Lunar Regolith at Tranquillity Base

Shoemaker, E. M.; Hait, Milan; Swann, G. A.; Schleicher, David; Dahlem, D. H.; Schaber, G. G.; Sutton, R. L.

doi:10.1126/science.167.3918.452

Cited by 70 publications

(53 citation statements)

References 5 publications

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“…The upper microns of the regolith, corresponding to the optical surface viewed by VNIR spectrometers, is overturned within $ 10 4 years. At a depth of $ 10 cm the turnover rate is on the order of 0.5 Ga (Shoemaker et al, 1970;Gault et al, 1974;McEwen et al, 1997). Thus, the amount of mixing between the basaltic ejecta deposits of DHCs and the underlying feldspathic basin ejecta will depend on the thickness of the DHC ejecta deposits.…”

Section: Discussionmentioning

confidence: 99%

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

Whitten

Head

2015

Planetary and Space Science

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Section: Discussionmentioning

confidence: 99%

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

Whitten

Head

2015

Planetary and Space Science

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“…In contradistinction the absolute magnitude of fluence in soils taken from within the nuclear active zone demands relatively rapid turnover times to depths an order of magnitude greater than 500 g cm -2 • The result of the analysis shows that the stratification time and the deep turnover time must be comparable (rx-;::::,rxf 3 ) to produce the differences in fluences which we observe in the regolith. The solution demands that the probability of mixing to depths of several kg cm -2 is commensurate with the probability of mixing to 0.5 kg cm -2 • This result is in sharp contradiction to that obtained by any of the depth-cratering rate models currently in use (Shoemaker et al, 1970;Soderblom and Lebovsky, 1972;Gault et al, 1974). These models assume a power law which gives a rapid decrease in the rate of cratering with depth, and relates the rate of deep cratering to that of shallow cratering.…”

Section: Uniform Mixing Modelmentioning

confidence: 91%

“…Such a reduction in the neutron flux would imply a quiescent regolith which has remained undisturbed within the nuclear active zone for periods of time comparable to the age of the mare. (2) Alternatively, the probability of mixing to depths of several hundred g cm -2 must be nearly identical to the probability of mixing to several kg cm -2 in contrast to the depth-rate of mixing relationships which are currently in use (Shoemaker et al, 1970;Soderblom and Lebovsky, 1972;Gault et al, 1974). If this is the case, the projectiles responsible for mixing to great depths are not the same population as the small projectiles responsible for cratering and mixing to depths less than 100 g cm -2 • The disagreement between these conclusions and more conventional ideas concerning the physical processes in the lunar regolith may result from the failure to consider the lateral transport of materials in the mixing model.…”

