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NASA NTRS1986

The origin of dwarf galaxies, cold dark matter, and biased galaxy formation

A reexamination is conducted of the formation of dwarf, diffuse, metal-poor galaxies due to supernova-driven winds, in view of data on the systematic properties of dwarfs in the Local Group and Virgo Cluster. The critical condition for global gas loss as a result of the first burst of star formation is that the virial velocity lie below an approximately 100 km/sec critical value. This leads, as observed, to two distinct classes of galaxies, encompassing the diffuse dwarfs, which primarily originate from typical density perturbations, and the normal, brighter galaxies, including compact dwarfs, which can originate only from the highest density peaks. This furnishes a statistical biasing mechanism for the preferential formation of bright galaxies in denser regions, enhancing high surface brightness galaxies' clustering relative to the diffusive dwarfs.

Dekel, A.↗

NASA NTRS1986

A nonmagmatic origin of group-IIE iron meteorites

New neutron activation data on 10 elements in 12 IIE and IIE-related irons lead to a reclassification of several irons. Seymchan and Lonaconing are removed from IIE, and Leshan added. Four IIE members are designated IIE-An to call attention to some anomalous properties. The eight normal IIE members define element-Ni trends generally similar to those in the nonmagmatic group IAB; the small negative slopes on W-Ni and Ir-Ni diagrams are strongly indicative of a nonmagmatic origin of the IIE irons. It is proposed that IIE irons like IAB irons originated as individual pools of impact-produced melt in the near-surface region of a chondritic parent body. The positive As-Ni and Au-Ni trends are the only evidence suggesting fractional crystallization, but their slopes are lower than those in magmatic group IIIAB, and only slightly higher than those of Cu and Sb in IAB. It is suggested that the S and C contents of the IIE precursor materials were much lower than those of the IAB precursors, thus higher temperatures were required to generate enough metallic melt to segregate into pools. These higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions.

Wasson, J. T.↗

NASA NTRS1987

Possible origin of some channels on Alba Patera, Mars

Several alternative models were proposed for the origin and mode of formation of channels and valley networks on Martian volcanoes, notably Hecates Tholus, Ceraunius Tholus, and Alba Patera. Early interpretations of Mariner 9 and Viking images suggested that these features on Alba were lava channels, while those on Ceraunius Tholus were interpreted as fluvial or volcanic debris channels. Subsequent mapping of Tyrrehna Patera and Hecate Tholus has suggested that pyroclastic activity may have characterized eruptions on these volcanoes, and that at least for Hecates the channels were probably formed by fluvial erosion of unconsolidated ash deposits on the flanks of the volcano. As part of a continuing program to better understand the eruptive history of the young volcanic centers on Mars, numerous channels were identified on the flanks of Alba Patera that resemble the channels on Hecates. As a result, the possibility is being explored that some of the small channels on the flanks of Alba Patera may be fluvial in origin and potentail water sources and modes of formation are being explored.

Postawko, S. E.↗

NASA NTRS1986

Nebular chemistry and theories of lunar origin

The cosmic history of planetary matter is traced from nucleosynthesis through accretion in an attempt to understand the origin of the moon. It is noted that nebular processes must be considered in any theory of lunar origin and that planetary differentiation and volcanism determine the final character of lunar rocks. The moon's unique blend of nebular components suggests that the earth and moon accreted from the same mix of components as the proto-moon orbited the proto-earth, with the earth winning and the moon progressively losing, its solar complement of the components.

Larimer, John W.↗

NASA NTRS1986

Origin of large magnetic fluctuations in the magnetosheath of Venus

The origin of large-amplitude hydromagnetic waves in the Venus magnetosheath downstream of the quasi-parallel bow shock is investigated by means of numerical simulations. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O(+) ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also discussed.

Winske, D.↗

NASA NTRS1987

Cleopatra Patera on Venus - Venera 15/16 evidence for a volcanic origin

The nonconcentric nature, anomalous depth, and terraced morphology of the nested craters that compose Cleopatra Patera are more closely analogous to volcanic craters (calderas) than multiring impact structures. Associated deposits northeast and downslope of the Patera first recognized on Venera 15/16 radar images are interpreted as volcanic plains related to, and perhaps cogenetic with, Cleopatra. The plains lavas probably originated as effusions from a radial rift zone or ring fissures. Rim deposits surrounding Cleopatra are asymmetric along the structural fabric of the region, also indicating fissure-type eruptions. Finally, a volcanic origin not only is easily reconciled with the tectonic setting of the Patera, it is almost required by the correlation between the Patera and regional structural trend.

