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PubMed1991

Slope variations on the surface of Phobos.

It has been suggested that slope fluctuations on the scale of pixel dimensions could be determined by statistical photoclinometry. A closer study of the surface of Phobos reveals variations in the scattering properties of single particles and micro-structures formed by the particles. In the present context, the photoclinometric method of brightness moments is extended to account for these variations by allowing statistical fluctuations in the phase function of the assumed Lommel-Seeliger scattering law. The mean slope on the investigated regions of Phobos has been found to vary from approximately 12 degrees on a 61m scale to approximately 7 degrees on a 216-272m scale. On the same scales, a value of the order of 2% has been obtained for the standard deviation of the scattering phase function. Hints of a fractal-like scale-invariance have been noticed in the covariance function of brightness.

Astronomy↗
PubMed1991

Interpretation of the surface brightness of Phobos.

Analysis of disk resolved images of Phobos obtained by the Phobos 2 spacecraft allows us to study the surface scattering law and albedo variations. From low phase angle images we find variations in local geometric albedo approximately 10%, with a correlation length approximately 1km. The scattering law is reasonably well matched by the recent proposed LPI (Lumme et al. 1990a) model, which allows us to deduce a small scale (approximately 1 mm) surface roughness (approximately 0.5), defined here as the rms. tangent of the local surface normal relative to the mean surface normal in the Duxbury (1991) model of Phobos. This value is very close to what has been found for Mercury and the Moon.

Astronomy↗
PubMed1993

Fractionation of hydrogen and deuterium on Venus due to collisional ejection.

The fractionation factor f is important for interpreting the current escape fluxes of H and D on Venus and how the D/H ratio has evolved. The escape flux is currently governed by the two processes of charge exchange and collisional ejection by fast oxygen atoms. Using a best-fit parameterized equation for the O-H scattering angle phase function, more accurate branching ratios for the oxygen ion dissociation and including the effects of the initial energy and momentum of the ions and electrons, as well as for the hydrogen and deuterium gas, we have reanalyzed the collisional ejection process. Our analysis produces improved values for the efficiency of H and D escape as a function of the ionospheric temperature. From our results we propose the reduction of the hydrogen flux for collisional ejection from 8 to 3.5 x 10(6) cm-2 s-1. Assuming that collisions leading to escape occur mostly in the region between 200 and 400 km, the revised D/H fractionation factor due to collisional ejection is 0.47, where previously the process had been considered completely discriminating against deuterium escape (or f approximately 0.) The resulting deuterium flux is 3.1 x 10(4) cm-2 s-1, roughly 6 times the flux due to charge exchange, making collisional ejection the dominant escape mechanism for deuterium on Venus.

Atmosphere↗
PubMedDate not supplied

Meteorite organics in planetary environments: hydrothermal release, surface activity, and microbial utilization.

Up to 50% of the organics in the Murchison meteorite, possibly including some of the polymer, is released in high temperature and pressure aqueous environments, to 350 degrees C and 250 bar, that simulate submarine volcanic, hydrothermal or impact-induced conditions. Meteorite organics of prebiotic significance, such as nonanoic acid, glycine, and pyrene survive the hydrothermal conditions. The released material is surface active with surface pressures up to 19.8 x 10(-3) N m-1, and exhibits an extended surface tension isotherm which suggests a mixture of amphiphilic components. One component, nonanoic acid, is shown to form vesicles. The materials extracted under mild conditions, at 120 degrees C, are nutrients for the humic acid bacterium Pseudomonas maltophilia and efficient nutrients for the oligotroph Flavobacterium oryzihabitans, demonstrating the capability of microorganisms to metabolize extraterrestrial organics.

Carbon↗
PubMed1986

A directed search for extraterrestrial laser signals.

This paper analyzes the technical feasibility of interstellar communication at infrared frequencies, both in its own right and in comparison with communication at radio frequencies. The analysis considers both the practical and fundamental limits affecting communication over interstellar distances and concludes that for specified transmitter and receiver locations communications at infrared and radio frequencies can be equally effective. On this basis a search for extraterrestrial signals at infrared wavelengths is equally as valid as any planned microwave effort. Work is now in progress to adapt a 10 micrometers heterodyne spectrometer to search for CO2 laser signals from 200 nearby stars.