Section: Uniform Mixing Modelmentioning

confidence: 99%

Apollo 17 neutron stratigraphy ?Sedimentation and mixing in the lunar regolith

Curtis

Wasserburg

1975

The Moon

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Abstract. We have measured shifts in the isotopic abunctances of Gd and Sm in soils from the Apollo 17 deep drill stem and calculated the neutron fluence from these measurements. The measurements show two well defined regions of nearly constant fluence: (1) a thick deep section with a very large neutron fluence, and (2) a thinner shallow region with a small fluence. This depth dependence is most plausibly described by a model of rapid accumulation in the last 100-200 m.y., the layered structure reflecting accumulations of isotopically homogeneous source material. This interpretation is compatible with a variety of other characteristics of the soils, including the spallation produced 126 Xe normalized to target element abundances.An alternative model of deposition, followed by irradiation without mixing, followed by shallow mixing will quantitatively describe the data. The model yields an age of 1.25 AE for the bottom of the drill stem. This model was rejected because of the implausible requirement that the soils from the drill stem be accumulated from a source of unirradiated material.The uniformity of various properties of soils provides criteria for defining major stratigraphic intervals in the drill stem which differ from those identified by the Preliminary Examination Team.Neutron fluences measured on shallow and deep soils from all lunar landing sites have been normalized to irradiation in an arbitrary standard chemical environment. We infer from histograms of the normalized fluences that there is a distinct difference in neutron fluence between shallow and deep samples which implies a general vertical stratification of neutron fluence in the lunar regolith.The regolith can be divided into three vertical regions: (1) a well mixed surface layer, ~ 100 g cm-2 thick, with an average fluence of 2.3 x 10 16 n cm-2 , (2) a poorly mixed zone extending from I 00 g cm-2 to at least 500 g cm-2 with an average fluence of 3.5 x 10 16 n cm-2 , and (3) a deep layer of lightly irradiated materials ( < 10 16 n cm-2 ). Analysis of this stratification, using a vertical mixing model, indicates that the probability of mixing to several hundred g cm-2 is comparable to the probability of mixing to several kg cm-2 • This is in contrast to the depth-cratering rate models which have been inferred from crater size frequency distributions using a power law. Alternatively, this discrepancy can be resolved if the true 157 Gd capture rate is t of the value calculated by Lingenfelter et al. (I 972).The most plausible interpretation is that vertical mixing models are not an adequate description of relatively rare deep cratering events which result in significant lateral heterogeneity and addition of unirradiated material to the lunar surface.

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“…Two explanations are possible a) The flattening is an artifact similar to that observed in large scale crater counts on the lunar surface (Gault, 1969, Shoemaker et. al., 1970.…”

Section: Interpretat1 Onsmentioning

confidence: 90%

Micrometeorite craters on lunar rock surfaces

Hörz¹,

Hartung²,

Gault³

1971

J. Geophys. Res.

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Detailed optical stereomicroscope study of seven lunar rocks (12006, 12017, 12021, 12038, 12047, 12051, 12073) has been made. Common microcraters on crystalline rock surfaces consist of a central glass‐lined ‘pit’ surrounded by a crush‐zone or ‘halo’ of microfractured crystalline material, both of which lie within a ‘spall’ area produced by the impact event. Crater diameters measured range from smaller than 0.1 mm up to several millimeters. The ratios of halo diameter to pit diameter and spall diameter to pit diameter average 2.3 and 4.5, respectively. The glass lining of most pits is melted host rock. On the basis of laboratory cratering experiments that yield glass‐lined pits, at least 95% of the microcraters observed are interpreted as the impacts of primary cosmic particles moving at relative velocities greater than 10 km/sec. A sharp demarcation line between cratered and completely uncratered rock surfaces indicates that parts of some rocks were buried in the lunar soil. The presence of ‘ropy splashes’ and ‘welded’ dust near the soil line of the rock is evidence of secondary impacts related to primary impacts occurring in the soil near the rock. Microcratering on the millimeter scale is the dominant process causing erosion of rock surfaces exposed to the lunar environment. Microcrater populations on glass‐covered rock surfaces are not ‘equilibrium’ populations, otherwise the cratering process would have removed the glass coatings entirely. The size distribution of these microcraters corresponds with the interplanetary particle mass distribution and indicates that on the log cumulative flux versus log particle mass curve a negative slope of greater than one exists down to masses in the 10−7–10−8 gram range. Using currently accepted particle‐flux data based on satellite‐borne experiments, a minimum exposure time for the glass surface on rock 12073 is 103 years. The populations of millimeter‐sized microcraters on the most cratered surfaces of all rocks examined are ‘equilibrium’ populations. The minimum time required to achieve the state of equilibrium on the rocks studied is about 105 years. The mean survival times of the seven rocks are calculated to 0.5–3.5×166 years using empirical data on rock destruction by meteroid impact. A similar approach is used to derive a minimum erosion rate of 0.2–0.4 mm in 106 years. However, all these data are heavily dependent on the interplanetary particle flux applied and thus are highly tentative.

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Lunar Regolith at Tranquillity Base

Cited by 70 publications

References 5 publications

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

Apollo 17 neutron stratigraphy ?Sedimentation and mixing in the lunar regolith

Micrometeorite craters on lunar rock surfaces

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