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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Stratification in the lunar regolith - A preliminary view

Although our knowledge of lunar regolith stratification is incomplete, several categories of thick and thin strata have been identified. Relatively thick units average 2 to 3 cm in thickness, and appear surficially to be massive. On more detailed examination, these units can be uniformly fine-grained, can show internal trends, or can show internal variations which apparently are random. Other thick units contain soil clasts apparently reworked from underlying units. Thin laminae average approximately 1 mm in thickness; lenticular distribution and composition of some thin laminae indicates that they are fillets shed from adjacent rock fragments. Other dark fine-grained well-sorted thin laminae appear to be surficial zones reworked by micrometeorites. Interpretations of stratigraphic succession can be strengthened by the occurrence of characteristic coarse rock fragments and the orientation of large spatter agglutinates, which are commonly found in their original depositional orientation.

Duke, M. B.↗
NASA NTRS1976

Physical properties of interplanetary grains

Morphological analyses of micrometeorite craters found on lunar rocks and laboratory simulation experiments are used to formulate a meteoritic interplanetary dust particle for optical scattering calculations that is roughly spherical and has a density of 2g cm/3. The model particle has chondritic elemental abundances and also contains a high content of finely dispersed carbon.

Brownlee, D. E.↗
NASA NTRS1976

The journey to Jupiter

The Pioneer missions to Jupiter are reviewed, and the observations made by the two probes are discussed. The spacecraft are described along with their launches, instrument packages, and trajectories. The major results of the micrometeorite, plasma, magnetic-field, and charged-particle experiments are summarized. The structure of Jupiter's magnetosphere is illustrated, spectroscopic and photometric observations by the Pioneers are discussed, and measurements of the Jovian atmosphere and gravitational field are examined. The measured masses and densities of the Galilean satellites are given, and future prospects for the Pioneer spacecraft are noted, including the possibility that Pioneer 10 may still be in contact with earth when it crosses the boundary between the solar system and interstellar space.

Hall, C. F.↗
NASA NTRS1976

Petrology of lunar rocks and implication to lunar evolution

Recent advances in lunar petrology, based on studies of lunar rock samples available through the Apollo program, are reviewed. Samples of bedrock from both maria and terra have been collected where micrometeorite impact penetrated the regolith and brought bedrock to the surface, but no in situ cores have been taken. Lunar petrogenesis and lunar thermal history supported by studies of the rock sample are discussed and a tentative evolutionary scenario is constructed. Mare basalts, terra assemblages of breccias, soils, rocks, and regolith are subjected to elemental analysis, mineralogical analysis, trace content analysis, with studies of texture, ages and isotopic composition. Probable sources of mare basalts are indicated.

Ridley, W. I.↗
NASA NTRS1976

Exospheric density measurements from the drag-free satellite Triad

The Triad satellite is maintained within 1 mm of its proof mass (a small metal ball) in a purely gravitational orbit, using a drag-free control system, named Discos, as the disturbance compensation system. The Discos proof mass is surrounded by an outer shell which holds fuel tanks and cold gas jets. The shell shields the proof mass from such nongravitational forces as radiation pressure, atmospheric drag, and micrometeorite impact. Whenever these forces displace the outer shell relative to the proof mass, an error signal is generated by a capacitive bridge sensor, and the satellite is propelled by gas jets to remain centered on the proof mass when the error reaches 1 mm. Local atmospheric densities near 800 km were measured, using ball position data, and the observed accelerations were corrected for the effects of solar radiation pressure. The measured densities reveal a greater dependence on latitude than that indicated by balloon satellites, and also a dependence on longitude. Only a small dependence on Kp, however, was observed in the region where the measurements were made.

Moe, K.↗
NASA NTRS1976

Formation chronology for C2 meteorites

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 initial Pu-244/U-238 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 initial Pu-244/U-238 ratio was less than 0.004, the meteorites cannot have existed in their present form for 4.6 billion years. 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.

Macdougall, J. D.↗
NASA NTRS1976

Ferromagnetic resonance studies of lunar core stratigraphy

We first review the evidence which links the characteristic ferromagnetic resonance observed in lunar fines samples with agglutinatic glass produced primarily by micrometeorite impacts and present new results on Apollo 15, 16, and 17 breccias which support this link by showing that only regolith breccias contribute significantly to the characteristic FMR intensity. We then provide a calibration of the amount of Fe metal in the form of uniformly magnetized spheres required to give our observed FMR intensities and discuss the theoretical magnetic behavior to be expected of Fe spheres as a function of size. Finally, we present FMR results on samples from every 5 mm interval in the core segments 60003, 60009, and 70009. These results lead us to suggest: (1) that secondary mixing may generally be extensive during regolith deposition so that buried regolith surfaces are hard to recognize or define; and (2) that local grinding of rocks and pebbles during deposition may lead to short scale fluctuations in grain size, composition, and apparent exposure age of samples.

