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Extralunar materials in Apollo 16 soils and the decay rate of the extralunar flux 4.0 Gy ago
EARTH AND PLANETARY SCIENCE LETTERS 17 (1972) 79-83. NORTH-HOLLAND PUBLISHING COMPANY
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EXTRALUNAR
MATERIALS
IN APOLLO
OF THE EXTRALUNAR
16 S O I L S A N D T H E D E C A Y R A T E FLUX 4.0 GY AGO
P.A. BAEDECKER. C.-L. CHOU, L.L. SUNDBERG and J.T. WASSON Department of Chemistry and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024, USA
Received 13 October 1972
The concentration of extralunar materials in the Apollo 16 regolith is about 3.5%, about 50% higher than that observed at the Apollo 14 site, and three times higher than values at mare landing sites. The integrated flux of extralunar materials is 2.5 times higher at the Apollo 16 than at the Apollo 14 site. These data support the hypothesis that the flux of extralunar materials decreased rapidly between 4.0 and 3.7 Gy ago, and demonstrate the presence of appreciable unaccreted materials near 1 AU 600 my after the formation of the solar system. Assumed ages of 3.91 and 4.00 Gy for the Apollo 14 and 16 sites yield a half-life of tbAsshort-lived population of about 45 _+ 15 my. The lunar accretion rate 4.00 Gy ago was about 2.4 × 10 - 6 g cm -2 y r - I , about 700 times greater than the current accretion rate of 3.5 Z 10 -9 g c m - 2 yr -1 .
That siderophilic-element concentrations are considerably higher in samples o f lunar soils than in lunar crystalline rocks is attributed to the accretion o f material of chondritic composition to the Moon [ 1]. Studies of these elements from the Apollo 1 1 - 1 5 and Luna 16 missions showed that the older the regolith, the greater the concentration of extralunar material [2]. The lowest concentrations were observed at the 3.26-Gy Apollo 12 site; the highest at the 3.91-Gy Apollo 14 site. Age estimates for the Apollo 16 site based on crater density indicate an age similar to that at the Apollo 14 site [3]. Preliminary 3 9 A r - 4 0 A r ages of a number o f small ( 2 - 4 mm) rock fragments from Apollo 16 soil samples indicate an age of about 4.00 Gy (L. Husain and O.A. Schaeffer, private communication); we will assume that this is the best age for the Apollo 16 site. Papanastassiou and Wasserburg [4] report a R b - S r crystallization age of 3.48 Gy for rock 68415, and have an unpublished recrystallization age o f 3.93 G y for rock 65015. That the regolith at the Descartes site is significantly older than that at the Fra Mauro site is consistent with our studies, which show higher siderophflic-element concentrations at Descartes.
It seems reasonable to define a regolith age of mare landing sites as the priod elapsed since the last major igneous activity. Similarly, it seems reasonable to take the time of the Imbrium impact as the age o f the Apollo 14 site. For the Apollo 16 site, present knowledge does not allow the assignment of a major event, the dating o f which would determine the age. The best approach at the moment is a statistical one. The data o f Husain and Schaeffer cited above show a tendency for the maximum ages of soil particles to cluster near 4.0 Gy. This implies that a major fraction of the rocks comminuted to make the regolith formed at about that time, and seems to justify the assumption that the onset of regolith production was reasonably sharp. The two R b - S r ages (which are precise to -+ 0.01 Gy) do not support either a sudden onset, or an Apollo 1 6 - A p o l l o 14 age difference as great as 90 my. It will be of interest to see what picture emerges as additional precise ages are determined. Undoubtedly some conclusions of this paper will have to be revised. Oberbeck and Quaide [5] observed that regolith depth as estimated from crater morphology increases with increasing integrated flux of crater-forming extralunar material. Baedecker et al. [6] showed that the
80
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
data of Oberbeck and Quaide [5] indicated that regolith depth was proportional to the integrated craterproducing flux to the 0 . 5 8 5 power. By assuming that the integrated flux of crater-forming materials was proportional to the integrated flux of extralunar material indicated by siderophilic-element concentrations, Baedecker et al. [6] were able to derive the relationship: log x2 = 2.41 log [x] 2 [X]l Xl
(1)
where [x] is net siderophilic-element concentration, X is integrated flux, and the subscripts designate different lunar locations. The assumption of proportionality is supported by an intercomparison of crater density data with our trace element data [2]. Our Apollo 16 data for eight trace elements are given in table 1. Duplicate determinations will be run on samples 65500 at a later date. All elements were determined by radiochemical neutron activation analysis. Details of the procedures are given in Baedecker et al. [2] and earlier publications cited therein. The precision of the procedure estimated by replicate studies of USGS standard rocks follows: Ni, + 3%; Zn, -+ 6%, Ga, + 4%, Ge, • 6%, Cd, + 9%; In, + 5%; Ir, + 25%; (limits are 70% confidence intervals on individual determinations). The precision of our Au procedure has not been adequately tested. At present our replicates generally agree only to within -+ 20% of the mean. Replicates on Ir in lunar soils agree better than expected on the basis of standard rock studies; a more realistic error limit for these samples is -+ 10%. We have no data on the concentration of these elements in the local crystalline rocks at the Apollo 16 site. However, at other sites the concentrations in local rocks are substantially lower than soil concentrations for all elements except Ga. (At the Apollo 14 site some crystalline rocks have rather high siderophilie-element contents. These seem to be remelted regolith samples - see Baedecker et al. [2] ). Thus, Ga in the soil appears to be dominantly of lunar origin. The source of the Zn, Cd and In in the soil is not well known. At most sites, it appears that no more than half can be of extralunar origin, and that lunar materials exist which are Zn-, Cd- and possibly In-rich. In soil samples from the Apollo 1 1 - 1 4 missions the dominant In source was terrestrial contamination. The dominant source of Ni, Ge, Ir and Au in the soil ap-
