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1.
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.  相似文献   

2.
Amendments to a quantitative scheme developed by T.R. McGetchin et al. (1973) for predicting the distribution of ejecta from lunar basins yield substantially thicker estimates of ejecta, deposited at the basin rim-crest and at varying ranges byond, than does the original model. Estimates of the total volume of material ejected from a basin, illustrated by Imbrium, also are much greater. Because many uncertainties affect any geometric model developed primarily from terrestrial analogs of lunar craters, predictions of ejecta thickness and volume on the Moon may range within at least an order of magnitude. These problems are exemplified by the variability ofT, thickness of ejecta at the rim-crest of terrestrial experimental craters. The proportion ofT to crater rim-height depends critically upon scaled depth-of-burst and whether the explosive is nuclear or chemical.  相似文献   

3.
The impact of aluminum projectiles onto high-alumina terrestrial basalt blocks at 13–15 km s?1 in the presence of a variable magnetic field has been studied. Low-frequency search coil data show that plasma is produced. causing local compression of the ambient field. Although field production is expected, it was not detectable with the existing apparatus. Measurements of the remanence of the shocked basalt show that magnetization was acquired in the material near the craters. The acquired remanence is predominantly soft, but also contains a component not demagnetized by 500 Oe AC field treatment. Material shocked in a 10-Oe vertical field exhibits inverse dependence of magnetization upon distance from the crater center. Examination of the shocked basalt in thin section reveals a general lack of shock metamorphism in the material surrounding the crater, except for the presence of a high-pressure melt glass which was splashed onto the crater walls. Micro-probe analyses show that the glass is a whole-rock melt of fairly uniform composition, and is contaminated with aluminum from the projectile. The mineralogical data support the view that the acquired magnetization is shock remanence, since negligible shock heating occurred in the magnetized material. These results bear on the problem of lunar magnetism, suggesting that shock effects or possibly thermoremanence in ejecta fragments may be responsible for part of the magnetization of the lunar surface.  相似文献   

4.
The settling trends of 318 lunar mare craters are compared with predictions of numerical finite-element models in order to determine the creep response of the upper lunar mare crust. No settling is evident in craters smaller than 5 km in diameter. Settling rates of larger craters increase as function of crater size in a manner suggesting a non-linear lunar creep response corresponding to the power law ε? = 8.3 · 10 ?34 σ2 where έ is the strain rate and σ is the differential stress. However, the observed nonlinearity is probably an apparent nonlinearity resulting from the temperature induced viscosity decrease with depth due to a lunar crustal temperature gradient of 3° C/km and a creep activation energy of 20 kcal/mole. It is concluded that creep in the lunar medium is essentially Newtonian, and that the effective viscosity of the upper lunar mare crust is (1.6 ± 0.3) · 1025 poise.  相似文献   

5.
A review of cratering data and available semi-empirical calculations suggests that the variation of ejecta thickness,t, with increasing range from lunar craters may be approximately modelled by the expression: t=0.14R0.74(r/R?3.0 wherer is range from the center of the crater andR, the crater radius, all in meters. This equation has been used to estimate the thickness of ejecta deposits at each of the Apollo sites contributed from the large multi-ringed frontside lunar basins. Predicted average thickness of Imbrium ejecta at Apollo 15 is 812 m; at Apollo 14, 130 m; at Apollo 17, 102 m; and at Apollo 16, 50 m. Since the sequence of formation of these basins is known, the stratigraphic column resulting from superimposed ejecta blankets can be calculated. Results suggest that pre-Nubium crustal material at upland Apollo sites lies at depths greater than 280 (Apollo 14) to 1940 m (Apollo 17). Predicted stratigraphic sections for the Apollo sites are tabulated.  相似文献   

6.
In this paper a theory is evaluated to describe the development of the lunar crater population with time under the bombardment by meteoroids and solar wind. Starting from a general mass distribution law a differential equation has been established and solved separately for meteoroid impact and solar wind bombardment. The theory permits the calculation of absolute formation ages of the lunar surface as well as the particle flux, supposing the crater distributions on the moon have been measured. As an important result it includes a D−2 equilibrium crater distribution law (D =crater diameter), actually measured in Mare Tranquillitatis and Oceanus Procellarum. Additionally, the exponential decrease of particle flux with time is confirmed.  相似文献   

