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Ejecta from large craters on the moon: Discussion
EJECTA FROM LARGE CRATERS ON THE MOON: DISCUSSION 13-15 March 1973, Defense Nuclear Agency Report 3151P2, 2 (1973) 79 8 M D. Nordyke, Nuclear craters and preliminary theory of the mechanics of explosive crater formation, J Geophys Res 66 (1961) 3439. 9 R B. Baldwin, The measure of the Moon (Univ. Chicago Press, 1963). 10 R.B. Baldwin, Ranger VII and gravaty scaling of lunar craters, Science 157 (1967) 546. 11 T R. McGetchm and J.W. Head, Lunar cander cones, Science 180 (1973) 68.
271
12 J H. Mackin, Ongm of lunar maria, Geol Soc Am Bull. 80 (1969) 735 13 N.M. Short and M.L. Foreman, Thickness of impact crater ejecta on the lunar surface, Modern Geol 3 (1972) 69 14 M R Dence, R A F Gleveand A.G. Plant, The Imbrlum basin and its ejecta, Abstracts of Papers, Submitted to the 5th Lunar Science Conference, Lunar So Inst, NASA (1974) 165 15 H J. Moore, D.H Scott and C A Hodges, Multi-ringed basins, 5th Lunar Scl. Conf. (Lunar Scaence Institute Houston, 1974).
E J E C T A FROM LARGE CRATERS ON THE MOON: D I S C U S S I O N M. SETTLE 1 Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambrtdge, Mass (USA) J.W. HEAD Department o f Geologwal Setences, Brown Universzty, Providence, R.I. (USA) and T.R. McGETCHIN 2 Department of Earth and Planetary Sctences, Massachusetts Institute o f Technology, Cambrtdge, Mass (USA) Received August 2, 1974
In commenting upon the problems involved in estimating the rim thickness of ejecta for large lunar craters, Pike [ 1] has correctly identified the less certain part of the expression describing ejecta thickness developed by McGetchln et al. [2] : t = 0.14 R 0-74 (r/R) -3 0
(1)
where t is average ejecta thickness at a radial range r measured from the center of a crater with radius R, all in meters. This equation is made up of two factors, one specifying the thickness of the rim ejecta deposit (T) of a crater of size R : T = 0.14 R 0.74
(2)
1 Presently at. Terrestrial Sciences Laboratory, Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01730. 2 Presently at: Geosclences Branch, Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87544.
and the other which describes the radial throning of the ejecta deposit'
t ~ (r/R) -3 0
(3)
The approach employed in constructing this model (eq. 1) was to compare ejecta studies made for nuclear and chemical explosive cratering events, small-scale laboratory cratering experiments, natural meteorite impacts, and semi-empirical cratering models which had been developed for lunar craters Thickness estimates based upon photographic studies o f burial of pre-existing topography were not considered in detail since such studies are forced to make a series of rather tenuous assumptions in order to assign an estimated change in crater depth to a single cratering event. The resulting empirical relation between T and R for large lunar craters (i.e. radii greater than 10 2 m) is founded upon measurements made for two nuclear cratering events (Teapot Ess, Jangle U) a terrestlal im-
272 pact crater (Meteor Crater, Arizona) and semi-quantitative cratering models for large lunar craters (Copernicus, and Imbrlum).* Thus eq. 2 spans a range of crater sizes and encompasses a variety of methodologies including experimental and modelling studies. The experimental data which can be used in deriving an expresslon for rim thickness T as a function of crater radius R is limited by consideration of scaled burst depth [3], type of explosive [4] and an intentional desire on the part of McGetchin et al [2] to avoid biasing the final expression towards the results of a single methodology. (Note that the explosion crater data employed by McGetchln et al [2] comes from near-surface nuclear bursts in agreement with Pike's [1] tentative conclusion that such craters provide the best explosion analog of the rim thickness/rim height relationship associated with large lunar craters.) The limited number of lunar ejecta modelling studies and terrestrial impact craters surrounded by ejecta deposits neccessarily narrowed the data base upon which eq. 2 was constructed. Alternatively, Pike's [1 ] approach is based upon the relationship between rim height above local ground level (/7) and crater size (R) which can be determined photometrically for a large number of lunar and experimental craters. Crater rim heights represent a combination of structural uplift and the ejecta thickness deposited upon the rim. Pike [1 ] recognizes that the ratio of rim thickness (T)/rim height (h) determined for terrestrial explosion craters as well as terrestrial impact craters may not necessarily be applicable to lunar craters spanning a range of sizes. In addition to a difference in gravity the lack of an atmosphere and the widespread presence of relatively unconsolidated surface deposits may result in a much wider range of T/h for the lunar case. However, a more severe limitation on the determination of rim thickness for large lunar craters arises from slumping of the crater wall which enlarges the crater and exposes successively thinner thicknesses of ejecta as the apparent rim deposit. The percentage of radial growth of the crater will always be smaller than the relative change In rim thickness * Pike [1] has pointed out amblgmtles in our citation of the value of T for the crater Copermcus, on p 228 [2]. T is mcorrectly hsted in meters, rather than feet The correct value was used in calculations and is entered as such (with a slight offset due to drafting error) in fig. l [ 1 ]
M. SETTLE ET AL. for an Incremental amount of slump, such that'
AR/R < A r/r.