pears to be extralunar materials. There is no evidence for a lunar source which can account for as much as 10% of the soil concentrations of these elements, with the exception of Ni at the Apollo 12 site, where the high-Mg rocks have Ni concentrations about 30% of those in the mature soil 12070. The siderophilic-element concentrations in the four soil samples are quite similar - more similar than at lunar sites other than Apollo 11. The extreme concentrations are found in the two soils from a 10-m crater on the lowest bench of Stone Mountain. The high concentrations found in 66081 are consistent with its location on the southwest wall of the crater, shadowed from South Ray Crater ejecta. The lower concentrations in 66041 (by a factor of about 0.75) probably indicate a dilution of the mature soil by fresh ejecta from South Ray Crater. The bulk composition of the Apollo 16 soils indicates that anorthositic materials contribute about 80% of the mass, with the remainder mainly a KREEP-type basalt [7]. The PET report [7] also notes that opaques are unusually low relative to other landing sites. On the other hand, some KREEP basalts contain a few hundred ppm of Ni [7, 8]. We assume that the indigenous concentrations of these elements in the Apollo 16 soils are about the same as those observed at the mare landing sites, i.e., 30 ppm Ni, 20 ppb Ge, 0.060 ppb Ir and 0.040 ppb Au. These values may have to be altered slightly after accurate data on crystalline rocks is obtained. It seems unlikely that any value other than Ni will undergo significant change, however, or that a change in Ni will appreciably affect the relative siderophilic-element pattern. We have averaged the siderophilic-element data for the four soils and subtracted the indigeneous values in order to obtain net concentrations. In our early work we followed the practice of Ganapathy et al. [ 1], and assumed that the extralunar material had concentrations of these elements similar to those in CI chondrites. In Baedecker et al. [2], however, we pointed out that the ratios of some elements differed from the literature CI ratios by as much as a factor of 1.3, and defined the following extralunar material concentrations: Ni, 1.49%; Ge, 29.5 ppm; It, 634 ppb; and Au, 208; the Ir value is set equal to the reported concentration in anhydrous CI-chondrite material. We use the Baedecker et al. [2] values in this paper, and obtain the magnitude of the extralunar component in the
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
81
Table 1 Replicate and mean concentrations of eight trace elements in soils from Apollo 16.
Sample
Ni (ppm) Zn (ppm) Ga (ppm) Ge (ppm) Cd (ppb) In (ppb) Ir (ppb) Au (ppb) mean repl mean repl. mean repl. mean repl. mean repl. mean repl. mean repl. mean repl.
60501,32 65500,4 66041,24 66081,21
540,450 491 455,476 705,623
495 491 470 660
24,21 26 25,23 23,22
23 26 24 23
5.1 5.6 4.8, 5.1 5.4, 5.1
5.1 5.6 5.0 5.2
1.32, 1.17 1.25 1.07, 1.00 1.65, 1.16
1.25 106,99 1.25 100 1.04 89,77 1 . 4 1 78,78
102 100 83 78
13,12 16 17 15
13 16 17 15
19,14 14 15 24,19
17 14 15 22
8.5, 7.5 8.1 11.6 *, 5.9 11.9, 9.3
Results of less than usual accuracy, given one-half weight in determination of mean. Apollo 16 softs by dividing these into the net soft concentrations. The resulting concentrations are plotted in fig. 1. The error bar attached to each element is a 70% confidence limit on the mean calculated from the individual data. A mean weighted by the inverse square of these errors is shown by the horizontal line. The resulting value, 3.47%, is about 50% higher than that observed at the Apollo 14 site at Fra Mauro, and about three times higher than values for the mare landing sites. The shaded area around the horizontal line shows 70% confidence limits on the mean extralunar concentration calculated by treating the mean concentration for each element as a single value. Integrated fluxes calculated by eq. (1) from mean soil concentrations at the first five Apollo landing sites are plotted against regolith age in fig. 2. All values are normalized to an Apollo 12 integrated flux of unity. Going backwards in time a slow increase is observed through Apollo 11 followed by a steep rise to the Apollo 14 and 16 points. This change o f slope is consistent with the viewpoint expressed by various workers [2, 9, 10] that two distinct populations o f extralunar materials have fallen on the Moon during the past 4.0 Gy: a short-lived and a long-lived population (there is no way at this time to rule out the possibility that more than two populations may be involved). Baedecker et al. [2] used integrated flux data for the Apollo 1 1 - 1 4 softs to estimate the half-lives o f these populations. Since the contribution o f the shortlived population was clearly discernible only in the Apollo 14 soil, they were forced to assume a half-life for the long-lived population in order to calculate half-lives for the short-lived population. Their rather wide limits of < 3 0 - 5 0 my reflect different choices
3
6
E
5
'
°:I
4 ..........
