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Crater frequency age determinations for the proposed Apollo 17 site at Taurus-Littrow
EARTH AND PLANETARY SCIENCE LETTERS 18 (1973) 102-108. NORTH-HOLLAND PUBLISHING COMPANY
[]
CRATER
FREQUENCY
AGE DETERMINATIONS
FOR THE
PROPOSED APOLLO 1 7 SITE A T T A U R U S - L I T T R O W Ronald GREELEY Physics Department, University of Santa Clara, Santa Clara, Calif. 95053, USA Donald E. GAULT* Max-Planck Institut far Kernphysik, 69 Heidelberg 1, Postfach 1248, West Germany Received 29 August 1972 Revised version received 1 November 1972 Crater frequency distributions determined for surfaces in the Taurus-Littrow region of the Moon and compared with crater counts and radiometric age dates for Apollo 11, 12 and 14 landing sites indicate that the surface of the proposed Apollo 17 landing site was formed between 2.5 and 2.8 by ago.
1. Introduction The site for the last manned lunar landing in the Apollo series has been selected in a valley on the southwestern edge of the Taurus Mountains about 70 km southeast of Mare Serenitatis (fig. 1). The site was selected partly because it displays a variety o f structures and because the geologic units appear to span a wide range in time. A major problem in lunar studies is the determination of ages [1] of different units by remote means. Lunar crater counts have been employed previously in attempts to derive ages [ 2 - 4 ] . On the assumption that the vast majority of craters are o f impact origin, relative age dates can be obtained by counting craters; basically, old surfaces should have more and larger craters than young surfaces. In theory, if factors such as meteoritic flux are known, and it is assumed that conditions have been constant over geologic time, then it should be possible to obtain dates of formation from crater counts. Recent studies, however, show that the flux has not been constant [5] and that estimates o f current rates are probably anomalously high [6]. * Present adress: Space Science Division, NASA-Ames Research Center, Moffett Field, Calif. 94035.
Returned lunar samples are helping to resolve the problems concerning meteoritic flux and are providing a means of empirically calibrating crater count data. Radiometric age dates for selected lunar samples represent the date of formation o f the geologic unit sampied. A t the time of formation the surfaces of the units were uncratered; through time they have served as impact counters, recording each impact. Each Apollo site of a different age represents a datum time plane with a specific crater frequency distribution. However, crater frequency distributions must be correlated only with the radiometric age dates representing the surface of the unit that has been crater counted. Gault and Greeley [7] have established crater frequency distributions calibrated to lunar sample ages for Apollo 11, 12, and 14 landing sites. It should now be possible to determine age dates from crater counts for areas not sampled if the dates for those areas can be bracketed by the Apollo crater frequency distributions. In this report, we attempt to determine age dates for some of the units in and around the proposed Taurus-Littrow landing site (Apollo 17) by correlating their crater frequency distributions with the established dates and crater counts of Apollo 11, 12, and 14 landing sites.
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
103
Fig. 1. Earth-based telescopic view of the eastern part of Mare Serenitatis, showing the dark outer annulus (Unit 1) and very dark material northwest of the proposed Apollo 17 landing site (arrow), and Unit 2.
2. Taurus-Littrow area F o u r different units photographed by Lunar Orbiter and Apollo were selected in the area for analysis (fig. 2): Unit I is a mare unit in Serenitatis restricted
to the dark annulus (fig. 1.) of the outer margin of the basin. This unit is interpreted by E1 Baz [8] as the last of two mare-filling episodes; it is considered to be Eratosthenian in age by Wilhelms and McCauley [9]. Crater frequency data (fig. 3) are based on 16,966 cra-
104
R. Greeley, D.E. Gault, Craterfrequency age determinations, Apollo 17 site
Fig. 2. Metric camera photograph (AS 15 1113) of the Taurus-Littrow area with Units 3 - 4 identified; Unit 2 is north, off edge of photograph (see fig. 1); Unit 4, including the proposed landing site, is the dark, low area between massif units of the Taurus Mountains; white cross marks the landing site. ters counted over an area o f 1234.8 km 2. Unit 2, (fig. 1) on the southeastern edge of Mare Serenitatis, is composed of plains-forming material similar to the Apennine Bench o f the Imbrium Basin [ 10] and is considered Imbrian in age [9, 10]. Crater frequency data (fig. 3) are based on 3,029 craters counted over an area of 542.7 km 2. Unit 3 (fig. 2) is essentially the
same as Unit 2 except that the surface has a softened, subdued appearance. The larger craters are somewhat indistinct and there is a paucity of smaller craters. This unit is considered to be layered, with craters (developed on layers representing older surfaces) projecting through the present surface. The underlying layer is considered to be Imbrian in age and has been cov-
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
105
-I
~E - 2 E
>_L) :7
uJ - 3
,,o, ,,= r~ bJ
:3
rr 0 0
o
2
-I
-2
-3
1.0
2.0
3.0 LOGIo CRATER DIAMETER, rn
2.0
5.0
Fig. 3. Crater size frequency distribution curves for Units 1 - 4 ; d a t u m lines for 1% and 10% saturation, and for crater counts representing production curves from Apollo 11, 12 and 14 are shown. Solid symbols are average n u m b e r of craters per square km; open symbols are cumulative n u m b e r o f craters per square kin. Dashed line is production curve for craters f o r m e d on ejecta blanket o f Copernicus [ 16].
