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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

The role of rim slumping in the modification of lunar impact craters

The role of rim slumping in the formation of terraces on interior crater walls and consequent modifications of the shapes of large lunar impact craters is evaluated. Impact excavation cavities were reconstructed by means of a terrace restoration model for twelve lunar craters ranging in diameter from 19 to 137 km. For craters up to 30 km, reconstructed cavity depths are comparable to or greater than cavity depths extrapolated from nonterraced smaller craters, and in the 30 to 70 km diameter range, the reconstructed cavity depths are significantly greater. For craters larger than 70 km, however, the reconstructed cavity depths are consistently less than those predicted from small crater morphometry, indicating a significant additional contribution from ejecta fallback and basement rebound to the modification of the shape of impact craters greater than 70 km.

Settle, M.↗
NASA NTRS1979

Geology of the lunar farside crater Necho

The ejecta morphology and distribution as well as the interior materials of the 30-km-diameter, Necho lunar crater is discussed and the effects of regional preimpact topography on the formation structure and modification of the crater are described. Three types of ejecta: hummocky, lineated, and smooth, that are concentrated in a band extending from the rim crest to a distance of less than one crater diameter are examined. The crater's interior, divided into floor units of textured, bubbled, and smooth material is analyzed and hypotheses of their origin are presented. The last area of study is the preimpact setting, characterized by a series of large, degraded, pre-Nectarian craters. Results showed that the Necho, in comparison with surrounding craters, was the freshest, insuring that the data from its excavation was preserved without a significant amount of degradation.

Gifford, A. W.↗
NASA NTRS1979

Atmospheric effects on Martian ejecta emplacement

The paper presents analytical descriptions of crater growth and numerical calculations of aerodynamic drag to evaluate the possible effects of drag on impact crater ejecta emplacement on Mars. The critical particle size below which ejecta deposition is restricted in range increases with crater size; models of ejecta trajectories in the current Martian atmosphere under hydrostatic equilibrium reveal critical particle diameters ranging from 0.4 to 20 cm, noting that ejecta approaching the critical particle size may impact with crater radius of the excavation crater rim. Ejecta larger than the critical particle size are undecelerated and form secondary impact craters modified by the later arriving decelerated ejecta cloud; thus, ejecta emplacement will be multiphased, but the process depends on the ejecta size distribution.

Schultz, P. H.↗
NASA NTRS1979

The Apollo 17 drill core - Petrologic systematics and the identification of a possible Tycho component

Modal data support a five-unit stratigraphy for the Apollo 17 drill core. The upper unit E (0-22 cm depth) is marked by high content of fused soil, brown glass, and mare basalt fragments. This unit corresponds with a portion of the core excavated and refilled within the last 2 m.y. The underlying unit D (22071 cm depth) has a low abundance of fused soil (i.e., low maturity) and is rich in coarse (less than 200 microns) mare fragments. A large section of the core, unit C (71-224 cm depth), is finer-grained, more mature (richer in agglutinates), more feldspathic and has more highland lithic, mineral and glass fragments than unit D. The next underlying unit, B (224-256 cm depth), has yellow/colorless KREEP glasses with a high Si, low-alkali composition unlike the common Apollo 15 or Apollo 17 KREEP series. The petrologic (fused soil) and Is/FeO maturity of this layer is also lower than the units above and below. The deepest unit, A (256-284 cm depth), is marked by its relatively higher maturity and lower yellow/colorless KREEP glass content. The most prominent petrographic/stratigraphic indicators are the pyroxene-rich immature mare unit D and the abundance of KREEP glass in unit B. This KREEP glass is distinctive petrographically and compositionally, and is probably exotic to the Apollo 17 site. It is suggested here that the KREEP glass in unit B is derived from Tycho, which implies widespread distribution of KREEP on the lunar nearside.

Vaniman, D. T.↗
NASA NTRS1979

History of the Apollo 17 deep drill string during the past few million years

Measurements of the cosmogenic radionuclides 22-Na, 26-Al and 53-Mn in the Apollo 17 deep drill string 70002-70009 have shown that this core sample was taken in an area of considerable recent surface activity. Measurements of 22-Na indicate that the core had not been significantly altered during its collection and subsequent handling, while measurements of the 26-Al and 53-Mn showed a very substantial excess in these isotopes in the top 25 cm and a depletion below about 50 cm. The analysis of the data indicates that a crater excavation occurred to a depth of 50 to 60 cm at the site of the deep drill string no longer that 0.5 m.y. ago, and probably 0.3 m.y. ago. Since that time, the crater has filled to a depth of about 25 cm with near-surface material which contains high concentrations of 26-Al and 53-Mn. This recent activity at the site of the Apollo 17 drill string places firm time constraints on the models developed by other investigators which describe the history of this site.

