contributions for different elements are more readily compared if abundances are normalized to Cl chondrites, as in Fig. 2. The results of an unweighted least squares analysis of the Cl-normalized data, using Au as the independent variable, are entered in Table 2. Three elements, Ir, Re and Sb, are strongly correlated with Au, and have near zero

negligibly

2-

I

Bi

I

I

I

I

I

I

l

I-

.

1 I ’ 4 I--

O-

. l

1

I-

.

O Se l-

O-

I-

4

O Au ( Percent

Cl

Abundance)

Fig. 2. Variation of ‘meteoritic’ elementsrelativeto Au in 12033 KREEP fractions. Elemental abundances have been normalized to Cl carbonaceous chondrite abundances. The slope of the regressionline for any element representsits ratio relative to Au in the KREEP meteoritic component. Five elements (Ir, Re, Sb, Se, Zn) clearly show a contribution from meteoritic material. The remaining elements (Bi and Ag) show no detectable meteoritic contribution. The siderophile elements Sb, Re and Ir in the KREEP meteoritic component are present in roughly 4 their cosmic abundances, relative to Au; volatile elements are much more strongly depleted. The intercept at Au = 0 approximates the indigenousabundance in KREEP. Negative intercepts for Ir, Re and Sb, though individually not statistically significant, suggest that the indigenousAu in KREEP is about 0.1 to O-2ppb. 10

JOHN W. MORUAN,R. GANAPATW, J. C. LATJLand EDWARDAxnzus

146

Table 2. Regression analysis of correlationof ‘meteoritic’ elements with Au (abundancesnormalized to Cl ohondrites) P, correlation coetlkient

Element

0.420 f 0.044 0.446 f 0.018

0.989 0.998

Ir Re

0.489 0.176 0.196 -0.023 -0.067

0.987 0.989 0.942 -0.208 -0.619

Sb Se Zn -48 Bi

Intercept (ppb)

Slope

f f f f f

-0.09 -0*019

0.056 0.019 0.049 0.077 0.085

-0.052 41 1.0 2.6 0.65

f 0.39 f 0.014

& 0.204 f8 f 0.3 i 0.6 f 0.22

intercepts. While not significantly different from zero, in all three cases their intercepts are negative, suggesting that the indigenous Au abundance in pristine KREEP material is 0.1 to 0.2 ppb, and not zero as assumed in our analysis. This does not affect the estimate of the meteoritic component, however, which is based upon the slope of the line. Two other elements, Zn and Se, are also strongly correlated with Au, but have significant positive intercepts corresponding to 1 ppm and 41 ppb, respectively. The last two elements considered, Ag and Bi, show no significant corre-

lation with Au, and only upper limits can be determined for their abundances relative to Au in the meteoritic component. Comparison with meteorites. In order to compare the meteoritic component with that of other lunar material and with known meteorite classes, it is convenient to use the Au-normalized values obtained from the regression analysis (Table 2). Au provides a useful reference standard because it condenses from a gas of cosmic Au/Fe ratio with the bulk of the nickel-iron (ANDERS, 1971) and is only slightly fractionated relative to Fe in cosmochemical processes, judging from the constancy of Au/Fe ratios in meteorites (CROCKETet al., 1967; EHIUNN et al., 1970; EHMANN and GILLUM, 1972; SCOTT, 1972). Similarly normalized abundances for the major chondrite groups and for Type I and Type IVA iron meteorites are given in Table 3. Table 3. Elemental abundances in meteorites* and 12033 KREEP fragments relative to ‘cosmic’ abundances and normalized to Au Ir Meteorites 1.07 Cl 1.12 c2 1.24 c3,4 1.14 H L 0.96 LL 0.70 E4 0.47 0.67 E5,6 Irons, I 0.48 Irons, IVA 0.08-1.1 Lunar sample 12033 KREEPt 0.42

Re

Sb

0.96 I.07 1.23 1.33 1.05 0.83 0.52 0.76 0.53 0.11-2.2

0.97 0.60 0.58 0.42 0.43 0.40 0.54 0.46 0.19 0.003-0.017

0.45

0.49

Se

Zn

I.02 0.55 0.39 0.25 0.39 0.39 0.40 0.35 0.00018 <0~0002

1.20 0.40 0.28 0.098 0.15 0.17 0.55 0.029 0.0068 (0.7

0.18

* From &&iON (1971). t Mean values calculated from slope of regressionlines.

