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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2022

Radiation-induced D/H Exchange Rate Constants in Aliphatics Embedded in Water Ice

Gas-phase and solid-state chemistry in low-temperature interstellar clouds and cores leads to a D/H enhancement in interstellar ices, which is eventually inherited by comets, meteorites, and even planetary satellites. Hence, the D/H ratio has been widely used as a tracer for the origins of extraterrestrial chemistry. However, the D/H ratio can also be influenced by cosmic rays, which are ubiquitous and can penetrate even dense interstellar molecular cores. The effects of such high-energy radiation on deuterium fractionation have not been studied in a quantitative manner. In this study, we present rate constants for radiation-induced D-to-H exchange for fully deuterated small (1–2 C) hydrocarbons embedded in H2O ice at 20 K and H-to-D exchange for the protiated forms of these molecules in D2O ice at 20 K. We observed larger rate constants for H-to-D exchange in the D2O ice versus D-to-H exchange in H2O ice, which we have attributed to the greater bond strength of C–D versus C–H. We find that the H-to-D exchange rate constants are smaller for protiated methane than ethane, in agreement with bond energies from the literature. We are unable to obtain rate constants for the unsaturated and reactive hydrocarbons ethylene and acetylene. Interpretation of the rate constants suggest that D/H exchange products are formed in abundance alongside radiolysis products. We discuss how our quantitative and qualitative data can be used to interpret the D/H ratios of aliphatic compounds observed throughout space.

Ice destruction↗
NASA NTRS2023

The Dielectric Constant of Sea Water and Extension to High Salinity

Accurate knowledge of the dielectric constant of sea water is important for remote sensing of surface parameters such as sea surface temperature (SST) and sea surface salinity (SSS). The advent of sensors in space, SMOS [1], Aquarius [2] and SMAP [3] capable of measuring SSS motivated modern measurements [4,5] and modelling [5,6] of the dielectric constant at L-band (1.4 GHz). In the past, the range of salinity included in the data used to create these models has been restricted to values typically encountered in the open ocean (e.g., less than 40 psu). However, there are many smaller water bodies with much higher salinity. Notable examples are the Great Salt Lake in Utah with salinity on the order of 180 psu and Garabogazköl lagoon in Turkmenistan with even higher salinity. Unfortunately, existing models for the dielectric constant can’t necessarily just be extended to higher values of salinity. The problem is that the polynomials in salinity and temperature used to represent the unknown parameters in the models are not constrained outside the range of SSS and SST used to determine their coefficients. While the models for the dielectric constant may be very good within that range, outside that range they can lead to unrealistic behavior. Research is underway to develop a model that represents the dielectric constant well over the ocean and behaves well at high salinity. In preparation for possible wideband remote sensing of salinity [7,8 ,9], the laboratory measurements made at 1.413 GHz [4,5] are being repeated at 0.707 GHz (P-band) and the plan is to include values of high salinity (50, 100, 150 psu).

D.M. Le Vine↗
NASA NTRS2022

Model for Dielectric Constant of Seawater based on L-band Measurements with Conductivity by Definition

This article reports an improvement in the model for the dielectric constant of seawater used to fit laboratory measurements of the dielectric constant at the L-band. The new model (dielectric constant as a function of salinity, temperature, and frequency) is based on the response of a polar molecule proposed by Debye and fits the same measurement as reported in earlier work but uses a functional form for conductivity, σ(S,T) , that is given by the definition of salinity. The new version of this model fits the data well and has the advantages that the relaxation time constant is allowed to be a function of temperature and salinity and is well behaved when extrapolated to high salinities.

L-band↗
NASA NTRS2022

Status of the Dielectric Constant of Sea Water at L-Band for Remote Sensing of Salinity

The model expressing the dielectric constant of sea water at microwave frequencies as a function of salinity and temperature is an important element in remote sensing of sea surface salinity. It is also important independently as a description of the physical properties of salt water. A major milestone was the development in the late 1970s by Klein and Swift of a model based on laboratory measurements at L- and S-band and a functional form supported by theory for polar molecules and previous work on freshwater. Much of the subsequent work has focused on measurements at higher frequency and determining model parameters tuned to apply for applications, such as remote sensing of sea surface temperature (SST). Interest in the dielectric constant at 1.4 GHz (L-band) increased again with the development of soil moisture ocean salinity (SMOS) and Aquarius to measure salinity from space, but there have been few new measurements at L-band and often confusion regarding the applicability of new models at 1.4 GHz. The objective of this article is to compare available models in the context of how well they represent the dielectric constant of sea water at 1.4 GHz. Among the criteria applied will be the recent measurements at the George Washington University of the dielectric constant at 1.4 GHz

Microwave Remote Sensing↗
NASA NTRS2024

The Dielectric Constant at P-Band for Salinity from 0 to 150 pss

Measurements have been made at P-band (0.707 GHz) to construct a model for the dielectric constant of sea water and extend the model for the dielectric constant to high salinity (50-150 pss). The measurements are part of research to develop a model for the dielectric constant suitable for future wide-bandwidth remote sensing of salinity and for application to water bodies such as the Great Salt Lake with salinity significantly above that found in the open ocean. Measurements have been made at temperatures from 2 to 30 °C and salinity from 0 to 138 pss. The data have been fit to a Debye model for the dielectric constant with a single resonance as has been employed at L-band (1.413 GHz) where remote sensing of salinity is currently done. Comparison with contemporary models developed from data at L-band indicates that the L-band model and new P-band model do well at both frequencies for salinity less than 50 pss but at higher values of salinity the L-band models diverge from the data. The data has also been used to test at high salinity the mathematical relationship between salinity and conductivity which is the basis for the practical salinity scale (pss).

