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Fossil track and thermoluminescence studies of Luna 20 material
Qeoahlmlca et Chmochimica Acts, 1973, Vol. 37. pp. 825 to MO. Pergamon Press. Printed In Northern Ireland
Fossil track and ~e~ol~~n~
studiesofLuna20 material
Cl. Caoz~z, R. WALEER and D. ZIMMERMAN Laboratory for Space Physics, Washington University, St. Louis, Missouri 63130 (.Rec&ve&12 Deemher 1972; accepted in revised form 26 Jaruu;cly1973) &&a&--Track densities in 85 feldspar crystals from L-2009 range from 2.5 x 10s/omBto >lO*/cma. This track distribution represents an int5rmedi&3 case between wh& have been previously defined as lightly and heavily irradiat5d soils and sugge&3 that the Luna 20 sample consists of a m&ure of a mature, heavily irradiat5d component with another, lightly irradiat5d component. Using a two component mixing model, the age of the lightly irradiated component is -270 x lad yr. It is possible, but by no mesns certain, that this is associated with the formation of the GraterApollo&s C. At -200°C the ratio of natural TL to that induced by a standard irradiation is similar to that in Apollo 12 and 14 cams below -7 cm. This confirms that most of the Luna 20 sample represents sub-surface m&&al.
and burial history of individual lunar crystals can be partially deciphered by nuclear track studies and rne~u~rnen~ of the~olu~escen~e. The former technique gives an integrated record while the latter is most sensitive to the recent position of the sample below the surface of the lunar regolith. The study of these phenomena in the Luna 20 sample was of interest, a priori, for several reasons. Perhaps the most important was the possibility of demonstrating that the Sun was much more active a;t the beginning of the solar system. This ha,s been suggested from &stronomic&l me~~ements made on other star systems, but has never been proven for our own Sun. U~ortun&tely, as will be seen from the body of this report, the Luna 20 sample has apparently little informrttion to contribute on this point. The measurement of the recent position of the sample in the regolith would also cl&Fy the ambiguous depth situation o&used by the partial slling and presumed subsequent mixing of the Luna 20 core. THE IRRADIATION
RE~UX~ OF TRACK DENSITY MEASUREXEMENTB ON SOILS FROMPREVIOUSMIEWIONS Track density distributions vary widely in different lunar soil samples. In a mature soil track densities typically vary from 107/cms to >10Q/cm8, with the overwhelming majority of crystals having densities in excess of 108/cma. Although both galactic cosmic rays and solar flare particles contribute to the track record, the latter dominate in the high-density crystals. This conclusion is based on consideration of the absolute magnitudes of the track densities end is further demonstrated directly in small crystals by the observation of track gradients produced by the rapidly falling energy spectrum characteristic of solar flrafepszticles. The variation between cryst&ls is due to differences in the time at which they were added to the soil and also to their burial history. This latter effect is pclrticulerly important since the production rate of solar flare tracks drops several orders of magnitude in a distanoe of 1 mm from the surface. The presence of high trltck densities in crystals removed from up to 2.6 m 826
8%6
G. CROZAZ, R. WALKERand
Il. ZIMXERMAN
below the lunar surface shows that these crystals were once at or near the surface-. a fact independently demonstrated, of course, from studies of solar wind gas in t,hr: same samples. Many different track groups have studied the irradiation record of soil samples from the various lunar missions. A complete resume of this work is beyond the scope of the present paper. In some cases specific groups use very different observational techniques and have either employed different track counting criteria or made measurements on quite different size fractions of the soil. To give an internally consistent picture we present here only comparisons with our own prior work in which we use essentially identical techniques. In our recent paper for the Third Lunar Science Conference ( CROZAZet al., 1972) we made a distinction between ‘heavily irradiated’ and ‘lightly irradiated’ soils. Although most lunar samples fall in the first category as manifested by the fraction of grains with track