0.20

A8 1.07 0.37 0.52 0.15 0.35 0.32 0.37 0.13 0.10 <0*002-0.03 so.15

Bi 0.99 0.44 0.27 0.026 0.029 0.067 0.35 0.028 0.0022 < 0.002 so.17

Lunar crater Copernicus: search for debris of impcu?tingbody at Apollo 12 site

147

It is immediately apparent that the KREEP meteori& component shows a decidedly ffactionated p&tern r&her than a primitive (Al-chon~i~-be) pattern. The ‘refrrtofory’ elements, Ir and Re, are depleted relative to An; the volatile elements, Se and Zn, are depleted relative to the refractory elements, and only (quite low) upper limits are found for the remaining volatile elements Ag snd Bi. Sb is depleted rel&ive to Au, but is present in about the s&me amount w fr and Re. The normalized &bund&nces of Ir and Re immediately limit the choice of impacting mlbterial to the Type I and Type IVA irons and ensWite chondrites. But E4 chondrites are too rich in the Eivolatile elements, Sb to Bi. This leaves only irons and E5, 6 ohondrites as possible matches for the meteoritic component in KREEP. Vola~~~~t~ losses. It is unlikely that the KREEP meteoritic pattern has been si~~c~ntly altered by vol&til~&tion dnring impact. S~O~ELD and MEYER (1972) have concluded from Pb isotope data that some 36 per cent of the Pb in 12033 soil was lost in a discrete event 0.9 AE ago (probably the Copernican impact). It, appears that the loss of the remaining volatile elements was of the same order or smaller. Tha.llium provides & suitable check, because it is more volatile than Pb (LARIMER, 1967, 1973) and tends to follow the less volatile Rb and Cs in m~gm~tic processes on the Moon (MORUAKet al., 1972a). The Tl/Cs ratio in 12033 KREEP is about a factor of 2 to 3 lower than that in Apollo 14 samples which were not involved in the 0.9 AE event, and a factor of ~2 higher than tha,t in rock 12013 samples (Fig. 4, MORGANet at., 1972a). Thus it seems that loss of Tl was moderate; no more than a factor of 3 ftnd probably a good deal less. Loss of elements less volatile than Tl (Sb, Ag, Bi, Zn, In) should have been smaller still. Loss of elements of greater or uncertain volatility (Se, Br, Cd) is harder to estimate. Judging from s, comparison with Apollo 14 samples (Table 1 of MORGANet al., 1972b), it is nof likely to have exceeded a factor of 2-3. This is consistent with observations from nuclear explosions, where elements of comparable vol&tility also fr~ction&~ only by moderate factors (BJ~R~E~STEDTand EDVARSON,1963). Studies on Apollo 11 glass spherules also suggest that volatiles recondense on expansion and cooling of the tieball. Volatile elements such as Na, K, and P were enriched by factors of 16 to 64 in spherule rims, relative to the centers (KURATand KEIL, 1972). Leaching experiments on Apollo 11 soil also showed & surfa,ce con~ntr~tion of volatiles, as expected for recondensation (LAULet al., 197X). Search for a Copernican component. The crater Copernicus was excavated in the ejecta blanket of Mare Imbrium (the Fra Mauro Formation) and underlying bedrock. The meteoritic pattern of the Copernican projectile will therefore be superimposed upon the much earlier imprint of the Imbrian impacting body, and Pre-Imbrian plane~sim~l debris. It is interesting, therefore, to comprtre the =EEP meteoritic component with that associated with similar high-alkali material collected at Fra Mauro during the Apollo 14 mission (GANAPATHY et al., 1972; MORGANet al., 1972b). For a more rigorous comparison, the Fret Mauro analyses have been corrected for the indigenous component of KREEP-like m&e&l based on our regression analysis. Exceptions were made in the case of Ag and Bi, where the correlations were uncertain, and the estimated indigenous component may be too high. Here the actual uncorrected abundances have been nsed to give upper limits on the meteoritic contribution for these two elements.

148

JOHN

W.

MORUAN, R. GANAPATEY, J. C. LAUL and EDWARD ANIZRS

The meteoritic patterns are compared in Fig. 3. The KREEP meteoritic component is strikingly similar to that of 4 microbreccias picked from 14321 and of 2 light norites (from soils 14142 and 14146), except for being marginally richer in Sb, Se and Zn. The agreement with dark norites (from 14151 and 14154) is somewhat less good, though the broad fractionation trends are similar; moderate depletion of Ir and Re, and strong depletion in volatiles. It appears that the same peculiar meteoritic component predominates in glassy KREEP from Copernicus and crystalline KREEP from Fra Mauro, although the two sites are separated by 410 km, and lie at different distances from the rim of Nare Imbrium (200 and 540 km). Two sources must have contributed to this component, in unknown proportions : Pre-Imbrian planetesimal debris from the ancient highlands regolith, and the Imbrian body itself (GAXAPATHY et al., 1972; NOR&AN et al., 1972b).