Fruchter, J. S.↗
NASA NTRS1979

Are early magnesium-rich basalts widespread on the moon

Orbital X-ray data indicate that high-magnesium material systematically occurs along mare/terra boundaries in all the lunar nearside basins for which there is orbital X-ray coverage. These peripheral units appear to represent the earliest stages of volcanism within each of the basins, although they were not necessarily emplaced at the same time in lunar history. High-magnesium subsurface layers excavated by post-mare impact craters in central Mare Crisium and Mare Fecunditatis suggest that the peripheral units are the only visible edges of a basin-wide basalt layer, largely buried by younger less-magnesian flows. This evidence suggests that early magnesium-rich mare basalts are widespread on the moon and that their volumes may be greatly underestimated. Abundant early mare deposits of this type would place severe constraints on models of mare basalt petrogenesis.

Andre, C. G.↗
NASA NTRS1979

Structural deformation at the Flynn Creek impact crater, Tennessee - A preliminary report on deep drilling

The geologic and core drilling studies described in the present paper show that the Flynn Creek crater has such distinctive morphological features as a broad flat hummocky floor; large central peak; locally terraced crater walls; uplifted, as well as flat-lying rim segments; and a surrounding ejecta blanket. The major structural features include a shallow depth of total brecciation and excavation as compared with apparent crater diameter; a thin breccia lens underlain by a thin zone of disrupted strata; concentric ring fault zones in inner rim, beneath crater wall, and outer crater floor regions; a large central uplift underlain by a narrow dipping zone of deeply disrupted strata; faulted, folded, brecciated, and fractured rim strata; and uplifted rim strata, which dip away from the crater, and flat-lying rim strata, which terminate as inward dipping rocks.

Roddy, D. J.↗
NASA NTRS1979

Evidence for ancient mare volcanism

The results of the present study suggest that dark-haloed impact craters are indicative of excavated mafic materials, originally emplaced as volcanic units, but subsequently buried by impact ejecta deposits from large craters and basins. Consequently, mapped light plains units should be viewed as soil units that may not reflect the origin of the underlying rock. Some observations which support these conclusions are examined.

Schultz, P. H.↗
NASA NTRS1979

Sample fields of the Viking landers, physical properties, and aeolian processes

Surface sampler activities on Mars during the Viking extended mission are considered, including excavation of deep trenches, construction of conical piles of materials, backhoe touchdown experiments, and acquisition of contiguous pictures of the surface beneath number 2 terminal descent engines using mirrors. Results of the Physical Properties Investigation that are relevant to aeolian processes are also discussed. Both pictures and surface sampler data indicate that the surface materials in the sample fields of the Viking landers may be grouped, in order of increasing strength, into drift material, crusty to cloddy material, blocky material and rocks.

Moore, H. J.↗
NASA NTRS1980

The role of fallback ejecta in the modification of impact craters

The physical processes responsible for the deposition of fragments of target material ejected from the transient cavity of an impact crater within the crater are discussed and quantities of fallback ejecta deposited in impact craters formed on different planetary surfaces are estimated. Consideration is given to the relative importance of in-flight particle collisions and atmospheric particle deceleration, and a model of the aerodynamic deceleration of fallback ejecta is developed. When applied to Meteor Crater in Arizona, the model predicts 5% of the primary ejecta, with an average fragment size of 1.4 cm, to have fallen back into the crater, in agreement with empirical studies of the crater fallback deposit, thus indicating atmospheric deceleration to be the principal mechanism for fallback deposition on earth. The model also predicts the proportion of primary ejecta to be deposited as fallback to increase with excavation cavity diameter on the earth. Model estimates also suggest that more than half of the primary ejecta from large-scale Venusian cavities should be redeposited as fallback, while relatively small amounts of fallback are expected for lunar and Martian craters.

Settle, M.↗
NASA NTRS1979

Scaling and the start-up phase of space industrialization

By terrestrial standards very little mass is needed to construct the space portion of a 10,000 megawatt (10 GW) power system. Use of lunar materials makes it reasonable to consider alternatives to silicon solar cells for conversion of sunlight to electricity and thereby avoid present major problems associated with solar cell production. Machinery needed on the moon to excavate lunar materials and deliver them to a transport system, to beneficiate lunar materials, to produce glasses and ceramics from lunar materials and to chemically process lunar materials into their major oxides and elements are minor mass fractions of the total mass of equipment needed in space to produce an SPS. In addition the processing equipment can throughput several hundred times their own mass each year with very little requirement for makeup mass from earth.

Criswell, D. R.↗
NASA NTRS1980

Formation of complex impact craters - Evidence from Mars and other planets

An analysis of the depth vs diameter data of Arthur (1980), is given along with geomorphic data for 73 Martian craters. The implications for the formation of complex impact craters on solid planets is discussed. The analysis integrates detailed morphological observations on planetary craters with geologic data from terrestrial meteorite and explosion craters. The simple to complex transition for impact craters on Mars appears at diameters in the range of 3 to 8 km. Five features appear sequentially with increasing crater size, flat floors, central peaks and shallower depths, scalloped rims, and terraced walls. This order suggests that a shallow depth of excavation and a rebound mechanism have produced the central peaks, not centripetal collapse and deep sliding. Simple craters are relatively uniform in shape from planet to planet, but complex craters vary considerably. Both the average onset diameter for complex impact craters on Mars and the average depth of complex craters vary inversely with gravitational acceleration on four planets.