106
R. Greeley, D.E. Gault, Craterfrequency age determinations, Apollo 1 7 site
ered with an upper layer of Copernican age [9]. The blanketing layer is part of the Sulpicius Gallus Fomlation and is possibly pyroclastic in origin [11]. Crater frequency data (fig. 3) are based on 9,538 craters counted over an area of 277.5 km 2. Unit 4 (fig. 2) is a very dark unit occupying low regions within the Taurus Mountains southeast of Mare Serenitatis. It has one of the lowest albedos of any unit of regional extent on the Moon and is considered to be pyroclastic in origin [8]. Small 'cinder cones' visually observed and photographed during Apollo 15 may be sources for the unit. The area analyzed for this unit is that immediately surrounding the proposed landing site. Crater frequency data (fig. 3) are based on 866 craters counted over an area of 76.2 km 2. Fig. 3 shows crater frequency distributions for each of the four areas examined. Shown with the crater counts are the 1% and 10% saturation lines [6] and datum lines representing crater count production curves for Apollo 11, 12, and 14 landing sites [7]. Unit 1, the mare unit of Eratosthenian age, has a crater frequency age corresponding to that of Apollo 12. Unit 2 is the older mare bench material of Imbrian age and has a crater frequency age corresponding almost exactly to that of Apollo 11. The crater frequency distribution for Unit 3 shows two production curves and two saturation curves. In the larger size range (greater than 100 m) the crater distribution coincides with Unit 2 and Apollo 11. In the smaller size ranges, however, the distribution appears to increase into a second production curve, indicating an age much younger than Apollo 12. This dual distribution can be best explained as representing partial rejuvenation of a cratered surface on which the smaller craters were obliterated and the larger craters were subdued, but still discernible. Such a process is reasonably explained by a blanketing of ash or other pyroclastic material, as has been proposed on the basis of surface geomorphology [8, 11]. Unit 4 (fig. 3) is the proposed Apollo 17 landing site. Its crater frequency distribution has a slight 'kink' similar, but less pronounced, to that for Unit 3. Even though the crater statistics are rather poor for craters smaller than about 50 m and larger than 180 m as a result of the limited photography available to us, the distribution is in the correct size range to establish saturation and production curves.