Housley, R. M.↗
NASA NTRS1976

Surface exposure indices of lunar soils - A comparative FMR study

The intensity of the characteristic FMR resonance of a lunar soil normalized to its total iron content (Is/FeO) was tested for use as its index of relative surface exposure age. The characteristic resonance is due to fine-grained metal that is the result of micrometeorite impact at the lunar surface. In this study, Is/FeO was measured for the below 250 micron size fraction of 152 soils and for the 90-150 micron size fraction of 88 soils. With this large suite of samples it was possible to correlate Is/FeO with a variety of other indices of relative surface exposure age (N, C, trapped He-4, trapped Ar-36, petrographic and magnetic agglutinates, and mean grain size) and thus not only affirm that Is/FeO is an index of relative surface exposure age but also consider the relative merits of the various surface exposure indices.

Morris, R. V.↗
NASA NTRS1976

Lead isotopic studies of lunar soils - Their bearing on the time scale of agglutinate formation

Fines (smaller than 75 microns) and bulk soil were studied to analyze loss of volatile lead; losses of the order of 10% to 30% radiogenic lead during the production of agglutinates are assessed. Lead isotope data from fine-agglutinate pairs are analyzed for information on the time scale of micrometeorite bombardment, from the chords generated by the data in concordia diagrams. Resulting mean lead loss ages were compared to spallogenic gas exposure ages for all samples. Labile parentless radiogenic Pb residing preferentially on or in the fines is viewed as possibly responsible for aberrant lead loss ages. Bulk soils plot above the concordia curve (in a field of excess radiogenic Pb) for all samples with anomalous ages.

Church, S. E.↗
NASA NTRS1976

Microcraters and solar flare tracks in crystals from carbonaceous chondrites and lunar breccias

Results of microcrater and solar-flare track studies of grains from the interiors of five carbonaceous chondrites and one lunar 'soil breccia' are combined to investigate variations in solar flare intensity, variations in micrometeorite particle flux, characteristics of interplanetary submicron particles, and the probable region in space where the precompaction irradiation occurred. The thickness turnover rate, material loss rate, and other parameters of the regolith-like surface where the meteorites must have formed are determined to a certain extent by considering published data on solar-wind and spallogenic species in the meteorites in conjunction with the present data. It is found that: (1) most irradiation features are most plausibly explained in terms of a regolith origin; (2) the shape of the solar-flare energy spectrum has not changed over the last 4.2 billion years; (3) the flux of small micrometeoroids about 4.2 billion years ago cannot have been more than 4 to 20 times higher than the flux today; and (4) parent-body regolith turnover rates were not extremely high.

Goswami, J. N.↗
NASA NTRS1977

Microcraters on lunar samples

The lunar microcrater phenomenology is described. The morphology of the lunar craters is in almost all aspects simulated in laboratory experiments in the diameter range from less than 1 nu to several millimeters and up to 60 km/s impact velocity. An empirically derived formula is given for the conversion of crater diameters into projectile diameters and masses for given impact velocities and projectile and target densities. The production size frequency distribution for lunar craters in the crater size range from approximately 1 nu to several millimeters in diameter is derived from various microcrater measurements within a factor of up to 5. Particle track exposure age measurements for a variety of lunar samples have been performed. They allow the conversion of the lunar crater size frequency production distributions into particle fluxes. The development of crater populations on lunar rocks under self-destruction by subsequent meteoroid impacts and crater overlap is discussed and theoretically described. Erosion rates on lunar rocks on the order of several millimeters per 10 yr are calculated. Chemical investigations of the glass linings of lunar craters yield clear evidence of admixture of projectile material only in one case, where the remnants of an iron-nickel micrometeorite have been identified.

Fechtig, H.↗
NASA NTRS1977

The radiation history of material returned by the Soviet automatic stations Luna 16 and Luna 20, according to track studies

Fission tracks formed by the vH (very heavy) nuclei group of solar and galactic cosmic rays have been studied in silicate minerals of the lunar regolith returned by the Luna 16 and Luna 20 unmanned spacecraft. It is shown that the material in the Luna 16 core sample, from a typical mare region of the lunar surface, has undergone stronger irradiation by cosmic rays than material returned a highland region by Luna 20. A low-irradiation component (about 10 percent of the total number of crystals) has been found in the Luna 20 core sample materials, which can possibly be attributed to material added to the main bulk of the regolith in the formation of the crater Apollonius C. From the track density distribution of crystals, as a function of depth in the regolith core sample, it follows that the process of formation of the upper layer of the regolith, both for the lunar mare and for the highland region, includes sequential layering of finely crushed crystalline matter and subsequent mixing of it by micrometeorite bombardment. A portion of the crystals with a very high track density may be a component added to the lunar surface from outer space.

