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At least 451 records · Page 25
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NASA NTRS2024

Heterogeneous Outgassing Regions Identified on Active Centaur 29P/Schwassmann–Wachmann 1

Centaurs are transitional objects between primitive trans-Neptunian objects and Jupiter-family comets. Their compositions and activities provide fundamental clues regarding the processes affecting the evolution of and interplay between these small bodies. Here we report observations of centaur 29P/Schwassmann–Wachmann 1 (29P) with the James Webb Space Telescope (JWST). We identified localized jets with heterogeneous compositions driving the outgassing activity. We employed the NIRSpec mapping spectrometer to study the fluorescence emissions of CO and obtain a definitive detection of CO 2 for this target. The exquisite sensitivity of the instrument also enabled carbon and oxygen isotopic signatures to be probed. Molecular maps reveal complex outgassing distributions, such as jets and anisotropic morphology, which indicate that 29P’s nucleus is dominated by active regions with heterogeneous compositions. These distributions could reflect that it has a bilobate structure with compositionally distinct components or that strong differential erosion takes place on the nucleus. As there are no missions currently planning to visit a centaur, these observations demonstrate JWST’s unique capabilities in characterizing these objects.

Sara Faggi↗
NASA NTRS1991

Physical properties of lava flows on the southwest flank of Tyrrhena Patera, Mars

Tyrrhena Patera (TP) (22 degrees S, 253.5 degrees W), a large, low-relief volcano located in the ancient southern highlands of Mars, is one of four highland paterae thought to be structurally associated with the Hellas basin. The highland paterae are Hesperian in age and among the oldest central vent volcanoes on Mars. The morphology and distribution of units in the eroded shield of TP are consistent with the emplacement of pyroclastic flows. A large flank unit extending from TP to the SW contains well-defined lava flow lobes and leveed channels. This flank unit is the first definitive evidence of effusive volcanic activity associated with the highland paterae and may include the best preserved lava flows observed in the Southern Hemisphere of Mars. Flank flow unit averages, channelized flow, flow thickness, and yield strength estimates are discussed. Analysis suggests the temporal evolution of Martian magmas.

Crown, David A.↗
NASA NTRS2010

Ultraviolet+Infrared Star Formation Rates: Hickson Compact Groups with Swift and SPitzer

We present Swift UVOT ultraviolet (UV; 1600-3000 A) data with complete three-band UV photometry for a sample of 41 galaxies in 11 nearby (<4500 km/s) Hickson Compact Groups (HCGs) of galaxies. We use UVOT uvw2-band (2000A) photometry to estimate the dust-unobscured component, SFR(sub uv), of the total star formation rate, SFR(sub TOTAL). We use Spitzer MIPS 24 micron photometry to estimate SFR(sub IR), the component of SFR(sub TOTAL) that suffers dust extinction in the UV and is re-emitted in the IR. By combining the two components, we obtain SFR(sub TOTAL) estimates for all HCG galaxies. We obtain total stellar mass, M(sub *) estimates by means of Two Micron All Sky Survey K(sub s)-band luminosities, and use them to calculate specific star formation rates, SSFR is identical with SFR(sub TOTAL)/ M (sub *). SSFR values show a clear and significant bimodality, with a gap between low (approximately <3.2 x 10(exp -11) / yr) and high-SSFR (approximately > 1.2 x lO)exp -10)/yr) systems. We compare this bimodality to the previously discovered bimodality in alpha-IRAC, the MIR activity index from a power-law fit to the Spitzer IRAC 4.5-8 micron data for these galaxies. We find that all galaxies with alpha-IRAC <= 0 (> 0) are in the high- (low-) SSFR locus, as expected if high levels of star-forming activity power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. Consistent with this finding, all elliptical/SO galaxies are in the low-SSFR locus, while 22 out of 24 spirals / irregulars are in the high-SSFR locus, with two borderline cases. We further divide our sample into three subsamples (I, II, and III) according to decreasing H I richness of the parent galaxy group to which a galaxy belongs. Consistent with the SSFR and alpha-IRAC bimodality, 12 out of 15 type I (11 out of 12 type III) galaxies are in the high- (low-) SSFR locus, while type II galaxies span almost the full range of SSFR values. We use the Spitzer Infrared Nearby Galaxy Survey (SINGS) to construct a comparison subsample of galaxies that (1) match HCG galaxies in J-band total galaxy luminosity and (2) are not strongly interacting and largely isolated. This selection eliminates mostly low-luminosity dwarfs and galaxies with some degree of peculiarity, providing a substantially improved, quiescent control sample. Unlike HCG galaxies, galaxies in the comparison SINGS subsample are continuously distributed both in SSFR and alpha-IRAC, although they show ranges in SFR(sub TOTAL) values, morphologies and stellar masses similar to those for HCG systems. We test the SSFR bimodality against a number of uncertainties, and find that these can only lead to its further enhancement. Excluding galaxies belonging to HCGs with three giant galaxies (triplets) leaves both the SSFR and the alpha-IRAC bimodality completely unaffected. We interpret these results as further evidence that an environment characterized by high galaxy number densities and low galaxy velocity dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star formation processes in galaxies and favoring a fast transition to quiescence.

