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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The D to H ratio on Titan and the planets: Implications for origin and evolution of planetary atmospheres

Measurements of deuterated methane show that Titan's atmosphere is enriched by at least several times in deuterium compared to the major planets. Potential causative factors for this enrichment are condensation to form tropospheric methane clouds, fractionation occuring over a hypothetical CH4-C2H2 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan and fractionation occuring as a result of the evolution of Titan's atmosphere. The greater part of the observed fractionation is probably derived from the formation of Titan and the subsequent evolution of Titan atmosphere driven by photochemistry. The latter process is developed here for the first time. The D/H ratio in a planetary atmosphere is one readily available measure of the origin and evolution of the hydrogen bearing volatiles on the planet. Comparison between D/H ratio in the inner solar system and the outer solar system may pose important constraints on current theories.

Pinto, J. P.↗
NASA NTRS1986

Measurements of physical properties of model Titan atmospheres

One aspect of the study of Titan's atmosphere is the elucidation of the chemical and physical nature of the aerosols. In order to facilitate this, a program to produce laboratory synthesized model materials for Titan's aerosol and to study their chemical and physical properties is now in progress. Various processes, including electric discharge, photolysis by ultraviolet light, and irradiation by energetic particles, will be used to produce the materials. A first set of experiments where a nominal Titan mixture (97%N2, 3% CH4, 0.2% H2) was subjected to pulsed high temperature shocks yielded a reddish brown waxy solid. This material was subjected to pyrolysis/gas chromatography, a technique that has been proposed as a method for analysis of the Titan aerosols. Preliminary results show the material to consist of simple hydrocarbons but little else, at least up to temperatures of 600 C. Since the material was colored, compounds other than those mentioned above must be present.

Scattergood, T. W.↗
NASA NTRS1986

Energy yields in the prebiotic synthesis of hydrogen cyanide and formaldehyde

Prebiotic experiments are usually reported in terms of carbon yields, i.e., the yield of product based on the total carbon in the system. These experiments usually involve a large input of energy and are designed to maximize the yields of product. However, large inputs of energy result in multiple activation of the reactants and products. A more realistic prebiotic experiment is to remove the products of the activation step so they are not exposed a second time to the energy source. This is equivalent to transporting the products synthesized in the primitive atmosphere to the ocean, and thereby protecting them from destruction by atmospheric energy sources. Experiments of this type, using lower inputs of energy, give energy yields (moles of products/joule) which can be used to estimate the relative importance of the different energy sources on the primitive earth. Simulated prebiotic atmospheres containing either CH4, CO or CO2 with N2, H2O and variable amounts of H2 were subjected to a high frequency Tesla coil. Samples of the aqueous phase were taken at various time intervals from 1 hr to 7 days, and the energy yields were obtained by extrapolation to zero time. The samples were analyzed for HCN with the cyanide electrode and for H2CO by chromotropic acid. The spark energy was estimated by calorimetry. The temperature rise in an insulated discharge flask was compared with the temperature rise from a resistance heater in the same flask. These results will be compared with calculated production rates of HCN and H2CO from lightning and a number of photochemical processes on the primitive Earth.

Stribling, R.↗
NASA NTRS1986

Halogen occultation experiment (HALOE) performance verification test procedure

The Performance Verification Test Procedure is given for the Halogen Occultation Experiment (HALOE) instrument, which is being developed in house at the Langley Research Center for the Upper Atmosphere Research Satellite (UARS). This procedure is used for comprehensive performance testing of the HALOE instrument which occurs before, during, and after flight environmental tests. The radiometric performance tests include noise, drift, linearity, instantaneous field-of-view, cal wheel gas cell characterization, and self thermal emissions. Pointer/tracker performance tests include sun sensor performance, gimbal performance, control system performance, and boresight alignment. In addition, the instrument is tested functionally in simulated orbit sequences and all command operating modes are exercised. The data analysis required for each test is specified and pass/fail criteria are given where applicable. This test will fully demonstrate the HALOE instrument's ability to achieve science mission requirements. The HALOE instrument is a gas correlation radiometer that measures vertical distribution of eight upper atmospheric constituents: O3, HCl, HF, NO, CH4, H2O, NO2, and CO2.