..........
'
.................................
.................
....................... ' .................................................... '
..................................................
o
•
~
1.51 I
4 Apollo- 16 soils
I
I
i
l
I
Ni Q Zr Au Fig. 1. Magnitude of extralunar component in Apollo 16 soils estimated by dividing net concentrations of four siderophilic elements by their concentrations in a material similar to CI chondrites. The stippled area is a 70% confidence interval.
2O o lop
~ \~
~T, z2-64 Myr
m
o
~--~--__~
40
\
.
T'6L
38 3.6 3.4 Reg01ilh 0ge (Gyr)
,5
3.2
Fig. 2. Integrated flux of extralunar material at six lunar landing sites plotted against regolith age. Two populations (one providing a nearly constant flux, the other a flux with a half-life of about 45 ± 15 my) are necessary to account for the change in integrated flux with time.
8.0 8.1 7.9 10.6
82
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
of the longer half-life. Providing the relative ages of the Apollo 14 and 16 regoliths are known, we can make improved estimates of this half-life. In table 2 we list half-life estimates for different assumed regolith ages and half-lives of the long-lived population. Inspection of this table shows that the latter quantity has essentially no effect on the half-life of the shortlived population of extralunar materials; the difference in age of the regolith contributes the major uncertainty. Before proceeding, we should discuss three other potential sources of error in the half-life estimation: (1) the error limits on the assigned concentrations of extralunar materials; (2) error limits on the constant in eq. (1); and (3) the possibility that some portion of the extralunar material at the two sites is from an older regolith. A fourth possibility, that the Imbrium projectile contributed a substantial amount of siderophiles to the Apollo 14 site, seems unlikely for reasons outlined by Baedecker et al. [2]. The uncertainties associated with errors in concentrations are shown as error bars in fig. 2. The systematic errors resulting from a 5% increase and decrease (see [6] ) in the constant in eq. (1) are shown by the square and triangular symbols for Apollo 14 and 16. The errors from this source at the other landing sites are negligible. The extreme half-lives allowed by these two types of errors are 34 and 64 my, and are shown on fig. 2. The possibility of contamination by older regoliths is a serious problem. For the Apollo 16 site we have no data bearing on the question. At the Apollo 14 site we have obtained data on the Cone-Crater soil 14141 [ 11]. After allowing for contamination of 14141 by the local, mature soil, we estimate that the upper limit of the siderophilic-element contents of subregolith materials is about 25% of those observed in the mature soils 14163 and 14259, and that it is reasonably likely that their contents are negligible in subregolith materials. It seems that a similar situation prevails at the Apollo 16 site, and we assume that the amount of siderophilic elements contributed by older regolith materials is negligible here as well. This possibility will have to be checked as additional data become available. Despite these different sources of uncertainty, we believe that the existence of a short-lived population of extralunar materials is now established. The half-
life of this flux of material is 45 + l 5 my if the difference in age of the Apollo 14 and 16 sites is 90 my, as currently available data indicate. It is not possible to assign an uncertainty to the age difference at this time. This half-life may be compared with Shoemaker's [12] limits of 4 5 - 1 1 0 my, and Baldwin's [13] 2 5 0 - m y estimate. The Earth was subjected to the same flux as the Moon, and it is important that models for the accretionary formation of planets account for such an intense bombardment 600 my after the formation of the solar system. It seems likely, in fact, that the half-life was determined by the rate at which the Earth was sweeping up material from the region near 1 AU. Earlier, when the Earth was much smaller, the rate at which debris was depleted from the local portion of the solar system was also much smaller; i.e., the effective halflife was much longer. By the time the Earth reached a substantial fraction of its present size, the half-life had reached the value recorded in the Apollo 14 and Apollo 16 regoliths. The 45-my half-life which we estimate is about 10 times longer than that estimated [14, 15] for material in Earth-crossing orbits. If the half-life range we have estimated is correct, it probably corresponds to the rate at which material is perturbed i n t o Earthcrossing orbits from the regions between Venus and Earth, or between Earth and Mars. If the dominant extralunar component at the