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At least 145 records · Page 8
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NASA NTRS2011

Carbonaceous Meteorites Contain a Wide Range of Extraterrestrial Nucleobases

All terrestrial organisms depend on nucleic acids (RNA and DNA), which use pyrimidine and purine nucleobases to encode genetic information. Carbon-rich meteorites may have been important sources of organic compounds required for the emergence of life on the early Earth; however, the origin and formation of nuc1eobases in meteorites has been debated for over 50 y. So far, the few nuc1eobases reported in meteorites are biologically common and lacked the structural diversity typical of other indigenous meteoritic organics. Here, we investigated the abundance and distribution of nucleobases and nucleobase analogs in formic acid extracts of 12 different meteorites by liquid chromatography-mass spectrometry. The Murchison and Lonewolf Nunataks 94102 meteorites contained a diverse suite of nucleobases, which included three unusual and terrestrially rare nucleobase analogs; purine, 2,6-diminopurine, and 6,8-diaminopurine. In a parallel experiment, we found an identical suite of nucleobases and nucleobase analogs generated in reactions of ammonium cyanide. Additionally, these nucleobase analoge were not detected above our parts-per-billion detection limits in any of the procedural blanks, control samples, a terrestrial soil sample, and an Antarctic ice sample. Our results demonstrate that the purines detected in meteorites are consistent with products of ammonium cyanide chemistry, which provides a plausible mechanism for their synthesis in the asteroid parent bodies, and strongly supports an extraterrestrial origin. The discovery of new nucleobase analogs in meteorites also expands the prebiotic molecular inventory available for constructing the first genetic molecules.

Callahan, Michael P.↗
NASA NTRS2018

Closed Loop Guidance Trade Study for Space Launch System Block-1B Vehicle

NASA is currently building the Space Launch System (SLS) Block-1 launch vehicle for the Exploration Mission 1 (EM-1) test flight. The design of the next evolution of SLS, Block-1B, is well underway. The Block-1B vehicle is more capable overall than Block-1; however, the relatively low thrust-to-weight ratio of the Exploration Upper Stage (EUS) presents a challenge to the Powered Explicit Guidance (PEG) algorithm used by Block-1. To handle the long burn durations (on the order of 1000 seconds) of EUS missions, two algorithms were examined. An alternative algorithm, OPGUID, was introduced, while modifications were made to PEG. A trade study was conducted to select the guidance algorithm for future SLS vehicles. The chosen algorithm needs to support a wide variety of mission operations: ascent burns to LEO, apogee raise burns, trans-lunar injection burns, hyperbolic Earth departure burns, and contingency disposal burns using the Reaction Control System (RCS). Additionally, the algorithm must be able to respond to a single engine failure scenario. Each algorithm was scored based on pre-selected criteria, including insertion accuracy, algorithmic complexity and robustness, extensibility for potential future missions, and flight heritage. Monte Carlo analysis was used to select the final algorithm. This paper covers the design criteria, approach, and results of this trade study, showing impacts and considerations when adapting launch vehicle guidance algorithms to a broader breadth of in-space operations.

Von der Porten, Paul↗
NASA NTRS1987

Thermomechanical Properties Indicate Degree of Epoxy Cure

Glass-transition temperature and density increase as reaction proceeds. Cured epoxy resin found to be related to extent of cure. In addition to providing insight into chemical reactions of curing, relationships show potential for process monitoring and control in fabrication of strong, lightweight composite parts.

Cizmecioglu, Muzaffer↗
NASA NTRS2000

Triaxial Burke-Schumann Flames with Applications to Flame Synthesis

The problem of a flame generated by three coaxial flows is solved by extending the Burke-Schumann methodology to include a third stream. The solution is particularly relevant to flame synthesis wherein multiple tubes are often employed either to introduce inert as a diffusion barrier or to introduce more than two reactants. The general problem is solved where the inner and outer tubes contain reactants and the middle tube contains either an inert or a third reactant. Relevant examples are considered and the results show that the triaxial Burke-Schumann flame can be substantially more complicated than the traditional Burke-Schumann flame. When the middle flow is inert the flame temperature is no longer constant but increases axially, reaching a maximum at the flame centerline. At the exit the flame does not sit on the tube exit but instead resides between the inner and outer tubes, resulting in an effective barrier for particle build-up on the burner rim. For the case of a third reactant in the middle flow, synthesis chemistry where the inner reaction is endothermic and the outer reaction is exothermic is considered. In addition to showing the flame temperature and flame shape, the results identify conditions wherein reaction is not possible due to insufficient heat transfer from the outer flame to support the inner flame reaction.