Tzanavaris, P.↗
NASA NTRS1986

The evolution and growth of Al2O3 scales on beta-NiAl

The formation and growth of Al2O3 scales on (beta)-NiAl were studied using electron microscopy and other analytical techniques to gain an understanding of the oxidation properties of (beta)-NiAl and of alumina-forming alloys, in general. The transient and mature stages of oxidation were studied as well as the transformation stage during which the oxide scale transforms from metastable Al2O3 phases to the thermodynamically stable alpha-Al2O3 phase. The transient oxidation stages were studied at 800 deg C and for short times at 1100C. At 800C, the scales consist predominantly of delta-Al2O3 which forms by cation vacancy ordering in the defective spinel lattice of gamma-Al2O3. At 1100C, a fast-growth morphology of theta-Al2O3 forms as a surface layer over delta-Al2O3. For both oxidation temperatures, the scales are often epitaxially oriented with respect to the metal. The transient scales grow by outward cation diffusion as evidenced by surface growth morphologies. The transformation to alpha-Al2O3 occurs within 1 hour at 1100C by a nucleation and radial growth process. The large volume decrease associated with the transformation results in a highly strained alpha-Al2O3 microstructure. A change in scale growth mechanism from outward cation to inward anion diffusion allows transient surface morphologies to be smoothed by surface diffusion. The mature stage of oxidation involves the growth of an alpha-Al2O3 scale having the lacey morphology formed as a result of the gamma yields alpha transformation. Growth of the scale occurs by counterdiffusion along grain boundaries resulting in ridges formed by impingement of alpha-Al2O3 nuclei during the transformation stage. Also scale growth occurs by inward oxygen diffusion through healed cracks; the cracks result from transformation stresses. The measured growth rates of scales having the lacey morphology are an order of magnitude less than fine-grained alpha-Al2O3 scales. Metal orientations were found to have a large effect on oxide morpholoies during all stages of oxidation.

Doychak, J. K.↗
NASA NTRS2012

Vesta Mineralogy after Dawn Global Observations

The Dawn mission has completed its mapping phases at Vesta and millions of spectra have been acquired by the Visible and InfraRed Mapping Spectrometer, VIR(1). VIR characterizes and maps the mineral distribution on Vesta -strengthening the Vesta HED linkage- and provides new insights into Vesta s formation and evolution(2,3). VIR spectra are dominated by pyroxene absorptions near 0.9 and 2.0 m and large thermal emission beyond 3.5 m. Although almost all surface materials exhibit howardite-like spectra, some large regions can be interpreted to be richer in eucritic (basaltic) material and others richer in diogenititic (Mg-orthopyroxenitic) material. The Rheasilvia basin contains Mg-pyroxene-rich terrains for example. Vesta' s surface shows considerable diversity at local scales. Many bright and dark areas(3,4) are associated with various geological features and show remarkably different morphology. Moreover, VIR detected statistically significant, but weak, variations at 2.8 m that have been interpreted as indicating the presence of OH-bearing phases on the surface(5). The OH distribution is uneven with large regions lacking this absorption feature. Associations of 2.8 m band with morphological structures indicate complex process responsible for OH. Vesta exhibits large spectral variations that often correlate with geological structures, indicating a complex geological and evolutionary history, more similar to that of the terrestrial planets than to other asteroids visited by spacecrafts.