Mauldin, L. E., III↗
NASA NTRS1985

Hydrogen species

Measurements of the members of the HO(x) family (OH, HO2, and H2O2) and their major source gases, H2O, and CH4 are discussed. Emphasis is placed on measurements which were made since the 1982 World Meteorologic Organization (WMO) report. Measurement techniques, available data, an assessment of data reliability, and a comparison of the data with theoretical distributions of stratospheric HO(x) species predicted from one and two dimensional photochemical models are discussed.

Schiff, H. I.↗
NASA NTRS1985

Oxidant abundances in rainwater and the evolution of atmospheric oxygen

H2O2 is probably the dominant oxidant in rainwater at O2 levels lower than 0.01-0.001 present atmospheric levels (PAL), and the magnitude of its dissolved flux is sensitive to the concentration of the trace gases NO, CO, and CH4 in the atmosphere, as well as to the partial pressure of atmospheric CO2. The earlier interpretation of the paleosol data in terms of the ratio of p-O2/p-CO2 in the atmosphere must be modified by taking into account the effects of photochemically produced oxidants and reactants at p-O2 of les than 0.01 PAL.

Kasting, J. F.↗
NASA NTRS1985

Far-infrared and submillimeter brightness temperatures of the giant planets

The brightness temperatures of Jupiter, Saturn, Uranus, and Neptune were measured in the 35-1000 micron range with the 3-m NASA Infrared Telescope Facility (at wavelengths greater than 350 microns) and with the Kuiper Airborne Observatory (at wavelengths less than 350 microns). The data indicate the presence in Jupiter's spectrum of excess radiation (compared to theoretical models) at 300-400 microns. In addition, slightly less flux was observed from Saturn at 200 microns than predicted by atmospheric models, which suggests the possible presence of an unmodeled absorber. The submillimeter fluxes from Uranus and Neptune appear to be most consistent with low mixing ratios (less than 1 percent) of CH4 in their deep atmospheres.

Hildebrand, R. H.↗
NASA NTRS1985

Time dependent chemistry in dense molecular clouds. I - Grain surface reactions, gas/grain interactions and infrared spectroscopy

For the fist time, a time-dependent model is described which includes the role of grains in the production of molecules in dense clouds including ion-molecule gas phase chemistry. The approach provides information regarding the coupling between the two phases. Although the coupling between the two chemistries is extremely strong, the two domains maintain their own identities. While H2O, CH4, and NH3 are made efficiently, with a high production rate on grains and released back to the gas phase, the gas phase is essentially responsible for the formation of CO, a very stable molecule which may or may not react on grains with atomic oxygen and may or may not form CO2.

Dhendecourt, L. B.↗
NASA NTRS1985

Spectra of simulated lightning on Venus, Jupiter, and Titan

Laser-induced plasmas in various gas mixtures were used to simulate lightning in other planetary atmospheres. This method of simulation has the advantage of producing short-duration, high-temperature plasmas free from electrode contamination. The laser-induced plasma discharges in air are shown to accurately simulate terrestrial lightning and can be expected to simulate lightning spectra in other planetary atmospheres. Spectra from 240 to 880 nm are presented for simulated lightning in the atmospheres of Venus, earth, Jupiter, and Titan. The spectra of lightning on the other giant planets are expected to be similar to that of Jupiter because the atmospheres of these planets are composed mainly of hydrogen and helium. The spectra of Venus and Titan show substantial amounts of radiation due to the presence of carbon atoms and ions and show CN Violet radiation. Although small amounts of CH4 and NH3 are present in the Jovian atmosphere, only emission from hydrogen and helium is observed. Most differences in the spectra can be understood in terms of the elemental ratios of the gas mixtures. Consequently, observations of the spectra of lightning on other planets should provide in situ estimates of the atmospheric and aerosol composition in the cloud layers in which lightning is occurring. In particular, the detection of inert gases such as helium should be possible and the relative abundance of these gases compared to major constituents might be determined.