Chao, B. H.↗
NASA NTRS2009

Energetic Processing of Interstellar Ices: A Route to Complexity

More than 140 gas-phase molecules have been detected in the interstellar (IS) medium or in circumstellar environments including inorganics, organics, ions, and radicals. The significant abundance of large, complex organic molecules, and families of isomers in these regions makes the origin and formation history of these species the subject of debate. Observationally determined condensed-phase species are H2O, CO, CO2, NH3 and CH30H, with CH4, HCOOH, OCS, OCN-, H2CO and NH4(+) present at trace levels. These ices can undergo energetic processing with cosmic rays or far-UV photons to form larger complex organics with abundance levels that make them undetectable in icy mantles. Once warmed, however, it is likely that these complex species would enter the gas-phase where they might be detected by Herschel or Alma. Understanding the role of radiation chemistry and thermal processing of ices and identifying new products are the goals of our laboratory research. In the Cosmic lee Laboratory at NASA Goddard Space Plight Center, we can study both the photo-and radiation chemistries of ices from 8 -- 300 K. Using dear- and mid-IR spectroscopy we can follow the destruction of primary molecules and the formation of radicals and secondary products as a function of energetic processing. During warming we can monitor the trapping of species and the results of any thermal chemistry. An overview of recent and past work will focus on complex secondary radiation products from small condensed-phase IS species. Likely reactions include dimerization, isomerization, H-addition and H-elimination. Another focus of our work is the development of reaction schemes for the formation of complex molecules and the use of such schemes to predict new molecules awaiting detection by Herschel and Alma.

Moore, Marla H.↗
NASA NTRS2003

The Stable Isotope Fractionation of Abiotic Reactions: A Benchmark in the Detection of Life

One very important tool in the analysis of biogenic, and potentially biogenic, samples is the study of their stable isotope distributions. The isotope distribution of a sample depends on the process(es) that created it. One important application of the analysis of C & N stable isotope ratios has been in the determination of whether organic matter in a sample is of biological origin or was produced abiotically. For example, the delta C-13 of organic material found embedded in phosphate grains was cited as a critical part of the evidence for life in 3.8 billion year old samples. The importance of such analysis in establishing biogenicity was highlighted again by the role this issue played in the recent debate over the validity of what had been accepted as the Earth s earliest microfossils. These kinds of analysis imply a comparison with the fractionation that one would have seen if the organic material had been produced by alternative, abiotic, pathways. Could abiotic reactions account for the same level of fractionation? Additionally, since the fractionation can vary between different abiotic reactions, understanding their fractionations can be important in distinguishing what reactions may have been significant in the formation of different abiological samples (such as extraterrestrial samples). There is however, a scarcity of data on the fractionation of carbon and nitrogen by abiotic reactions. In order to interpret properly what the stable isotope ratios of samples tell us about their biotic or abiotic nature, more needs to be known about how abiotic reactions fractionate C and N. Carbon isotope fractionations have been studied for a few abiotic processes. These studies presumed the presence of a reducing atmosphere, focusing on reactions involving spark discharge, W photolysis of reducing gas mixtures, and cyanide polymerization in the presence of ammonia. They did find that the initial products showed a depletion in I3C with values in the range of a few per mil to as low as -60 % (potentially comparable to that which accompanies the biosynthesis of organic matter). We need to understand what kind of fractionations are observed with reactions under the non-reducing or mildly reducing conditions now thought to be present on the early Earth. While nitrogen is receiving increased attention as a tool for these kinds of analyses, almost nothing is known about the isotope fractionation that one would expect for abiotic sources of fixed/reduced nitrogen. This project will measure the fixation from a series of abiotic reactions that may have been present on the early Earth (and other terrestrial planets) and produced organic material that could have ended up in the rock record. The work will look at a number of reactions, under a non- reducing, or mildly reducing, atmosphere, covering sources of prebiotic organic C & N from shock heating, to photochemistry, to hydrothermal reactions. Some reactions that we plan to study are; Shock heating of a non-reducing atmosphere to produce CO and NO (in collaboration with Chris McKay), formation of formaldehyde (and related compounds) from COY the formation of ammonia from nitrogen oxides (ultimately from NO) by ferrous iron reduction, and the hydrothermal synthesis of compounds including the hydrocarboxylation/hydrocarbonylation reaction (in collaboration with George Cody), reactions of oxalate to form hydrocarbons and other oxygenated compounds and the formation of lipids from oxalic/formic acid (in collaboration with Tom McCollom), and reactions of carbon monoxide & carbon dioxide with N2, ammonia or nitritehitrate to form hydrogen cyanide, nitriles, ammonia/amines and nitrous