ChristinaDeSanctis, Maria↗
NASA NTRS2024

Experimentally Modeling the Emergence of Prebiotically Plausible Phospholipid Vesicles

The prebiotic emergence of protocells is an important part of any origins of life scenario. Although fatty-acid-based vesicles are well studied, how they transitioned to phospholipid vesicles is uncertain. Herein, we show that cyclic-phospholipids formed from fatty acids and glycerol could have played a role by generating a heterogeneous library of vesicles with diverse morphologies and tolerance to a range of metal ions, temperature, and pH. The cyclic phosphate moiety facilitates the natural emergence of vesicles composed of diacyl-phospholipids to become part of the chemical evolutionary process. Competing emergent properties of the various systems (facilitated by additives) could have led to an early preference of the sn-1,2-acyl-substitution on the glycerol backbone coincidental with extant biology. Thus, cyclic-phospholipids could have played a significant role not only in early prebiotic protocellular chemistry but also in facilitating the chemical evolution of protocells from the structurally simple to the functionally more complex.

Sunil Pulletikurti↗
NASA NTRS1992

A phylogenetic analysis of the myxobacteria: basis for their classification

The primary sequence and secondary structural features of the 16S rRNA were compared for 12 different myxobacteria representing all the known cultivated genera. Analysis of these data show the myxobacteria to form a monophyletic grouping consisting of three distinct families, which lies within the delta subdivision of the purple bacterial phylum. The composition of the families is consistent with differences in cell and spore morphology, cell behavior, and pigment and secondary metabolite production but is not correlated with the morphological complexity of the fruiting bodies. The Nannocystis exedens lineage has evolved at an unusually rapid pace and its rRNA shows numerous primary and secondary structural idiosyncrasies.

Non-NASA Center↗
NASA NTRS1994

Forced free-shear layer measurements

Detailed three-dimensional three-component phase averaged measurements of the spanwise and streamwise vorticity formation and evolution in acoustically forced plane free-shear flows have been obtained. For the first time, phase-averaged measurements of all three velocity components have been obtained in both a mixing layer and a wake on three-dimensional grids, yielding the spanwise and streamwise vorticity distributions without invoking Taylor's hypothesis. Initially, two-frequency forcing was used to phase-lock the roll-up and first pairing of the spanwise vortical structures in a plane mixing layer. The objective of this study was to measure the near-field vortical structure morphology in a mixing layer with 'natural' laminar initial boundary layers. For the second experiment the second and third subharmonics of the fundamental roll-up frequency were added to the previous two-frequency forcing in order to phase-lock the roll-up and first three pairings of the spanwise rollers in the mixing layer. The objective of this study was to determine the details of spanwise scale changes observed in previous time-averaged measurements and flow visualization of unforced mixing layers. For the final experiment, single-frequency forcing was used to phase-lock the Karman vortex street in a plane wake developing from nominally two-dimensional laminar initial boundary layers. The objective of this study was to compare measurements of the three-dimensional structure in a wake developing from 'natural' initial boundary layers to existing models of wake vortical structure.

Leboeuf, Richard L.↗
NASA NTRS2004

Fitting Flux Ropes to a Global MHD Solution: A Comparison of Techniques

Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a sub-class of coronal mass ejections (CMEs) in the solar wind. However, it has proven difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply five different fitting techniques to two hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in "blind tests", for which no information, other than time series, was provided. From the results, we infer the following: 1) Accuracy decreases markedly with increasingly glancing encounters; 2) Correct identification of the boundaries of the flux rope can be a significant limiter; 3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.

Riley, Pete↗
NASA NTRS1979

Gaseous halos - Implications for cluster, galaxy, and radio-source evolution

Substantial amounts of enriched gas produced during the early stages of galactic evolution can be stably contained within massive extended halos and released into the intracluster medium by galaxy collisions at a relatively late epoch. If most of the intracluster gas in rich clusters originates in this manner, the onset of ram-pressure stripping can be delayed until z is no more than about 0.5. Prior to this, such clusters contain many spiral and gas-rich elliptical galaxies, thereby accounting for the unusually large proportion of blue galaxies found in distant clusters. Implications are drawn for X-ray observations of rich clusters and for the morphology of extended extragalactic radio sources.