densities > 10s/cm2 (80 to 100 per cent), some soils are clearly much less irradiated. Notable examples are the coarse-grained layer of the Apollo 12 core (12025 and 12028) and Sample 14141 from Cone Crater. In the former, we found no crystals with p > 10B/cmaand in the latter only 10 per cent. In both these cases we conoluded that most of the individual particles had not been cycled to the surface and that their stored tracks were due predominantly to galactic cosmic rays which penetrate to much greater depths than solar flare particles. We found only occasionally samples falling between these two extremes. The track record was also shown by us to correlate very well with other indications of the age of a lunar soil. Samples with high average track densities were shown to have small median grain sizes and higher concentrations of glassy aggregates, methane and A+ than samples with low track densities. TRACK STUDIESOF LUNA 20 SAMPLES Feldspar crystals of a 10 mg aliquot were mounted in epoxy resin, polished, and subsequently etched in a solution of NaOH (6 g NaOH, 8 g HsO) to reveal tracks. The track densities quoted were our total pit counts as measured on photographs taken with a scanning electron microscope. As previously described (CROZAZet al., 1971) total pit counts give track densities up to a factor of 2 higher than plastic replica counts when a length criterion of L > L max/6 is imposed. The data obtained on 86 crystals of Luna 20 are shown in Fig. 1 which also shows similar, previous data for two Luna 16 samples. Also shown for comparison in Fig. 2 are our data for different layers of the Apollo 12 core tube. Six yellowish-green crystals, subsequently shown to be pyroxenes, were also mounted and etched in an acid solution [2 parts HP (48 per cent) to 1 part of H&$0* (80 per cent) to 4 parts of water]. The track development in these crystals was imperfect, with most of the tracks appearing extremely well aligned. Track densities for these crystals were, respectively, 4 x 104/ems, 1 x lOS/cms, 4 x 106/ cm2, 5 x lOS/cm2, 1 x 10B/cm2 and 2 x 107/cm2. Although these appear to be considerably lower than the average for the feldspar grains, we cannot be sure, without further work, that the tracks were revealed with 100 per cent efficiency by our etching procedure.
Fossiltrack and thermoluminescence studiesof Luna 20 material
LUNA
16
LUNA
827
20
G-14A-14
... ... * ; .
Fig. 1. Distributionof track densitiesin Luna 20 feldspaxscompared to the distributionin Luna 16 feldqers. The &fiowsindicateorystalawith molvably high tmok deneitics. THERMOLUMINESCENCE MICASUREMENTS Thermolu~escence (TL) me~~ements were made on a O-8 mg Luna 20 bulk sample. At low glow curve temperatures around 200°C the ratio of natural TL to that induced by a standard irradiation (N/A) was OG. This is similar to Apollo 12 and 14 cores below about 7 cm (deep core vaIues range from Od5 to 1.0) and is very different from the values 0 to O-2 found in the first 4 cm. Although this result shows that the Luna 20 sample was indeed predominantly removed from depth, we cannot give a precise value for the average depth, or indeed say whether the core material was thoroughly mixed on its return from the Moon. If the degree of mixing is important for other investigations, it could probably be determined by extensive measurements of TL in single grams. Compared to typical Apollo 12 and 14 glow curves from the standard irradiations, the Luna 20 material is of average brightness and typical glow curve shape below -4OO’C, but above 400% emits substantially less light. Presumably the material has fewer of the deeper electron traps as a result of a mineralogio difference. The glow curves are in fact similar to those previously found in the ano~ho~tic ‘genesis rook’ 16415. D~~ussxo~
OF RESULTS
Only 67 per cent of the feldspar grains of the Luna 20 sample have traok densities greater than lO+m 2. Further, some 9 per cent have densities lower than 107/cmS. These track densities are lower than those we have previously found for
828
G. Caozu, It.WUKICR and D. ZIMMERMAN
APOLLO
I
.
12 CORE .
.
-
*.
!
.
*.
.. :.
.
** .*
-
* .
f2:
. * *
. , . *
.
I = 14
Fig. 2. Distribution of track densities as a function of depth in feldspsra from the Apolto 12 double core. The Roman numerals indicctter various stra&igmphic units tw ~~~~~~~~~~~~L~~S~LEP~EL~SPINBBYE~YINA~ONTEAB~ (1970).
