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At least 1,801 records · Page 100
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NASA NTRS1978

The primitive solar accretion disk and the formation of the planets

The author develops the idea that the formation of the solar system was triggered by the explosion of a supernova near a compressed interstellar cloud, which was further compressed by the supernova ejecta until it went over the threshold for gravitational collapse. During the collapse it is expected that the cloud would fragment into much smaller pieces. The principle source of friction in the collapsing nebula is taken to be turbulent viscosity, the required stirring having been supplied possibly by meridional circulation currents. The theory can be shown to account for how a great deal of condensed matter in the form of cometary bodies could be put into elliptical orbits extending toward 100,000 AU, the region of the Oort reservoir.

Cameron, A. G. W.↗
NASA NTRS1978

Conference on Protostars and Planets, University of Arizona, Tucson, Ariz., January 3-7, 1978, Proceedings

These papers deal with star formation, the origin of the solar system, and the possible formation of planets associated with other stars. Specific topics include chemical and isotopic anomalies in meteorites, laboratory analogs of interstellar dust, properties of dark globules, locations of newly formed stars in molecular clouds, O-star formation, observations of star-formation regions near supernova remnant W44, and equilibrium condensation of supernova ejecta. Other contributions discuss radio observations of molecular clouds associated with T Tauri stars, rotational velocities of pre-main-sequence stars, observational studies of star formation, characteristics of a possible preplanetary disk around the pre-main-sequence star MWC 349, calculations of the collapse of a rotating dusty protostellar cloud, a theory on the dynamics of dust particles in an incompressible turbulent fluid, and an analysis of the dynamical collapse of a model presolar nebula.

Gehrels, T.↗
NASA NTRS1979

Magnetic field and shock effects and remanent magnetization in a hypervelocity impact experiment

The impact of aluminum projectiles onto high-alumina terrestrial basalt blocks at 13-15 km/s in the presence of a variable magnetic field is studied. Plasma production but not field production was detected, and characteristics of the remanence and the shocked basalt are reported. Mineralogical data suggest that the magnetization acquired in the material near the craters is shock remanence. The experimental results might indicate that shock effects or possibly thermoremanence in ejecta fragments, may be responsible for part of the magnetization of the lunar surface.

Srnka, L. J.↗
NASA NTRS1979

Evidence for a massive X-ray halo around Markarian 541

Mkn 541, a Type I Seyfert galaxy, has been detected as an X-ray source with the rotating modulation collimator on SAS 3. The galaxy, which is identified as the optical counterpart of 4U 2351+06, has an X-ray flux of 3 x 10 to the -11th erg/sq cm per sec, which corresponds to a luminosity of 2 x 10 to the 44th erg/s at 240 Mpc. Evidence is presented that the X-rays are emitted in a volume of diameter 150 kpc. A variety of possible emission mechanisms are investigated, and it is suggested that the X-rays are caused by thermal bremsstrahlung from 500 billion solar masses of hot gas trapped in the gravitational well of Mkn 541, which implies that the mass of the galaxy exceeds 5 trillion solar masses. The hot gas can be explained as the accumulated ejecta of the Seyfert nucleus.

Cash, W.↗
NASA NTRS1979

Relative age of Camelot crater and crater clusters near the Apollo 17 landing site

Topographic profiles and depth-diameter ratios from the crater Camelot and craters of the central cluster in the Apollo 17 landing area suggest that these craters are of the same age. Therefore, layers that can be recognized in the deep-drill core and that can be identified as ejecta deposits from Camelot or from the cluster craters should yield similar emplacement ages.

Lucchitta, B. K.↗
NASA NTRS1979

Differential aeolian redistribution rates on Mars

Data from the Viking Orbiters and Landers, which have been monitoring the Martian aeolian environment since 1976, are employed to study erosion rates and the redistribution of rocks and soil deposits on the planet. Examination of the Martian cratering history yields a computed erosion rate of about one meter per billion yr. The extent of deflation between ejecta deposits and the surrounding terrain, and the origin and age of Martian windblown drifts are also discussed. The study indicates that approximately the present aeolian environment has prevailed on the planet for a significant fraction of geological time.

