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NASA NTRS2000

Interpreting Assemblages with Titanite (Sphene): It Does not have to be Greek to You.

Assemblages with titanite, pyroxene(s), olivine, ilmenite, magnetite, and quartz can be used to constrain the intensive and compositional variables that operate during crystallization. Such assemblages are relatively rare in metamorphic rocks, but they are more common in igneous rocks and more frequently reported in plutonic than volcanic rocks. We used the program QUILF, enhanced with thermodynamic data for titanite, to compute stable reactions among titanite (CaTiOSiO4), Fe-Mg-Ti ilmenite and magnetite (hereafter ilmenite and magnetite), Ca-Mg-Fe pyroxenes and olivine, and quartz, and to evaluate some of the factors that control titanite stability. Calculations at 1, 3, and 6 Kbar and 650, 850, 1100 0 C, in the system CaO - MgO - FeO Fe2O3 - TiO2 - SiO2, suggest that the reactions: Augitc + Ilmenite = Titanite + Magnetite + Quartz and Augite + Ilmenite + Quartz = Titanite + Orthopyroxene, impose well defined fugacity of O2, alpha(sub SiO2), , and compositional restrictions to the assemblages: (1) Titanite + Magnetite + Quartz, (2) Titanite + Orthopyroxene, (3) Augite + Ilmenite, and consequently titanite stability. From our calculations in this system we can draw the following general conclusions: (1) The assemblage Titanite + Magnetite + Quartz is always a good indicator of relatively high fugacity of O2, and it is likely more common in relatively iron-rich bulk compositions and for decreasing temperature and pressure conditions. (2) At high temperatures (>= 650 C) titanite is not stable in quartz-saturated rocks that contain the assemblage Orthopyroxene + Augite + Ilmenite + Magnetite. (3) In quartz-saturated rocks the coexistence of titanite and magnetite with either orthopyroxene or olivine requires a confluence of conditions relating bulk composition, fugacity of O2, and slow cooling. Thus, such assemblages must be rare. (4) Regardless of T and fugacity of O2 conditions, and bulk-composition, titanite is not stable in quartz-absent rocks that contain Olivine + Orthopyroxene + Augite + Ilmenite + Magnetite. Decreasing temperature and pressure conditions appear to favor titanite crystallization, thus, it is not unsurprising that titanite is frequently observed in slowly cooled rocks, albeit, in association with amphibole. We argue that the titanite + amphibole association is likely favored by high water activity, regardless of oxygen fugacity. Because water activity increases during crystallization of a pluton, the association titanite + amphibole, and consequently titanite, is likely to be more common in plutonic rocks than in volcanic rocks.

Xirouchakis, Dimitrios M.↗

NASA NTRS1999

Phase Separation Kinetics in Isopycnic Mixtures of H2O/CO2/Ethoxylated Alcohol Surfactants

Ternary mixtures of H2O and CO2 with ethoxylated alcohol (C(sub i)E(sub j)) surfactants form three coexisting liquid phases at conditions where two of the phases have equal densities (isopycnic phases). Isopycnic phase behavior has been observed for mixtures containing C8E5, C10E6, and C12E6 surfactants, but not for those mixtures containing either C4E1 or C8E3 surfactants. Pressure-temperature (PT) projections for this three-phase equilibrium were determined for H2O/CO2/C8E5 and H2O/CO2/C10E6 mixtures at temperatures from approximately 25 to 33 C and pressures between 90 and 350 bar. Measurements of the microstructure in H2O/CO2/C12E6 mixtures as a function of temperature (25-31 C), pressure (63.1-90.7 bar), and CO2 composition (0-3.9 wt%) have also been carried out to show that while micellar structure remains essentially un-changed, critical concentration fluctuations increase as the phase boundary and plait point are approached. In this report, we present our first measurements of the kinetics of isopycnic phase separation for ternary mixtures of H2O/CO2/C8E5.