7.
The axisymmetric distribution of stress, internal energy and particle velocity resulting from the impact of an iron meteoroid with a gabbroic anorthosite lunar crust has been calculated for the regime in which shock-induced melting and vaporization takes place. Comparison of impact flow fields, with phase changes in silicates taken into account, with earlier results demonstrate that in the phase change case when the 15-km/s projectile has penetrated some two projectile radii into the moon, the peak stress in the flow is ～0.66 Mbar at a depth of 66 km, and the stress has decayed to ～66 kbar at a depth of 47 km. Rapid attenuation occurs because of the high rarefaction velocity of the high-pressure phases associated with a 35% (zero-pressure) density increase. This feature of the phase-change flow tends to strongly concentrate the maximum shock pressures along the meteoroid trajectory (axis) and makes the conical zone along which high internal energy deposition occurs, both shallow and narrow. Examination of the gravitational energies required to excavate larger craters on the moon indicates the importance of gravity forces acting during the excavation of craters having radii in the range greater than ～2 – ～140 km. It is observed that the “hydrodynamic” energy vs. crater radius relation approaches those for various “gravitational” energy vs. radius relations at the radii values corresponding to the larger mare basins. Cratering energy values in the range of (1.0 – 9.4) · 1032 erg are inferred on this basis for the Imbrium crater. Using these values and the criteria that all rocks exposed to ～100 kbar or greater shock pressures are included in the ejecta (some of which falls back) implies that the maximum depth of sampling expected to be represented within the Apollo collection lies in the range 148–328 km.  相似文献   

8.
The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=106 abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.  相似文献   

9.
Nanophase Fe metal grains (np-Fe°) are a product of space weathering, formed by processes related to meteorite impacts, and solar-wind sputtering on airless planetary bodies, such as the Moon. Iron isotopes of lunar soils are fractionated during these processes, and the np-Fe° in the finest (<10 μm), mature, size fractions of the soil become enriched in heavier isotopes by ∼0.3‰ in 56Fe/54Fe in comparison to the bulk rocks (0.03±0.05‰), from which the soil was formed. A positive correlation of δ56Fe values with the soil maturity index, IS/FeO, suggests that the high δ56Fe values reflect production of nanophase Fe metal that is produced by space weathering that occurs on airless planetary bodies. Furthermore, the enrichment of δ56Fe in the smallest size fraction of lunar soils supports a model for creation of np-Fe° through vapor deposition induced by micrometeorites, as well as that by solar-wind sputtering.  相似文献   

10.
Magnetic hysteresis loops and the derived hysteresis ratios RH and RI are used to classify the various natural dilute magnetic materials. RI is the ratio of saturation isothermal remanence (IR) to saturation (IS) magnetization, and RH is the ratio of remanent coercive force (HR) to coercive force (HC). The RH and RI values depend on grain size, the characteristics of separate size modes in mixtures of grains of high and low coercivity, and the packing characteristics. Both RH and RI are affected by thermochemical alterations of the ferromagnetic fraction. Hysteresis loop constriction is observed in lunar samples, chondrite meteorites and thermochemically-altered basaltic rocks, and is due to mixtures of components of high and low coercivity. Discrete ranges of RH and RI for terrestrial and lunar samples and for chondrite meteorites provide for a classification of these natural materials based on their hysteresis properties.  相似文献   

11.
Most of the known pit craters in Hawaii occur along the East and Southwest Rift Zones of Kilauea volcano. The pit craters typically are either astride a single rift zone fracture or between a pair of rift zone fractures. These fractures are prominent in the pit crater walls. The pit craters are elliptical in plan view, with their major diameters ranging from 8 to 1140 m. They range in depth from 6 m to 186 m. They typically develop with initially steep, locally overhanging walls, but as the walls collapse, the craters fill with talus and become shaped like inverted elliptical cones. None of the craters apparently formed as eruptive vents, although some have been subsequently filled by lava. Devil's Throat is the best-exposed pit crater along the East Rift Zone. It is sited at a `waist' between two east-striking zones of ground cracks; the spacing between the crack zones decreases towards Devil's Throat. East-striking fractures are also prominent in the pit crater walls. Pit craters along the Southwest Rift Zone typically are elongate in plan view along the direction of the rift, have large caves at their bases along the long axes of the craters, and are smaller than those of the East Rift Zone. Some closely spaced pits there have coalesced to form a trough. Based on our observations and mechanical considerations, we infer that pit craters form by stoping over an underlying large-aperture rift zone fracture, and not by piston-like collapse over broad magma bodies or voids. Flow of magma along the underlying fracture may remove stoped blocks and prevent the fracture from being choked with debris. This mechanism is consistent with pit crater location, ground crack patterns, the preferred orientation of fractures in pit crater walls, and pit crater geometry (both in map view and cross-section). The mechanism also fits with observations of stoping into a gaping rift fracture that conducted lava from Kilauea caldera during the 1920s. Additionally, the ratio of pit crater width to depth of 0.5 to 2 is consistent with pit craters forming over a nearly vertical opening mode fracture.  相似文献   

