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NASA NTRS1988

The evolution of glutathione metabolism in phototrophic microorganisms

The low molecular weight thiol composition of a variety of phototropic microorganisms is examined in order to ascertain how evolution of glutathione (GSH) production is related to the evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols (RSH) to fluorescent derivatives (RSmB) which were analyzed by high performance liquid chromatography (HPLC). Significant levels of GSH were not found in green sulfur bacteria. Substantial levels were present in purple bacteria, cyanobacteria, and eukaryotic algae. Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide. Many of the organisms also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability which was quenched by treatment with 2-pyridyl disulfide or 5,5 prime-bisdithio - (2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototropic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

Fahey, Robert C.↗

NASA NTRS1988

Cosmological parameters and evolution of the galaxy luminosity function

The relationship between the observed distribution of discrete sources of a flux limited sample, the luminosity function of these sources, and the cosmological model is discussed. It is stressed that some assumptions about the form and evolution of the luminosity function must be made in order to determine the cosmological parameters from the observed distribution of sources. Presented is a method to test the validity of these assumptions using the observations. It is shown how, using higher moments of the observed distribution, one can determine, independently of the cosmological model, all parameters of the luminosity function except those describing evolution of the density and the luminosity of the luminosity function. These methods are applied to the sample of approximately 1000 galaxies recently used by Loh and Spillar to determine a value of the cosmological density parameter Omega approx = 1. It is shown that the assumptions made by Loh and Spillar about the luminosity function are inconsistent with the data, and that a self-consistent treatment of the data indicates a lower value of Omega approx = 0.2 and a flatter luminosity function. It should be noted, however, that incompleteness in the sample could cause a flattening of the luminosity function and lower the calculated value of Omega and that uncertainty in the values of these parameters due to random fluctuations is large.

Caditz, David↗

NASA NTRS1988

Convergence of Galerkin approximations for operator Riccati equations: A nonlinear evolution equation approach

An approximation and convergence theory was developed for Galerkin approximations to infinite dimensional operator Riccati differential equations formulated in the space of Hilbert-Schmidt operators on a separable Hilbert space. The Riccati equation was treated as a nonlinear evolution equation with dynamics described by a nonlinear monotone perturbation of a strongly coercive linear operator. A generic approximation result was proven for quasi-autonomous nonlinear evolution system involving accretive operators which was then used to demonstrate the Hilbert-Schmidt norm convergence of Galerkin approximations to the solution of the Riccati equation. The application of the results was illustrated in the context of a linear quadratic optimal control problem for a one dimensional heat equation.

Rosen, I. G.↗

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

Origin and evolution of fluctuations in the solar wind - Helios observations and Helios-Voyager comparisons

Hour-averaged data from the Helios and Voyager spacecraft are used here to investigate the origin and evolution of low-frequency interplanetary fluctuations from 0.3 to 20 AU. The previously observed evolution toward a less purely Alfvenic state with increasing heliocentric distance is shown to occur more rapidly in the inner heliosphere and in low-speed as well as high-speed streams. It is concluded that outward-traveling flucutations are predominantly generated by the sun, but that in situ turbulence, most likely due to stream shear, generates fluctuations with both inward and outward senses of correlation.

Roberts, D. A.↗

NASA NTRS1987

The evolution of a coronal streamer prior to mass ejection

A model describing the quasi-static evolution of a coronal helmet streamer as it is inflated with excess mass was developed. The model produces a sequence of magnetostatic equilibria for a magnetic field configuration that includes a current sheet in the equatorial plane as well as distributed volume currents. The equilibrium solutions are given as Legendre series, and the height at which the current sheet starts is adjusted iteratively to maintain constant flux in the closed-field region. The model was fit to SMM coronagraph/polarimeter observations showing the slow growth of a coronal streamer prior to its disruption in association with an eruptive prominence and mass ejection on August 18, 1980. The results suggest that the early phase of this event is consistent with the quasi-static evolution of the corona in response to the slow addition of mass to the closed-field region of the streamer.

Wolfson, Richard↗

NASA NTRS1988

Evidence for a scaling solution in cosmic-string evolution

Numerical simulations are used to study the most fundamental issue of cosmic-string evolution: the existence of a scaling solution. Strong evidence is found that a scaling solution does indeed exist. This justifies the main assumption on which the cosmic-string theories of galaxy formation is based. The main conclusion coincides with that of Albrecht and Turok (1985) but the results are not consistent with theirs. In fact, the results indicate that the details of string evolution are very different from the standard dogma.

Bennett, David P.↗

NASA NTRS1988

On wind-type flows in astrophysical jets. III - Temporal evolution of perturbations and the formation of shocks

The temporal evolution of disturbances in a spherically symmetric polytropic wind from a central object is studied. Such disturbances may be due to localized momentum addition/subtraction, as, for example, by MHD waves, heating/cooling mechanisms in the outflow, or localized deviations from spherical symmetric expansion. The evolution of an initial perturbed state to a continuous or discontinuous final equilibrium state, as predicted by previous analytic calculations for stationary flows, is followed. It is shown that some of the predicted discontinuous equilibrium states are not physically accessible, while the attainment of the remaining equilibrium states depends on both the temporal and the spatial parameters characterizing the perturbation. The results are derived for solar conditions, but in fact can be applied to outflows in other astrophysical systems. In particular, applications to the solar wind and flows in astrophysical jets are discussed.

