F. Horz

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.

Surface features on glass spherules from the Luna 16 sample

Two ellipsoidal spherules approximately 0.5 mm in diameter were studied in detail using a scanning electron microscope. A variety of surface features were observed: vesicles, mounds, dimples, streaks, ridges, grooves, accretion phenomena, and high-speed impact craters. The diameters of 27 glass-lined pits formed by impact on one spherule range from less than 1μm to approximately 50μm. Intermediate-sized glass-lined pits surrounded by concentric fractures demonstrate the transition between larger craters that have both a pit and a spall zone and generally smaller craters that have only a pit. Assuming all craters showing evidence of impact-related melting or flow are the result of primary impacts, the differential mass spectrum of impacting meteoroids in the range 10−11 to 10−10 g is in good agreement with a spectrum based on satellite-borne particle-detecting experiments.

Micrometeorite craters and related features on lunar rock surfaces

Abstract Craters in the 0.4 mm and larger size class were observed on six Apollo 12 whole rock surfaces (12017, 12021, 12038, 12047, 12051 and 12073). Craters on crystalline surfaces are characterized by a central, glass-lined cavity, a concentric zone of shock fractured, high albedo material and a concentric spallation area. The crater geometries observed are similar to craters produced on glasses and crystalline materials in the laboratory with projectile velocities exceeding 10 km/sec. The high projectile velocities required and the presence of a distinct demarcation line between cratered and uncratered surfaces on individual rocks indicate that most of the microcraters are produced by primary cosmic particles. These discrete impact events account for most of the erosion and fragmentation of lunar surface rocks.