Astronomy↗
PubMed1986

Review of methodology and technology available for the detection of extrasolar planetary systems.

Anyone undertaking an interstellar voyage might wish to be assured of the existence of a safe planetary harbor at the other end! Aside from the obvious interest of the participants in this Symposium, astronomers and astrophysicists are also eager to detect and study other planetary systems in order to better understand the formation of our own Solar System. Scientists involved in the search for extraterrestrial intelligence argue that planets suitable for the evolution of life may abound elsewhere within our own Milky Way Galaxy. On theoretical grounds, they are probably correct, but they lack any observational support. For in spite of decades of claimed astrometric detections of planetary companions and the recent exciting and tantalizing observations from the IRAS satellite and the IR speckle observations of Van Biesbroeck 8 and other cool stars, there is no unambiguous proof for the existence of another planetary system beyond our own. In this paper we review the various methods for detecting extrasolar planets and briefly describe the Earth and space based technology currently available and discuss the near-term plans to implement these different search techniques. In each case an attempt is made to identify the limiting source of systematic error inherent to the methodology and to assess the potential for technological improvements.

Astronomical Phenomena↗
PubMed1989

Radio frequency interference at Jodrell Bank Observatory within the protected 21 cm band.

Radio frequency interference (RFI) will provide one of the most difficult challenges to systematic Searches for Extraterrestrial Intelligence (SETI) at microwave frequencies. The SETI-specific equipment is being optimized for the detection of signals generated by a technology rather than those generated by natural processes in the universe. If this equipment performs as expected, then it will inevitably detect many signals originating from terrestrial technology. If these terrestrial signals are too numerous and/or strong, the equipment will effectively be blinded to the (presumably) weaker extraterrestrial signals being sought. It is very difficult to assess how much of a problem RFI will actually represent to future observations, without employing the equipment and beginning the search. In 1983 a very high resolution spectrometer was placed at the Nuffield Radio Astronomy Laboratories at Jodrell Bank, England. This equipment permitted an investigation of the interference environment at Jodrell Bank, at that epoch, and at frequencies within the 21 cm band. This band was chosen because it has long been "protected" by international agreement; no transmitters should have been operating at those frequencies. The data collected at Jodrell Bank were expected to serve as a "best case" interference scenario and provide the minimum design requirements for SETI equipment that must function in the real and noisy environment. This paper describes the data collection and analysis along with some preliminary conclusions concerning the nature of the interference environment at Jodrell Bank.

Astronomy↗
PubMed1989

VLSI processors for signal detection in SETI.

The objective of the Search for Extraterrestrial Intelligence (SETI) is to locate an artificially created signal coming from a distant star. This is done in two steps: (1) spectral analysis of an incoming radio frequency band, and (2) pattern detection for narrow-band signals. Both steps are computationally expensive and require the development of specially designed computer architectures. To reduce the size and cost of the SETI signal detection machine, two custom VLSI chips are under development. The first chip, the SETI DSP Engine, is used in the spectrum analyzer and is specially designed to compute Discrete Fourier Transforms (DFTs). It is a high-speed arithmetic processor that has two adders, one multiplier-accumulator, and three four-port memories. The second chip is a new type of Content-Addressable Memory. It is the heart of an associative processor that is used for pattern detection. Both chips incorporate many innovative circuits and architectural features.

Astronomy↗
PubMedDate not supplied

Nonconscious intelligence in the universe.

Animals lacking humanoid intelligence have evolved systems indistinguishable in function, if not in structure, from systems built by humans. Although radio communication has never been verified in animals, it is completely feasible biologically. If such systems are present in non-intelligent organisms on other planets, then our chances of detecting life in the universe by current SETI methods are greatly enhanced.

Animal Communication↗
PubMed1992

An isotopic biogeochemical study of the Green River oil shale.