Lesemann, Markus↗

NASA NTRS2000

Glass/Jamming Transition in Colloidal Aggregation

We have studied colloidal aggregation in a model colloid plus polymer system with short-range attractive interactions. By varying the colloid concentration and the strength of the attraction, we explored regions where the equilibrium phase is expected to consist of colloidal crystallites in coexistance with colloidal gas (i.e. monomers). This occurs for moderate values of the potential depth, U approximately equal to 2-5 kT. Crystallization was not always observed. Rather, over an extended sub-region two new metastable phases appear, one fluid-like and one solid-like. These were examined in detail with light scattering and microscopy techniques. Both phases consist of a near uniform distribution of small irregular shaped clusters of colloidal particles. The dynamical and structural characteristics of the ergodic-nonergodic transition between the two phases share much in common with the colloidal hard sphere glass transition.

Segre, Philip N.↗

NASA NTRS2000

Martian Magmatic-Driven Hydrothermal Sites: Potential Sources of Energy, Water, and Life

Magmatic-driven processes and impact events dominate the geologic record of Mars. Such recorded geologic activity coupled with significant evidence of past and present-day water/ice, above and below the martian surface, indicate that hydrothermal environments certainly existed in the past and may exist today. The identification of such environments, especially long-lived magmatic-driven hydrothermal environments, provides NASA with significant target sites for future sample return missions, since they (1) could favor the development and sustenance of life, (2) may comprise a large variety of exotic mineral assemblages, and (3) could potentially contain water/ice reservoirs for future Mars-related human activities. If life developed on Mars, the fossil record would presumably be at its greatest concentration and diversity in environments where long-term energy sources and water coexisted such as at sites where long-lived, magmatic-driven hydrothermal activity occurred. These assertions are supported by terrestrial analogs. Small, single-celled creatures (prokaryotes) are vitally important in the evolution of the Earth; these prokaryotes are environmentally tough and tolerant of environmental extremes of pH, temperature, salinity, and anoxic conditions found around hydrothermal vents. In addition, there is a great ability for bacteria to survive long periods of geologic time in extreme conditions, including high temperature hydrogen sulfide and sulfur erupted from Mount St. Helens volcano. Our team of investigators is conducting a geological investigation using multiple mission-derived datasets (e.g., existing geologic map data, MOC imagery, MOLA, TES image data, geophysical data, etc.) to identify prime target sites of hydrothermal activity for future hydrological, mineralogical, and biological investigations. The identification of these sites will enhance the probability of success for future missions to Mars.

Anderson, R. C.↗

NASA NTRS2000

Viscous and Thermal Effects on Hydrodynamic Instability in Liquid-Propellant Combustion

A pulsating form of hydrodynamic instability has recently been shown to arise during the deflagration of liquid propellants in those parameter regimes where the pressure-dependent burning rate is characterized by a negative pressure sensitivity. This type of instability can coexist with the classical cellular, or Landau, form of hydrodynamic instability, with the occurrence of either dependent on whether the pressure sensitivity is sufficiently large or small in magnitude. For the inviscid problem, it has been shown that when the burning rate is realistically allowed to depend on temperature as well as pressure, that sufficiently large values of the temperature sensitivity relative to the pressure sensitivity causes the pulsating form of hydrodynamic instability to become dominant. In that regime, steady, planar burning becomes intrinsically unstable to pulsating disturbances whose wavenumbers are sufficiently small. In the present work, this analysis is extended to the fully viscous case, where it is shown that although viscosity is stabilizing for intermediate and larger wavenumber perturbations, the intrinsic pulsating instability for small wavenumbers remains. Under these conditions, liquid-propellant combustion is predicted to be characterized by large unsteady cells along the liquid/gas interface.