the lunar soils that we have characterized as ‘heavily irradiated’. Conversely, the traok densities are considerably higher than those in ‘lightly irradiated’ lunar soils such as the coarse-grained layer of the Apollo 12 double core or the Cone Crater sample. The other ~~rae~r~tics reported for the Luna 20 sample are consistent with the traok density being intermediate between heavily and lightly irradiated. Although the average grain size is small, the report by VINOGRADOV (1972) mentions that many more large fragments are found in the Luna 20 sample than in the Luna 16 sample, which is classified by us as ‘heavily irradiated’. Similarly, the fraction of glassy ag~eg~tes is smaller in the Luna 20 sample than in the Luna 16 sample or other samples olassified by us as ‘heavily irradiated’. VINOGRADOV (1972)notes that a considerable fraotion of the Lnna 20 material may come from the crater Apollonius C which is described as, “comparatively young (possibly Copernioan)“. When a layer of soil has been deposited and then
Fossiltrack and thermoluminescence studiesof Luna 20 material
829
lam relatively undisturbed before being collected (14141) or covered over (1202612028), then a characteristic distribution of track densities is produced that makes it meaningful to calculate a track exposure age (CROZAZet al., 1972). The description of the Luna 20 sampling is somewhat ambiguous but can be interpreted as meaning that the deepest penetration of the core was ~20 cm. If we arbitrarily divide the Luna 20 crystals into a lightly irradiated and a heavily irradiated component, with the dividing line fixed at a track density of 2 x 10*/cma, then the distribution of track densities for the less irradiated component is consistent with what would be expected from galactic cosmic rays acting on a 20 cm thick layer for a period of ~270 x lo6 years. This could possibly be a measure of the formation age of Apollonius C. However, the reader is warned that this number is strongly model dependent and cannot be taken, by itself, as a firm measure of the age of the crater. In the two component mixing model described above -+ of the total sample falls in the lightly irradiated category. STEELE and SMITH(1973) have shown that LTJNA20 feldspara of the type studied here are remarkably homogeneous in composition. It thus appears that the two irradiation categories do not correspond to any other obvious separation into two components and, to that extent, the separation remains arbitrary. An intense early solar activity might manifest itself in having highland samples more heavily irradiated, on the average, than younger mare samples. The Luna 20 sample, having been added recently to the regolith, unfortunately provides no handle on this problem. The report by VINO~RAIIOV(1972) mentioned that our Soviet colleagues had found very low track densities in the olivine fraction of the Luna 20 sample. Due to the smallness of our sample and the paucity of olivine crystals, we have not been able to contribute any information on this point. Possibly the low track densities observed by us in pyroxene crystals are related to the Soviet observation; however, much more work would have to be done on the etching characteristics of these particular pyroxenes before this could be established. Additional material is needed in order to investigate this point. Acknowledgemmte-It ie a pleasureto acknowledge the help of S. Srrrro~ and P. SWAN in performing the measurements reported here. We also wish to thank D. YTJHASfor valuable conversations. This work would have been impossible without the cooperation of our Soviet colleagues,who in the beat tradition of the internationalcharacterof lunar scienceso generously provided us with samples. This work was supported by NASA Grant No. NGL 26-008-066.
REFERENUES BEERXANN C., Caovlz G., DROZD R., HOEENBERQC. M., RALSTONC., WALKER R. M. and YUELAS D. (1972)Rare gas and particle track studies of Apollo 16 samples: Hadley rille and special
soils (abstract). In: The ApouO 15 Samples, (editors J. W. Chamberlain and C. Watkins), pp. 329-332. Lunar Science Institute. CROZAZG., WATXER. R. and WOOI+J-M D. (1971) Nuclear track studies of dynamic surface processeson the moon and the constancy of solar activity. Proo. Second Lunar Sci. Conf., Beochim. Coemochirn.Acto Swppl. f&2543-2668. M.I.T. Preaa CROV~ZG., DROZD R., HOJXENBERUC. M., HOYT H. P., JR., RAUB D., WALKER R. M. and
830
G. CROZ~Z, I%. WALKER and D. ZIMMERMAN
YUFUS D. (1972) Solar flare and galactic cosmic ray studios of Apollo 14 and 15 samples. Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, 2917-2931. M.I.T. Press. LUNAR SABSPLEPRELIMINARYEXAMINATIONTEAM (1970) Preliminary examination of lunar samples from Apollo 12. Science 167,1325-1339. STEELE I. M. and SMITE J. V. (1973) Compositional and X-ray data for Luna 20 feldspar. Geoohim. Coemochim. Acta 37, 1075-1077. VINOORADOVA. P. (1972) Preliminary data on the lunar soil collected by the Luna 20 unmanned spacecraft. Geokhbiya 7, 763-774. English translation Geochim. Cosmoehim. Acta 3’7, 721729 (1973).