3. Crater-frequency age determinations The crater production curves (fig. 3) derived from crater-frequency distributions [7] for the Apollo 11 and 14 landing sites yield crater populations which are greater by factors of 2.5 and 10, respectively, than the production curve for the Apollo 12 site. If the basalts returned from the three sites are considered to be locally derived so that their R b - S r ages represent a measure of length of time the respective surfaces have been recording the impacts of meteoritic bodies, then a basis is established for fornmlating empiral temporal variations in the flux of crater-forming bodies. In order to minimize the range of ages at each landing site that might be caused by systematic interlaboratory differences in age determinations, only R b - S r results reported from the Lunatic Asylum [ 1 2 - 1 5 ] were used to derive [7] the empirical relationships. It is significant that flux models based on the assumption of a single source or origin for the impacting bodies, whether the source provided either a constant flux or an exponentially decaying flux, cannot be fitted to the crater-frequency radiometric-age data; only flux models assumed to be made up from two or more sources of bodies (e.g., constant flux plus exponentially decaying flux, two or more exponentially decaying fluxes, etc.) can be fitted to the raw data. However, due to the rather large range in the radiometric ages reported for each of the landing sites, there is still considerable latitude in the empirical relationships one may derive and there are consequently large variations in age-estimates one obtains when extrapolating outside the range of tire basic data. Only by imposing special boundary conditions (e.g., mass-rate of accretion, total mass accreted, etc.) is it possible to restrict the limits for meaningful solutions for the empirical solutions and narrow the scope of the ecceptable flux models. In this manner 'best' estimates were derived for the temporal variations in the flux of crater-forming bodies. Dependent, therefore, on: (1) the functional relationships and boundary conditions used to describe the temporal variations in the cratering rate; and (2) the radiometric ages selected to be representative for the Apollo 1 1, 12, and 14 landing sites used to calibrate the crater-frequency ages [7], ages may be obtained for the surficial layer of Unit 4 varying from
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
3.16 to less than 0.5 by. Our 'best' estimate for the age of the surficial layer of Unit 4 (proposed Apollo 17 landing site) is between 2.5 and 2.8 by. The surficial layer of Unit 3 has a very low crater-frequency age and appears to be less (perhaps substantially less) than 2 by. Indeed, the crater-frequency age for Unit 3 appears to be (fig. 3) virtually identical to that for the ejecta blanket of Copernicus. If the age of Copernicus has been correctly inferred to be 0.85 by (17, 18), Unit 3 must correspondingly be approximately the same very low age. Table 1 summarizes the correlation of crater frequency distributions for units of Taurus-Littrow with crater counts from Apollo 11, 12, and 14 landing sites and the available radiometric age dates of Apollo sampies. Shown also is the correlation of the relative age dates of the Taurus-Littrow area and the Apollo sites. Relative age dates derived from independent sources and crater count data are in agreement. Returned samples from Taurus-Littrow by Apollo 17 should allow reffmement of these crater frequency age determinations. Caution must be exercised, however, in evaluating the radiometric dates from the returned samples for correlation with crater count data. Although the landing site is on Unit 4, the Taurus Mountains are within a few kilometers of the site and material from them may very easily be mixed with the younger plains-forming material. The Taurus Mountains have been relative age-dated as Pre-Imbrian, some of the oldest material on the Moon. Another possibility of 'age contamination' is the excavation by impact of rocks underlying Unit 4. Thus, the crater count date must be correlated only with age dates obtained from samples of Unit 4. Table 1 Relative ages correlated with ages derived from crater counts for units in the proposed Taurus-Littrow Apollo 17 landing site.
Unit 1 Unit 2 Unit 3 (surfacelayer) Unit 4 (surface layer)
Relative age*
Age in years
Eratosthenian Imbrian Copernican Copernican
~3.3 -3.7 <2 2.5
X 109** × 109** × 109:~ - 2.8 × 109:~
* Wilheims and McCauley (1971). ** Correlated with Rb-Sr ages for Apollo 11 and 12 sites. :~ Extrapolated from crater counts and radiometric ages for Apollo 11, 12, and 14 landing sites.
107
Acknowledgements We thank T. Bunch, D. Stt~ffler, and J. Guest for their critical review of the manuscript. We particularly wish also to thank G. Yamakoshi for her tedious but diligent and exacting efforts in obtaining crater frequency distributions.