'Au'Ir'Re'Sb'Se'Zn'Ag'Bi' 'Au'Ir'Re'Sb'Se'Zn'I\g'Bi'~~

Fig. 3. Comparison of the meteoritic oomponents in 12033 KREEP fractions and in Apollo 14 material of similar high-alkali composition. Abundances relative to Cl chondrites have been normalized to Au to remove effects due to differing absolute amounts of meteoritic material. Each error bar represents the standard deviation of the ratio for a single sample. The KREEP pattern is similar to that of the two light norites and four microbreccias; Ir, Re and Sb are present in about equal amonnts, but are depleted relative to Au by roughly a factor of 2. Dark norites do not resemble KREEP quite so closely. In all 4 cases volatiles are strongly depleted relative to siderophiles, though somewhat less so in KREEP than in the Apollo 14 samples. The slight, but almost uniform, enrichment of KREEP in Ir, Re, Sb, Se and Zn, relative to the two light norites and four microbreccias, may reflect admixture of projectile material from the Copernican impact.

Lunar crater

Copernicus: searchfor debrisof impactingbody at Apollo 12 site

149

It is curious that the patterns are so similar, though the 12033 material must have passed through at least one more major impact (Copernicus). At most, the slight enrichments in Ir, Re, Sb, Se and Zn in 12033 (relative to the Apollo 14 microbreccias and light norites) might be attributed to the Copernican event. One might arguethat this enrichment represents contamination by micrometeorites in the 3 AE interval between the Imbrian and Copernican impacts. Actually this explanation requires rather implausible conditions. The contamination can occur only in the 0.6 AE interval between the Imbrian impact and Procellarian flooding, at which time the contaminated layer (no more than 15 m thick, judging from the thickness of the regolith at Fra Mauro) is buried under a few hundred meters of mare basalts. In the subsequent Copernican impact, the thin, contaminated layer must be cleanly exhumed and hurled to the Fra Mauro site, accompanied by no more than an equal mass of micrometeorite-free material from underlying Imbrian ejecta or overlying mare basalt. The great bulk of the overlying material must be left behind. It might also be argued that the enrichment represents recent micrometeorite contamination at the Apollo 12 site. This is rather improbable. Soil 12033 has a very short surface exposure age ( ~40 Myr; CROZAZet al., 1971), and hence a very low micrometeorite content (LAULetal., 1971). Moreover, our KREEP samples were subjected to the same rigorous ultrasonic cleaning as were the Apollo 14 samples. It also seems unlikely that chance contamination would occur in just the right amounts in all four samples to give the self-consistent regressions shown in Fig. 2. Alternatively, it might be suggested that the slight enrichments are due to a systematic error (or contamination) in the Au determinations. This seems improbable, as it would increase all elements proportionally, whereas it appears that the elements are increased by the same absolute amounts despite differences of more than a factor of two in their abundances when normalized to Au. Although the apparent enrichments of Ir, Re, Sb, Se and Zn relative to Au are barely significant individually, they are present in all elements where comparisons may be made. Summary. At this point it may be appropriate to summarize our current conclusions. (1) 12033 KREEP contains a meteoritic component in which refractory siderophile elements (Ir and Re) and volatiles (Sb, Se, Zn, Ag, Bi) are depleted relative to Au. (2) This component does not match the elemental abundance pattern of most meteorite classes, but resembles the meteoritic pattern observed in a number of norites and microbreccias from Apollo 14. (3) There is only a subtle, inconspicuous difference between the Apollo 12 and Apollo 14 pattern that could be attributed to the Copernican projectile: a very slight enrichment of Ir, Re, Sb, Se and Zn relative to Au. Ubiquity of KREEP-related meteoritic component

It is remarkable that essentially identical meteoritic components occur in KREEP from two locations, Apollo 12 and Apollo 14. MORGANet a,?. (1972b) have discussed two possible origins for this component: the Imbrian body and Pre-Imbrian planetesimals. Of these, they consider the former somewhat more likely by a small margin. Apart from the compositional pattern, the high abundance of noble metals is very characteristic of this component. The Au content in Apollo 14 KREEP is 2-3 times higher than that in ‘normal’ lunar soils, which contain the equivalent of 16-2 per cent Cl chondrite material. Apparently the abundance of projectile material in the Imbrian ejecta was unusually high. Future work will show whether this high abundance is a truly general characteristic (in which case it implies a low impact velocity of the Imbrian or Pre-Imbrian bodies) or whether it is merely a freak,