Margolis, Stephen B.↗

NASA NTRS1999

Massive Chromite in the Brenham Pallasite and the Fractionation of Cr During the Crystallization of Asteroidal Cores

Large (greater than or equal to 2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group a pallasites (PMG), closely related to high-Au IIIAB irons, Pallasites seem to have formed by the intrusion of a highly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-group pallasites, an indication that Cr contents were high in the intruding liquid and that Cr behaved as an incompatible element during the crystallization of the IIIAB magma, contrary to expectations based on the negative IIIAB Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the interstices between olivine grains, the site normally occupied by metal in Brenham and other pallasites. The massive chromite may have formed as a late cumulus phase; because Fe-Ni was also crystallizing, its absence in the chromite-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite shows the opposite zoning, with the highest FeO contents at grain edges adjacent to olivine. The observed gradients are those expected to form by Fe-Mg exchange between olivine and chromite during slow cooling at subsolidus temperatures. Compared to normal Brenham, contents of phosphoran olivine and phosphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in PMG Molong and in high-Au IIIAB Bear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, implying that, in the PMG, the Mg was extracted from the olivine during high-temperature reaction with the precipitating chromite. There are other circumstantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in die residual magma rising from an initial value of about 300 micrograms/g to a value around 700 micrograms/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most probable one to be that they reflect sampling artefacts resulting from analysts avoiding visible chromite (and the commonly associated phase FeS) when choosing metal samples.

Wasson, John T.↗

NASA NTRS2001

Retrieval of Polar Stratospheric Cloud Microphysical Properties from Lidar Measurements: Dependence on Particle Shape Assumptions

A retrieval algorithm has been developed for the microphysical analysis of polar stratospheric cloud (PSC) optical data obtained using lidar instrumentation. The parameterization scheme of the PSC microphysical properties allows for coexistence of up to three different particle types with size-dependent shapes. The finite difference time domain (FDTD) method has been used to calculate optical properties of particles with maximum dimensions equal to or less than 2 mu m and with shapes that can be considered more representative of PSCs on the scale of individual crystals than the commonly assumed spheroids. Specifically. these are irregular and hexagonal crystals. Selection of the optical parameters that are input to the inversion algorithm is based on a potential data set such as that gathered by two of the lidars on board the NASA DC-8 during the Stratospheric Aerosol and Gas Experiment 0 p (SAGE) Ozone Loss Validation experiment (SOLVE) campaign in winter 1999/2000: the Airborne Raman Ozone and Temperature Lidar (AROTEL) and the NASA Langley Differential Absorption Lidar (DIAL). The 0 microphysical retrieval algorithm has been applied to study how particle shape assumptions affect the inversion of lidar data measured in leewave PSCs. The model simulations show that under the assumption of spheroidal particle shapes, PSC surface and volume density are systematically smaller than the FDTD-based values by, respectively, approximately 10-30% and approximately 5-23%.

Reichardt, J.↗

NASA NTRS2001

Atomic Oxygen Abundance in Molecular Clouds: Absorption Toward Sagittarius B2

We have obtained high-resolution (approximately 35 km/s) spectra toward the molecular cloud Sgr B2 at 63 micrometers, the wavelength of the ground-state fine-structure line of atomic oxygen (O(I)), using the ISO-LWS instrument. Four separate velocity components are seen in the deconvolved spectrum, in absorption against the dust continuum emission of Sgr B2. Three of these components, corresponding to foreground clouds, are used to study the O(I) content of the cool molecular gas along the line of sight. In principle, the atomic oxygen that produces a particular velocity component could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in the UV illuminated surface layer (PDR) of a cloud, and in an atomic (H(I)) gas halo. For each of the three foreground clouds, we estimate, and subtract from the observed O(I) column density, the oxygen content of the H(I) halo gas, by scaling from a published high-resolution 21 cm spectrum. We find that the remaining O(I) column density is correlated with the observed (13)CO column density. From the slope of this correlation, an average [O(I)]/[(13)CO] ratio of 270 +/- 120 (3-sigma) is derived, which corresponds to [O(I)]/[(13)CO] = 9 for a CO to (13)CO abundance ratio of 30. Assuming a (13)CO abundance of 1x10(exp -6) with respect to H nuclei, we derive an atomic oxygen abundance of 2.7x10(exp -4) in the dense gas phase, corresponding to a 15% oxygen depletion compared to the diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination of the PDR contribution to the O(I) column density. The PDR regions should contain O(I) but not (13)CO, and would thus be expected to produce an offset in the O(I)-(13)CO correlation. Our data do not show such an offset, suggesting that within our beam O(I) is spatially coexistent with the molecular gas, as traced by (13)CO. This may be a result of the inhomogeneous nature of the clouds.