12.
Both lunar transient phenomena and 222Rn/210Po anomalies observed by Apollo-15 and -16 orbital alpha spectrometers display preferences for certain kinds of locations: rims of circular maria and craters with central peaks and/or dark floors. If these classes of observations are due to lunar gas venting, why are these types of locations preferred? The hypothesis offered is that these are locations at which cracks or channels exist extending deep enough into the moon to tap lunar volatile reservoirs. Possible channels include circumferential cracks around circular maria, old lava tubes for dark-floor and volcanic central peak craters, and shattered subsurface rock structure for impact central peak craters.  相似文献   

13.
Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than 14° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10?6 gauss cm3 gm?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.  相似文献   

14.
The Zerga meteorite, an LL6 ordinary chondrite found at Aouelloul crater in 1973, is a small fragment of a larger mass whose pre-atmospheric radius was most likely between 20 and 125 cm. A typical amphoterite, it is a monomict breccia that has undergone at least one recrystallization episode.3He and21Ne contents define a shielding-corrected, cosmic ray exposure age of 21–24 × 106 years and the26Al content is consistent with a terrestrial age ?500,000 years (2σ limit). The K—Ar gas-retention age is 3.1 × 109 years. The meteorite's areal association with the impact crater is merely coincidental. A new K-Ar age of the glassy impactite found at Aouelloul dates the crater at 3.1 ± 0.3 × 106 years, sensibly indistinguishable from the recently determined age of nearby Tenoumer crater (2.5 ± 0.5 × 106years). The similar ages of these two impact craters, and their almost perfect linear alignment with a third, morphologically similar crater (Temimichat Ghallaman) over a distance of 600 km, suggests a simultaneous triple impact occasioned by the disruption of a large meteorite moving on a very shallow atmospheric trajectory. If so, the concomitant low impact angles may be responsible for the unusually shallow original depths inferred for two of the craters from gravity data.  相似文献   

15.
207Pb/206Pb of “low temperature sited” (LTS) lead as reported by Silver (1975) increases with40Ar/36Ar of trapped argon in thirteen samples from lunar maria. This strongly supports an earlier conclusion by (1972) that large (40Ar/36Ar)T ratios represent ancient regolith records, and provides a rough (40Ar/36Ar)T timescale.The erasure of (40Ar/36Ar)T records in surface soils by the excavation of deep-seated, “fresh” bedrock and by erosion of particle surfaces via ion sputtering must have been counteracted by conserving processes in the regolith. Two such processes are relatively well understood: agglutinate formation and the excavation and comminution of soil breccias which have preserved an ancient (40Ar/36Ar)T record. The frequency distribution of (40Ar/36Ar)T in 82 “soils” from all Apollo missions suggests a third process, which requires that sizeable “pockets” of ancient regolith materials including soils have survived deep turnover for billions of years.Large-scale mobility of LTS lead throughout all of the regolith does not appear to occur.Inert gas ions with sufficient energy for trapping may have reached the lunar surface more than 3 b.y. ago.The Apollo 11 microbreccias appear to have been formed more than 3 b.y. ago from regoliththen extant on the surface.  相似文献   

16.
Strudel scours are craters in the sea floor as much as 25 m wide and 6 m deep, that are excavated by vertical drainage flow during the yearly spring flooding of vast reaches of shorefast ice surrounding arctic deltas; they form at a rate of about 2.5 km?2 y?1. We monitored two such craters in the Beaufort Sea and found that in relatively unprotected sites they fill in by deposition from bedload in 2 to 3 years. Net westward sediment transport results in sand layers dipping at the angle of repose westward into the strudel-scour crater, whereas the west wall of the crater remains steep to vertical. At the bottom the crater traps almost all bedload: sand, pebbles, and organic detritus. As infilling progresses, the materials are increasingly winnowed, and bypassing must occur. Over a 20m wide sector, an exposed strudel scour trapped 360 m3 of bedload during two seasons; this infilling represents a bedload transport rate of 9 m3 y?1 m?1. This rate should be applicable to a 4.5-km wide zone with equal exposure and similar or shallower depth. Within this zone, the transport rate is 40,500 m3 y?1, similar to estimated longshore transport rates on local barrier beaches. Based on the established rate of cut and fill, all the delta-front deposits should consist of strudel-scour fill. Vibracores typically show dipping interbedded sand and lenses of organic material draped over steep erosional contacts, and an absence of horizontal continuity of strata—criteria that should uniquely identify high-latitude deltaic deposits. Given a short 2- to 3-year lifespan, most strudel scours seen in surveys must be old and partially filled. The same holds true for ice gouges and other depressions not adjusted to summer waves and currents, and therefore such features record events of only the past few years. In view of such high rates of bottom reworking of the shallow shelf, any human activities causing turbidity, such as dredging, would have little effect on the environment. However, huge amounts of transitory material trapped by long causeways planned for offshore development would result in major changes in the environment.  相似文献   