References and notes [ 1] Geochronology (the dating of geologic events and rock formations) is expressed in either radiometric age dates (years before present) or relative ages, in which stratigraphic units are older or younger than others, with no expression as to how much older or younger they may be. Relative time units are assigned proper names. A relative time scale has been established for lunar surface units; from youngest to oldest, they are: Copernican System, Eratosthenian System, Imbrian System, and PreImbrian. [2] T.J. Kreiter, Dating lunar surface features by using crater frequencies, Publ. Astron. Soc. Pacific 72 (1960) 393. [3] W.K. Hartmann, Preliminary note on lunar cratering rates and absolute time scales, Icarus 12 (1970) 131. [4] W.K. Hartmann, Lunar cratering chronology, Icarus 13 (1970) 299. [5 ] E.M. Shoemaker, Origin of fragmental debris on the lunar surface and the history of bombardment of the Moon, Publ. Inst. Invest. Geol., Univ. Barcelona, 25 (1971) 27. [6] D.E. Gault, Saturation and equilibrium conditions for impact cratering on the lunar surface: Criteria and implications, Radio Sci. 5 (1970) 273. [7] D.E. Gault and R. Greeley, Crater frequency distributions for the Apollo 11, 12, and 14 landing sites correlated with radiometric age-dates (in preparation). [8] F. El Baz, The cinder fields of the Taurus Mountains, NASA SP-289 (1972) 25-66. [9] D.E. Wilhelms and J.F. McCauley, Geologic map of the near side of the Moon, U.S. Geol. Surv. Map 1-703 (1971). [10] M.H. Carr, Sketch map of the region around candidate Littrow Apollo landing sites, NASA SP-289 (1972) 25 -63. [ 11] M.H. Carr, Geologic map of the Mare Serenitatis of the Moon, U.S. Geol. Surv. Map 1-489 (1966). [12] D.A. Papanastassiou, G.J. Wasserburg and D.S. Burnett, The Rb-Sr ages of lunar rocks from the Sea of Tranquillity, Earth Planet. Sci. Letters 8 (1970) 1. [13] D.A. Papanastassiou and G.J. Wasserburg, Lunar chronology and evolution from Rb-Sr studies of Apollo 11 and 12 samples, Earth Planet. Sci. Letters 11 (1971) 37. [ 14] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr ages of igneous rocks from the Apollo 14 mission and the
108
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
age of the Fra Mauro formation, Earth Planet. Sci. Letters 12 (1971) 36. [15] G.J. Wasserburg and D.A. Papanastassiou, Apollo 15: age of the Marsh of Decay; lunar crust and mantle formation, Earth Planet. Sci. Letters 13 (1971) 97. [ 16] R. Greeley and D.E. Gault, Endogenetic craters interpreted from crater counts on the inner wall of Copernicus, Science 171 (1971) 477.
[ 17] L.T. Silver, U - T h - P b isotope systems in Apollo 11 and 12 regohthic materials and a possible age for the Copernicus impact event, EOS 52 (1971) 534 (abstract). {18] R.O. Pepin, J.G. Bradley, J.C. Dragon and L.E. Nyquist, K.-Ar dating of lunar soils: Apollo 1.2, Apollo 14 and Luna 16, Third Lunar Sci. Conf., Lunar Sci. Inst. 88 (1972) 602.
[]
CRATER
FREQUENCY
AGE DETERMINATIONS
FOR THE
PROPOSED APOLLO 1 7 SITE A T T A U R U S - L I T T R O W Ronald GREELEY Physics Department, University of Santa Clara, Santa Clara, Calif. 95053, USA Donald E. GAULT* Max-Planck Institut far Kernphysik, 69 Heidelberg 1, Postfach 1248, West Germany Received 29 August 1972 Revised version received 1 November 1972 Crater frequency distributions determined for surfaces in the Taurus-Littrow region of the Moon and compared with crater counts and radiometric age dates for Apollo 11, 12 and 14 landing sites indicate that the surface of the proposed Apollo 17 landing site was formed between 2.5 and 2.8 by ago.
1. Introduction The site for the last manned lunar landing in the Apollo series has been selected in a valley on the southwestern edge of the Taurus Mountains about 70 km southeast of Mare Serenitatis (fig. 1). The site was selected partly because it displays a variety o f structures and because the geologic units appear to span a wide range in time. A major problem in lunar studies is the determination of ages [1] of different units by remote means. Lunar crater counts have been employed previously in attempts to derive ages [ 2 - 4 ] . On the assumption that the vast majority of craters are o f impact origin, relative age dates can be obtained by counting craters; basically, old surfaces should have more and larger craters than young surfaces. In theory, if factors such as meteoritic flux are known, and it is assumed that conditions have been constant over geologic time, then it should be possible to obtain dates of formation from crater counts. Recent studies, however, show that the flux has not been constant [5] and that estimates o f current rates are probably anomalously high [6]. * Present adress: Space Science Division, NASA-Ames Research Center, Moffett Field, Calif. 94035.