Thirty-five different samples from three different sulfur cycles were examined in this stratigraphically oriented study of the Shell 22x-l well (U.S.G.S. C177 core) in the Piceance Basin, Colorado. Carbon isotopic compositions of constituents of Green River bitumens indicate mixing of three main components: products of primary photoautotrophs and their immediate consumers (delta approximately -30% vs PDB), products of methanotrophic bacteria (delta approximately -85%), and products of unknown bacteria (delta approximately -40%). For individual compounds synthesized by primary producers, delta-values ranged from -28 to -32%. 13C contents of individual primary products (beta-carotane, steranes, acyclic isoprenoids, tricyclic triterpenoids) were not closely correlated, suggesting diverse origins for these materials. 13C contents of numerous hopanoids were inversely related to sulfur abundance, indicating that they derived both from methanotrophs and from other bacteria, with abundances of methanotrophs depressed when sulfur was plentiful in the paleoenvironment. gamma-Cerane coeluted with 3 beta(CH3),17 alpha(H),21 beta(H)-hopane, but delta-values could be determined after deconvolution. gamma-Cerane (delta approximately -25%) probably derives from a eukaryotic heterotroph grazing on primary materials, the latter compound (delta approximately -90%) must derive from methanotrophic organisms. 13C contents of n-alkanes in bitumen differed markedly from those of paraffins generated pyrolytically. Isotopic and quantitative relationships suggest that alkanes released by pyrolysis derived from a resistant biopolymer of eukaryotic origin and that this was a dominant constituent of total organic carbon.

Alkanes↗
PubMed1995

Molecular indicators for palaeoenvironmental change in a Messinian evaporitic sequence (Vena del Gesso, Italy). II: High-resolution variations in abundances and 13C contents of free and sulphur-bound carbon skeletons in a single marl bed.

The extractable organic matter of 10 immature samples from a marl bed of one evaporitic cycle of the Vena del Gesso sediments (Gessoso-solfifera Fm., Messinian, Italy) was analyzed quantitatively for free hydrocarbons and organic sulphur compounds. Nickel boride was used as a desulphurizing agent to recover sulphur-bound lipids from the polar and asphaltene fractions. Carbon isotopic compositions (delta vs PDB) of free hydrocarbons and of S-bound hydrocarbons were also measured. Relationships between these carbon skeletons, precursor biolipids, and the organisms producing them could then be examined. Concentrations of S-bound lipids and free hydrocarbons and their delta values were plotted vs depth in the marl bed and the profiles were interpreted in terms of variations in source organisms, 13 C contents of the carbon source, and environmentally induced changes in isotopic fractionation. The overall range of delta values measured was 24.7%, from -11.6% for a component derived from green sulphur bacteria (Chlorobiaceae) to -36.3% for a lipid derived from purple sulphur bacteria (Chromatiaceae). Deconvolution of mixtures of components deriving from multiple sources (green and purple sulphur bacteria, coccolithophorids, microalgae and higher plants) was sometimes possible because both quantitative and isotopic data were available and because either the free or S-bound pool sometimes appeared to contain material from a single source. Several free n-alkanes and S-bound lipids appeared to be specific products of upper-water-column primary producers (i.e. algae and cyanobacteria). Others derived from anaerobic photoautotrophs and from heterotrophic protozoa (ciliates), which apparently fed partly on Chlorobiaceae. Four groups of n-alkanes produced by algae or cyanobacteria were also recognized based on systematic variations of abundance and isotopic composition with depth. For hydrocarbons probably derived from microalgae, isotopic variations are well correlated with those of total organic carbon. A resistant aliphatic biomacromolecule produced by microalgae is, therefore, probably an important component of the kerogen. These variations reflect changes in the depositional environment and early diagenetic transformations. Changes in the concentrations of S-bound lipids induced by variations in conditions favourable for sulphurization were discriminated from those related to variations in primary producer assemblages. The water column of the lagoonal basin was stratified and photic zone anoxia occurred during the early and middle stages of marl deposition. During the last stage of the marl deposition the stratification collapsed due to a significant shallowing of the water column. Contributions from anaerobic photoautotrophs were apparently associated with variations in depth of the chemocline.

Animals↗
PubMed1991

Structural investigation of protein kinase C inhibitors.

The phospholipid and Ca2+ dependent protein kinase (PKC) plays an essential role in a variety of cellular events. Inhibition of PKC was shown to arrest growth in tumor cell cultures making it a target for possible antitumor therapy. Calphostins are potent inhibitors of PKC with high affinity for the enzyme regulatory site. Structural characteristics of calphostins, which confer the inhibitory activity, are investigated by comparing their optimized structures with the existing models for PKC activation. The resulting model of inhibitory activity assumes interaction with two out of the three electrostatic interaction sites postulated for activators. The model shows two sites of hydrophobic interaction and enables the inhibitory activity of gossypol to be accounted for.