Summers, David P.↗
NASA NTRS1996

Oligomerization of uridine phosphorimidazolides on montmorillonite: a model for the prebiotic synthesis of RNA on minerals

The 5'-phosphorimidazolide of uridine reacts on Na(+)-montmorillonite 22A in aqueous solution to give oligomers as long as 7 mers. The maximum chain length increases to 9 mers and the overall oligomer yield increases when 9:1 ImpU, A5' ppA mixtures react under the same conditions. The oligomer yield and maximum chain length decreases with the structure of the added pyrophosphate in the order A5' ppA > A5' ppU > U5' ppU. Structure analysis of individual oligomer fractions was performed by selective enzymatic hydrolyses followed by HPLC analysis of the products. The regioselectivity for 3',5'-bond formation is 80-90% in the 9:1 ImpU, A5' ppA reaction, a percentage comparable to that observed in the 9:1 ImpA, A5' ppA reaction. Oligomerization of ImpU is inhibited by addition of dA5' ppdA, and MeppA. No oligomers containing A5' ppU were products of the 9:1 ImpU,A5' ppA reaction, a finding consistent with the simple addition of the ImpU to the A5' ppA and not the rearrangement of an ImpU-A5' ppA adduct. Concentrations of lysine or arginine which were close to that of the ImpU did not inhibit oligomer formation. Treatment of Na(+)-montmorillonite with 1 M arginine yielded arginine-montmorillonite, an amino acid-mineral adduct which did not catalyze ImpU oligomerization. Neither the 4-9 mers formed in the 9:1 ImpU, A5' ppA reaction nor the 4-9 mers formed by the base hydrolysis of poly(U) served as templates for the formation of oligo(A)s.

Non-NASA Center↗
NASA NTRS1990

Solid state processing for high temperature alloys and composites

A variety of solid state powder processing methods have been examined for their potential to produce high temperature materials. These techniques included solid state diffusion, mechanical alloying, reaction milling, and exothermic reactions. While both mechanical alloying and exothermic reactions are capable of producing high temperature composites in addition to yielding solid solution and precipitation hardened alloys, reaction milling can only be used to make composite materials. Although at present this method has only been applied to make carbide strengthened Al, recent work indicates that reaction milling is capable of yielding high strength NiAl-AlN composites.

Whittenberger, J. Daniel↗
NASA NTRS1995

Photodeposition of amorphous polydiacetylene films from monomer solutions onto transparent substrates

Polydiacetylenes are a very promising class of polymers for both photonic and electronic applications because of their highly conjugated structures. For these applications, high-quality thin polydiacetylene films are required. We have discovered a novel technique for obtaining such films of a polydiacetylene derivative of 2-methyl-4-nitroaniline using photodeposition from monomer solutions onto UV transparent substrates. This heretofore unreported process yields amorphous polydiacetylene films with thicknesses on the order of I micron that have optical quality superior to that of films grown by standard crystal growth techniques. Furthermore, these films exhibit good third-order nonlinear optical susceptibilities; degenerate four-wave mixing experiments give x(3) values on the order of 10(exp -8) - 10(exp -7) esu. We have conducted masking experiments which demonstrate that photodeposition occurs only where the substrate is directly irradiated, clearly indicating that the reaction occurs at the surface. Additionally, we have also been able to carry out photodeposition using lasers to form thin polymer circuits. In this work, we discuss the photodeposition of polydiacetylene thin films from solution, perform chemical characterization of these films, investigate the role of the substrate, speculate on the mechanism of the reaction, and make a preliminary determination of the third-order optical nonlinearity of the films. This simple, straightforward technique may ultimately make feasible the production of polydiacetylene thin films for technological applications.