(4)
Thus any scheme to compensate for slumping will increase rim thicknesses to pre-slump conditions and simultaneously decrease crater radius. If the degree of slumping AR/R is constant over a large range of crater size as suggested by Pike [1], then a relationship such as eq 2 between T and R will need to be displaced upward towards greater thicknesses If, on the other hand, the degree of slumping Increases with increasing crater size, the correction must correspondingly increase with increasing crater size and the slope of eq 2 will increase. The difficulty of Inferring original rim geometry and the associated uncertainty In determining the true radius of the crater of excavation for basin-sized events pose the most serious problem in inferring a relationship between T and R on the basis of apparent exposed &mensions. Pike properly points out that all these considerations make it impossible for any model to predict ejecta thickness to a precision of tenths of a meter [1]. Indeed, McGetchin et al. [2] have taken great care in discussing the application of the model to the moon to round thickness estimates to the nearest tens of meters where possible, and to stress that the model can only predict average ejecta thickness, not true accumulations at given points on the moon. Fig. 1 compares Pike's three alternative equations for average ejecta rim thickness (P1, P2, P3) to the original eq. 1 developed by McGetchin et al. [2]. Also shown is the decay of such rim deposits for the specific case of Imbrium with the radius at 485 km taken as the original crater of excavation. Current work on the most recent basin event, Orientale [5], suggests that by analogy the Imbrlum ring at R = 335 km represents a central peak ring and that the ring at R = 485 km is the rim of the original crater. Pike [1 ] does not discount this possibility. For the case of Imbrlum (R = 485 km), Pike's alternative models P1 and P2 predict thicknesses at various Apollo sites which vary approximately 2 0 - 4 0 % from values predicted by MSH (eq. 1). Such variations would not be resolvable in the area of discontinuous ejecta deposition where local concentrations of ejecta in the form of rays and secondary craters contrast markedly from areas which are untouched by lateral sedimentation. Nor would such discrepancies be rec-
EJECTA FROM LARGE CRATERS ON THE MOON. DISCUSSION [
i
i
i
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1
i
i
iiiii]
i
i
111111
I0,000
polio 15
W
I00
MSH t = 0 14 R 074 (r/R) "30
I0I
04m I0 km
PIt
• 311 RO3'3(r/R) " 3 °
P2
t • 3505R°:~40(r/R) -30
1
I I Itllll
"~l ~
\P3 ~pI \
I l I IIIIJl 1 105m 106m I00 km I 0 0 0 kin CRATER RADIUS
1 I III
107rn
Fig 1 Rim thickness shown as a function of crater size by left hand curves with positive slope. Ejecta thicknesses predicted by different models for the case of Imbrium represented by right-handed curves with negative slope (assuming cube root decay [2]) P1, P2, P3 represent alternative models of Pike [ 1 ], MSH represents original model of McGetchm et al [2] All thicknesses in meters. ognizable in the area o f continuous ejecta deposition where pre-existing topography preferentially thins and concentrates the ejecta blanket. Alternatively model P3, which is based upon the T/R relationship observed for smaller experimental craters, predicts thicknesses an order of magnitude greater than any of the other models. At the Apollo 15 site P3 predicts a thickness seven times greater than an estimate based on the size of the smallest telescopically detectable crater [6], while at Apollo 14, P3 predicts a thickness more than eleven times that suggested for Imbrium ejecta by seismic measurements [7]. Finally, recent study of the regional geology o f the Cayley Formation in the Apollo 16 region [8,9] falls to account for the 370-m deposit o f Imbrlum ejecta predicted by P3 at this radial range from Imbrium's center. Such consistent overestimations strongly suggest that such large rim ejecta thickness estimates are unrealistic for large lunar craters. In conclusion, it is interesting to note the convergence of Pike's alternative models P1 and P2 [ 1 ] and
273
the original model of McGetchin et al. [2] for ejecta rim thickness in the range o f crater size representative of lunar basin-forming craters. P1 and P2 are based upon dimensional studies o f lunar craters while the model of McGetchin et al. is founded upon terrestrial experimental and impact craters and previous modelhng studies o f lunar cratering. Pike's alternative approach offers truly alternative thicknesses in the range of crater sizes 5 km < R < 100 km Determination o f the variability of rim ejecta deposits in this size range is vital in unravelhng the local geology of highland sites whose regional histories have been strongly influenced by such events. Furthermore, the development o f different approaches in the continuing study of regional variations of the quantity o f material transported b y basin-forming impact events is clearly important in recognition of the dominance of this single process in the evolution of the lunar surface [5, 11 ]. Ultimately such model predictions will converge and be used to quantitatively relate observed depositional morphologies to the mechanics of excavation and deposition involved in the geologic process of impact cratering [9, 10].