150

JOHN W. MORUA.N,R. GAXAPATHY,J. C. LAUL and EDWARD

ANDEI~S

resulting from preferential ejection of projectile material to these two sites. (At least one norite from the Apollo 16 site contains similarly high abundances of meteoritic elements; U. KRXHENB%BL et al., unpublished work.) Nature of Cope&can projectile The absence of a prominent meteoritic component attributable to Copernicus admits of three explanations. (a) The Copernican ray crossing the Apollo 12 site received less than its share of projectile material. (b) The projectile was chemically inconspicuous, e.g. a Ca-rich achondrite. This is rather unlikely. The Copernican projectile must have been several km in diameter, and although it seems likely that some differentiated meteorite parent bodies have achondritic shells several km thick (by analogy to the Earth and Moon), it is quite improbable that km-sized fragments of these shells are ejected on impact. Moreover, an achondritic projectile would leave the small excess of volatiles in Apollo 12 KREEP unexplained. (c) The projectile had a high impact velocity and was therefore distributed over a very large mass of ejecta. Until Copernican ejecta from other sites become available, we cannot tell whether (a) or (c) applies. However, there is one argument in favor of (c). When a single sample is drawn at random from a large population, it is more likely to be typical than atypical. Thus, with no landings near Copernicus scheduled for the foreseeable future, it may be worth exploring the consequences of the assumption that Apollo 12 KREEP is indeed representative of average Copernican ejecta, and that the slight apparent enrichment of Ir, Re and volatiles (Fig. 3) comes from the Copernican projectile. We recognize that projectile and target material do not mix uniformly during a cratering event. Lightly shockedejecta on the craterrim receiveless than their share of projectilematerial, and impact melts on the crater floor receive more than their share. However, we are concerned only with the small fraction of heavily shocked ejecta that are thrown over great distances. For our treatment to be valid it is only necessarythat the average projectile content of this material be similar to that for the whole of the ejecta. We cannot prove from first principles that this should be so, but can merely point to some empirical evidence in support of this view. (1) Apollo 14 breccias and glasses are consistently high in meteoritic elements. On the assumption that the average meteoritic content is typical of the entire Imbrian event, the impact velocity calculated from crateringtheory is 3 to 10 km/set, close to previous estimates by other means (MORGANe.?al., 1972b). (2) The impact velocity for the Henbury meteorite, calculated from the Ni content of impact glass, is 3 km/set, in good agreement with estimates by M. A. Dence (private communication, 1972).

The ‘Copernican’ (1) meteoritic component in Apollo 12 KREEP glass is appreciably smaller than the meteoritic components in most other strongly shocked lunar materials : Apollo 11 and 15 anorthosites ; Apollo 14 and 15 norites, breccias and glasses (MORGAN et a.!., 1972a, b; U. KRXHENBUHL et al., unpublishedwork). Possibly this is due to some trivial reason, such as inefficient mixing of projectile material, or biased sampling. It seems worthwhile, however, to pursue the alternative assumption, that the low abundance is representative of the entire impact; and to use it to

Lunm crater Copernicus: search for debris of impacting body at Apollo 12 sita

161

some characteristics of the Copernican impact. The consistenoy of these characteristics with other evidence will serve as a test of this assumption. From oratering theory (OPI~, 1961), the impact velocity w is related to the ratio of eroded mass M to projectile mass ~1,by

infer

il+

= kw/(p/s)‘la

(1)

where s is the crushing strength of lunar rock (assumed to be 9 x lo* dynes/cm2), p is its density (3.3 g/cm3), and k is a dimensionless factor varying between 2 and 5, which can be evaluated graphically from data given by opik. The ratio M/,u can be obtained by chemical analysis of a representative sample of ejects; it is essentially the reciprocal of the mass fraction of meteoritic material in the ejecta: p/(M + ,u) M ,u/M. B ecause the amount of meteoritic material is always estimated via a trace element (e.g. Au), the abundance of this trace element in the meteorite must also be known. In the present case, the situation is made more complicated by the presence of an ancient meteoritic component (typified by Apollo 14 microbreccias and light norites) that must be subtracted from the gross meteoritic pattern in Apollo 12 KREEP to obtain the net ‘Copernican’ component. We shall use the subscripts 4, 2 and C for Apollo 14, Apollo 12 and Copernicus. Suppose the to-be-excavated mass m4 of Pre-Copernican material has concentrations [Aula and [El, of Au and a largely meteoritic element E. The amount of E is simply ([E]$[Au],)[Au],m,. The Copernican projectile adds a mass m,, containing [El, and [Au],. The amount of E in the final mixture of ejecta, of mass mz, is ([E].J[Au]J[Au],m,. Putting [E]/[Au] = R and m, = rnz - m,, we obtain [Au], = mz[Az and