Fossil track and ~e~ol~~n~
studiesofLuna20 material
Cl. Caoz~z, R. WALEER and D. ZIMMERMAN Laboratory for Space Physics, Washington University, St. Louis, Missouri 63130 (.Rec&ve&12 Deemher 1972; accepted in revised form 26 Jaruu;cly1973) &&a&--Track densities in 85 feldspar crystals from L-2009 range from 2.5 x 10s/omBto >lO*/cma. This track distribution represents an int5rmedi&3 case between wh& have been previously defined as lightly and heavily irradiat5d soils and sugge&3 that the Luna 20 sample consists of a m&ure of a mature, heavily irradiat5d component with another, lightly irradiat5d component. Using a two component mixing model, the age of the lightly irradiated component is -270 x lad yr. It is possible, but by no mesns certain, that this is associated with the formation of the GraterApollo&s C. At -200°C the ratio of natural TL to that induced by a standard irradiation is similar to that in Apollo 12 and 14 cams below -7 cm. This confirms that most of the Luna 20 sample represents sub-surface m&&al.
and burial history of individual lunar crystals can be partially deciphered by nuclear track studies and rne~u~rnen~ of the~olu~escen~e. The former technique gives an integrated record while the latter is most sensitive to the recent position of the sample below the surface of the lunar regolith. The study of these phenomena in the Luna 20 sample was of interest, a priori, for several reasons. Perhaps the most important was the possibility of demonstrating that the Sun was much more active a;t the beginning of the solar system. This ha,s been suggested from &stronomic&l me~~ements made on other star systems, but has never been proven for our own Sun. U~ortun&tely, as will be seen from the body of this report, the Luna 20 sample has apparently little informrttion to contribute on this point. The measurement of the recent position of the sample in the regolith would also cl&Fy the ambiguous depth situation o&used by the partial slling and presumed subsequent mixing of the Luna 20 core. THE IRRADIATION
RE~UX~ OF TRACK DENSITY MEASUREXEMENTB ON SOILS FROMPREVIOUSMIEWIONS Track density distributions vary widely in different lunar soil samples. In a mature soil track densities typically vary from 107/cms to >10Q/cm8, with the overwhelming majority of crystals having densities in excess of 108/cma. Although both galactic cosmic rays and solar flare particles contribute to the track record, the latter dominate in the high-density crystals. This conclusion is based on consideration of the absolute magnitudes of the track densities end is further demonstrated directly in small crystals by the observation of track gradients produced by the rapidly falling energy spectrum characteristic of solar flrafepszticles. The variation between cryst&ls is due to differences in the time at which they were added to the soil and also to their burial history. This latter effect is pclrticulerly important since the production rate of solar flare tracks drops several orders of magnitude in a distanoe of 1 mm from the surface. The presence of high trltck densities in crystals removed from up to 2.6 m 826
8%6
G. CROZAZ, R. WALKERand
Il. ZIMXERMAN
below the lunar surface shows that these crystals were once at or near the surface-. a fact independently demonstrated, of course, from studies of solar wind gas in t,hr: same samples. Many different track groups have studied the irradiation record of soil samples from the various lunar missions. A complete resume of this work is beyond the scope of the present paper. In some cases specific groups use very different observational techniques and have either employed different track counting criteria or made measurements on quite different size fractions of the soil. To give an internally consistent picture we present here only comparisons with our own prior work in which we use essentially identical techniques. In our recent paper for the Third Lunar Science Conference ( CROZAZet al., 1972) we made a distinction between ‘heavily irradiated’ and ‘lightly irradiated’ soils. Although most lunar samples fall in the first category as manifested by the fraction of grains with track densities > 10s/cm2 (80 to 100 per cent), some soils are clearly much less irradiated. Notable examples are the