Lis, D. C.↗

NASA NTRS1994

Amorphous to Amorphous Form Transitions of Water Ice and Astrophysical Implications

We have combined Selected Area Electron Diffraction (SAED) and cryogenic techniques in an instrumental configuration that allows observing the structure of vapor deposited ice as it evolves during warmup. The ice is deposited in-situ inside an Hitachi H-500 H transmission electron microscope at a base pressure of 1-5 x 10(exp -7) torr on a thin amorphous carbon substrate at 15K or 86K and warmed up at a rate of 1-2 K/min. We find a progression of amorphous forms and well defined amorphous to amorphous transitions. Apart from the well known low-density form of ice, we confirm the presence of a high-density form and find a third amorphous form that coexists with cubic ice. We will report too on the amorphous to crystalline transition and the implications of these results for radical diffusion and gas retention observed in laboratory analog studies of interstellar and cometary ices.

Jenniskens, Peter↗

NASA NTRS2000

Viscous Analysis of Pulsating Hydrodynamic Instability and Thermal Coupling Liquid-Propellant Combustion

A pulsating form of hydrodynamic instability has recently been shown to arise during liquid-propellant deflagration in those parameter regimes where the pressure-dependent burning rate is characterized by a negative pressure sensitivity. This type of instability can coexist with the classical cellular, or Landau form of hydrodynamic instability, with the occurrence of either dependent on whether the pressure sensitivity is sufficiently large or small in magnitude. For the inviscid problem, it has been shown that, when the burning rate is realistically allowed to depend on temperature as well as pressure, sufficiently large values of the temperature sensitivity relative to the pressure sensitivity causes like pulsating form of hydrodynamic instability to become dominant. In that regime, steady, planar burning becomes intrinsically unstable to pulsating disturbances whose wave numbers are sufficiently small. This analysis is extended to the fully viscous case, where it is shown that although viscosity is stabilizing for intermediate and larger wave number perturbations, the intrinsic pulsating instability for small wave numbers remains. Under these conditions, liquid-propellant combustion is predicted to be characterized by large unsteady cells along the liquid/gas interface.

Margolis, Stephen B.↗

NASA NTRS2002

The Speciation of Sulfur in an Ocean on Europa

Stability of native sulfur, iron sulfides, and aqueous sulfur compounds is evaluated at assumed P-T conditions of the Europa's ocean floor. Pyrite, gypsum, and ferric hydroxides can coexist in contact with sulfate-rich oceanic water. Additional information is contained in the original extended abstract.

Zolotov, M. Yu.↗

NASA NTRS1997

Upper Tropospheric/Lower Stratospheric Distribution of Reactive Nitrogen and Ozone

In-situ measurements of a large number of trace chemicals from upper troposphere/lower stratosphere (UT/LS) were performed with the NASA DC-8 aircraft during February/March 1994 over the Pacific Ocean (10 S to 60 N). Mixing ratios in the UT were relatively low in the warm tropical and subtropical air south of the polar jetstream (approx. =28 N) but increased sharply with latitude in the cold polar air north of the jetstream. At about 45 N, high concentrations of PAN (300 ppt) coexisted with extremely low (approx. = 20 ppt) concentrations of NOx. Elevated NOx levels in the UT did not always correspond to continental outflow conditions. Deepest penetrations into the stratosphere (550 ppb O3, 279 ppb NOx, and 350 K potential temperature) corresponded to a region that has been defined as the 'lowermost stratosphere' (LS) by Holton et al. Analysis of data shows that the mixing ratios of long-lived tracer species (e.g., CH4, HNO3, NOy, CFCs, HCFCs) are linearly correlated with those of O3 and N2O. A delta-NOY/delta-O3 of 0.0054 ppb/ppb and delta-NOy/delta-N2O of -0.081 ppb/ppb is in good agreement with other reported measurements from the DC-8. These slopes are however, somewhat steeper than those reported from the ER-2 studies. We find that the reactive nitrogen budget in the UT/LS is largely balanced with shortfalls that are no greater than 15%. A number of oxygenated species (e.g., acetone, H2O2) are present and may provide an important in-situ source of HOx in the UT/LS region.