Apollo 16 site is the same material which veneered the Earth late in its accretionary history, it is of great interest to examine the composition of this matter. We estimate "apparent" extralunar concentrations of Zn, Cd and In to be 4,5, 7.0 and 14.0% CI equivalent- all higher than the values plotted in fig. 1 for the siderophilic elements. Although this might suggest that the short-lived population was enriched in volatiles relative to CI chondrites, the interpretation is ambiguous because there is some evidence for the presence of lunar materials which are enriched in these elements. The strongest conclusion we can make is in regard to the ratio of one siderophilic element to another: these ratios are the same in the short-lived population, the long-lived population and the CI chondrites to within a factor of about 2.0 (or 1.5 in ratios other than Ge/Ir). The abundances of the siderophiles in the short-lived population were probably
83
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
Table 2 Comparison of haft-life estimates for the long-lived and short-lived populations of extralunar matter. The age of the Apollo 14 regolith is taken to be 3.91 Gy. Regolith age, Apollo 16 3.97 Gy
4.00 Gy
4.03 Gy
Half-life, long-lived
Haft-•ed, short-lived
Half-life, long-lived
Half-life, short-lived
Half-life, long-lived
Half-life short-lived
(my) .o 2000
(my) 30.3 30.1 29.6
(my) ~ 2000
(my) 45.1 45,1 44.5
(my) oo 2000
(my) 60.7 60.2
1000
59.4
1000
1000
at least half as large as chondritic values; if they were as m u c h as a factor o f three lower, least-squares fitting o f bulk chemical soil data should be able to resolve the presence o f an extralunar end member. If we assume a CI c o m p o s i t i o n , a regolith depth o f 4.0 m at the Apollo 12 site, a mean regolith density of 2.0 g c m - 3, a constant flux for the long-lived population and a 45-my half-life for the short-lived popul a t i o n , our data yield fluxes o f 2.4 X 1 0 - 6 and 3.5 × 10 - 9 g cm - 2 yr - 1 for the short-lived and longlived populations 4.0 Gy ago.
Acknowledgements We are grateful to R. Bild, R. Glimp, E. Grudewicz, J. K a u f m a n and K. R o b i n s o n for assistance. N e u t r o n irradiations were c o m p e t e n t l y handled by A. Voigt o f the Ames L a b o r a t o r y , D. Rusling o f the N o r t h r o p Space Center, J. Brower o f U C L A , and their associates. This research was s u p p o r t e d in part by N A S A grant N G R 0 5 - 0 0 7 - 2 9 1 and N S F grant G A - 3 2 0 8 4 .
References
[1] R. Ganapathy, R.R. Keays, J.C. Laul and E. Anders, Trace elements in Apollo 11 lunar rocks: Implications for meteorite influx and origin of moon, Geochim. Cosmochim. Acta, Suppl. 1, Proc. Apollo 11 Lunar Sci. Conf. (1970) 1117. [2] P.A. Baedecker, C.-L. Chou and J.T. Wasson, The extralunar component in lunar soils and breccias, Geochim. Cosmochim. Acta, Suppl. 3, Proc. Third Lunar Sci. Conf. (1972) in press.
[3] D.J. Milton and C.A. Hodges, Geologic maps of the Descartes region of the moon, U.S. Geol. Surv. Map 1-748 (1972). [4] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr age of a crystalline rock from Apollo 16, Earth Ptanet. Sci. Letters 16 (1972) in press. [5] V.R. Oberbeck and W.L. Quaide, Genetic implications of lunar regolith thickness variations, Icarus 9 (1968) 446. [6] P.A. Baedecker R. Schaudy, J.L. Elzie, J. Kimberlin and J.T. Wasson, Trace element studies of rocks and soils from Oceanus Procellarum and Mare TranquiUitatis, Geochim. Cosmochim. Acta, Suppl. 2, Proc. Second Lunar Sci. Conf. (1971) 1037. [7] Apollo 16 Preliminary Examination Team, The Apollo 16 lunar samples: A petrographic and chemical description of samples from the lunar highlands, Science (1972) in press. [8] A.J. Gancarz, L.A. Albee and A.A. Chodos, Comparative petrology of Apollo 16 sample 68415 and Apollo 14 samples 14276 and 14310, Earth Planet. Sci. Letters 16 (1972) [9] W.K. Hartmann, Lunar cratering chronology, Icarus 13 (1970) 299. [10] L.A. Soderblom and L.A. Lebofsky, Technique for rapid determination of relative ages of lunar areas from orbital photography, J. Geophys. Res. 77 (1972) 279. [11] J.T. Wasson, C.-L. Chou, R.W. Bild and P.A. Baedecker, Extralunar materials in Cone-Crater soil 14141, Geochim. Cosmochim. Acta (1973) submitted. [12] E.M. Shoemaker, Cratering history and early evolution of the moon, in: C. Watkins, ed., Lunar Science-Ill (1972) 696. [13] R.B. Baldwin, On the history of lunar impact cratering: The absolute time scale and the origin of planetesimals, Icarus 14 (1971) 36. [14] E.J. Opik, Collision probabilities with the planets and distribution of interplanetary matter, Proc. Irish Acad. 53A (1951) 165. [15] G.W. WetheriU and J.G. Williams, Evaluation of the Apollo asteroids as sources of stone meteorites J. Geophys. Res. 73 (1968) 635.