Norman, C.↗
NASA NTRS1994

The RSA survey of dwarf galaxies, 1: Optical photometry

We present detailed surface photometry, based on broad B-band charge coupled device (CCD) images, of about 80 dwarf galaxies. Our sample represents approximately 10% of all dwarf galaxies identified in the vicinity of Revised Shapley-Ames (RSA) galaxies on high resolution blue photographic plates, referred to as the RSA survey of dwarf galaxies. We derive global properties and radial surface brightness profiles, and examine the morphologies. The radial surface brightness profiles of dwarf galaxies, whether early or late type, display the same varieties in shape and complexity as those of classical giant galaxies. Only a few are well described by a pure r(exp 1/4) law. Exponential profiles prevail. Features typical of giant disk galaxies, such as exponential profiles with a central depression, lenses, and even, in one case (IC 2041), a relatively prominent bulge are also found in dwarf galaxies. Our data suggest that the central region evolves from being bulge-like, with an r(exp 1/4) law profile, in bright galaxies to a lens-like structure in dwarf galaxies. We prove detailed surface photometry to be a helpful if not always sufficient tool in investigating the structure of dwarf galaxies. In many cases kinematic information is needed to complete the picture. We find the shapes of the surface brightness profiles to be loosely associated with morphological type. Our sample contains several new galaxies with properties intermediate between those of giant and dwarf ellipticals (but no M32-like objects). This shows that such intermediate galaxies exist so that at least a fraction of early-type dwarf ellipticals is structurally related to early-type giants instead of belonging to a totally unrelated, disjunct family. This supports an origin of early-type dwarf galaxies as originally more massive systems that acquired their current morphology as a result of substantial, presumable supernova-driven, mass loss. On the other hand, several early-type dwarfs in our sample are merger candidates. Merger events may lead to anisotropic velocity distributions in systems of any luminosity, including dwarfs. The RSA sample of dwarf galaxies is more likely to contain mergers because, in contrast to earlier dwarf galaxy surveys that have focused on clusters and rich groups of galaxies, the RSA dwarfs are typically located in low density environments. The occurrence of mergers among dwarf galaxies is of interest in connection with the rapid evolution of faint blue galaxy counts at redshift z less than 1 which suggests that dwarf galaxies were about five times more numerous in the recent past. Finally, our sample contains several examples of late-type dwarfs and 'transition' types that are potential precursors of nucleated early-type dwarfs. All the above processes--mass loss, mergers, astration--are likely to have contributed to the formation of the current population of diffuse early-type dwarfs. A few new redshifts of dwarf galaxies are reported in this paper.

Vader, J. Patricia↗
NASA NTRS1996

MASTER: An Orbiter for the Detailed Study of Vesta

MASTER (Mainbelt Asteroid Exploration/Rendezvous), a Discovery-class orbiter, will carry out a global geological and geochemical survey of Vesta. Primary goals include determining Vesta's mean density and interior structure through detailed mapping of the gravity field, imaging surface morphology at 3-m resolution, mapping mineralogy between 0.4 and 2.5 microns, and determining abundances of key elements through X-ray and gamma-ray spectroscopy. Spectroscopic evidence indicates the presence of basaltic lava flows on Vesta and suggests that impact basins may have exposed mantle materials. These possibilities, combined with the likelihood that Vesta is the ultimate source of HED meteorites, makes this asteroid an important target for a comprehensive orbiter mission. MASTER's global survey, combined with ongoing studies of HED meteorites, will be a major step in understanding the chemical, thermal, and geological evolution of Vesta. A particularly attractive opportunity involves a launch in June 2003 with arrival at Vesta in November 2009. The orbital phase of the mission is scheduled to last one year. A flyby of another mainbelt asteroid enroute to Vesta may be possible.

Veverka, J.↗
NASA NTRS1992

The oldest records of photosynthesis

There is diverse, yet controversial fossil evidence for the existence of photosynthesis 3500 million years ago. Among the most persuasive evidence is the stromatolites described from low grade metasedimentary rocks in Western Australia and South Africa. Based on the understanding of the paleobiology of stromatolites and using pertinent fossil and Recent analogs, these Early Archean stromatolites suggest that phototrophs evolved by 3500 million years ago. The evidence allows further interpretation that cyanobacteria were involved. Besides stromatolites, microbial and chemical fossils are also known from the same rock units. Some microfossils morphologically resemble cyanobacteria and thus complement the adduced cyanobacterial involvement in stromatolite construction. If cyanobacteria had evolved by 3500 million years ago, this would indicate that nearly all prokaryotic phyla had already evolved and that prokaryotes diversified rapidly on the early Earth.

Non-NASA Center↗
NASA NTRS2004

Fitting Flux Ropes to a Global MHD Solution: A Comparison of Techniques

Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a subclass of CMEs in the solar wind. However, it has proven difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply 5 different fitting techniques to 2 hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in "blind tests", for which no information, other than the time series, was provided. F rom the results, we infer the following: (1) Accuracy decreases markedly with increasingly glancing encounters; (2) Correct identification of the boundaries of the flux rope can be a significant limiter; and (3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.