Singh, H. S.↗

NASA NTRS2002

Large Eddy Simulation of Cryogenic Injection Processes at Supercritical Pressure

This paper highlights results from the first of a series of hierarchical simulations aimed at assessing the modeling requirements for application of the large eddy simulation technique to cryogenic injection and combustion processes in liquid rocket engines. The focus is on liquid-oxygen-hydrogen coaxial injectors at a condition where the liquid-oxygen is injected at a subcritical temperature into a supercritical environment. For this situation a diffusion dominated mode of combustion occurs in the presence of exceedingly large thermophysical property gradients. Though continuous, these gradients approach the behavior of a contact discontinuity. Significant real gas effects and transport anomalies coexist locally in colder regions of the flow, with ideal gas and transport characteristics occurring within the flame zone. The current focal point is on the interfacial region between the liquid-oxygen core and the coaxial hydrogen jet where the flame anchors itself.

Oefelein, Joseph C.↗

NASA NTRS2002

Use of TRMM Data to Test an Improved Parameterization of Stratiform Precipitation

We successfully implemented our EAUcup parameterization of convection in our most recent geodesic-grid version of the Colorado State University General Circulation Model (CSU GCM). Tendencies due to convection and large-scale cloud microphysics are computed using the same time-step, allowing smoother interactions between convective and large-scale moist processes. Figures illustrate our most recent progress on the simulation of convective and stratiform precipitations for January. Comparisons against TRMM precipitation radar (PR) data would reveal a systematic overestimation of the simulated monthly-mean convective rainfall over well-known areas of deep convection. One major deficiency in the global distribution of stratiform precipitation simulated in the CSU GCM is the lack of stratiform precipitation over land in the tropics. This result strongly disagree with Fowler and Randall who show that convective and stratiform precipitations always coexist and that their ratio to the total precipitation is about 0.5. We believe that stratiform precipitation over land evaporates before reaching the ground because the lower troposphere remains too dry in response to the lack of convective detrainment occurring immediately above the Planetary Boundary Layer (PBL). This issue is further investigated at present. We made significant progress in the development of our parameterization of fractional cloudiness. As an example of our ongoing research effort, a figure shows the global distribution of the cloud ice mixing ratio and corresponding fractional area of ice clouds at 250 hPa. These results were obtained using our latitude-longitude grid version of the CSU GCM. There is a nice correspondence between areas of high (low) values of the cloud ice mixing ratio and the cloud fraction. A nice correspondence between the global distribution of the cloud water mixing ratio and fraction of water clouds in the lower troposphere can also be found. We are pursuing this effort, but in the geodesic version of the GCM. We hope to be able to make comparison of the simulated precipitation against TRMM precipitation data in the near future.

Randall, David A.↗

NASA NTRS1998

The Impact of Deviation from Michaelis-Menten Saturation on Mathematical Model Stability Properties

Based on purely abstract ecological theory, it has been argued that a system composed of two or more consumers competing for the same resource cannot persist. By analysis on a Monod format mathematical model, Hubble and others demonstrated that this assertion is true for all but very special cases of such competing organisms which are determined by an index formed by a grouping of. the parameters which characterize the biological processes of the competing organisms. In the laboratory, using a bioreactor, Hansen and Hubble obtained confirmatory results for several cases of two competing species, and they characterized it as "qualitative confirmation" of the assertion. This result is amazing, since the analysis required the exact equality of the hey index, and it seems certain that no pair of organism species could have exactly equal values. It is quite plausible, however, that pairs of organism species could have approximately equal indices, and the question of how different they could be and still have coexistence of the two (or more) presents itself. In this paper, the pursuit of this question and a compatible resolution is presented.