Paley, M. S.↗
NASA NTRS1984

Mononitration of durene

Reaction conditions under which the nitration of durene give predominantly, and in high yield, mononitrodurene (I), rather than dinitrodurene were found. The nitrating agent was No2+RF6-. Nitration with nitrosulfuric acid also gave mononitrodurene; however, byproducts such as 2.3.5.6-Me4C6HCH2C6H2Me3-2,4,5, were also formed. The NO2 PF6- gave an intermediate additional compound with durene. The reaction carried out with 3.6-dideuterodurene in D2O shows no isotope effect.

Hanna, S. B.↗
NASA NTRS1986

Cooperation of catalysts and templates

In order to understand how self-reproducing molecules could have originated on the primitive Earth or extraterrestrial bodies, it would be useful to find laboratory models of simple molecules which are able to carry out processes of catalysis and templating. Furthermore, it may be anticipated that systems in which several components are acting cooperatively to catalyze each other's synthesis will have different behavior with respect to natural selection than those of purely replicating systems. As the major focus of this work, laboratory models are devised to study the influence of short peptide catalysts on template reactions which produce oligonucleotides or additional peptides. Such catalysts could have been the earliest protoenzymes of selective advantage produced by replicating oligonucleotides. Since this is a complex problem, simpler systems are also studied which embody only one aspect at a time, such as peptide formation with and without a template, peptide catalysis of nontemplated peptide synthesis, and model reactions for replication of the type pioneered by Orgel.

White, D. H.↗
NASA NTRS1995

Very low temperature formaldehyde reactions and the build-up of organic molecules in comets and interstellar ices

We have investigated thermally promoted reactions of formaldehyde (H2CO) in very low temperature ices. No such reactions occurred in ices of pure formaldehyde. However, addition of trace amounts of ammonia (NH3) were sufficient to catalyze reactions at temperatures as low as 40 K. Similar reactions could take place in interstellar ices and in Comets and produce considerable amounts of organic molecules.

Schutte, W. A.↗
NASA NTRS1999

Ultrasound Imaging System Implementation and Ignition Protocol for the Microgravity Smoldering Combustion (MSC) Experiments

The Microgravity Smoldering Combustion (MSC) experiment is a study of the smolder characteristics of porous combustible materials in a microgravity environment. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder, both in microgravity and normal earth gravity. Experiments have been conducted aboard the NASA Space Shuttle in the Get Away Special Canister (GAS-CAN), an apparatus requiring completely remote operation. Future GAS-CAN experiments will utilize an ultrasound imaging system (UIS) which has been incorporated into the MSC experimental apparatus. Thermocouples are currently used to measure temperature and reaction front velocities. A less intrusive method is desirable, however, as smolder is a very weak reaction and it has been found that heat transfer along the thermocouple is sufficient to affect the smolder reaction. It is expected that the UIS system will eventually replace the existing array of thermocouples as a non-intrusive technique without compromising data acquisition. The UIS measures line of sight permeability, providing information about the reaction front position and extent. Additionally, the ignition sequence of the MSC experiments has been optimized from previous experiments to provide longer periods of self-supported smolder. An ignition protocol of a fixed power to the igniter for a fixed time is now implemented. This, rather than a controlled temperature profile ignition protocol at the igniter surface, along with the UIS system, will allow for better study of the effect of gravity on a smolder reaction.

Walther, David C.↗
NASA NTRS1988

Computed potential energy surfaces for chemical reactions

The minimum energy path for the addition of a hydrogen atom to N2 is characterized in CASSCF/CCI calculations using the (4s3p2d1f/3s2p1d) basis set, with additional single point calculations at the stationary points of the potential energy surface using the (5s4p3d2f/4s3p2d) basis set. These calculations represent the most extensive set of ab initio calculations completed to date, yielding a zero point corrected barrier for HN2 dissociation of approx. 8.5 kcal mol/1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional Transition State Theory and a method which utilizes an Eckart barrier to compute one dimensional quantum mechanical tunneling effects. It is concluded that the lifetime of the HN2 species is very short, greatly limiting its role in both termolecular recombination reactions and combustion processes.

