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At least 145 records · Page 8
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NASA NTRS1984

Chemical evolution in space

Most of the complex molecules in interstellar space ar probably contained in small, frozen interstellar dust grains which are about as old as the earth and have been photochemically converted into large organic molecules. These molecules' maximum molecular weight is limited only by the approximately 0.1-micron grain size. Their evolution leads from cool, evolved stellar atmospheres' formation of seedlings to destruction through incorporation into the material of new stars. Organic dust constitutes about 0.1 percent of the total mass of the Milky Way, far outweighing any estimates of total planetary mass in the Galaxy. Because comets may be virtually pure, aggregated interstellar dust, they offer a source of interstellar organic material for detailed study.

Greenberg, J. M.↗
NASA NTRS1985

Soluble minerals in chemical evolution. I - Adsorption of 5-prime-AMP on CaSO4 - A model system

The adsorption of 5-prime-AMP onto solid CaSO4-2H2O was studied in a saturated suspension as a function of pH and electrolyte concentration. The adsorption is pH-dependent and is directly correlated with the charge on the 5-prime-AMP molecule which is determined by the state of protonation of the N-1 nitrogen of the purine ring and the phosphate oxygens. It is proposed that the binding that occurs between the nucleotide and the salt is electrostatic in nature. The adsorption decreases with increasing ionic strength of the solution which means that in a fluctuating environment of wetting and drying cycles, a biomolecule similar to 5-prime-AMP could be expected to desorb during the drying phase. The results indicate that CaSO4-2H2O can serve as a concentrating surface for biomolecules. The significance of this is discussed with regard to the possible role of soluble minerals and their surfaces in a geochemical model consistent with the evolution of the earth and the origin of life.

Orenberg, J. B.↗
NASA NTRS1986

Second Symposium on Chemical Evolution and the Origin of Life

Recent findings by NASA Exobiology investigators are reported. Scientific papers are presented in the following areas: cosmic evolution of biogenic compounds, prebiotic evolution (planetary and molecular), early evolution of life (biological and geochemical), evolution of advanced life, solar system exploration, and the Search for Extraterrestrial Intelligence (SETI).

Donald L Devincenzi↗
NASA NTRS1986

Synthesis and analysis in studies of chemical evolution

Studies of the various processes that may have given rise to life on the Earth have demonstrated the appropriateness of an approach that makes use of analysis and synthesis. Analysis of extraterrestrial samples in the form of meteorites has demonstrated the presence of several precursors of biomolecules, most notably a full suite of nucleic acid bases and nucleotides of biological significance. These species were determined after exhaustive extraction of the sample and subsequent analysis using HPLC, GC, MS, and GC-MS. Procedural blanks indicate that these molecules are likely not the result of contamination during the extraction and analysis process. Similar species were found as products of spark discharge experiments in atmospheres thought to mimic primitive Earth conditions. These results indicate that the basic chemistry underlying these syntheses is common, and that life may not be unique to the Earth. Studies underway in the laboratory make use of proton nuclear magnetic resonance spectroscopy as a probe to assess associations between selected amino acids and any of several nucleotides comprising their genetic code and genetic anticode sequences. These studies demonstrate a clear selectivity by the anticode sequences, thus confirming the hydrophobicity studies performed by Lacey et al. These studies further support the contention that life is likely a natural result of the physics and chemistry of the universe.

Ponnamperuma, C.↗
NASA NTRS1986

Clay energetics in chemical evolution

Clays have been implicated in the origin of terrestrial life since the 1950's. Originally they were considered agents which aid in selecting, concentrating and promoting oligomerization of the organic monomeric substituents of cellular life forms. However, more recently, it has been suggested that minerals, with particular emphasis on clays, may have played a yet more fundamental role. It has been suggested that clays are prototypic life forms in themselves and that they served as a template which directed the self-assembly of cellular life. If the clay-life theory is to have other than conceptual credibility, clays must be shown by experiment to execute the operations of cellular life, not only individually, but also in a sufficiently concerted manner as to produce some semblance of the functional attributes of living cells. Current studies are focussed on the ability of clays to absorb, store and transfer energy under plausible prebiotic conditions and to use this energy to drive chemistry of prebiotic relevance. Conclusions of the work are applicable to the role of clays either as substrates for organic chemistry, or in fueling their own life-mimetic processes.

Coyne, L. M.↗
NASA NTRS1986

Paleosols and the chemical evolution of the atmosphere

The chemistry and mineralogy of soils reflects the chemistry of the atmosphere below which they develop. Today the presence of a cover of land plants can obscure the signature of a highly oxidizing atmosphere. Prior to the advent of higher land plants ca. 400 m.y. ago the obscuring effect of a biological interface should have been minor. It can be shown that under such conditions the behavior of ferrous iron during weathering depends on the relationship between the ratio of the net concentration of oxidants to the concentration of acids in rainwater, and on the ratio of the oxygen demand to the acid demand for complete weathering of the parent rock. An analysis of the behavior of iron in some 15 paleosols between 1.0 and 2.9 b.y. in age showed some time ago that iron was retained, or largely retained, in paleosols developed on rocks with a low ratio of oxidant demand to acid demand (R value) and was lost from paleosols with a high R value. The results indicate that between 1.1 and 2.9 b.y.b.p. the ratio of the net concentration of oxidants to CO2 in rainwater was less than ca. 1/600 of the present-day value. Work during the past year has sought to extend the data base for paleosols and to define more precisely the relationships between the composition of paleosols and the chemistry of the atmosphere below which they developed.

