Comets

Vacuum ultraviolet observations from sounding rockets and satellite observatories of the gaseous comae of several comets are reviewed. The earliest of these led to discovery of the hydrogen envelope extending for millions of km from the nucleus. Subsequent observations of H I Lyman alpha, the OH (0,0 band and the oxygen resonance triplet provided strong evidence for the water-ice model of the cometary nucleus. Several species were discovered in the coma including C, C(+), CO, S, and CS. High resolution spectroscopy and the spatial variation of the observed emissions provide means to elucidate the production and excitation mechanisms of these species. The similarity of the spectra of the half dozen comets observed to date argues for a common, homogeneous composition (with the exception of dust and CO) of the cometary ice and a minimal effect on the neutral species due to molecular collisions in the inner coma

Essays on institutional persistence, contract structure, and authority

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Economics, 2008.Includes bibliographical references (p. 154-158).This thesis is a collection of essays that uses theoretical and experimental methods to explore institutional persistence, contract structure, and authority. Chapter One studies the informational and efficiency properties of institutions that form to reduce moral hazard. While in the short run such mechanisms may be optimal, in the long run inefficient institutions may persist because information about changes in the underlying environment is lost. Using experimental and theoretical methods, it analyzes a market with high quality and low quality products that are indistinguishable without a costly certification process. Sellers in the market make endogenous production decisions and are heterogeneous in their levels of moral hazard leading to two possible equilibria-non-certifying and certifying-that vary in both efficiency and information about the underlying environment. The certifying equilibrium, which does not carry information about changes in the distribution of sellers, does not adjust when the underlying environment changes, perpetuating a market structure that makes all market participants weakly worse off. Chapter Two studies how changes in contract structure may help preserve antiquities. Most countries prohibit the export of certain antiquities. This practice often leads to illegal excavation and looting for the black market, which damages objects and destroys important aspects of the archaeological record. Chapter Two argues that many of the goals for export bans could be better accomplished through the use of long-term leases which would raise revenue for the country of origin while preserving national long-term ownership rights. Chapter Three uses experiments to study how control rights are distributed in a setting with incentive conflicts.(cont.) It shows that while effort levels are consistent with theoretical predictions, principals retain control rights even when it is strongly in their interest to delegate. Chapter Three also documents a differential response to authority by gender. As agents, women have strong fairness preferences resulting in diminished effort in asymmetric treatments but higher effort in symmetric ones. As principals, women are more likely to transfer authority when it is efficient to do so.Tom S. Wilkening.Ph.D

Access to metastable complex ion conductors via mechanosynthesis: Preparation, microstructure and conductivity of (Ba,Sr)LiF3 with inverse perovskite structure

Highly metastable Ba1−xSrxLiF3 (0 < x ≤ xmax ≈ 0.4) with an inverse perovskite structure analogous to that of BaLiF3 was synthesized by soft mechanical treatment of BaF2 and LiF together with SrF2 at ambient temperature. Ex as well as in situX-ray powder diffraction (XRPD) measurements show that heat treatment at 393 K initiates the decomposition of the mixed phase into BaLiF3, LiF and (Sr,Ba)F2. Structural details of the metastable compound (Ba,Sr)LiF3 were investigated by ultrafast 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Interestingly, five magnetically inequivalent F sites were identified which correspond to fluorine anions coordinated by a variable number of Ba and Sr cations, respectively. Details from XRPD and NMR spectroscopy are discussed with respect to the formation mechanisms and thermal stability of the as prepared fluorides. Impedance spectroscopy is used to characterize (long-range) ionic transport properties. Results are compared with those obtained recently on mechanosynthesized BaLiF3

Second-generation probes for biosynthetic intermediate capture : towards a comprehensive profiling of polyketide assembly

Malonyl carba(dethia) N-decanoyl cysteamine methyl esters and novel acetoxymethyl esters were utilised as second-generation probes for polyketide intermediate capture. The use of these tools in vivo led to the characterisation of an almost complete set of biosynthetic intermediates from a modular assembly line, providing a first kinetic overview of intermediate processing leading to complex natural product formation

Discriminating changes in intracellular NADH/NAD+ levels due to anoxicity and H2 supply in R. eutropha cells using the Frex fluorescence sensor