Walch, Stephen P.↗
NASA NTRS2003

Theoretical Studies of the Extra-terrestrial Chemistry of Biogenic Elements and Compounds

Results are presented on the following:(A) Ab initio quantum chemical studies of reactions in astrophysical ices.Theoretical electronic structure calculations were used to investigate reactions between formaldehyde (H2CO) and both hydrogen cyanide (HCN) and isocyanide (HNC) in search of other favorable reactions such as ammonia-formaldehyde addition, which was found in a recent theoretical study to be strongly enhanced when it occurs within cold ices.The present study examines further reactions between this product and H2CO in ices.(B) Heterogeneous hydrogenation of CO and H2CO on icy grain mantles.Formaldehyde (H2CO) and methanol (CH30H) are thought to be produced in the interstellar medium by the successive hydrogenation of carbon monoxide (CO) on grain surfaces. In the gas phase, the steps in which H adds to CO and H2CO possess modest barriers and are too inefficient to account for the observed abundances. Recent laboratory work has confirmed that formaldehyde and methanol are formed when H atoms are deposited on CO ice at 12 K. The present study employed ab initio quantum chemical calculations to investigate the impact of water ice on the sequential hydrogenation of CO.(C) Ice-bound condensed-phase reactions involving formic acid (HCOOH), methylenimine (CH2NH), hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and ammonia ( 3) were investigated in order to characterize possible pathways to larger organic species that are efficient at the cold temperatures prevalent in cometary nuclei and the interstellar medium. (D) Pathways to glycine and other amino acids in ultraviolet-irradiated ices determined via quantum chemical modeling.(E) Photoionization in ultraviolet processing of astrophysical ice analogs at cryogenic temperatures.

Woon, David E.↗
NASA NTRS1994

Peroxy radical measurements with NCAR's chemical amplifier

The present NCAR instrument for HO2/RO2 measurements has been described previously. It is based on the reactions involving HO2, RO2, and HO radicals with CO and NO. Since (HO2) + (RO2) + (HO) is much greater than (HO) for most atmospheres, it is useful as a peroxy radical detector. Operation of the instrument depends on the creation of a chemical chain reaction which is initiated as HO2 and RO2 radicals in ambient air encounter added NO gas; this forms an NO2 molecule and an HO or RO radical: HO2(RO2) + NO yields HO(RO) + NO2. RO radicals react relatively efficiently with O2 to form an HO2 radical, and subsequently an HO-radical, by reaction with NO. CO gas added to the reaction chamber during part of the operating cycle, recycles the HO to HO2; HO + CO (+O2) yields HO2 + CO2. The reaction sequence may form several hundred NO2 molecules per HO2 (RO2) originally present, before chain termination occurs. The added CO is replaced by N2 addition periodically so that the chain reaction is suppressed, and a 'blank' signal resulting from NO2, O3 and possibly other NO2-forming species (non-chain processes) in ambient air is recorded. The difference between the signal with and without CO is proportional to the peroxy radical concentration. The NO2 produced is monitored using a sensitive luminol chemiluminescence detector system. In the NCAR instrument the length of the amplification chain is determined using a stable source of HO2 radicals (H2O2 thermal decomposition); the ratio of the signal seen with CO present to that with N2 present gives the sensitivity of the instrument to HO2 (molecules of NO2 formed/peroxy radical). The instrument is automated to carry out in hourly repeated cycles: (1) chain length determination; (2) NO2 calibration; and (3) linearity check on the response of signals. One minute averages of signals are normally recorded. The sensitivity of the instrument to detect peroxy radicals is in the pptv range. The present instrument has operated continuously (24 hr/day) in the field studies which extended over a period of several weeks. The major advantages of this instrument are as follows: (1) its relative simplicity; (2) low power requirements; and (3) its rapid response to all types of peroxy radicals--HO2, CH3O2 and the higher alkyl and acyl peroxy radicals; however not all RO2 species generate HO2 radicals with perfect efficiency and hence have somewhat lower response/molecule than HO2 radicals.

Cantrell, Christopher↗
NASA NTRS1983

Sintering of reaction bonded silicon nitride

A process to produce sintered reaction-bonded Si3N4 (SRBSN) articles has been developed. This process consists of the addition of an appropriate sintering aid to reaction-bonded Si3N4 followed by sintering between 1780 and 2000 C, using an over pressure of nitrogen. The principal advantage of this process is the low sintering shrinkages of 5 to 10 percent. The properties and microstructure of two SRBSN systems sintered with MgO and Y2O3 additives are described and were found to be comparable to corresponding hot-pressed Si3N4 systems. Examples of applications of both systems are illustrated, demonstrating near net shape fabrication capability of the process.

Mangels, J. A.↗