References 1 F.J. Pike, Electa from large craters on the Moon Comments on the geometric model of McGetchm et al., Earth Planet. Sci Lett 23 (1974) 265. 2 T.R. McGetchin, M Settle and J.W. Head, Radial thickness variation m impact crater ejecta implications for lunar basra deposits, Earth Planet Sci. Lett. 20 (1973) 226. 3 V.R. Oberbeck, Laboratory simulation of impact cratering with high explosives, J Geophs Res 76 (1971) 5732 4 D E Burton, C M Snell and J B Bryan, Computer design of high-explosive experiments to simulate subsurface nuclear detonation, Nuclear Technology (1974) m press 5 J.W Head, Orientale multi-ringed basra interior and lmphcatlons for the petrogenesls of lunar highland samples, The Moon (1974) m press 6 R E Eggleton, Thickness of the Apenmman Series m the Landsberg region of the Moon, Astrogeologlc Stu&es Annual Progress Report to NASA August 25, 1961 to August 24, 1962, U S. Geol. Survey (1963) 19 7 R L Kovach, J.S Watkms and T Landers, Actwe seismic experiment, Apollo 14 Preliminary Science Report, NASA Spec Paper SP-272 (1971) 163 8 J.W Head, Stratigraphy of the Descartes Region (Apollo 16): implications for the origin of samples, The Moon (1974) in press 9 V.R Oberbeck, F. Horz, R.H Mornson and W L Quaide,
274 Emplacement of the Cayley Formation, NASA Tech. Memo, TM-X-62,302 (1973). 10 J W Head, M Settle and T R. McGetchm, Radial thickness variations of impact crater ejecta: morphologlc considerations (m preparation)
M. SETTLE ET AL 11 K A Howard, D E Wilhelms and D.H. Scott, Lunar basin formation and Highland stratigraphy, Rev. Geophys Space ScL (1974) in press.
271
12 J H. Mackin, Ongm of lunar maria, Geol Soc Am Bull. 80 (1969) 735 13 N.M. Short and M.L. Foreman, Thickness of impact crater ejecta on the lunar surface, Modern Geol 3 (1972) 69 14 M R Dence, R A F Gleveand A.G. Plant, The Imbrlum basin and its ejecta, Abstracts of Papers, Submitted to the 5th Lunar Science Conference, Lunar So Inst, NASA (1974) 165 15 H J. Moore, D.H Scott and C A Hodges, Multi-ringed basins, 5th Lunar Scl. Conf. (Lunar Scaence Institute Houston, 1974).
E J E C T A FROM LARGE CRATERS ON THE MOON: D I S C U S S I O N M. SETTLE 1 Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambrtdge, Mass (USA) J.W. HEAD Department o f Geologwal Setences, Brown Universzty, Providence, R.I. (USA) and T.R. McGETCHIN 2 Department of Earth and Planetary Sctences, Massachusetts Institute o f Technology, Cambrtdge, Mass (USA) Received August 2, 1974
In commenting upon the problems involved in estimating the rim thickness of ejecta for large lunar craters, Pike [ 1] has correctly identified the less certain part of the expression describing ejecta thickness developed by McGetchln et al. [2] : t = 0.14 R 0-74 (r/R) -3 0
(1)
where t is average ejecta thickness at a radial range r measured from the center of a crater with radius R, all in meters. This equation is made up of two factors, one specifying the thickness of the rim ejecta deposit (T) of a crater of size R : T = 0.14 R 0.74
(2)
1 Presently at. Terrestrial Sciences Laboratory, Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01730. 2 Presently at: Geosclences Branch, Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87544.