Returned lunar samples are helping to resolve the problems concerning meteoritic flux and are providing a means of empirically calibrating crater count data. Radiometric age dates for selected lunar samples represent the date of formation o f the geologic unit sampied. A t the time of formation the surfaces of the units were uncratered; through time they have served as impact counters, recording each impact. Each Apollo site of a different age represents a datum time plane with a specific crater frequency distribution. However, crater frequency distributions must be correlated only with the radiometric age dates representing the surface of the unit that has been crater counted. Gault and Greeley [7] have established crater frequency distributions calibrated to lunar sample ages for Apollo 11, 12, and 14 landing sites. It should now be possible to determine age dates from crater counts for areas not sampled if the dates for those areas can be bracketed by the Apollo crater frequency distributions. In this report, we attempt to determine age dates for some of the units in and around the proposed Taurus-Littrow landing site (Apollo 17) by correlating their crater frequency distributions with the established dates and crater counts of Apollo 11, 12, and 14 landing sites.
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
103
Fig. 1. Earth-based telescopic view of the eastern part of Mare Serenitatis, showing the dark outer annulus (Unit 1) and very dark material northwest of the proposed Apollo 17 landing site (arrow), and Unit 2.
2. Taurus-Littrow area F o u r different units photographed by Lunar Orbiter and Apollo were selected in the area for analysis (fig. 2): Unit I is a mare unit in Serenitatis restricted
to the dark annulus (fig. 1.) of the outer margin of the basin. This unit is interpreted by E1 Baz [8] as the last of two mare-filling episodes; it is considered to be Eratosthenian in age by Wilhelms and McCauley [9]. Crater frequency data (fig. 3) are based on 16,966 cra-
104
R. Greeley, D.E. Gault, Craterfrequency age determinations, Apollo 17 site
Fig. 2. Metric camera photograph (AS 15 1113) of the Taurus-Littrow area with Units 3 - 4 identified; Unit 2 is north, off edge of photograph (see fig. 1); Unit 4, including the proposed landing site, is the dark, low area between massif units of the Taurus Mountains; white cross marks the landing site. ters counted over an area o f 1234.8 km 2. Unit 2, (fig. 1) on the southeastern edge of Mare Serenitatis, is composed of plains-forming material similar to the Apennine Bench o f the Imbrium Basin [ 10] and is considered Imbrian in age [9, 10]. Crater frequency data (fig. 3) are based on 3,029 craters counted over an area of 542.7 km 2. Unit 3 (fig. 2) is essentially the
same as Unit 2 except that the surface has a softened, subdued appearance. The larger craters are somewhat indistinct and there is a paucity of smaller craters. This unit is considered to be layered, with craters (developed on layers representing older surfaces) projecting through the present surface. The underlying layer is considered to be Imbrian in age and has been cov-
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
105
-I
~E - 2 E
>_L) :7
uJ - 3
,,o, ,,= r~ bJ
:3
rr 0 0
o
2
-I
-2
-3
1.0
2.0
3.0 LOGIo CRATER DIAMETER, rn
2.0
5.0
Fig. 3. Crater size frequency distribution curves for Units 1 - 4 ; d a t u m lines for 1% and 10% saturation, and for crater counts representing production curves from Apollo 11, 12 and 14 are shown. Solid symbols are average n u m b e r of craters per square km; open symbols are cumulative n u m b e r o f craters per square kin. Dashed line is production curve for craters f o r m e d on ejecta blanket o f Copernicus [ 16].
106
R. Greeley, D.E. Gault, Craterfrequency age determinations, Apollo 1 7 site
ered with an upper layer of Copernican age [9]. The blanketing layer is part of the Sulpicius Gallus Fomlation and is possibly pyroclastic in origin [11]. Crater frequency data (fig. 3) are based on 9,538 craters counted over an area of 277.5 km 2. Unit 4 (fig. 2) is a very dark unit occupying low regions within the Taurus Mountains southeast of Mare Serenitatis. It has one of the lowest albedos of any unit of regional extent on the Moon and is considered to be pyroclastic in origin [8]. Small 'cinder cones' visually observed and photographed during Apollo 15 may be sources for the unit. The area analyzed for this unit is that immediately surrounding the proposed landing site. Crater frequency data (fig. 3) are based on 866 craters counted over an area of 76.2 km 2. Fig. 3 shows crater frequency distributions for each of the four areas examined. Shown with the crater counts are the 1% and 10% saturation lines [6] and datum lines representing crater count production curves for Apollo 11, 12, and 14 landing sites [7]. Unit 1, the mare unit of Eratosthenian age, has a crater frequency age corresponding to that of Apollo 12. Unit 2 is the older mare bench material of Imbrian age and has a crater frequency age corresponding almost exactly to that of Apollo 11. The crater frequency distribution for Unit 3 shows two production curves and two saturation curves. In the larger size range (greater than 100 m) the crater distribution coincides with Unit 2 and Apollo 11. In the smaller size ranges, however, the distribution appears to increase into a second production curve, indicating an age much younger than Apollo 12. This dual distribution can be best explained as representing partial rejuvenation of a cratered surface on which the smaller craters were obliterated and the larger craters were subdued, but still discernible. Such a process is reasonably explained by a blanketing of ash or other pyroclastic material, as has been proposed on the basis of surface geomorphology [8, 11]. Unit 4 (fig. 3) is the proposed Apollo 17 landing site. Its crater frequency distribution has a slight 'kink' similar, but less pronounced, to that for Unit 3. Even though the crater statistics are rather poor for craters smaller than about 50 m and larger than 180 m as a result of the limited photography available to us, the distribution is in the correct size range to establish saturation and production curves.
