William L. Quaide

Genetic implications of Lunar regolith thickness variations

Abstract Results of the analyses of the distribution of thickness of the regolith in 12 Lunar Orbiter sites are presented. Each of the sites analyzed has one of four type of thickness distributions characterized by approximate median values of 3.3, 4.6, 7.5, and 16 meters. The regolith thickness correlates directly with cratering density, and the average regolith thickness computed from volumes of all craters in an area is in excellent agreement with the measired average thickness determined through use of normal and concentric craters. The form of the distributions of thickness determined from crater ejecta distribution maps are also in excellent agreement with the forms of measured distributions. The results indicate that most of the craters considered and the regolith must be of impact origin. Variations in thickness of the regolith in different areas of the lunar surface reflect differences in elapsed time since the production of new rock surfaces. An idealized history of the evolution of the regolith suggests that the regolith changes with time from an initially coarse-grained, poorly sorted thin deposit of nearly uniform thickness to a thick, fine-grained deposit with nearly equal areas of each thickness interval. Evidence is presented which supports the view that the substrate rocks are of volcanic flow origin, and that flow activity occured repeatedly in any one place but came to a close at different times in different regions of the Moon. Apollo sampling procedures are reviewed and it is concluded that the most readily available samples which can be related to specific selenological events with the highest degree of confidence would be those collected from the blocky ejecta on the rims of concentric craters. The Lunar Orbiter III P11 potential Apollo landing site should be considered as a prime target because of the abundance of craters with blocky rims and because it is the only Apollo site which differs in age from the other potential Apollo sites.

Debye-Scherrer investigations of experimentally shocked silicates.

Small ballistic ranges were used to perform controlled laboratory shock experiments on 12 selected silicates [quartz (30–310 kb), oligoclase (30–340 kb), andesine (40–100 kb), olivine (80–500 kb), forsterite (50–150 kb), enstatite (60–150 kb), biotite (10–90 kb), hornblende (50–150 kb, garnet (40–160 kb), kunzite (60–150 kb), beryl (60–140 kb), topaz (60–150 kb)]. At least 4 pressure points per mineral are available.Debye-Scherrer investigations of shocked materials revealed a gradual lattice breakdown of crystalline matter under shock. Individual mineral species behave selectively. Sheet silicates break down very easily, followed by tecto-silicates. Chain-, ino- and ortho-silicates are of considerably higher shock resistance. Depending on the mineral species, the first sign of shock damage is evidenced in the long range order at 20–70 kb. At intermediate pressures (100–200 kb) the long range order is essentially destroyed with the short range order heavily disturbed. At pressures exceeding 300 kb tecto-silicates are completely collapsed. The degree of internal fragmentation is strongly related to shock pressure, thus providing a sensitive tool for absolute pressure calibration of shocked materials.The internal fragmentation is structurally controlled, leading to polycrystalline aggregates of strongly preferred orientation. The grain size distribution of the fragmentation products is highly heterogeneous. The mechanisms leading to fragmentation as evidenced by the X-ray patterns are highly complex. The formation of high pressure polymorphs is discussed.Though application of the new results to naturally shocked rocks may have some limitations, the usefulness of Debye-Scherrer investigations in the study of shocked materials is demonstrated.

On the origin of the lunar smooth-plains

Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater.Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.

GRAVITATIVE EFFECTS ON LUNAR IMPACT STRUCTURES

Lunar craters from hypervelocity impacts and modifications by gravity sliding, noting other mechanisms for cratering and modification

Development of the mare regolith: Some model considerations

Mare regolith is fragmental debris of variable thickness that lies upon fractured bedrock. Its origin by impact comminution of primarily local basaltic rocks is widely accepted, but the consequences of such an origin are not appreciated fully. This investigation uses results obtained in an earlier Monte Carlo study by Oberbecket al. (1973) to shed light on those consequences by evaluating regolith growth and mixing as a function of time. Results reported are for average cases and must be used with caution. Each small area of the lunar surface has experienced a unique history and results based on averages may have no application to specific cases. Consideration of average processes is useful, nevertheless, when this limitation is kept in mind. The study demonstrates that regolith growth is self regulated and has the same trend and nearly the same terminal growth rates whatever the history of bombardment: rapid initial accumulation followed by diminishing rates of growth. Mixing and all other processes investigated are growth regulated. Mixing increases as growth slows, but never to the extent that the regolith is homogenized. Because the average regolith is never homogenized, products of growth regulated processes are preserved in the stratigraphy. Differences in material properties are to be expected in vertical sections of the regolith, therefore, but this model is not sufficiently refined to permit prediction of all possible trends. It does indicate, however, that deeper levels contain thinner depositional units, lesser quantities of meteoritic and exotic components, and more debris derived from shallow levels in the mare basalts than material in near surface layers. Additionally, neutron fluence production is regulated by the growth process, but because rates of growth do not differ much over the last aeon, whatever the total age or early bombardment history, values of surface fluence may be similar in many areas whatever their age.