and the other which describes the radial throning of the ejecta deposit'
t ~ (r/R) -3 0
(3)
The approach employed in constructing this model (eq. 1) was to compare ejecta studies made for nuclear and chemical explosive cratering events, small-scale laboratory cratering experiments, natural meteorite impacts, and semi-empirical cratering models which had been developed for lunar craters Thickness estimates based upon photographic studies o f burial of pre-existing topography were not considered in detail since such studies are forced to make a series of rather tenuous assumptions in order to assign an estimated change in crater depth to a single cratering event. The resulting empirical relation between T and R for large lunar craters (i.e. radii greater than 10 2 m) is founded upon measurements made for two nuclear cratering events (Teapot Ess, Jangle U) a terrestlal im-
272 pact crater (Meteor Crater, Arizona) and semi-quantitative cratering models for large lunar craters (Copernicus, and Imbrlum).* Thus eq. 2 spans a range of crater sizes and encompasses a variety of methodologies including experimental and modelling studies. The experimental data which can be used in deriving an expresslon for rim thickness T as a function of crater radius R is limited by consideration of scaled burst depth [3], type of explosive [4] and an intentional desire on the part of McGetchin et al [2] to avoid biasing the final expression towards the results of a single methodology. (Note that the explosion crater data employed by McGetchln et al [2] comes from near-surface nuclear bursts in agreement with Pike's [1] tentative conclusion that such craters provide the best explosion analog of the rim thickness/rim height relationship associated with large lunar craters.) The limited number of lunar ejecta modelling studies and terrestrial impact craters surrounded by ejecta deposits neccessarily narrowed the data base upon which eq. 2 was constructed. Alternatively, Pike's [1 ] approach is based upon the relationship between rim height above local ground level (/7) and crater size (R) which can be determined photometrically for a large number of lunar and experimental craters. Crater rim heights represent a combination of structural uplift and the ejecta thickness deposited upon the rim. Pike [1 ] recognizes that the ratio of rim thickness (T)/rim height (h) determined for terrestrial explosion craters as well as terrestrial impact craters may not necessarily be applicable to lunar craters spanning a range of sizes. In addition to a difference in gravity the lack of an atmosphere and the widespread presence of relatively unconsolidated surface deposits may result in a much wider range of T/h for the lunar case. However, a more severe limitation on the determination of rim thickness for large lunar craters arises from slumping of the crater wall which enlarges the crater and exposes successively thinner thicknesses of ejecta as the apparent rim deposit. The percentage of radial growth of the crater will always be smaller than the relative change In rim thickness * Pike [1] has pointed out amblgmtles in our citation of the value of T for the crater Copermcus, on p 228 [2]. T is mcorrectly hsted in meters, rather than feet The correct value was used in calculations and is entered as such (with a slight offset due to drafting error) in fig. l [ 1 ]
M. SETTLE ET AL. for an Incremental amount of slump, such that'
AR/R < A r/r.
(4)
Thus any scheme to compensate for slumping will increase rim thicknesses to pre-slump conditions and simultaneously decrease crater radius. If the degree of slumping AR/R is constant over a large range of crater size as suggested by Pike [1], then a relationship such as eq 2 between T and R will need to be displaced upward towards greater thicknesses If, on the other hand, the degree of slumping Increases with increasing crater size, the correction must correspondingly increase with increasing crater size and the slope of eq 2 will increase. The difficulty of Inferring original rim geometry and the associated uncertainty In determining the true radius of the crater of excavation for basin-sized events pose the most serious problem in inferring a relationship between T and R on the basis of apparent exposed &mensions. Pike properly points out that all these considerations make it impossible for any model to predict ejecta thickness to a precision of tenths of a meter [1]. Indeed, McGetchin et al. [2] have taken great care in discussing the application of the model to the moon to round thickness estimates to the nearest tens of meters where possible, and to stress that the model can only predict average ejecta thickness, not true accumulations at given points on the moon. Fig. 1 compares Pike's three alternative equations for average ejecta rim thickness (P1, P2, P3) to the original eq. 1 developed by McGetchin et al. [2]. Also shown is the decay of such rim deposits for the specific case of Imbrium with the radius at 485 km taken as the original crater of excavation. Current work on the most recent basin event, Orientale [5], suggests that by analogy the Imbrlum ring at R = 335 km represents a central peak ring and that the ring at R = 485 km is the rim of the original crater. Pike [1 ] does not discount this possibility. For the case of Imbrlum (R = 485 km), Pike's alternative models P1 and P2 predict thicknesses at various Apollo sites which vary approximately 2 0 - 4 0 % from values predicted by MSH (eq. 1). Such variations would not be resolvable in the area of discontinuous ejecta deposition where local concentrations of ejecta in the form of rays and secondary craters contrast markedly from areas which are untouched by lateral sedimentation. Nor would such discrepancies be rec-