coarse-grained layer of the Apollo 12 core (12025 and 12028) and Sample 14141 from Cone Crater. In the former, we found no crystals with p > 10B/cmaand in the latter only 10 per cent. In both these cases we conoluded that most of the individual particles had not been cycled to the surface and that their stored tracks were due predominantly to galactic cosmic rays which penetrate to much greater depths than solar flare particles. We found only occasionally samples falling between these two extremes. The track record was also shown by us to correlate very well with other indications of the age of a lunar soil. Samples with high average track densities were shown to have small median grain sizes and higher concentrations of glassy aggregates, methane and A+ than samples with low track densities. TRACK STUDIESOF LUNA 20 SAMPLES Feldspar crystals of a 10 mg aliquot were mounted in epoxy resin, polished, and subsequently etched in a solution of NaOH (6 g NaOH, 8 g HsO) to reveal tracks. The track densities quoted were our total pit counts as measured on photographs taken with a scanning electron microscope. As previously described (CROZAZet al., 1971) total pit counts give track densities up to a factor of 2 higher than plastic replica counts when a length criterion of L > L max/6 is imposed. The data obtained on 86 crystals of Luna 20 are shown in Fig. 1 which also shows similar, previous data for two Luna 16 samples. Also shown for comparison in Fig. 2 are our data for different layers of the Apollo 12 core tube. Six yellowish-green crystals, subsequently shown to be pyroxenes, were also mounted and etched in an acid solution [2 parts HP (48 per cent) to 1 part of H&$0* (80 per cent) to 4 parts of water]. The track development in these crystals was imperfect, with most of the tracks appearing extremely well aligned. Track densities for these crystals were, respectively, 4 x 104/ems, 1 x lOS/cms, 4 x 106/ cm2, 5 x lOS/cm2, 1 x 10B/cm2 and 2 x 107/cm2. Although these appear to be considerably lower than the average for the feldspar grains, we cannot be sure, without further work, that the tracks were revealed with 100 per cent efficiency by our etching procedure.
Fossiltrack and thermoluminescence studiesof Luna 20 material
LUNA
16
LUNA
827
20
G-14A-14
... ... * ; .
Fig. 1. Distributionof track densitiesin Luna 20 feldspaxscompared to the distributionin Luna 16 feldqers. The &fiowsindicateorystalawith molvably high tmok deneitics. THERMOLUMINESCENCE MICASUREMENTS Thermolu~escence (TL) me~~ements were made on a O-8 mg Luna 20 bulk sample. At low glow curve temperatures around 200°C the ratio of natural TL to that induced by a standard irradiation (N/A) was OG. This is similar to Apollo 12 and 14 cores below about 7 cm (deep core vaIues range from Od5 to 1.0) and is very different from the values 0 to O-2 found in the first 4 cm. Although this result shows that the Luna 20 sample was indeed predominantly removed from depth, we cannot give a precise value for the average depth, or indeed say whether the core material was thoroughly mixed on its return from the Moon. If the degree of mixing is important for other investigations, it could probably be determined by extensive measurements of TL in single grams. Compared to typical Apollo 12 and 14 glow curves from the standard irradiations, the Luna 20 material is of average brightness and typical glow curve shape below -4OO’C, but above 400% emits substantially less light. Presumably the material has fewer of the deeper electron traps as a result of a mineralogio difference. The glow curves are in fact similar to those previously found in the ano~ho~tic ‘genesis rook’ 16415. D~~ussxo~
OF RESULTS
Only 67 per cent of the feldspar grains of the Luna 20 sample have traok densities greater than lO+m 2. Further, some 9 per cent have densities lower than 107/cmS. These track densities are lower than those we have previously found for
828
G. Caozu, It.WUKICR and D. ZIMMERMAN
APOLLO
I
.
12 CORE .
.
-
*.
!
.
*.
.. :.
.
** .*
-
* .
f2:
. * *
. , . *
.
I = 14
Fig. 2. Distribution of track densities as a function of depth in feldspsra from the Apolto 12 double core. The Roman numerals indicctter various stra&igmphic units tw ~~~~~~~~~~~~L~~S~LEP~EL~SPINBBYE~YINA~ONTEAB~ (1970).