[]
EXTRALUNAR
MATERIALS
IN APOLLO
OF THE EXTRALUNAR
16 S O I L S A N D T H E D E C A Y R A T E FLUX 4.0 GY AGO
P.A. BAEDECKER. C.-L. CHOU, L.L. SUNDBERG and J.T. WASSON Department of Chemistry and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024, USA
Received 13 October 1972
The concentration of extralunar materials in the Apollo 16 regolith is about 3.5%, about 50% higher than that observed at the Apollo 14 site, and three times higher than values at mare landing sites. The integrated flux of extralunar materials is 2.5 times higher at the Apollo 16 than at the Apollo 14 site. These data support the hypothesis that the flux of extralunar materials decreased rapidly between 4.0 and 3.7 Gy ago, and demonstrate the presence of appreciable unaccreted materials near 1 AU 600 my after the formation of the solar system. Assumed ages of 3.91 and 4.00 Gy for the Apollo 14 and 16 sites yield a half-life of tbAsshort-lived population of about 45 _+ 15 my. The lunar accretion rate 4.00 Gy ago was about 2.4 × 10 - 6 g cm -2 y r - I , about 700 times greater than the current accretion rate of 3.5 Z 10 -9 g c m - 2 yr -1 .
That siderophilic-element concentrations are considerably higher in samples o f lunar soils than in lunar crystalline rocks is attributed to the accretion o f material of chondritic composition to the Moon [ 1]. Studies of these elements from the Apollo 1 1 - 1 5 and Luna 16 missions showed that the older the regolith, the greater the concentration of extralunar material [2]. The lowest concentrations were observed at the 3.26-Gy Apollo 12 site; the highest at the 3.91-Gy Apollo 14 site. Age estimates for the Apollo 16 site based on crater density indicate an age similar to that at the Apollo 14 site [3]. Preliminary 3 9 A r - 4 0 A r ages of a number o f small ( 2 - 4 mm) rock fragments from Apollo 16 soil samples indicate an age of about 4.00 Gy (L. Husain and O.A. Schaeffer, private communication); we will assume that this is the best age for the Apollo 16 site. Papanastassiou and Wasserburg [4] report a R b - S r crystallization age of 3.48 Gy for rock 68415, and have an unpublished recrystallization age o f 3.93 G y for rock 65015. That the regolith at the Descartes site is significantly older than that at the Fra Mauro site is consistent with our studies, which show higher siderophflic-element concentrations at Descartes.
It seems reasonable to define a regolith age of mare landing sites as the priod elapsed since the last major igneous activity. Similarly, it seems reasonable to take the time of the Imbrium impact as the age o f the Apollo 14 site. For the Apollo 16 site, present knowledge does not allow the assignment of a major event, the dating o f which would determine the age. The best approach at the moment is a statistical one. The data o f Husain and Schaeffer cited above show a tendency for the maximum ages of soil particles to cluster near 4.0 Gy. This implies that a major fraction of the rocks comminuted to make the regolith formed at about that time, and seems to justify the assumption that the onset of regolith production was reasonably sharp. The two R b - S r ages (which are precise to -+ 0.01 Gy) do not support either a sudden onset, or an Apollo 1 6 - A p o l l o 14 age difference as great as 90 my. It will be of interest to see what picture emerges as additional precise ages are determined. Undoubtedly some conclusions of this paper will have to be revised. Oberbeck and Quaide [5] observed that regolith depth as estimated from crater morphology increases with increasing integrated flux of crater-forming extralunar material. Baedecker et al. [6] showed that the
80
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
data of Oberbeck and Quaide [5] indicated that regolith depth was proportional to the integrated craterproducing flux to the 0 . 5 8 5 power. By assuming that the integrated flux of crater-forming materials was proportional to the integrated flux of extralunar material indicated by siderophilic-element concentrations, Baedecker et al. [6] were able to derive the relationship: log x2 = 2.41 log [x] 2 [X]l Xl
(1)
where [x] is net siderophilic-element concentration, X is integrated flux, and the subscripts designate different lunar locations. The assumption of proportionality is supported by an intercomparison of crater density data with our trace element data [2]. Our Apollo 16 data for eight trace elements are given in table 1. Duplicate determinations will be run on samples 65500 at a later date. All elements were determined by radiochemical neutron activation analysis. Details of the procedures are given in Baedecker et al. [2] and earlier publications cited therein. The precision of the procedure estimated by replicate studies of USGS standard rocks follows: Ni, + 3%; Zn, -+ 6%, Ga, + 4%, Ge, • 6%, Cd, + 9%; In, + 5%; Ir, + 25%; (limits are 70% confidence intervals on individual determinations). The precision of our Au procedure has not been adequately tested. At present our replicates generally agree only to within -+ 20% of the mean. Replicates on Ir in lunar soils agree better than expected on the basis of standard rock studies; a more realistic error limit for these samples is -+ 10%. We have no data on the concentration of these elements in the local crystalline rocks at the Apollo 16 site. However, at other sites the concentrations in local rocks are substantially lower than soil concentrations for all elements except Ga. (At the Apollo 14 site some crystalline rocks have rather high siderophilie-element contents. These seem to be remelted regolith samples - see Baedecker et al. [2] ). Thus, Ga in the soil appears to be dominantly of lunar origin. The source of the Zn, Cd and In in the soil is not well known. At most sites, it appears that no more than half can be of extralunar origin, and that lunar materials exist which are Zn-, Cd- and possibly In-rich. In soil samples from the Apollo 1 1 - 1 4 missions the dominant In source was terrestrial contamination. The dominant source of Ni, Ge, Ir and Au in the soil ap-