Arvidson, R.↗
NASA NTRS1978

The nature of rays and sources of highland material in Mare Crisium

Although highland fragments are rare in the Luna 24 core, they should provide an important source of information on the sequence of formation and lithology of lunar east-limb basins. The numerous rays that cross the Crisium basin suggest several sources of highland materials. Both deposition of primary ejecta and reworking of local material were most likely responsible for the formation of ray systems in Mare Crisium. In addition to the fact that Luna 24 landed near the uprange end of a Giordano Bruno ray, incomplete sampling of the uppermost layer and formation of the ray by reworking of in situ regolith suggest that its materials may be scarce or not present in the core. Highland fragments present are most likely to be representative of several highland sources.

Maxwell, T. A.↗
NASA NTRS1978

Implications from Luna 24 sample 24170 to U-Pb evolution in the lunar mantle

The concentrations of U, Th and Pb, as well as the Pb isotopic composition, have been determined for a Luna 24 sample from the Mare Crisium area. A relatively recent disturbance to the U-Pb system during meteorite impact or burial in hot ejecta is suggested by the findings. According to three-stage U-Pb evolution model calculations, the source cumulates of the Luna 24 basalt evolved in a U-238/Pb-204 environment much lower than that of the Apollo mare basalts. A laterally heterogeneous lunar magma ocean, U-Pb fractionation during cumulate formation, and/or lack of KREEP addition to the Luna 24 basalt are also reflected in the calculations.

Unruh, D. M.↗
NASA NTRS1978

Feldspathic granulite 79215 - Limitations on T-fo2 conditions and time of metamorphism

Feldspathic granulite 79215, an annealed polymict breccia which has a bulk composition between anorthositic gabbro and gabbroic anorthosite, contains numerous oxide complexes in the matrix. An Ar-39-Ar-40 stepwise heating experiment gives a well-defined plateau corresponding to an age of 4.03 + or - 0.02 AE. The polmict character of this breccia and the variability of the complexes suggest that they formed as a consequence of reactions between spinel-rich clasts and matrix under the high-T low-P conditions of an ejecta blanket. The duration of annealing is estimated to have been less than 10 million yr; the absence of a KREEP component may indicate an inhomogeneous distribution of this component at the lunar surface at 4.0 AE.

Mcgee, J. J.↗
NASA NTRS1978

Core 74001/2 - Grain size and petrology as a key to the rate of in-situ reworking and lateral transport on the lunar surface

A suite of samples from the double drive tube 74001/2 has been studied. Material from this core mainly consists of orange and black droplets interpreted to be volcanic pyroclastic ejecta. Grain size analysis indicates that this material is very homogeneous in its grain size properties. It is also the finest and best sorted suite of soil samples in the lunar collection having a mean grain size of 40 microns and a mean standard deviation of 1.75 phi. The upper 5.5 cm of this core has apparently undergone in situ reworking by meteorites over a period of about 10 million years. This reworked zone contains 'exotic' grains including basalt, mineral fragments, vitric breccias, and agglutinates. One type of agglutinate is unique and has been made primarily from orange and black glass droplets melted and welded together by micrometeorite impacts. Other agglutinates are made mainly from basaltic fragments and minerals. The amount of 'exotic' material added to the core combined with an estimate of the location of the source areas for the 'exotic' material allows us to estimate that a maximum of about 0.36 gm per sq cm of regolith surface is added from a radius of about 1 meter in 10 million years. Furthermore, no more than about 0.01 gm per sq cm of regolith surface is added from a radius of about 100 meters in 10 million years.

Mckay, D. S.↗
NASA NTRS1978

Regolith depositional history at Shorty Crater

Nuclear particle track measurements in the 68 cm double drive tube 74002-74001 indicate that the whole core was deposited in one event some 10 m.y. ago. Significant reworking of the soil only occurred down to a few cm from the lunar surface since this event. Complementary investigations in this core by other groups are discussed. Most of the evidence available leads to a two stage model in which the orange and black soils collected at Shorty Crater were first irradiated for approximately 25 m.y. at some depth and then were deposited only a few m.y. ago as an overturned ejecta blanket.

Crozaz, G.↗
NASA NTRS1978

The magnetization of the lunar crust as deduced from orbital surveys

A two-dimensional magnetic anomaly map of a region of intense crustal magnetization near Van de Graaff crater has been constructed from Apollo 15 and 16 subsatellite vector magnetometer observations. The mapping method used limits coverage to relatively low altitudes (65-80 km) over the Van de Graaff region along the Apollo 15 track and over the central far side highlands along the Apollo 16 track; a simple differencing technique was used to remove external noise contributions. An approximate inversion of this map using a surface plate model for source region geometry has revealed at least four different sources. No correlation between the lateral locations of these source regions and surface morphology was found; the directions of magnetization seem to be unrelated except for a strong north-south depletion. A rough antipodal correlation of the two most strongly magnetized regions yet mapped on the moon with the Imbrium and Orientale basins gives mild support to the ejecta deposit hypothesis suggested by Strangway et al. (1973) for formation of anomaly source regions.