3. Crater-frequency age determinations The crater production curves (fig. 3) derived from crater-frequency distributions [7] for the Apollo 11 and 14 landing sites yield crater populations which are greater by factors of 2.5 and 10, respectively, than the production curve for the Apollo 12 site. If the basalts returned from the three sites are considered to be locally derived so that their R b - S r ages represent a measure of length of time the respective surfaces have been recording the impacts of meteoritic bodies, then a basis is established for fornmlating empiral temporal variations in the flux of crater-forming bodies. In order to minimize the range of ages at each landing site that might be caused by systematic interlaboratory differences in age determinations, only R b - S r results reported from the Lunatic Asylum [ 1 2 - 1 5 ] were used to derive [7] the empirical relationships. It is significant that flux models based on the assumption of a single source or origin for the impacting bodies, whether the source provided either a constant flux or an exponentially decaying flux, cannot be fitted to the crater-frequency radiometric-age data; only flux models assumed to be made up from two or more sources of bodies (e.g., constant flux plus exponentially decaying flux, two or more exponentially decaying fluxes, etc.) can be fitted to the raw data. However, due to the rather large range in the radiometric ages reported for each of the landing sites, there is still considerable latitude in the empirical relationships one may derive and there are consequently large variations in age-estimates one obtains when extrapolating outside the range of tire basic data. Only by imposing special boundary conditions (e.g., mass-rate of accretion, total mass accreted, etc.) is it possible to restrict the limits for meaningful solutions for the empirical solutions and narrow the scope of the ecceptable flux models. In this manner 'best' estimates were derived for the temporal variations in the flux of crater-forming bodies. Dependent, therefore, on: (1) the functional relationships and boundary conditions used to describe the temporal variations in the cratering rate; and (2) the radiometric ages selected to be representative for the Apollo 1 1, 12, and 14 landing sites used to calibrate the crater-frequency ages [7], ages may be obtained for the surficial layer of Unit 4 varying from
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
3.16 to less than 0.5 by. Our 'best' estimate for the age of the surficial layer of Unit 4 (proposed Apollo 17 landing site) is between 2.5 and 2.8 by. The surficial layer of Unit 3 has a very low crater-frequency age and appears to be less (perhaps substantially less) than 2 by. Indeed, the crater-frequency age for Unit 3 appears to be (fig. 3) virtually identical to that for the ejecta blanket of Copernicus. If the age of Copernicus has been correctly inferred to be 0.85 by (17, 18), Unit 3 must correspondingly be approximately the same very low age. Table 1 summarizes the correlation of crater frequency distributions for units of Taurus-Littrow with crater counts from Apollo 11, 12, and 14 landing sites and the available radiometric age dates of Apollo sampies. Shown also is the correlation of the relative age dates of the Taurus-Littrow area and the Apollo sites. Relative age dates derived from independent sources and crater count data are in agreement. Returned samples from Taurus-Littrow by Apollo 17 should allow reffmement of these crater frequency age determinations. Caution must be exercised, however, in evaluating the radiometric dates from the returned samples for correlation with crater count data. Although the landing site is on Unit 4, the Taurus Mountains are within a few kilometers of the site and material from them may very easily be mixed with the younger plains-forming material. The Taurus Mountains have been relative age-dated as Pre-Imbrian, some of the oldest material on the Moon. Another possibility of 'age contamination' is the excavation by impact of rocks underlying Unit 4. Thus, the crater count date must be correlated only with age dates obtained from samples of Unit 4. Table 1 Relative ages correlated with ages derived from crater counts for units in the proposed Taurus-Littrow Apollo 17 landing site.