Dielectric Constant↗
NASA NTRS1976

Limit on the secular change of the gravitational constant based on studies of solar evolution

The past and present observable properties of the sun have been theoretically calculated on the assumption that the gravitational constant G increases or decreases with time. Consideration of Davis's experimental upper limit on the present solar neutrino flux and of terrestrial paleontological data sets a limit on the absolute rate of change of G, namely, the absolute value of the quotient of the change in the gravitational constant and the gravitational constant is less than 1/10 billion/yr, if other standard constants do not also change with time.

Chin, C.-W.↗
NASA NTRS1976

The optical constants of several atmospheric aerosol species - Ammonium sulfate, aluminum oxide, and sodium chloride

An investigation is conducted of problems which are related to a use of measured optical constants in the simulation of the optical constants of real atmospheric aerosols. The techniques of measuring optical constants are discussed, taking into account transmission measurements through homogeneous and inhomogeneous materials, the immersion of a material in a liquid of a known refractive index, the consideration of the minimum deviation angle of prism measurement, the interference of multiply reflected light, reflectivity measurements, and aspects of mathematical analysis. Graphs show the real and the imaginary part of the refractive index as a function of wavelength for aluminum oxide, NaCl, and ammonium sulfate. Tables are provided for the dispersion parameters and the optical constants.

Toon, O. B.↗
NASA NTRS1976

Scalar-tensor theory of gravitation with negative coupling constant

The possibility of a Brans-Dicke scalar-tensor gravitation theory with a negative coupling constant is considered. The admissibility of a negative-coupling theory is investigated, and a simplified cosmological solution is obtained which allows a negative derivative of the gravitation constant. It is concluded that a Brans-Dicke theory with a negative coupling constant can be a viable alternative to general relativity and that a large negative value for the coupling constant seems to bring the original scalar-tensor theory into close agreement with perihelion-precession results in view of recent observations of small solar oblateness.

Smalley, L. L.↗
NASA NTRS1982

The temperature dependence of the rate constant for the reaction of hydroxyl radicals with nitric acid

The rate constant for the reaction of hydroxyl radicals with nitric acid in the 225-443 K temperature range has been measured by means of the flash photolysis resonance fluorescence technique. Above 300 K, the rate constant levels off in a way that can only be explained by the occurrence of two reaction channels, of which one, operative at low temperatures, proceeds through the formation of an adduct intermediate. The implications of these rate constant values for stratospheric reaction constants is discussed.

Kurylo, M. J.↗
NASA NTRS1982

Rate constant for the reaction between OH and CS2 at 298 and 520 K

In an attempt to resolve discrepancies between published values of the rate constant for the reaction between the hydroxyl radical and carbon disulfide, the reaction has been studied in a discharge flow system by using resonance fluorescence for kinetic measurements and mass spectrometry for product analysis. On the basis of the measured rate constant for disappearance of OH, and measurements of the amount of carbonyl sulfide formed, it was estimated that for the reaction HO + CS2 yields HS + OCS, rate constant values are not greater than 3 x 10 to the -15th/cu cm per sec at 520 K and not greater than 7 x 10 to the -15th/cu cm per sec at 298 K, upper limits are specified because of the inability to isolate exclusively this reaction channel, and because of possible involvement of wall reactions. These results confirm the low values found for this rate constant in two very recent studies.

Leu, M.-T.↗
NASA NTRS1982

Shock tube measurements of growth constants in the branched chain formaldehyde-carbon monoxide-oxygen system

Exponential free radical growth constants were measured for formaldehyde carbon monoxide-oxygen systems by monitoring the growth of oxygen atom concentration as manifested by CO flame band emission. Data were obtained over the temperature range of 1200 to 2000 K. The data were analyzed using a formaldehyde oxidation mechanism involving 12 elementary reaction steps. The computed growth constants are roughly in accord with experimental values, but are much more temperature dependent. The data was also analyzed assuming formaldehyde is rapidly decomposed to carbon monoxide and hydrogen. Growth constants computed for the resulting carbon monoxide hydrogen oxygen mixtures have a temperature dependence similar to experiments; however, for most mixtures, the computed growth constants were larger than experimental values.