Monte Carlo calculations of lunar Regolith thickness distributions

Abstract Regolith thickness distributions associated with crater populations observed on selected maria surfaces have been calculated using a Monte Carlo computer technique. The calculations assume that the crater type produced and the volume of debris ejected and added to the growing regolith depends on the ratio of crater diameter and regolith thickness present at the time and place of formation of each crater. Calculated thickness distributions obtained are in agreement with those estimated using a previously described statistical method based on the morphology of small lunar craters. Additionally, the Monte Carlo calculations accurately predict the size frequency distributions of the same types of small, fresh lunar craters used in the statistical method. The model employed is therefore realistic. Furthermore, the model calculations presented are shown to have value (a) in predicting the thickness of the regolith from crater populations at various lunar sites, (b) relative dating applications in which crater populations are compared, and (c) in interpreting the origin and history of regolith deposits at specific locations.

Luna 9 Photographs: Evidence for a Fragmental Surface Layer

The morphological features of the lunar surface photographed by Luna 9 indicate a surficial layer of weakly cohesive to noncohesive frag mental material. Most of this material is finer than a centimeter and probably finer than a few millimeters, although objects of centimeter size and larger are plentiful.

Rilles, ridges, and domes—Clues to maria history

Abstract The lunar rilles, maria ridges, and maria domes are spacially associated with low areas of the lunar surface,the maria, and immediately adjacent terrae. The spacial association implies a genetic relationship. The morphology and structural setting of most of the rilles suggests that they are tectonic features of tensional origin produced by downward bending of maria borders during a stage of maria foundering. The domes and ridges were most likely produced by small central eruptions or by laccolithic intrusions. Most of the ridges appear to be volcanic structures which grew above dike feeders during the last stages of maria filling. Age relationships and structural settings of the rilles and ridges further suggest that the formation of these features took place in sequence, ridges and domes followed by rilles. It is suggested here that this sequence of events occurred repeatedly during a long history of maria growth and that the maria grew to their present sizes by successive stages of volcanic inundation and collapse. Circular maria probably resulted from a simple enlargement of initially circular depressions whereas the irregularly shaped maria resulted from a coalescence of two or more growing basins or depressions.

Impact metamorphism of lunar surface materials.

Silicate grains from Tranquillity Base have shock-induced features ranging from internal fragmentation through complete disruption of the lattice to thermal melting. Half the crystalline grains with diameters less than 125 micrometers have features of shock equivalent to those produced in the laboratory at pressures greater than about 40 kilobars. One quarter have features indicative of pressures greater than 90 kilobars. These properties together with great quantities of melt glass and aluminum-26 in the fine-grained material are indicative of repeated shock-loading by meteoritic bombardment over long periods of time.

Limnological studies and remote sensing of the Upper Truckee River sediment plume in Lake Tahoe, California-Nevada

Abstract Five studies of the Upper Truckee River sediment plume in Lake Tahoe were conducted in California-Nevada using aerial photography and simultaneous measurements in the lake. The aerial and “water truth” studies covered a range of river discharge conditions during the snowmelt-runoff period in the spring of 1971. Color and multispectral aerial photography allowed delineation of the extent and relative density of four to five units within each plume. Simple correlation coefficients are high between these units and measures of suspended sediment, dissolved inorganic carbon, and light penetration, as well as measures of primary productivity and heterotrophic activity. Correlations are inconsistent between the above variables and nutrients (N-NO 3 , P-PO 4 , and Fe) apparently due to biological utilization of the latter in Tahoes nutrient-poor environment. Two studies were conducted in the morning and afternoon of a single day; the plumes eastward shift during the day was recorded photographically and with limnological measurements. High correlations between sediment plumes and biological productivity coupled with evidence that silt particles and associated nutrients stimulate bacterial growth indicate that sediment plumes are accelerating the eutrophication of Lake Tahoe.