Gossypol↗
PubMed1988

Sulfur isotope effects associated with oxidation of sulfide by O2 in aqueous solution.

Normal sulfur isotope effects averaging epsilon = -5.2 +/- 1.4% (s.d.) were consistently observed for the oxidation of sulfide in aqueous solution. Reaction products were sulfate, thiosulfate and sulfite at pH 10.8-11 in distilled water; S0 was formed in two experiments with synthetic seawater at pH 8-9.5. Because the -5.2% normal isotope effect differs significantly from the previously measured +2% inverse effect associated with anaerobic oxidation of sulfide by photosynthetic bacteria, stable sulfur isotopic measurements are potentially useful for distinguishing aerobic vs. anaerobic sulfide oxidation in marine and freshwater sulfureta.

Bacteria, Anaerobic↗
PubMed1984

The chemical conditions on the parent body of the Murchison meteorite: some conclusions based on amino, hydroxy and dicarboxylic acids.

Amino and hydroxy acids have been identified in the Murchison meteorite. Their presence is consistent with a synthetic pathway involving aldehydes, hydrogen cyanide and ammonia in an aqueous environment (Strecker-cyanohydrin synthesis). From the various equilibrium and rate constants involved in this synthesis, four independent estimates of the ammonium ion concentrations on the parent body at the time of compound synthesis are obtained; all values are about 2 x 10(-3) M. Succinic acid and beta-alanine have also been detected in the Murchison meteorite. Their presence is consistent with a synthesis from acrylonitrile, hydrogen cyanide and ammonia. Using the equilibrium and rate constants for this synthetic pathway, and the succinic acid/beta-alanine ratio measured in the Murchison meteorite, an estimate of the hydrogen cyanide concentration of 10(-3) to 10(-2) M is obtained. Since hydrogen cyanide hydrolyzes relatively rapidly in an aqueous environment (t1/2 < 10(4) yrs) this high concentration implies a period of synthesis of organic compounds as short as 10(4) years on the Murchison meteorite parent body.

Acrylonitrile↗
PubMed1984

Prebiotic syntheses of purines and pyrimidines.

The work done in many laboratories during the last two decades has confirmed that hydrogen cyanide and cyanoacetylene are the two major precursors for the prebiotic synthesis of purines and pyrimidines, respectively. Although several different pathways for the synthesis of purines have been described, they are all variations of the initial mechanism proposed by Oró and Kimball, where hydrogen cyanide leads first to the formation of a 4,5-di-substituted imidazole derivative, and then to the closing of the purine ring with a C1 compound. A number of experiments have shown that purines and pyrimidines can also be obtained from methane, ammonia (nitrogen), and water mixtures, provided an activating source of energy (radiation, electric discharges, etc.) is available. However, in this case the yields are lower by about two orders of magnitude because of the intermediate formation of hydrogen cyanide and cyanoacetylene. The latter two compounds have been found in interstellar space, Titan and other bodies of the solar system. They were probably present in the primordial parent bodies from the solar nebula in concentrations of 10(-2) to 10(-3) M as inferred from recent calculations by Miller and coworkers obtained for the Murchison meteorite. These concentrations should have been sufficient to generate relatively large amounts of purine and pyrimidine bases on the primitive Earth.

Adenine↗
PubMed1984

On the stereoselective aminoacylation of RNA.

Aminoacylation of the "internal" 2'hydroxyl groups of poly(A) with the imidazolide of racemic DL-leucine resulted in the formation of an enantiomeric excess of the D-ester, whereas aminoacylation with the imidazolide of racemic N-3,5-dinitrobenzoyl-DL-leucine gave an enantiomeric excess of the L-ester. Comparison of these results with those obtained earlier for alanine shows that the larger side chain tends to favor the formation of the D-ester. High resolution proton magnetic resonance spectroscopy was performed on the D- and L-alanyl "internal" 2'-esters of 3'5'-ApA. The resonance of the side chain methyl group of the D-ester is upfield from that of the L-ester, and the separation of the peaks increases with a decrease in temperature.

Acylation↗