Brabbs, T. A.↗
NASA NTRS1985

Shock tube measurements of growth constants in the branched-chain ethane-carbon monoxide-oxygen system

Exponential free radical growth constants have been measured for ethane carbon monoxide oxygen mixtures by monitoring the growth of oxygen atom concentration as manifested by CO flame band emission. Data were obtained over the temperature range of 1200 to 1700 K. The data were analyzed using an ethane oxidation mechanism involving seven elementary reaction steps. Calculated growth constants were close to experimental values at lower temperatures, up to about 1400 K, but at higher temperatures computed growth constants were considerably smaller than experiment. In attempts to explain these results additional branching reactions were added to the mechanism. However, these additional reactions did not appreciably change calculated growth constants.

Brokaw, R. S.↗
NASA NTRS1987

Measurements of the dielectric constants for planetary volatiles

The model of Titan at present has the surface temperature, pressure, and composition such that there is a possibility of a binary ethane-methane ocean. Proposed experiments for future Titan flybys include microwave mappers. Very little has been measured of the dielectric properties of the small hydrocarbons at these radar frequencies. An experiment was conducted utilizing a slotted line to measure the dielectric properties of the hydrocarbons, methane to heptane, from room temperature to -180 C. Measurements of the real part of the dielectric constants are accurate to + or - 0.006 and the imaginary part (the loss tangent) of the liquids studied is less than or equal to 0.001. In order to verify this low loss tangent, the real part of the dielectric constant of hexane at 25 C was studied as a function of the frequency range of the slotted line system used. The dielectric constant of hexane at room temperature, between 500 MHz and 3 MHz, is constant within experimental error.

Anicich, Vincent G.↗
NASA NTRS1986

A GSFC alternative to the SLR MERIT constants

The use of the Lageos satellite to monitor the earth's orientation is examined. The derivations of long wavelength ocean tidal parameters, a geocentric gravitational constant of 398,600.436 cu km/ sq sec + or - 0.0001 cu km/sq sec, and Love numbers using Lageos laser ranging data are described. The uncertainties of the geopotential model, GEM-L2 of Lerch et al. (1982), are discussed. The calculation of polar motion using the Lageos constants is considered. The Lageos constants are tested by applying them to independent laser ranging data. It is determined that the new constants improve the rms of fit to independent Lageos data and improve the earth orientation parameters compared to VLBI data obtained during the IRIS project.

Christodoulidis, Demosthenes C.↗
NASA NTRS1988

Reaction rate constants of HO2 + O3 in the temperature range 233-400 K

The reaction rate constants of HO2 + O3 were measured in the temperature range 233-400 K using a discharge flow system with photofragment emission detection. In the range 233-253 K, the constants are approximately a constant value, and then increase with increasing temperature. This result suggests that the reaction may have two different channels. An expression representing the reaction rate constants is presented.

Wang, Xiuyan↗
NASA NTRS1988

Measurement of the Solar Constant (SOLCON)

Measurement of the Solar Constant (SOLCON) is a solar physics experiment of the Atmospheric Laboratory for Applications and Science (ATLAS 1) NASA mission scheduled for late 1990. The objectives of this investigation are: (1) to measure the absolute value of the solar constant with improved accuracy, and (2) to detect and measure long-term variations that may exist in the absolute value of the solar constant. The solar constant is the total irradiance of the sun at a distance of one astronomical unit. This will be measured directly in space by an absolute self-calibrating radiometer with an absolute accuracy estimated to be of the order of + or - 0.1 percent and a sensitivity better than 0.05 percent. Features of this radiometer are given.

Crommelynck, D.↗
NASA NTRS1988

Earth Radiation Budget Satellite extraterrestrial solar constant measurements - 1986-1987 increasing trend

From June 1986 through Nov 1987, the Earth Radiation Budget Satellite (ERBS) pyrheliometric measurements indicated that the solar constant was increasing approximately +0.02 percent per year. Earlier ERBS measurements indicated that the solar constant was declining approximately -0.03 percent per year during the 1984 through mid-1986 period. Since mid-1986 represents the beginning of solar cycle 22, it is believed that the reversal in the long-term solar constant trend may be linked to increased solar activity associated with the beginning of the 11-year sunspot cycle. The typical value of the solar constant was found to be 1365 Wm-2.

Lee, Robert B., III↗
NASA NTRS1990

Modeling a constant power load for nickel-hydrogen battery testing using SPICE

The effort to design and model a constant power load for the HST (Hubble Space Telescope) nickel-hydrogen battery tests is described. The constant power load was designed for three different simulations on the batteries: life cycling, reconditioning, and capacity testing. A dc-dc boost converter was designed to act as this constant power load. A boost converter design was chosen because of the low test battery voltage (4 to 6 VDC) generated and the relatively high power requirement of 60 to 70 W. The SPICE model was shown to consistently predict variations in the actual circuit as various designs were attempted. It is concluded that the confidence established in the SPICE model of the constant power load ensures its extensive utilization in future efforts to improve performance in the actual load circuit.

Bearden, Douglas B.↗