Blackwell, Charles↗

NASA NTRS2002

Experimental Investigation of Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Modern low-pressure turbine airfoils are subject to increasingly stronger pressure gradients as designers impose higher loading in an effort to improve efficiency and to reduce part count. The adverse pressure gradients on the suction side of these airfoils can lead to boundary-layer separation, particularly under cruise conditions. Separation bubbles, notably those which fail to reattach, can result in a significant degradation of engine efficiency. Accurate prediction of separation and reattachment is hence crucial to improved turbine design. This requires an improved understanding of the transition flow physics. Transition may begin before or after separation, depending on the Reynolds number and other flow conditions, has a strong influence on subsequent reattachment, and may even eliminate separation. Further complicating the problem are the high free-stream turbulence levels in a real engine environment, the strong pressure gradients along the airfoils, the curvature of the airfoils, and the unsteadiness associated with wake passing from upstream stages. Because of the complicated flow situation, transition in these devices can take many paths that can coexist, vary in importance, and possibly also interact, at different locations and instances in time. The present work was carried out in an attempt to systematically sort out some of these issues. Detailed velocity measurements were made along a flat plate subject to the same nominal dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil ('Pak-B'). The Reynolds number based on wetted plate length and nominal exit velocity, Re, was varied from 50;000 to 300; 000, covering cruise to takeoff conditions. Low, 0.2%, and high, 7%, inlet free-stream turbulence intensities were set using passive grids. These turbulence levels correspond to about 0.2% and 2.5% turbulence intensity in the test section when normalized with the exit velocity. The Reynolds number and free-stream turbulence level do not have a significant effect on the location of boundary-layer separation unless they are high enough to induce transition upstream of separation. The location and extent of the transition zone, in contrast, depend strongly on Re and TI. The beginning of reattachment closely follows the onset of transition. Under low free-stream turbulence conditions the boundary layer is laminar at separation and then begins to exhibit fluctuations in a finite frequency band in the shear layer over the separation bubble. These fluctuations are due to instability waves. The fluctuations grow in magnitude, higher harmonics are generated, and finally lead to a breakdown to turbulence. Transition begins in the shear layer, but quickly spreads to the near wall region and causes the boundary layer to reattach. The transition is rapid and the resulting turbulence contains a full range of high and low frequencies. Under high free-stream turbulence conditions, slowly growing low-frequency fluctuations are induced in the pretransitional boundary layer by the free-stream. The separation bubbles are considerably thinner than in the low TI cases, resulting in thinner boundary layers at the end of the test wall. At Re=50,000 and 100,000, the pre-transitional boundary layer separates at about the same location as in the low TI cases. Transition occurs through a bypass mode, begins upstream of the corresponding low-TI location, and proceeds in a manner similar to that of an attached boundary layer. Under high TI at Re=200,000 and 300,000, transition begins before separation. The boundary layer may separate, but if it does the separation bubble is very short and does not significantly affect the downstream development of the boundary layer. A comparison is made to previous work in a simulated cascade.

Hultgren, Lennart S.↗

NASA NTRS2002

Large Eddy Simulation of Cryogenic Injection Processes at Supercritical Pressure

Oefelein, Joseph C.↗

NASA NTRS2002

Geography From Another Dimension

The GEODESY software program is intended to promote geographical awareness among students with its remote sensing capabilities to observe the Earth's surface from distant vantage points. Students and teachers using GEODESY learn to interpret and analyze geographical data pertaining to the physical attributes of their community. For example, the program provides a digital environment of physical features, such as mountains and bodies of water, as well as man-made features, such as roads and parks, using aerial photography, satellite imagery, and geographic information systems data in accordance with National Geography Standards. The main goal is to have the students and teachers gain a better understanding of the unique forces that drive their coexistence. GEODESY was developed with technical assistance and financial support from Stennis Space Center's Commercial Remote Sensing Program Office, now known as the Earth Science Applications Directorate.

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