Trussoni, E.↗

NASA NTRS1988

The evolution of molecular clouds

The problem of the structure and evolution of molecular clouds is reviewed, with particular emphasis given to the relationship with star formation. The basic hypothesis is that magnetic fields are the primary agents for supporting molecular clouds, although damped Alfven waves may play an important role in the direction parallel to the field lines. This picture naturally leads to a conception of 'bimodal star formation'. It is proposed that high-mass stars form from the overall gravitational collapse of a supercritical cloud, whereas low-mass stars form from small individual cores that slowly condense by ambipolar diffusion from a more extended envelope until they pass the brink of graviational instability and begin to collapse dynamically from 'inside-out'. The evidence that the infall stage of protostellar evolution is terminated by the development of a powerful stellar wind is reviewed.

Shu, Frank H.↗

NASA NTRS1988

Evolution of polar stratospheric clouds during the Antarctic winter

The occurrence of Polar Stratospheric Clouds (PSCs), initially inferred from satellite measurements of solar extinction, have now also been noted by the recent scientific expeditions in the Antarctic. The presence of such clouds in the Antarctic has been postulated to play a significant role in the depletion of ozone during the transition from winter to spring. The mechanisms suggested involve both dynamical and chemical processes which, explicity or implicitly, are associated with the ice particles constituting the PSCs. It is, thus, both timely and necessary to investigate the evolution of these clouds and ascertain the nature and magnitude of their influences on the state of the Antarctic stratosphere. To achieve these objectives, a detailed microphysical model of the processes governing the growth and sublimation of ice particles in the polar stratosphere was developed, based on the investigations of Ramaswamy and Detwiler. The present studies focus on the physical processes that occur at temperatures below those required for the onset of ice deposition from the vapor phase. Once these low temperatures are attained, the deposition of water vapor onto nucleation particles becomes extremely significant. First, the factors governing the magnitude of growth and the growth rate of ice particles at various altitudes are examined. Second, the ice phase mechanisms are examined in the context of a column model with altitudes ranging from 100 to 5 mb pressure levels. The column microphysical model was used to perform simulations of the cloud evolution, using the observed daily temperatures. The effect due to the growth of the particles on the radiation fields are also investigated using a one dimensional radiative transfer model. Specifically, the perturbations in the longwave cooling and that in the shortwave heating for the late winter/early spring time period are analyzed.

Ramaswamy, V.↗

NASA NTRS1988

Synoptic and chemical evolution of the Antarctic vortex in winter and spring, 1987

The dynamical evolution of the vortex at least up to 50 mb is dominated by synoptic scales in the troposphere. In particular, there is a clear response when poleward extension of tropospheric anticyclones from latitudes of 40 and 50 S to 70 and 80 S occurs. This response is evident in isentropic potential vorticity maps, TOMS ozone fields and SAM II polar stratospheric clouds. An important feature of the high latitude Southern Hemisphere lower stratosphere is a transition at potential temperatures in the 390 to 400 K range. This transition, the vortopause, is clearly marked in aircraft profiles of O3, H2O, N2O and ClO at latitudes 68 to 72 S near the Antarctic peninsula, and also cross-sections of potential vorticity and potential temperature; above it, the isopleths are more closely spaced than below it. The aircraft measurements of H2O, O3, NO sub y, N2O, ClO, and whole air data are examined in material coordinates, theta and P sub theta, backed up by trajectory analysis. The evolution of the chemical mixing ratios is examined in these coordinates as a function of time from mid-August to late September. Conclusions are drawn about the rates of change and their causes. The meteorological and aircraft data are examined for evidence of the following kinds of motion with respect to the vortex: ingress of air aloft, subsidence, peeling off of air to lower latitudes, and folding of the vortopause. Conclusions are presented regarding the evidence for a chemical sink of ozone above and below theta = 400 K, and whether the vortex has a mass flow through it, or if the chemical sink operates on a fixed mass of air. Implications for mid-latitudes are briefly considered in the light of the conclusions.

Tuck, A. F.↗

NASA NTRS1987

Textural evolution of partially-molten planetary materials in microgravity

Recent Earth-based experiments examining the textural evolution of partially-molten rocks have revealed two important ways in which surface energy considerations affect magma. An initial experimental program addressing surface-energy effects on partially-molten materials in microgravity would involve simple, isothermal treatment of natural samples (meteorites, perioditic komatiite) at preselected temperatures in the melting range. Textural evolution would be assessed by time studies in which the only experiment variable would be run duration. Textural characterization of each sample would be done by quenching, recover, and sectioning for generally later, computer-aided interpretation of features.