the lunar soils that we have characterized as ‘heavily irradiated’. Conversely, the traok densities are considerably higher than those in ‘lightly irradiated’ lunar soils such as the coarse-grained layer of the Apollo 12 double core or the Cone Crater sample. The other ~~rae~r~tics reported for the Luna 20 sample are consistent with the traok density being intermediate between heavily and lightly irradiated. Although the average grain size is small, the report by VINOGRADOV (1972) mentions that many more large fragments are found in the Luna 20 sample than in the Luna 16 sample, which is classified by us as ‘heavily irradiated’. Similarly, the fraction of glassy ag~eg~tes is smaller in the Luna 20 sample than in the Luna 16 sample or other samples olassified by us as ‘heavily irradiated’. VINOGRADOV (1972)notes that a considerable fraotion of the Lnna 20 material may come from the crater Apollonius C which is described as, “comparatively young (possibly Copernioan)“. When a layer of soil has been deposited and then
Fossiltrack and thermoluminescence studiesof Luna 20 material
829
lam relatively undisturbed before being collected (14141) or covered over (1202612028), then a characteristic distribution of track densities is produced that makes it meaningful to calculate a track exposure age (CROZAZet al., 1972). The description of the Luna 20 sampling is somewhat ambiguous but can be interpreted as meaning that the deepest penetration of the core was ~20 cm. If we arbitrarily divide the Luna 20 crystals into a lightly irradiated and a heavily irradiated component, with the dividing line fixed at a track density of 2 x 10*/cma, then the distribution of track densities for the less irradiated component is consistent with what would be expected from galactic cosmic rays acting on a 20 cm thick layer for a period of ~270 x lo6 years. This could possibly be a measure of the formation age of Apollonius C. However, the reader is warned that this number is strongly model dependent and cannot be taken, by itself, as a firm measure of the age of the crater. In the two component mixing model described above -+ of the total sample falls in the lightly irradiated category. STEELE and SMITH(1973) have shown that LTJNA20 feldspara of the type studied here are remarkably homogeneous in composition. It thus appears that the two irradiation categories do not correspond to any other obvious separation into two components and, to that extent, the separation remains arbitrary. An intense early solar activity might manifest itself in having highland samples more heavily irradiated, on the average, than younger mare samples. The Luna 20 sample, having been added recently to the regolith, unfortunately provides no handle on this problem. The report by VINO~RAIIOV(1972) mentioned that our Soviet colleagues had found very low track densities in the olivine fraction of the Luna 20 sample. Due to the smallness of our sample and the paucity of olivine crystals, we have not been able to contribute any information on this point. Possibly the low track densities observed by us in pyroxene crystals are related to the Soviet observation; however, much more work would have to be done on the etching characteristics of these particular pyroxenes before this could be established. Additional material is needed in order to investigate this point. Acknowledgemmte-It ie a pleasureto acknowledge the help of S. Srrrro~ and P. SWAN in performing the measurements reported here. We also wish to thank D. YTJHASfor valuable conversations. This work would have been impossible without the cooperation of our Soviet colleagues,who in the beat tradition of the internationalcharacterof lunar scienceso generously provided us with samples. This work was supported by NASA Grant No. NGL 26-008-066.
REFERENUES BEERXANN C., Caovlz G., DROZD R., HOEENBERQC. M., RALSTONC., WALKER R. M. and YUELAS D. (1972)Rare gas and particle track studies of Apollo 16 samples: Hadley rille and special
soils (abstract). In: The ApouO 15 Samples, (editors J. W. Chamberlain and C. Watkins), pp. 329-332. Lunar Science Institute. CROZAZG., WATXER. R. and WOOI+J-M D. (1971) Nuclear track studies of dynamic surface processeson the moon and the constancy of solar activity. Proo. Second Lunar Sci. Conf., Beochim. Coemochirn.Acto Swppl. f&2543-2668. M.I.T. Preaa CROV~ZG., DROZD R., HOJXENBERUC. M., HOYT H. P., JR., RAUB D., WALKER R. M. and
830
G. CROZ~Z, I%. WALKER and D. ZIMMERMAN
YUFUS D. (1972) Solar flare and galactic cosmic ray studios of Apollo 14 and 15 samples. Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, 2917-2931. M.I.T. Press. LUNAR SABSPLEPRELIMINARYEXAMINATIONTEAM (1970) Preliminary examination of lunar samples from Apollo 12. Science 167,1325-1339. STEELE I. M. and SMITE J. V. (1973) Compositional and X-ray data for Luna 20 feldspar. Geoohim. Coemochim. Acta 37, 1075-1077. VINOORADOVA. P. (1972) Preliminary data on the lunar soil collected by the Luna 20 unmanned spacecraft. Geokhbiya 7, 763-774. English translation Geochim. Cosmoehim. Acta 3’7, 721729 (1973).