Hood, L. L.↗
NASA NTRS1978

Characteristics of the cratering process on small satellites and asteroids

Photographs of the two known natural satellites of Mars - Phobos and Deimos - illustrate the importance of impact cratering in the postformational histories of the two moons and, by inference, the asteroids and other small planetary satellites. The objectives of the present study are to investigate qualitatively the impact cratering process on small bodies, to apply some of these considerations to the observed surfaces of Phobos and Deimos, and to consider the asteroids briefly in the light of these results. The cratering process on small bodies is discussed in three stages: compression, excavation, and modification. Nonescaping and escaping ejecta are considered.

Cintala, M. J.↗
NASA NTRS1978

Experimental studies of oblique impact

Meteoritic materials most probably impact planetary bodies along oblique trajectories inclined less than 45 deg above their surfaces. Laboratory studies of hypervelocity impacts against rock and particulate media are presented that indicate important effects of obliquity on crater size, shape, and ejecta distribution. The effects are particularly important to crater size-frequency analyses and geologic interpretations of crater formations. Impacts at shallow incidence, which are not uncommon, lead to ricochet of the impacting object accompanied with some entrained excavated materials at velocities only slightly reduced from the pre-impact value.

Gault, D. E.↗
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

Ar-40/Ar-39 age of the Shergotty achondrite and implications for its post-shock thermal history

Ar-40/Ar-39 measurements are used to determine the age of the Shergotty achondrite and the chronology of the shock event responsible for the complete conversion of its plagioclase to maskelynite is discussed. Apparent ages are found to vary between 240 and 640 million years for the whole rock sample, with a plateau age of 254 million years for a maskelynite separate. The Rb-Sr age of 165 million years determined by Nyquist at al (1978) suggests that the maskelynite as well as the whole rock was incompletely degassed. Argon diffusion characteristics indicate a post-shock cooling time greater than 1000 years and a burial depth greater than 300 m for a thermal model of a cooling ejecta blanket of variable thickness. It is concluded that the shock event which degassed the argon and reset the Rb-Sr systematics occurred between 165 and 250 million years ago when the parent body experienced a collision in the asteroid belt.

Bogard, D. D.↗
NASA NTRS1979

Asteroidal regoliths

A physical model is developed for the evolution of regoliths on small bodies and applied to the asteroid and meteorite parent bodies. The model considers global deposition of that fraction of cratering ejecta that is not lost to space. In addition, it is applied to cases of both strong, cohesive bodies and to bodies of weak, unconsolidated materials. It is found that large, strong asteroids generate surficial regoliths of a few kilometers depth while strong asteroids smaller than 10-km diameter generate negligible regoliths. In conclusion, it is noted that the theory that substantial regoliths are produced predominantly by blanketing differs from earlier hypotheses that asteroidal regoliths might be thin or absent and that short surface exposure of asteroidal materials is due chiefly to erosion rather than blanketing.

Housen, K. R.↗
NASA NTRS1980

The evolution of supernova remnants in different galactic environments, and its effects on supernova statistics

Examination of the interaction between supernova (SN) ejecta and the various environments in which the explosive event might occur shows that only a small fraction of the many SNs produce observable supernova remnants (SNRs). This fraction, which is found to depend weakly upon the lower mass limit of the SN progenitors, and more strongly on the specfic characteristics of the associated interstellar medium, decreases from approximately 15 percent near the galctic center to 10 percent at Rgal approximately 10 kpc and drops nearly to zero for Rgal 15 kpc. Generally, whether a SNR is detectable is determined by the density of the ambient interstellar medium in which it is embeeede. The presence of large, low density cavities arpund stellar associations due to the combined effects of stellar winds and supernova shells strongly suggests that a large portion of the detectable SNRs have runway stars as their progenitors. These results explain the differences between the substantially larger SN rates in the galaxy derived both from pulsar statistics and from observations of SN events in external galaxies, when compared to the substantially smaller SN rates derived form galactic SNR statistics.

Kafatos, M.↗

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