pears to be extralunar materials. There is no evidence for a lunar source which can account for as much as 10% of the soil concentrations of these elements, with the exception of Ni at the Apollo 12 site, where the high-Mg rocks have Ni concentrations about 30% of those in the mature soil 12070. The siderophilic-element concentrations in the four soil samples are quite similar - more similar than at lunar sites other than Apollo 11. The extreme concentrations are found in the two soils from a 10-m crater on the lowest bench of Stone Mountain. The high concentrations found in 66081 are consistent with its location on the southwest wall of the crater, shadowed from South Ray Crater ejecta. The lower concentrations in 66041 (by a factor of about 0.75) probably indicate a dilution of the mature soil by fresh ejecta from South Ray Crater. The bulk composition of the Apollo 16 soils indicates that anorthositic materials contribute about 80% of the mass, with the remainder mainly a KREEP-type basalt [7]. The PET report [7] also notes that opaques are unusually low relative to other landing sites. On the other hand, some KREEP basalts contain a few hundred ppm of Ni [7, 8]. We assume that the indigenous concentrations of these elements in the Apollo 16 soils are about the same as those observed at the mare landing sites, i.e., 30 ppm Ni, 20 ppb Ge, 0.060 ppb Ir and 0.040 ppb Au. These values may have to be altered slightly after accurate data on crystalline rocks is obtained. It seems unlikely that any value other than Ni will undergo significant change, however, or that a change in Ni will appreciably affect the relative siderophilic-element pattern. We have averaged the siderophilic-element data for the four soils and subtracted the indigeneous values in order to obtain net concentrations. In our early work we followed the practice of Ganapathy et al. [ 1], and assumed that the extralunar material had concentrations of these elements similar to those in CI chondrites. In Baedecker et al. [2], however, we pointed out that the ratios of some elements differed from the literature CI ratios by as much as a factor of 1.3, and defined the following extralunar material concentrations: Ni, 1.49%; Ge, 29.5 ppm; It, 634 ppb; and Au, 208; the Ir value is set equal to the reported concentration in anhydrous CI-chondrite material. We use the Baedecker et al. [2] values in this paper, and obtain the magnitude of the extralunar component in the
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
81
Table 1 Replicate and mean concentrations of eight trace elements in soils from Apollo 16.
Sample
Ni (ppm) Zn (ppm) Ga (ppm) Ge (ppm) Cd (ppb) In (ppb) Ir (ppb) Au (ppb) mean repl mean repl. mean repl. mean repl. mean repl. mean repl. mean repl. mean repl.
60501,32 65500,4 66041,24 66081,21
540,450 491 455,476 705,623
495 491 470 660
24,21 26 25,23 23,22
23 26 24 23
5.1 5.6 4.8, 5.1 5.4, 5.1
5.1 5.6 5.0 5.2
1.32, 1.17 1.25 1.07, 1.00 1.65, 1.16
1.25 106,99 1.25 100 1.04 89,77 1 . 4 1 78,78
102 100 83 78
13,12 16 17 15
13 16 17 15
19,14 14 15 24,19
17 14 15 22
8.5, 7.5 8.1 11.6 *, 5.9 11.9, 9.3
Results of less than usual accuracy, given one-half weight in determination of mean. Apollo 16 softs by dividing these into the net soft concentrations. The resulting concentrations are plotted in fig. 1. The error bar attached to each element is a 70% confidence limit on the mean calculated from the individual data. A mean weighted by the inverse square of these errors is shown by the horizontal line. The resulting value, 3.47%, is about 50% higher than that observed at the Apollo 14 site at Fra Mauro, and about three times higher than values for the mare landing sites. The shaded area around the horizontal line shows 70% confidence limits on the mean extralunar concentration calculated by treating the mean concentration for each element as a single value. Integrated fluxes calculated by eq. (1) from mean soil concentrations at the first five Apollo landing sites are plotted against regolith age in fig. 2. All values are normalized to an Apollo 12 integrated flux of unity. Going backwards in time a slow increase is observed through Apollo 11 followed by a steep rise to the Apollo 14 and 16 points. This change o f slope is consistent with the viewpoint expressed by various workers [2, 9, 10] that two distinct populations o f extralunar materials have fallen on the Moon during the past 4.0 Gy: a short-lived and a long-lived population (there is no way at this time to rule out the possibility that more than two populations may be involved). Baedecker et al. [2] used integrated flux data for the Apollo 1 1 - 1 4 softs to estimate the half-lives o f these populations. Since the contribution o f the shortlived population was clearly discernible only in the Apollo 14 soil, they were forced to assume a half-life for the long-lived population in order to calculate half-lives for the short-lived population. Their rather wide limits of < 3 0 - 5 0 my reflect different choices
3
6
E
5
'
°:I
4 ..........