Schaber, Gerald G.↗

NASA NTRS1987

Tektite-like bodies at Lonar Crater, India - Implications for the origin of tektites

Homogeneous dense glass bodies (both irregular and splash form) with high silica contents (about 67 pct SiO2) occur in the vicinity of Lonar Crater, India. Their lack of microlites and mineral remnants and their uniform chemical composition virtually preclude a volcanic origin. They are similar to tektites reported in the literature. While such a close association of tektite-like bodies with impact craters is already known (Aouelloul Crater, Mauritania; Zhamanshin Crater, U.S.S.R.), the tektite-like bodies at Lonar Crater are unique in that they occur in an essentially basaltic terrain. Present geochemical data are consistent with these high silica glass bodies being impact melt products of two-thirds basalt and one-third local intertrappean sediment (chert). The tektite-like bodies of the impact craters Lonar, Zhamanshin, and Aouelloul are generally similar. Strong terrestrial geochemical signatures reflect the target rock REE patterns and abundance ratios and demonstrate their terrestrial origin resulting from meteorite impact, as has been suggested by earlier workers.

Murali, A. V.↗

NASA NTRS1986

RNA catalysis and the origins of life

The role of RNA catalysis in the origins of life is considered in connection with the discovery of riboszymes, which are RNA molecules that catalyze sequence-specific hydrolysis and transesterification reactions of RNA substrates. Due to this discovery, theories positing protein-free replication as preceding the appearance of the genetic code are more plausible. The scope of RNA catalysis in biology and chemistry is discussed, and it is noted that the development of methods to select (or predict) RNA sequences with preassigned catalytic functions would be a major contribution to the study of life's origins.

Orgel, Leslie E.↗

NASA NTRS1988

The atmospheres of the earth and the other planets: Origin, evolution and composition

The current understanding of the composition, chemistry, and structure of the atmospheres of the other planets and the origin, early history, and evolution of the earth's atmosphere is reviewed. The information on the atmospheres of the other planets is based on the successful Mariner, Viking, Pioneer, and Voyager missions to these planets. The information on the origin, early history, and evolution of the atmosphere, which is somewhat speculative, is largely based on numerical studies with geochemical and photochemical models.

Levine, Joel S.↗

NASA NTRS1987

Local and exotic components of primitive meteorites, and their origin

The chemical and isotopic compositions of chondrites and their implications for chondrite origins are considered, reviewing the results of recent theoretical and experimental investigations. Numerical data are compiled in tables and graphs and discussed in detail. Consideration is given to the properties and classifications of chondrites; components of local origin in the matrix, chondrules, Ca-Al-rich inclusions, and carbon and organic matter; exotic components such as O-16-enriched dust, nucleosynthetic anomalies, and extinct radionuclides; and isotopic anomalies of volatiles such as C, N, and the noble gases. The implications of the chondrite data for the stony fraction of comet nuclei are briefly indicated.

Anders, E.↗

NASA NTRS1988

The origin of the infrared luminosity in Centaurus A

The origin of the infrared luminosity in Centaurus A is studied using new tracking and data acquisition techniques which yield diffraction-limited profiles at both 50 and 100 microns. Ninety percent of the 100 micron flux is found to originate in a source which extends 5 kpc along the optical dust lane that bisects the galaxy; the remaining 10 percent comes from an unresolved source coincident with the active radio nucleus. The extended 100 micron emission profile is fully consistent with a uniform disk of thermally radiating particles; when combined with results of previous studies at shorter wavelengths, these measurements indicate that nearly all of the infrared luminosity is produced by thermally emitting grains which are heated by massive young stars distributed throughout the optically thick dust lane.

Joy, Marshall↗

NASA NTRS1988

Samples from Martian craters: Origin of the Martian soil by hydrothermal alteration of impact melt deposits and atmospheric interactions with ejecta during crater formation

The origin of the Martian soil is an important question for understanding weathering processes on the Martian surface, and also for understanding the global geochemistry of Mars. Chemical analyses of the soil will provide an opportunity to examine what may be a crustal average, as studies of loess on the Earth have demonstrated. In this regard the origin of the Martian soil is also important for understanding the chemical fractionations that have affected the composition of the soil. Several processes that are likely to contribute to the Martian soil are examined.