1 z[Au], a

C

(mz - md.WW + m&dAul~ = mdW4,. Substituting for [Au]~ from (2) into (3) and rearranging, we have

(2)

(3) (4)

Before using this equation to find iV/,u, we must know the composition of the Copernican projectile, so that we can specify [Au],. In terms of our working hypothesis, it seems that the composition was primitive (Cl, C2 or E3,4 chondrite), because the normalized abundances of the refractory elements Ir and Re (0.06 and O-05) are similar to those of the volatile elements Se and Zn (O-09 and 0.07). Let us do the calculation for Cl composition. Results for C2 or E3,4 composition will be essentially similar. With R, M 1-O (when normalized to cosmic abundances) and [Au], = 150 ppb (Cl value) we obtain a mean M/p = 1000 f 200. Substituting this vaIue in equation (l), with k = 4.4 we find w = 36 & 5 km/see. This velocity falls well within the range for ‘live’ comets (PORTER,1952), and is only slightly higher than that for 3 ‘cometary’ Apollo asteroids, probably extinct cometary nuclei (ANDERSand ARNOLD,1965). It is higher than velocities of meteorites or ‘asteroidal’ Apollo asteroids.

152

JOHNW. MORUAN, R. GANAPATHY, J. C. LAULand EDWARD ANDERS

Several other lines of evidence support the notion that the Copernican object was a comet. (1) It is widely believed that comets have a primitive, volatile-rich composition, similar to that of Cl chondrites .* No direct analysis of comet nuclei is yet available, but the micrometeorite component in the lunar regolith has Cl chondrite composition, and the influx rate calculated from these data (GANAPATHY et al., 1970) agrees within s, factor of two with the flux of (presumably cometary) meteors (DOHNANYI,1971). (2) WHITAKER(1966) and CPIK (1969) have proposed that craters with major ray systems are produced by comets, with the rays being caused by escaping volatilea. It is not clear that the bound water content (~20 per cent) of an ice-free Cl chondrite would suffice for this purpose, but perhaps the effect would be augmented by the low crushing strength which leads to shallower penetration. SUN (1968) has also suggested a cometary origin of ray craters, speci&ally for Copernicus, but some of the assumptions on which his argument was based no longer seem wholly justified. (3) The diameter of the Copernican projectile, calculated for the above velocity, falls in the cometary range. The projectile diameter d was calculated by the following expression derived from CPIK’S (1969) equations (6, 8 and 12) : d = 1.1134, [ ($&]

-1’2 (S/V4

where B, is the estimated diameter of Copernicus prior to slumping, 76 km (BALDWIN, 1963), 6 is the density of the object (~2.0 g cm-a), s, is frontal compressive strength and can be approximated by $s, or 1.5 x log dyne cm-2, and other symbols have the significance described earlier. For the mean velocity of 36 km/set calculated for the Copernican impact we find a diameter of 3.6 km. This value is in reasonable agreement with estimates of 3.6 to 9 km (~PIK, 1963) for cometary nuclei, as well as with more recent telescopic determinations (ROEMER,1968). All these srguments do not prove that the Copernican object was a cometary nucleus. There remains the trivial possibility that the KREEP glass at the Apollo 12 site picked up less than its share of projectile material. However, this stands in contrast with the consistent prominence of the ancient (Imbrian or Pre-Imbrian) meteoritic components in KREEP and anorthosite. A conclusive answer to this question will require further advances in cratering theory and more extensive sampling of ejecta from a single crster. AcknowIedg~ents-We are indebted to N. J. HWBARD, NASA Manned Spacecraft Center, for KBEEP sample 12033, 105. Valuable assistance was given by RUDY BANOVICRand F’~ANK QUINN. This work was supported in part by NASA Grant 14-001-167. REFERENCES ADAMSJ. B. and MCCORDT. B. (1971) Alteration of lunar optical properties: age and composition effects. Science171,567-571. ANIIERSE. (1971) Meteorites and the early solar system. Ann. Rev. A&m. Astrophys. 9, l-34. * ‘Live’ comets also contain large amounts of ices, such as HsO, NHs, etc. However, the ices last for only 102-lo3 revolutions, and since the dynamical lifetime of the comet against planetary impact is ~10’ revolutions, the probability that a comet still contained ices at the time of impact is only ~10~~. Thus the assumption of an ice-free, Cl chondrite composition seems justified.

Lunar crater Copernicus: search for debris of impacting body at Apollo 12 site

153

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R. CANAPATHY,

J. C. LAUL end EDWARD ANDERS

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