..........
'
.................................
.................
....................... ' .................................................... '
..................................................
o
•
~
1.51 I
4 Apollo- 16 soils
I
I
i
l
I
Ni Q Zr Au Fig. 1. Magnitude of extralunar component in Apollo 16 soils estimated by dividing net concentrations of four siderophilic elements by their concentrations in a material similar to CI chondrites. The stippled area is a 70% confidence interval.
2O o lop
~ \~
~T, z2-64 Myr
m
o
~--~--__~
40
\
.
T'6L
38 3.6 3.4 Reg01ilh 0ge (Gyr)
,5
3.2
Fig. 2. Integrated flux of extralunar material at six lunar landing sites plotted against regolith age. Two populations (one providing a nearly constant flux, the other a flux with a half-life of about 45 ± 15 my) are necessary to account for the change in integrated flux with time.
8.0 8.1 7.9 10.6
82
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
of the longer half-life. Providing the relative ages of the Apollo 14 and 16 regoliths are known, we can make improved estimates of this half-life. In table 2 we list half-life estimates for different assumed regolith ages and half-lives of the long-lived population. Inspection of this table shows that the latter quantity has essentially no effect on the half-life of the shortlived population of extralunar materials; the difference in age of the regolith contributes the major uncertainty. Before proceeding, we should discuss three other potential sources of error in the half-life estimation: (1) the error limits on the assigned concentrations of extralunar materials; (2) error limits on the constant in eq. (1); and (3) the possibility that some portion of the extralunar material at the two sites is from an older regolith. A fourth possibility, that the Imbrium projectile contributed a substantial amount of siderophiles to the Apollo 14 site, seems unlikely for reasons outlined by Baedecker et al. [2]. The uncertainties associated with errors in concentrations are shown as error bars in fig. 2. The systematic errors resulting from a 5% increase and decrease (see [6] ) in the constant in eq. (1) are shown by the square and triangular symbols for Apollo 14 and 16. The errors from this source at the other landing sites are negligible. The extreme half-lives allowed by these two types of errors are 34 and 64 my, and are shown on fig. 2. The possibility of contamination by older regoliths is a serious problem. For the Apollo 16 site we have no data bearing on the question. At the Apollo 14 site we have obtained data on the Cone-Crater soil 14141 [ 11]. After allowing for contamination of 14141 by the local, mature soil, we estimate that the upper limit of the siderophilic-element contents of subregolith materials is about 25% of those observed in the mature soils 14163 and 14259, and that it is reasonably likely that their contents are negligible in subregolith materials. It seems that a similar situation prevails at the Apollo 16 site, and we assume that the amount of siderophilic elements contributed by older regolith materials is negligible here as well. This possibility will have to be checked as additional data become available. Despite these different sources of uncertainty, we believe that the existence of a short-lived population of extralunar materials is now established. The half-
life of this flux of material is 45 + l 5 my if the difference in age of the Apollo 14 and 16 sites is 90 my, as currently available data indicate. It is not possible to assign an uncertainty to the age difference at this time. This half-life may be compared with Shoemaker's [12] limits of 4 5 - 1 1 0 my, and Baldwin's [13] 2 5 0 - m y estimate. The Earth was subjected to the same flux as the Moon, and it is important that models for the accretionary formation of planets account for such an intense bombardment 600 my after the formation of the solar system. It seems likely, in fact, that the half-life was determined by the rate at which the Earth was sweeping up material from the region near 1 AU. Earlier, when the Earth was much smaller, the rate at which debris was depleted from the local portion of the solar system was also much smaller; i.e., the effective halflife was much longer. By the time the Earth reached a substantial fraction of its present size, the half-life had reached the value recorded in the Apollo 14 and Apollo 16 regoliths. The 45-my half-life which we estimate is about 10 times longer than that estimated [14, 15] for material in Earth-crossing orbits. If the half-life range we have estimated is correct, it probably corresponds to the rate at which material is perturbed i n t o Earthcrossing orbits from the regions between Venus and Earth, or between Earth and Mars. If the dominant extralunar component at the