Newsom, Horton E.↗

NASA NTRS1988

Lunar mare ridges - Analysis of ridge-crater intersections and implications for the tectonic origin of mare ridges

Models proposed for the origin of lunar mare ridges belong to one of the three categories: volcanic, tectonic, and a combination of volcanic and tectonic. This paper presents the results of an analysis of ridge-crater intersections in Oceanus Procellarum and Maria Cognitum, Imbrium, and Serenitatis and examines the range of fault attitudes and displacements associated with mare ridge development. The results indicate tectonic, rather than volcanic, involvement. The evidence cited to support a volcanic origin for mare ridges is reassessed. In addition, plausible end-member tectonic models are presented.

Sharpton, Virgil L.↗

NASA NTRS1988

On the origin of the Orion and Monoceros molecular cloud complexes

A detailed model for the origin of the Orion and Monoceros cloud complexes is presented, showing that a single high-velocity H I cloud-galaxy collision can explain their main observed features. The collision generates massive shocked layers, and self-gravity can then provide the conditions for the transformation of these layers into molecular clouds. The clouds formed by the collision maintain the motion of their parental shocked gas and reach positions located far away from the plane. According to this model, both the Orion and Monoceros complexes were formed some 60 million yr ago, when the original shocked layer was fragmented by Galactic tidal forces.

Franco, J.↗

NASA NTRS1989

Geologic setting of an unusual Martian channel - Hypotheses on origin

Several areas within the Memnonia region of Mars have been proposed as candidate sites for a Mars sample return mission. Geologic studies of these areas made from computer-enhanced high-resolution Viking images show that an assemblage of rocks having different ages and compositions are accessible to a roving vehicle. In addition, evidence of recent fluvial episodes is indicated by small channels that incises materials emplaced during the Amazonian Period. The channel discussed in this paper is located near the highland-lowland boundary in the Memnonia MC-16SW quadrangle of Mars. It is of particular interest because of distinctive morphologic characteristics compared to other Martian channels. Although several hypotheses are advanced to explain the origin of the channel, none of the arguments are completely satisfactory. However, it is believed that water mobilized from ice in the subjacent regolith (hypothesis 1) or that condensate water from highly volatile ash flows (hypothesis 2) are the most likely origins for the channel.

Scott, D. H.↗

NASA NTRS1989

Origin and evolution of outer solar system atmospheres

The origin and evolution of the atmospheres of bodies in the outer solar system is studied on the basis of the abundances of key molecular species. Formation models in which significant infall of icy and rocky planetesimals accompanies planet formation is supported by the enrichment of methane and deuterated species from Jupiter and Neptune. The chemistry of the solar nebula and Titan are discussed. The prospects for obtaining information on the atmosphere of Triton from the Voyager 2 mission are considered. It is found that the mean density of the Pluto-Charon system implies an origin in the rather water-poor solar nebula.

Lunine, J. I.↗

NASA NTRS1989

On the origin of the Pluto-Charon binary

The normalized angular momentum density of Pluto-Charon (0.45) exceeds the critical value of 0.39 above which no stably rotating single object exists, suggesting a collisional origin for this binary. The effects of viscosity on Pluto's rotational stability and on the density of Charon are considered. Both a more or less dense Charon would be consistent with a collisional origin if one (the least massive) or both protoobjects were differentiated. It is noted that the angular momentum of the system requires the protoobjects to be comparably (if not equally) sized if off-center impact velocities vary between escape (about 1.3 km/s) and somewhat greater values (about 2.5 km/s) appropriate to Pluto's eccentric and inclined solar orbit.

Mckinnon, William B.↗

NASA NTRS1989

Nature and origin of matrix in the unique type 3 chondrite, Kakangari

The characteristics of the matrix of the Kakangari type-3 chondrite were investigated using high-resolution TEM and AEM. It was found that the textural and mineralogical characteristics of the fine-grained matrix of Kakangari are different from those of the other chondrite groups. Unlike the matrix phases present in the CV3 and CO3 chondrites, the matrix mineralogy of Kakangari contains clear information regarding its possible origin and thermal history. Data obtained are most consistent with an origin of Kakangari by annealing, without melting at high temperature (possibly during a chondrule-forming thermal event), of clusters or aggregates of ultrafine-grained amorphous or crystalline phases. The precursor materials were most likely to have been a mixture of nebula and presolar dust. Chondrules in Kakangari were probably formed from identical precursors to the matrix, but reached temperatures sufficiently high to produce melting.

Brearley, Adrian J.↗