The hydrogen-oxidizing “Knallgas” bacterium Ralstonia eutropha can thrive in aerobic and anaerobic environments and readily switches between heterotrophic and autotrophic metabolism, making it an attractive host for biotechnological applications including the sustainable H2-driven production of hydrocarbons. The soluble hydrogenase (SH), one out of four different [NiFe]-hydrogenases in R. eutropha, mediates H2 oxidation even in the presence of O2, thus providing an ideal model system for biological hydrogen production and utilization. The SH reversibly couples H2 oxidation with the reduction of NAD+ to NADH, thereby enabling the sustainable regeneration of this biotechnologically important nicotinamide cofactor. Thus, understanding the interaction of the SH with the cellular NADH/NAD+ pool is of high interest. Here, we applied the fluorescent biosensor Frex to measure changes in cytoplasmic [NADH] in R. eutropha cells under different gas supply conditions. The results show that Frex is well-suited to distinguish SH-mediated changes in the cytoplasmic redox status from effects of general anaerobiosis of the respiratory chain. Upon H2 supply, the Frex reporter reveals a robust fluorescence response and allows for monitoring rapid changes in cellular [NADH]. Compared to the Peredox fluorescence reporter, Frex displays a diminished NADH affinity, which prevents the saturation of the sensor under typical bacterial [NADH] levels. Thus, Frex is a valuable reporter for on-line monitoring of the [NADH]/[NAD+] redox state in living cells of R. eutropha and other proteobacteria. Based on these results, strategies for a rational optimization of fluorescent NADH sensors are discussed

An estimate of the global distribution of radon emissions from the ocean

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 31 (2004): L19104, doi:10.1029/2004GL021051.There is a need for improved estimates of the radon (222Rn) flux density from the ocean for use in the modeling and interpretation of atmospheric radon in global climate and air pollution studies. We use a modification of a frequently used model of gas transfer to generate global predictions of ocean radon flux density for each month of the year (climate averaged) on a 192 by 94 global grid. Compared with the often-used approximation of a constant radon flux from the ocean, the model's predictions indicate large variations over regions of the ocean (a factor of ten is not uncommon). For example, latitude bands near the equator and Southern Ocean are predicted to emit relatively high average radon flux compared with other latitude bands. The predicted annually-averaged flux density from the ocean is 0.0382 mBq m−2 s−1 (0.00182 atoms cm−2 s−1), smaller than some commonly-used estimates

Placental expression of angiopoietin-1, angiopoietin-2 and tie-2 during placental development in an ovine model of placental insufficiency-fetal growth restriction.

Fetal growth restriction (FGR) is associated with increased perinatal morbidity and mortality, and often results from functional placental insufficiency. Placentation requires extensive vasculogenesis and subsequent angiogenesis, in both maternal and fetal tissues. Angiopoietin-1 (Ang-1) and Angiopoietin-2 (Ang-2) are angiogenic growth factors expressed in the placenta, and compete for binding to a common receptor, Tunica interna endothelial cell kinase-2 (Tie-2). Our objective was to examine Ang-1, Ang-2 and Tie-2 expression in ovine placental tissue obtained from normal and FGR pregnancies throughout gestation. Fetal cotyledon and maternal caruncle tissue concentrations of Ang-1, Ang-2 and Tie-2 mRNA were assessed by real-time reverse transcriptase-polymerase chain reaction and protein concentrations were assessed by Western immunoblot analysis, at 55, 90 and 135 d gestational age (dGA). Concentrations of Ang-1, Ang-2 and Tie-2 mRNA in FGR fetal cotyledons were increased at 55 dGA, and Tie-2 mRNA concentrations were decreased in FGR fetal cotyledons and maternal caruncles at 135 dGA. Immunoblot analysis demonstrated increased concentrations of Ang-2 in the fetal cotyledon at 55 dGA, and lower concentrations at 135 dGA. In contrast, concentrations of Tie-2 were increased at 90 dGA, but tended to decrease at 135 dGA in FGR maternal caruncles. The changes observed during early- to mid-gestation may result in increased branching angiogenesis, but may also set the stage for increased nonbranching angiogenesis during late gestation, altered placental architecture and placental insufficiency that result in FGR

Multinuclear Solid-State NMR Study of Local Structure and Dynamics in Li0.7Nb3S4

The present work focuses on channel-structured Li0.7Nb3S4, where, among others, the question of the dimensionality of the diffusion pathway arises