Apollo 16 site is the same material which veneered the Earth late in its accretionary history, it is of great interest to examine the composition of this matter. We estimate "apparent" extralunar concentrations of Zn, Cd and In to be 4,5, 7.0 and 14.0% CI equivalent- all higher than the values plotted in fig. 1 for the siderophilic elements. Although this might suggest that the short-lived population was enriched in volatiles relative to CI chondrites, the interpretation is ambiguous because there is some evidence for the presence of lunar materials which are enriched in these elements. The strongest conclusion we can make is in regard to the ratio of one siderophilic element to another: these ratios are the same in the short-lived population, the long-lived population and the CI chondrites to within a factor of about 2.0 (or 1.5 in ratios other than Ge/Ir). The abundances of the siderophiles in the short-lived population were probably
83
P.A. Baedecker et al., Extralunar materials in Apollo 16 soils
Table 2 Comparison of haft-life estimates for the long-lived and short-lived populations of extralunar matter. The age of the Apollo 14 regolith is taken to be 3.91 Gy. Regolith age, Apollo 16 3.97 Gy
4.00 Gy
4.03 Gy
Half-life, long-lived
Haft-•ed, short-lived
Half-life, long-lived
Half-life, short-lived
Half-life, long-lived
Half-life short-lived
(my) .o 2000
(my) 30.3 30.1 29.6
(my) ~ 2000
(my) 45.1 45,1 44.5
(my) oo 2000
(my) 60.7 60.2
1000
59.4
1000
1000
at least half as large as chondritic values; if they were as m u c h as a factor o f three lower, least-squares fitting o f bulk chemical soil data should be able to resolve the presence o f an extralunar end member. If we assume a CI c o m p o s i t i o n , a regolith depth o f 4.0 m at the Apollo 12 site, a mean regolith density of 2.0 g c m - 3, a constant flux for the long-lived population and a 45-my half-life for the short-lived popul a t i o n , our data yield fluxes o f 2.4 X 1 0 - 6 and 3.5 × 10 - 9 g cm - 2 yr - 1 for the short-lived and longlived populations 4.0 Gy ago.
Acknowledgements We are grateful to R. Bild, R. Glimp, E. Grudewicz, J. K a u f m a n and K. R o b i n s o n for assistance. N e u t r o n irradiations were c o m p e t e n t l y handled by A. Voigt o f the Ames L a b o r a t o r y , D. Rusling o f the N o r t h r o p Space Center, J. Brower o f U C L A , and their associates. This research was s u p p o r t e d in part by N A S A grant N G R 0 5 - 0 0 7 - 2 9 1 and N S F grant G A - 3 2 0 8 4 .
References
[1] R. Ganapathy, R.R. Keays, J.C. Laul and E. Anders, Trace elements in Apollo 11 lunar rocks: Implications for meteorite influx and origin of moon, Geochim. Cosmochim. Acta, Suppl. 1, Proc. Apollo 11 Lunar Sci. Conf. (1970) 1117. [2] P.A. Baedecker, C.-L. Chou and J.T. Wasson, The extralunar component in lunar soils and breccias, Geochim. Cosmochim. Acta, Suppl. 3, Proc. Third Lunar Sci. Conf. (1972) in press.
[3] D.J. Milton and C.A. Hodges, Geologic maps of the Descartes region of the moon, U.S. Geol. Surv. Map 1-748 (1972). [4] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr age of a crystalline rock from Apollo 16, Earth Ptanet. Sci. Letters 16 (1972) in press. [5] V.R. Oberbeck and W.L. Quaide, Genetic implications of lunar regolith thickness variations, Icarus 9 (1968) 446. [6] P.A. Baedecker R. Schaudy, J.L. Elzie, J. Kimberlin and J.T. Wasson, Trace element studies of rocks and soils from Oceanus Procellarum and Mare TranquiUitatis, Geochim. Cosmochim. Acta, Suppl. 2, Proc. Second Lunar Sci. Conf. (1971) 1037. [7] Apollo 16 Preliminary Examination Team, The Apollo 16 lunar samples: A petrographic and chemical description of samples from the lunar highlands, Science (1972) in press. [8] A.J. Gancarz, L.A. Albee and A.A. Chodos, Comparative petrology of Apollo 16 sample 68415 and Apollo 14 samples 14276 and 14310, Earth Planet. Sci. Letters 16 (1972) [9] W.K. Hartmann, Lunar cratering chronology, Icarus 13 (1970) 299. [10] L.A. Soderblom and L.A. Lebofsky, Technique for rapid determination of relative ages of lunar areas from orbital photography, J. Geophys. Res. 77 (1972) 279. [11] J.T. Wasson, C.-L. Chou, R.W. Bild and P.A. Baedecker, Extralunar materials in Cone-Crater soil 14141, Geochim. Cosmochim. Acta (1973) submitted. [12] E.M. Shoemaker, Cratering history and early evolution of the moon, in: C. Watkins, ed., Lunar Science-Ill (1972) 696. [13] R.B. Baldwin, On the history of lunar impact cratering: The absolute time scale and the origin of planetesimals, Icarus 14 (1971) 36. [14] E.J. Opik, Collision probabilities with the planets and distribution of interplanetary matter, Proc. Irish Acad. 53A (1951) 165. [15] G.W. WetheriU and J.G. Williams, Evaluation of the Apollo asteroids as sources of stone meteorites J. Geophys. Res. 73 (1968) 635.