EJECTA FROM LARGE CRATERS ON THE MOON. DISCUSSION [
i
i
i
i iii1
1
i
i
iiiii]
i
i
111111
I0,000
polio 15
W
I00
MSH t = 0 14 R 074 (r/R) "30
I0I
04m I0 km
PIt
• 311 RO3'3(r/R) " 3 °
P2
t • 3505R°:~40(r/R) -30
1
I I Itllll
"~l ~
\P3 ~pI \
I l I IIIIJl 1 105m 106m I00 km I 0 0 0 kin CRATER RADIUS
1 I III
107rn
Fig 1 Rim thickness shown as a function of crater size by left hand curves with positive slope. Ejecta thicknesses predicted by different models for the case of Imbrium represented by right-handed curves with negative slope (assuming cube root decay [2]) P1, P2, P3 represent alternative models of Pike [ 1 ], MSH represents original model of McGetchm et al [2] All thicknesses in meters. ognizable in the area o f continuous ejecta deposition where pre-existing topography preferentially thins and concentrates the ejecta blanket. Alternatively model P3, which is based upon the T/R relationship observed for smaller experimental craters, predicts thicknesses an order of magnitude greater than any of the other models. At the Apollo 15 site P3 predicts a thickness seven times greater than an estimate based on the size of the smallest telescopically detectable crater [6], while at Apollo 14, P3 predicts a thickness more than eleven times that suggested for Imbrium ejecta by seismic measurements [7]. Finally, recent study of the regional geology o f the Cayley Formation in the Apollo 16 region [8,9] falls to account for the 370-m deposit o f Imbrlum ejecta predicted by P3 at this radial range from Imbrium's center. Such consistent overestimations strongly suggest that such large rim ejecta thickness estimates are unrealistic for large lunar craters. In conclusion, it is interesting to note the convergence of Pike's alternative models P1 and P2 [ 1 ] and
273
the original model of McGetchin et al. [2] for ejecta rim thickness in the range o f crater size representative of lunar basin-forming craters. P1 and P2 are based upon dimensional studies o f lunar craters while the model of McGetchin et al. is founded upon terrestrial experimental and impact craters and previous modelhng studies o f lunar cratering. Pike's alternative approach offers truly alternative thicknesses in the range of crater sizes 5 km < R < 100 km Determination o f the variability of rim ejecta deposits in this size range is vital in unravelhng the local geology of highland sites whose regional histories have been strongly influenced by such events. Furthermore, the development o f different approaches in the continuing study of regional variations of the quantity o f material transported b y basin-forming impact events is clearly important in recognition of the dominance of this single process in the evolution of the lunar surface [5, 11 ]. Ultimately such model predictions will converge and be used to quantitatively relate observed depositional morphologies to the mechanics of excavation and deposition involved in the geologic process of impact cratering [9, 10].
References 1 F.J. Pike, Electa from large craters on the Moon Comments on the geometric model of McGetchm et al., Earth Planet. Sci Lett 23 (1974) 265. 2 T.R. McGetchin, M Settle and J.W. Head, Radial thickness variation m impact crater ejecta implications for lunar basra deposits, Earth Planet Sci. Lett. 20 (1973) 226. 3 V.R. Oberbeck, Laboratory simulation of impact cratering with high explosives, J Geophs Res 76 (1971) 5732 4 D E Burton, C M Snell and J B Bryan, Computer design of high-explosive experiments to simulate subsurface nuclear detonation, Nuclear Technology (1974) m press 5 J.W Head, Orientale multi-ringed basra interior and lmphcatlons for the petrogenesls of lunar highland samples, The Moon (1974) m press 6 R E Eggleton, Thickness of the Apenmman Series m the Landsberg region of the Moon, Astrogeologlc Stu&es Annual Progress Report to NASA August 25, 1961 to August 24, 1962, U S. Geol. Survey (1963) 19 7 R L Kovach, J.S Watkms and T Landers, Actwe seismic experiment, Apollo 14 Preliminary Science Report, NASA Spec Paper SP-272 (1971) 163 8 J.W Head, Stratigraphy of the Descartes Region (Apollo 16): implications for the origin of samples, The Moon (1974) in press 9 V.R Oberbeck, F. Horz, R.H Mornson and W L Quaide,
274 Emplacement of the Cayley Formation, NASA Tech. Memo, TM-X-62,302 (1973). 10 J W Head, M Settle and T R. McGetchm, Radial thickness variations of impact crater ejecta: morphologlc considerations (m preparation)
M. SETTLE ET AL 11 K A Howard, D E Wilhelms and D.H. Scott, Lunar basin formation and Highland stratigraphy, Rev. Geophys Space ScL (1974) in press.