Pike, R. J.↗
NASA NTRS1980

Lunar nitrogen - Evidence for secular change in the solar wind

The lunar soil is an efficient trap for nitrogen in the solar wind. The concentration of nitrogen in lunar soils implies a flux of solar wind at least three times greater than that which is observed today. The implication is that the present-day flux is anomalously low. Soil samples from drill cores, and soil breccias excavated from buried levels of the regolith provide samples of ancient solar wind. From these samples it is apparent that the N-15/N-14 ratio in the solar wind has undergone a monotonic secular increase over the last 2.5 billion years, at a rate of about 15% per billion years. Nuclear reactions in the sun appear to be the most likely cause of the variation.

Clayton, R. N.↗
NASA NTRS1981

Radar mapping, archaeology, and ancient land use in the Maya lowlands

Data from the use of synthetic aperture radar in aerial survey of the southern Maya lowlands suggest the presence of very large areas drained by ancient canals for the purpose of intensive cultivation. Preliminary ground checks in several very limited areas confirm the existence of canals and raised fields. Excavations and ground surveys by several scholars provide valuable comparative information. Taken together, the new data suggest that Late Classic period Maya civilization was firmly grounded in large-scale and intensive cultivation of swampy zones.

Adams, R. E. W.↗
NASA NTRS1982

Longwall Guidance and Control Development

The longwall guidance and control (G&C) system was evaluated to determine which systems and subsystems lent themselves to automatic control in the mining of coal. The upper coal/shale interface was identified as the reference for a vertical G&C system, with two sensors (the natural backgound and the sensitized pick) being used to locate and track this boundary. In order to insure a relatively smooth recession surface (roof and floor of the excavated seam), a last and present cut measuring instrument (acoustic sensor) was used. Potentiometers were used to measure elevations of the shearer arms. The intergration of these components comprised the vertical control system (pitch control). Yaw and roll control were incorporated into a face alignment system which was designed to keep the coal face normal to its external boundaries. Numerous tests, in the laboratory and in the field, have confirmed the feasibility of automatic horizon control, as well as determining the face alignment.

Source record↗
NASA NTRS1982

A schematic model of crater modification by gravity

The morphology of craters found on planets and moons of the solar system is examined and a development model which can account for the observed crater characteristics is discussed. The prompt collapse of craters to form flat floors, terraced walls, and central peak structures is considered to be the result of an approximate Bingham plastic rheology of the material surrounding the crater. This rheology is induced dynamically by the strong incoherent acoustic 'noise' accompanying excavation of the crater. Central pits, peak rings, and other multiple symmetric-profile rings originate by oscillation of this fluid. Large craters with transient depths comparable to the lithosphere thickness are subject to collapse by fragmentation of the lithosphere as well as fluidization. The considered concepts are developed mathematically. A model emerges which appears capable of explaining most of the qualitative features of large impact structures.

Melosh, H. J.↗
NASA NTRS1980

Impact cratering in viscous targets - Laboratory experiments

To determine the effects of target yield strength and viscosity on the formation and morphology of Martian multilobed, slosh and rampart-type impact craters, 75 experiments in which target properties and impact energies were varied were carried out for high-speed motion picture observation in keeping with the following sequence: (1) projectile initial impact; (2) crater excavation and rise of ejecta plume; (3) formation of a transient central mound which generates a surge of material upon collapse that can partly override the plume deposit; and (4) oscillation of the central mound with progressively smaller surges of material leaving the crater. A dimensional analysis of the experimental results indicates that the dimensions of the central mound are proportional to (1) the energy of the impacting projectile and (2) to the inverse of both the yield strength and viscosity of the target material, and it is determined that extrapolation of these results to large Martian craters requires an effective surface layer viscosity of less than 10 to the 10th poise. These results may also be applicable to impacts on outer planet satellites composed of ice-silicate mixtures.

Greeley, R.↗
NASA NTRS1982

Distribution and provenance of lunar highland rock types at North Ray Crater, Apollo 16

In connection with its selenographic setting in the central lunar highlands, the Apollo 16 landing site in the Descartes area is highly important as a prime sampling area for rocks which formed as part of the primordial crust and as a key location for the analysis of the deformation and transport of crustal material by impact processes. The present investigation is concerned with the North Ray crater, which is located on the N-S running boundary between the smooth Cayley plains to the west and the Descartes mountains to the east. Attention is given to aspects of selenography and location of samples, the ejecta distribution of post-Cayley impact craters, sample classification, the frequency distribution of rock types in the North Ray Crater ejecta, an interpretation of compositional and age data, a model of the target stratigraphy and excavation of North Ray Crater, and implications for the emplacement and provenance of North Ray target rocks.

Stoeffler, D.↗