Kashkarov, L. L.↗
NASA NTRS1977

An upper limit for the flux of interstellar dust grains in the solar system

Data from the LEAM (Lunar Ejecta and Meteorite experiment, a micrometeorite detector at the Apollo 17 landing site) have been examined for evidence of interstellar (IS) dust grains traversing the solar system. The analysis technique considers IS grains approaching the solar system from the local solar apex. A model calculates the grains' hyperbolic orbits into the solar system and predicts the impact directions on the moon. The observations are then compared with the predicted impact directions to measure the IS dust flux. No evidence has been found (at the 97.5% confidence level) for a flux greater than about 6 hundred-thousandths per sq m/sec for particles of at least 2 by 10 to the -14th power g.

Tomandl, D. A.↗
NASA NTRS1977

12054 and 76215 - New measurements of interplanetary dust and solar flare fluxes

The mass distribution and flux of micrometeoroids, variations in solar activity, solar-wind erosion and solar-flare track production are discussed on the basis of lunar sample analyses. A bimodal size frequency distribution of micrometeorites is found; the ratio of the density of craters larger than 0.1 micron to the density of those larger than 500 microns is 50 to 100 million. Solar cosmic-ray track ages determined for the lunar samples through use of the model of Blanford et al. (1975) indicate no variation in solar activity over a period of 2 million years. Solar wind erosion is set at no more than 0.03 A per year.

Morrison, D. A.↗
NASA NTRS1977

Shock metamorphism of lunar and terrestrial basalts

Lonar Crater (India) basalt and lunar basalt 75035 were shock loaded under controlled laboratory conditions up to 1000 kbar, generally in a CO/CO2 (1:1) environment evacuated to 10 to the minus seventh power torr. The Kieffer et al. (1976) classification scheme of progressive shock metamorphism is found to apply to lunar basalts. The major shock features of the five classes that span the range 0 to 1000 kbar are described. Only three out of 152 basalt specimens show shock effects in their natural state as severe as Class 2 features. The scarcity of shocked basalt hand samples in contrast to the abundance of shock-produced agglutinates and homogeneous glass spheres in the lunar regolith indicates the dominant role of micrometeorite impact in the evolution of the lunar regolith. The overall glass content in asteroidal and Mercurian regoliths is considered.

Schaal, R. B.↗
NASA NTRS1977

Noble gas evidence for the depositional and irradiational history of 60010-60009 core soils

Isotopic abundances of the noble gases have been determined in grain size separates of eleven soils from different depths in the 60010-60009 double drive tube and in magnetic and plagioclase separates from a few of these soils. Data for the 60010 core are presented here. The entire core was deposited a maximum of approximately 125 m.y. ago as deduced from the Ar-38 cosmic ray exposure age of soil 60009,457. Soils in the topmost 12 cm of the core show loss of cosmogenic He-3 and Ne-21 and gain of trapped solar gases in proportion to the degree of surface reworking by micrometeorites as deduced from FMR data. A variety of compositional and irradiational evidence suggests that soils in the core were formed by mixing of three or more components during or immediately prior to core deposition less than about 125 m.y. ago. Based on cosmogenic noble gases and a variety of other data soils 60009,457 and 60010,3107 are similar (and possibly identical) to two of the end member soils which formed the mixture. More mature soils in the core, however, could not have matured in situ from these two soils because of significant differences in noble gas abundances and chemical composition.

Bogard, D. D.↗
NASA NTRS1977

Steady state, exposure age, and growth of agglutinates in lunar soils

Mineralogical evolution of lunar soils as a function of agglutinate content can be described with the help of a two-component model. Steady-state condition of exposed lunar soils can be achieved only by the balance of comminution, agglutination, and replenishment processes. Rate of replenishment depends on the meteoroid flux and the nature of the substrate which varies both in space and time. Rates of agglutination and comminution depend not only on the micrometeoritic flux but also on the chemical and modal composition of an exposed soil. It is concluded from modeling the above that steady-state condition of lunar soils is independent of its maturity or exposure age. Agglutinate content of a lunar soil can be used to estimate the relative exposure age of the soil only if the soil is in a non-steady-state condition.

Basu, A.↗
NASA NTRS1975

Agglutinates as indicators of lunar soil maturity - The rare gas evidence at Apollo 16

The weight percent of lunar agglutinic glass (glass-welded aggregates resulting from micrometeorite impacts) in Apollo 16 bulk soils is considered with respect to quantities of trapped rare gases, e.g., He-4, Ne-20, Ar-36, Kr-84, and Xe-132, in an effort to determine the cumulative time a given soil sample has spent exposed on the lunar surface. The agglutinic glass, which reacts strongly in a magnetic field, is obtained by the standard fluid (methanol)-magnetic technique. It is noted that the absolute concentrations of trapped rare gases all show a linear relationship with increasing agglutinic glass content.

Charette, M. P.↗