17.
The times at which phyllosilicate matrix and euhedral olivines became associated have been determined for five C2 meteorites. The ages, calculated from fission track densities on crystal surfaces, are based on an initial244Pu/238U ratio in the matrix material of 0.0154 at 4.6 b.y., and range from 4.22 b.y. for Nogoya to 4.42 b.y. for Murray. Unless the initial244Pu/238U ratio was less than 0.004, the meteorites cannot have existed in their present form for 4.6 × 109 yr. The measured ages place limits on the time when pre-compaction effects such as micrometeorite craters and solar flare tracks were produced, and they may approximately date the formation of the olivine crystals themselves.  相似文献   

18.
Accurate and precisely located self-potential (SP), temperature (T) and CO2 measurements were carried out in the summit area of Stromboli along 72 straight profiles. SP data were acquired every metre and T data every 2.5 m. CO2 concentrations were acquired with the same density as T, but only along seven profiles. The high density of data and the diversity of the measured parameters allows us to study structures and phenomena at a scale rarely investigated. The shallow summit hydrothermal activity (Pizzo–Fossa area) is indicated by large positive SP, T and CO2 anomalies. These anomalies are focused on crater faults, suggesting that the fracture zones are more permeable than surrounding rocks at Stromboli. The analysis of the distribution of these linear anomalies, coupled with the examination of the geologic, photographic and topographic data, has led us to propose a new structural interpretation of the summit of Stromboli. This newly defined structural framework comprises (1) a large Pizzo circular crater, about 350 m in diameter; (2) a complex of two concealed craters nested within the Pizzo crater (the Large and the Small Fossa craters), thought to have formed during the eruption of the Pizzo pyroclastites unit; the Small Fossa crater is filled with highly impermeable material that totally impedes the upward flow of hydrothermal fluids; and (3) The present complex of active craters. On the floor of the Fossa, short wavelength SP lows are organized in drainage-like networks diverging from the main thermal anomalies and converging toward the topographic low in the Fossa area, inside the Small Fossa crater. They are interpreted as the subsurface downhill flow of water condensed above the thermal anomalies. We suspect that water accumulates below the Small Fossa crater as a perched water body, representing a high threat of strong phreatic and phreatomagmatic paroxysms. T and CO2 anomalies are highly correlated. The two types of anomalies have very similar shapes, but the sensitivity of CO2 measurements seems higher for lowest hydrothermal flux. Above T anomalies, a pronounced high frequency SP signal is observed. Isotopic analyses of the fluids show similar compositions between the gases rising through the faults of the Pizzo and Large Fossa craters. This suggests a common origin for gases emerging along different structural paths within the summit of Stromboli. A site was found along the Large Fossa crater fault where high gas flux and low air contamination made gas monitoring possible near the active vents using the alkaline bottle sampling technique.  相似文献   

19.
Zusammenfassung Es wird gezeigt, dass wir einige irdische Vulkankrater kennen, deren morphologischer Aufbau nahezu identisch ist mit den typischen Formen der Mondkrater. Auf die Wichtigkeit dieses Befundes für das Problem der Entstehung der Mondkrater wird hingewiesen. Es ist zu vermuten, dass sich auf der Erde noch weitere Beispiele dieser Art werden finden lassen.
Summary It is shown, that some terrestrial volcanic craters are known, the morphological structure of which is practically identical with the typical shapes of lunar craters. The importance of this relation for the problem of the origin of lunar craters is pointed out. It may be expected, that some more examples of this kind might be found.
  相似文献   

20.
During the initial explosive phase of the eruption of Arenal volcano small projectiles were thrown a maximum distance of 5 km. Considering the effect of atmospheric drag these projectiles must have had initial velocities of at least 600 m/sec. For this velocity, the gas pressure in the magma chamber must have reached at least 4700 bars and the kinetic energy of the initial explosion is estimated as 2.4 ± 1.2 × 10a ergs. Had the effect of aerodynamic braking been ignored in making these calculations, as has always been done in the past, the calculated initial velocity would have been 220 m/sec; chamber pressure and kinetic energy estimates would thus be substantially lower. Clearly, velocities of ejecta, chamber pressures and kinetic energies for many explosive volcanic events have been seriously underestimated in the recent past, as has been the ability of overlying materials to contain, in certain cases, tremendous overpressures for short periods of time. A projectile with an initial velocity of 600 m/sec would have a maximum range of more than 200 km on the moon. Thus, the presence of far-reaching secondary crater fields on the moon cannot, at this time, be considered evidence for an impact origin of the parent crater. 600 m/sec is not the upper limit for initial velocities of volcanic ejecta. There is some indication that such velocities could reach values greater than 2 km/sec, suggesting that volcanic as well as impact mechanisms may be able to impart escape velocity to lunar materials.  相似文献   

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