Holland, H. D.↗
NASA NTRS1986

General constraints on the age and chemical evolution of the Galaxy

The formalism of Schramm and Wasserburg (1970) for determining the mean age of the elements is extended to develop as model-independent a range for the Galaxy's age as possible that takes all nuclear and meteoritic data uncertainties into account. A model-independent upper limit to that age is derived from an expansion of the equation giving the mean age of the elements in moments of the normalized effective nucleosynthesis rate. This limit depends only on the ratio of the mean time of formation of the elements to the total duration of nucleosynthesis, and on model-independent data. It is shown tht the Th-232/U-238. U-235/U-238, and Pu-244/U-238 chronometric pairs can give constraints on the relative rate of nucleosynthesis over the history of the synthesis of the solar system material.

Meyer, Bradley S.↗
NASA NTRS1987

Soluble minerals in chemical evolution. II - Characterization of the adsorption of 5-prime-AMP and 5-prime-CMP on a variety of soluble mineral salts

The adsorption of 5-prime-AMP and 5-prime-CMP is studied in the saturated solutions of several mineral salts as a function of pH, ionic strength, and surface area of the solid salt. It is suggested that the adsorption which results from the binding between the nucleotide molecule and the salt surface is due to electrostatic forces. The adsorption is reversible in nature and decreases with increasing ionic strength.

Chan, Stephen↗
NASA NTRS1987

Galactic chemical evolution - Z versus ln(1/mu) relationship

The ways in which a radial abundance gradient in a disk galaxy may be interpreted are reexamined, asking how metallicity Z depends upon gas mass fraction mu in analytic models characterized by a temporally varying history of metal-poor infall. Six different such models are studied, and it is shown that the metallicity versus gas fraction relationship is well approximated by an expression which is the analog of the Z = y ln(1/mu) relation for closed evolution. The approximation appears valid when the infall rate is nonneglible, but not so great that it dominates the gas budget.

Clayton, Donald D.↗
NASA NTRS1989

Chemical evolution of viscously evolving galactic discs

The ability of the Lin-Pringle (1987) model of galactic disk formation to reproduce the observed radial distributions of total gas surface density and metals in disk galaxies is investigated. It is found that a satisfactory fit is obtained provided that there exists an outer cut-off to the star-forming disk beyond which gas is allowed to viscously evolve. The metallicity gradient is then established by radial inflow of gas from beyond this cut-off.

Clarke, Catherine J.↗
NASA NTRS1989

Meteoritic silicon carbide and its stellar sources - Implications for galactic chemical evolution

Interstellar silicon carbide grains in meteorites provide a novel means for studying the carbon-star population of about 5 x 10 to the 9th years ago. Their C-12/C-13 ratios differ greatly from the solar value but resemble those of present-day csrbon stars, implying little change in the galactic C-13 inventory. Isotope data on nitrogen and silicon suggest that the silicon carbide grains come mainly from red giants, with small contributions from novae.

Ming, Tang↗
NASA NTRS1989

Present state and chemical evolution of the atmospheres of Titan, Triton, and Pluto

An evaluation is made of the current understanding of the atmospheres of Titan, Triton, and Pluto, as well as of theoretical models for their origin and evolution. All three atmospheres contain methane, while Titan, and probably Triton, have nitrogen. The primary driver in the evolution of the Titan atmosphere has been the irreversible photolysis of methane. If a surface reservoir of liquid methane exists to resupply the atmosphere, it is subject to enrichment in ethane due to the long-term photolysis of methane. The key issue in the origin and early evolution of Titan's atmosphere is the source of molecular nitrogen; two schemes for the conversion of ammonia to nitrogen have been considered.

Lunine, J. I.↗
NASA NTRS1991

Fourth Symposium on Chemical Evolution and the Origin and Evolution of Life

This symposium was held at the NASA Ames Research Center, Moffett Field, California, July 24-27, 1990. The NASA exobiology investigators reported their recent research findings. Scientific papers were presented in the following areas: cosmic evolution of biogenic compounds, prebiotic evolution (planetary and molecular), early evolution of life (biological and geochemical), evolution of advanced life, solar system exploration, and the Search for Extraterrestrial Intelligence (SETI).

Wharton, Robert A., Jr.↗
NASA NTRS1995

Physical and chemical evolution of reduced organic matter in the ISM

Icy mantles on interstellar grains have been a topic of study in airborne astronomy. Recent laboratory analog studies of the yield of organic residue from UV photolyzed ices have shown that this mechanism can be the most significant source of complex reduced organic matter in the interstellar medium. However, the total yield is a function of the occurrence of heating events that evaporate the ice, i.e. T is greater than 130 K, and the mechanism for such events is debated. Recently, we proposed that the recombination of radicals in the ice does not need high temperature excursions and, instead, occurs during a structural transformation of water ice at temperatures in the range 38 - 68 K.

Jenniskens, Peter↗