Watson, E. B.↗

NASA NTRS1988

Part 1: Aspects of lithospheric evolution on Venus. Part 2: Thermal and collisional histories of chondrite parent bodies

The geological evolution of distinctly different kinds of solar system objects is addressed. Venus has been observed over the past decade by orbital radars on both American and Soviet spacecraft. These surface measurements provide clues to the structure and evolution of the lithosphere. The parent bodies of chondritic meteorites, thought to resemble asteroids, represent the other end of the size spectrum of terrestrial objects. Their early thermal and collisional histories may be constrained by the chemical and textural record preserved in meteorite samples. Impact craters on Venus have been observed by the Soviet Venera 15/16 spacecraft. A formalism is presented by which the size-frequency distribution of impact craters may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years. The impact crater density reported from Venera observations, if valid for the entire Venus surface, indicates a mean volcanic flux no greater than 2 cu km/y, corresponding to a maximum average rate of resurfacing of about 4 km/b.y. For the lowest estimated mean crater retention age of the surface of Venus imaged by Venera 15/16, the rate of lithospheric recycling on Venus does not exceed 1.5 sq km/y. Ordinary chondrite meteorites show textural and chemical patterns indicative of varying intensities of thermal metamorphism. The conventional onion-shell model, which envisions highly metamorphosed material in the core and less intensely heated rocks near the surface, predicts an inverse relation between peak temperature and cooking rate, but none has been observed. A metamorphosed-planetesimal model is devised to explain this discrepancy, whereby heating occurs in planetesimals a few kilometers in radius which then accrete to form 100-km-radius parent bodies. Cooling rates are then randomly controlled by burial depth. Thermal and collisional constraints are examined, and the model is found to be applicable only to highly insulating Al-26-rich planetesimals that remain closely aggregated upon accretion. An alternative model is presented, in which onion-shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled. If reassembly times are short, then cooling rates would be determined by burial depth in the reaccreted parent body. This model, unlike previous ones, can explain both coherent and incoherent cooling of Breccia clasts by collisions during or after metamorphism, respectively.

Grimm, Robert E.↗

NASA NTRS1989

Evolution of cosmic string networks

A discussion of the evolution and observable consequences of a network of cosmic strings is given. A simple model for the evolution of the string network is presented, and related to the statistical mechanics of string networks. The model predicts the long string density throughout the history of the universe from a single parameter, which researchers calculate in radiation era simulations. The statistical mechanics arguments indicate a particular thermal form for the spectrum of loops chopped off the network. Detailed numerical simulations of string networks in expanding backgrounds are performed to test the model. Consequences for large scale structure, the microwave and gravity wave backgrounds, nucleosynthesis and gravitational lensing are calculated.

Albrecht, Andreas↗

NASA NTRS1988

Tectonic evolution of the Archaean high-grade terrain of South India

The southern Indian shield consists of three major tectonic provinces viz., (1) Dharwar Craton, (2) Eastern Ghat Mobile Belt, and (3) Pandyan Mobile Belt. An understanding of their mutual relations is crucial for formulating crustal evolution models. The tectonic evolution of these provinces is summarized.

Ramakrishnan, M.↗

NASA NTRS1989

Non-linear evolution of a second mode wave in supersonic boundary layers

The nonlinear time evolution of a second mode instability in a Mach 4.5 wall-bounded flow is computed by solving the full compressible, time-dependent Navier-Stokes equations. High accuracy is achieved by using a Fourier-Chebyshev collocation algorithm. Primarily inviscid in nature, second modes are characterized by high frequency and high growth rates compared to first modes. Time evolution of growth rate as a function of distance from the plate suggests this problem is amenable to the Stuart-Watson perturbation theory as generalized by Herbert.

Erlebacher, Gordon↗

NASA NTRS1987

Summary talk - XXIInd Rencontre de Moriond Workshop on Starbursts and Galaxy Evolution

The topics covered at the 1987 Moriond Astrophysics Conference are reviewed. Observations of supernova SN 1987A and the discovery of neutrinos from the supernova are examined. Questions concerning dark matter are discussed, including the possibility of neutrinos being candidates for dark matter if they have masses of 20 to 30 eV. In addition, questions concerning starbursts and galactic evolution addressed at the conference are summarized, including the role of starbursts and their relation to active galactic nuclei, the formation of ellipticals, the relation of IRAS galaxies to starburst galaxies, starburst energy flow, and galactic evolution in cosmology.

Schramm, David N.↗

NASA NTRS1988

The origin and evolution of atmospheric oxygen

This paper discusses the chemical processes involved in the evolution of the earth's atmospheric oxygen and ozone, as well as the sources, sinks, and transfer rates of oxygen in the present atmosphere. Special attention is given the evolution of atmospheric O3 as a function of the buildup of O2, with the results of calculations presented as the vertical profiles of O3, in terms of the present atmospheric level (PAL) oxygen values. Calculations show that the total O3 column density that is approximately half of the present level was reached when atmospheric oxygen level reached 0.1 PAL. At this level of ozone, the biological shielding of the earth's surface from the UV radiation is believed to have been achieved.

Levine, Joel S.↗