Borucki, W. J.↗
NASA NTRS1986

Evaluation of a storable fluorine based pilot for scramjets

An ignitor/pilot device for scramjet engines has been tested with C2H4, C2H6, and CH4 gaseous hydrocarbon fuels at Mach 2. The pilot was designed to operate with storable nontoxic propellants rather than hazardous pyrophorics; the pilot fluid is a mixture of ethylene, oxygen, and sulfur hexafluoride. The pilot reduced the minimum temperature at which all three hydrocarbon fuels ignited; a comparison was then conducted between combustion efficiency calculations for these test results and those of pilots using nitrogen in place of sulfur hexafluoride, and using silane-hydrogen (yielding equivalent energy). The nitrogen-substituted pilot exhibited the highest efficiency.

Diskin, G. S.↗
NASA NTRS1986

The ionosphere of Uranus - A myriad of possibilities

A one-dimensional model has been used to study the effects of exospheric temperature, methane and water influx, ionospheric outflow, and electron precipitation on the composition and structure of the ionosphere of Uranus. Peak ion concentrations range from 1000 to 1 million per cu cm with a wide variation in peak altitude, which depends strongly on the exospheric temperature. In all the cases considered, H(+) is the major ion in the topside ionosphere. At altitudes near or below the peak, H3(+) and CH5(+) can dominate, depending on the magnitude of CH4 and H2O influx. Atomic hydrogen column depths above the methane absorbing layer exceed 10 to the 17th per sq cm and can produce large (400 R) emissions of resonantly scattered Lyman-alpha. In the sunlit polar cap, electron precipitation with energy fluxes of 0.6 to 1.0 erg/sq cm s results in direct production of Lyman-alpha emissions that exceed 1 kR.

Chandler, M. O.↗
NASA NTRS1985

Clathrate hydrates in the solar system

Clathrate hydrates are crystalline compounds in which an expanded ice lattice forms cages that contain gas molecules. There are two principal hydrate structures. Structure I, with a 12 A cubic unit cell, contains 46 water molecules and 8 cages of two types, giving an ideal formula (for CH4) of CH4.5.75H2O. The actual formula contains somewhat more water as the cages are not completely filled. Other examples that form Structure I hydrates are C2H6, C2H4, C2H2, CO2, SO2, OCS, Xe, H2S. Structure II, with a 17 A cubic unit cell, contains 136 water molecules, and 8 large and 16 small cages. The ideal formula for CHCl3 is CHCL3.17H2O. Other examples of Structure II hydrates include C3H8, C2H5Cl, acetone, and tetrahydrofuran. Small molecules such as Ar, Kr and probably N2 and O2 also form a Structure II hydrate. The small molecules occupy both the large and small cages, giving an ideal formula of Ar.5.67H2O. The conditions of pressure and temperature for hydrate formation are discussed.

Miller, S. L.↗
NASA NTRS1986

A dynamically based transport parameterization for one-dimensional photochemical models of the stratosphere

The net vertical tracer flux in the stratosphere is due primarily to advection by the global-scale mean meridional circulation, not to diffusion by turbulent eddies. Using a simple model of this circulation, it is shown that the net flux can be approximated by a flux-gradient relationship in which the vertical 'eddy transport' coefficient is proportional to the square of the diabatic heating rate and inversely proportional to the rate at which meridional tracer gradients are destroyed by horizontal eddy mixing and chemical damping. Thus the transport coefficient will differ for tracers with differing chemical lifetimes. Profiles computed for various source gases (CH4 N2O, CF2Cl2, and CFCl3) show that this parameterization provides an improvement over conventional formulations that utilize a single transport coefficient for all species.