Unit 1 Unit 2 Unit 3 (surfacelayer) Unit 4 (surface layer)
Relative age*
Age in years
Eratosthenian Imbrian Copernican Copernican
~3.3 -3.7 <2 2.5
X 109** × 109** × 109:~ - 2.8 × 109:~
* Wilheims and McCauley (1971). ** Correlated with Rb-Sr ages for Apollo 11 and 12 sites. :~ Extrapolated from crater counts and radiometric ages for Apollo 11, 12, and 14 landing sites.
107
Acknowledgements We thank T. Bunch, D. Stt~ffler, and J. Guest for their critical review of the manuscript. We particularly wish also to thank G. Yamakoshi for her tedious but diligent and exacting efforts in obtaining crater frequency distributions.
References and notes [ 1] Geochronology (the dating of geologic events and rock formations) is expressed in either radiometric age dates (years before present) or relative ages, in which stratigraphic units are older or younger than others, with no expression as to how much older or younger they may be. Relative time units are assigned proper names. A relative time scale has been established for lunar surface units; from youngest to oldest, they are: Copernican System, Eratosthenian System, Imbrian System, and PreImbrian. [2] T.J. Kreiter, Dating lunar surface features by using crater frequencies, Publ. Astron. Soc. Pacific 72 (1960) 393. [3] W.K. Hartmann, Preliminary note on lunar cratering rates and absolute time scales, Icarus 12 (1970) 131. [4] W.K. Hartmann, Lunar cratering chronology, Icarus 13 (1970) 299. [5 ] E.M. Shoemaker, Origin of fragmental debris on the lunar surface and the history of bombardment of the Moon, Publ. Inst. Invest. Geol., Univ. Barcelona, 25 (1971) 27. [6] D.E. Gault, Saturation and equilibrium conditions for impact cratering on the lunar surface: Criteria and implications, Radio Sci. 5 (1970) 273. [7] D.E. Gault and R. Greeley, Crater frequency distributions for the Apollo 11, 12, and 14 landing sites correlated with radiometric age-dates (in preparation). [8] F. El Baz, The cinder fields of the Taurus Mountains, NASA SP-289 (1972) 25-66. [9] D.E. Wilhelms and J.F. McCauley, Geologic map of the near side of the Moon, U.S. Geol. Surv. Map 1-703 (1971). [10] M.H. Carr, Sketch map of the region around candidate Littrow Apollo landing sites, NASA SP-289 (1972) 25 -63. [ 11] M.H. Carr, Geologic map of the Mare Serenitatis of the Moon, U.S. Geol. Surv. Map 1-489 (1966). [12] D.A. Papanastassiou, G.J. Wasserburg and D.S. Burnett, The Rb-Sr ages of lunar rocks from the Sea of Tranquillity, Earth Planet. Sci. Letters 8 (1970) 1. [13] D.A. Papanastassiou and G.J. Wasserburg, Lunar chronology and evolution from Rb-Sr studies of Apollo 11 and 12 samples, Earth Planet. Sci. Letters 11 (1971) 37. [ 14] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr ages of igneous rocks from the Apollo 14 mission and the
108
R. Greeley, D.E. Gault, Crater frequency age determinations, Apollo 17 site
age of the Fra Mauro formation, Earth Planet. Sci. Letters 12 (1971) 36. [15] G.J. Wasserburg and D.A. Papanastassiou, Apollo 15: age of the Marsh of Decay; lunar crust and mantle formation, Earth Planet. Sci. Letters 13 (1971) 97. [ 16] R. Greeley and D.E. Gault, Endogenetic craters interpreted from crater counts on the inner wall of Copernicus, Science 171 (1971) 477.
[ 17] L.T. Silver, U - T h - P b isotope systems in Apollo 11 and 12 regohthic materials and a possible age for the Copernicus impact event, EOS 52 (1971) 534 (abstract). {18] R.O. Pepin, J.G. Bradley, J.C. Dragon and L.E. Nyquist, K.-Ar dating of lunar soils: Apollo 1.2, Apollo 14 and Luna 16, Third Lunar Sci. Conf., Lunar Sci. Inst. 88 (1972) 602.