Holton, J. R.↗
NASA NTRS1986

Radiative-convective equilibrium models of Uranus and Neptune

The present study of Uranus and Neptune radiative-convective equilibrium models gives emphasis to such aspects of the stratospheric energy balance as the influence of aerosol heating and convective penetration. The results obtained for Uranus imply that a 'continuum absorber' may be a significant factor in the stratosphere despite the great distance from the sun. The results obtained for Neptune show that such a continuum absorber could significantly contribute to the energy balance within a localized stratospheric region, although it probably cannot furnish sufficient power to account for the observed IR spectrum irrespective of its vertical distribution. Attention is accordingly given to the 'convective penetration' that could arise under such rapid vertical mixing that CH4's condensation cannot occur before the gas is carried above the condensation region.

Appleby, J. F.↗
NASA NTRS1986

World-wide increase in tropospheric methane, 1978-1983

Techniques used to assess methane concentration in the troposphere are described, and data obtained during the period from 1978 to 1983 are presented in detail. Tropospheric methane concentrations in remote locations averaged a yearly world-wide increase of 0.018 + or - 0.002 parts per million by volume (ppmv). Average world-wide tropospheric concentration of methane in dry air was 1.625 ppmv at the end of 1983 measured against an NBS standard certified as 0.97 ppmv. Contributing to this steady increase in methane concentration are increases in the source strengths from cattle and rice fields, which in turn result from CO, CH4 and HO coupling. Among the physical and chemical effects is an increase in greenhouse warming of about 0.04 C per decade.

Blake, D. R.↗
NASA NTRS1986

Global atmospheric concentrations and source strength of ethane

A study of the variation in ethane (C2H6) concentration between northern and southern latitudes over three years is presented together with a new estimate of its source strength. Ethane concentrations vary from 0.07 to 2 p.p.b.v. (parts per billion by volume) in air samples collected in remote surface locations in the Pacific (latitude 71 N-47 S) in all four seasons between September 1984 and June 1985. The variations are consistent with southerly transport from sources located chiefly in the Northern Hemisphere, further modified by seasonal variations in the strength of the reaction of C2H6 with OH radicals. These global data can be combined with concurrent data for CH4 and the laboratory reaction rates of each with OH to provide an estimate of three months as the average atmospheric lifetime for C2H6 and 13 + or - 3 Mtons for its annual atmospheric release.

Blake, D. R.↗
NASA NTRS1986

In situ gas and ion measurements at comet Halley

The Giotto space probe's neutral mass spectrometer experiment has determined the abundances and the chemical, elemental and isotopic compositions of gases and low energy ions in the coma of comet Halley. Preliminary results show water predominating, at about 80 percent by volume, with a density of 4.7 x 10 to the 7th molecules/cu cm at 1000 km and a photodestruction scale length of 39,000 km. Limits on the abundances of CO2, NH3 and CH4 relative to H2O are also obtained. An ion temperature change observation indicates a contact surface location at 47,000 + or - 200 km.

Krankowsky, D.↗
NASA NTRS1986

Performance and endurance tests of a multipropellant resistojet for space station auxiliary propulsion

This paper presents the results of an effort to demonstrate the technology readiness of a long-life multipropellant resistojet for space station auxiliary propulsion. Experiments were performed to evaluate the compatibility of grain-stabilized platinum tubes at temperatures up to 1400 deg C in environments of CO2, CH4, NH3, H2O, and H2. All samples tested showed extrapolated lifetimes in excess of 10,000 hr based on 10 percent mass loss as end-of-life. However, samples tested in an ammonia atmosphere at 1400 deg C showed severe pitting, which raised concerns about the compatibility of grain-stabilized platinum with ammonia-containing atmospheres. Additional tests showed that reducing the metal temperature to about 900 deg C (+ or - 100 deg C) significantly reduced this adverse effect.

Morren, W. E.↗