Riley, Pete↗
NASA NTRS1997

The Mars Pathfinder Mission

Mars Pathfinder, one of the first Discovery-class missions (quick, low-cost projects with focused science objectives), will land a single spacecraft with a microrover and several instruments on the surface of Mars in 1997. Pathfinder will be the first mission to use a rover, carrying a chemical analysis instrument, to characterize the rocks and soils in a landing area over hundreds of square meters on Mars, which will provide a calibration point or "ground truth" for orbital remote sensing observations. In addition to the rover, which also performs a number of technology experiments, Pathfinder carries three science instruments: a stereoscopic imager with spectral filters on an extendable mast, an alpha proton X ray spectrometer, and an atmospheric structure instrument/meteorology package. The instruments, the rover technology experiments, and the telemetry system will allow investigations of the surface morphology and geology at submeter to a hundred meters scale, the petrology and geochemistry of rocks and soils, the magnetic properties of dust, soil mechanics and properties, a variety of atmospheric investigations, and the rotational and orbital dynamics of Mars. Landing downstream from the mouth of a giant catastrophic outflow channel, Ares Vallis at 19.5 deg N, 32.8 deg W, offers the potential of identifying and analyzing a wide variety of crustal materials, from the ancient heavily cratered terrain, intermediate-aged ridged plains, and reworked channel deposits, thus allowing first-order scientific investigations of the early differentiation and evolution of the crust, the development of weathering products, and tile early environments and conditions on Mars.

Golombek, Matthew P.↗
NASA NTRS2001

Dendritic Alloy Solidification Experiment (DASE)

A space experiment, and supporting ground-based research, is proposed to study the microstructural evolution in free dendritic growth from a supercooled melt of the transparent model alloy succinonitrile-acetone (SCN-ACE). The research is relevant to equiaxed solidification of metal alloy castings. The microgravity experiment will establish a benchmark for testing of equiaxed dendritic growth theories, scaling laws, and models in the presence of purely diffusive, coupled heat and solute transport, without the complicating influences of melt convection. The specific objectives are to: determine the selection of the dendrite tip operating state, i.e. the growth velocity and tip radius, for free dendritic growth of succinonitrile-acetone alloys; determine the growth morphology and sidebranching behavior for freely grown alloy dendrites; determine the effects of the thermal/solutal interactions in the growth of an assemblage of equiaxed alloy crystals; determine the effects of melt convection on the free growth of alloy dendrites; measure the surface tension anisotropy strength of succinon itrile -acetone alloys establish a theoretical and modeling framework for the experiments. Microgravity experiments on equiaxed dendritic growth of alloy dendrites have not been performed in the past. The proposed experiment builds on the Isothermal Dendritic Growth Experiment (IDGE) of Glicksman and coworkers, which focused on the steady growth of a single crystal from pure supercooled melts (succinonitrile and pivalic acid). It also extends the Equiaxed Dendritic Solidification Experiment (EDSE) of the present investigators, which is concerned with the interactions and transients arising in the growth of an assemblage of equiaxed crystals (succinonitrile). However, these experiments with pure substances are not able to address the issues related to coupled heat and solute transport in growth of alloy dendrites.

Beckermann, C.↗
NASA NTRS2011

Thermal Properties and Evolution of a Coronal Cavity as Observed by the X-Ray Telescope on Hinode

Coronal cavities are voids in coronal emission often observed above high latitude filament channels. sometimes, these cavities have areas of bright x-ray emission in their centers (i.e. Hudson et al 1999). In this study, we use data from the x-ray Telescope (XRT) on Hinode to examine the thermal emission properties of a cavity observed during July 2008 that contains bright x-ray emission in its center. using ratios of XRT filters, we find evidence for elevated temperatures in the cavity center. The area of elevated temperature evolves from a ring-shaped structure at the be~inning of the observation, to an elongated structure two days later, finally appearing as a compact round source four days after the initial observation. We use a morphological model to fit the cavity emission, and find that a uniform structure running through the cavity does not fit the observations well. Instead, the observations are reproduced by modeling several short cavity "cores" with different parameters on different days. These changing core parameters may be due to some observed activity heating different parts of the cavity core at different times. We also find that core temperatures of 1.75 MK, 1.7 MK and 2.25 MK (for 19 July, 21 July and 23 July, respectively) in the model lead to structures that are consistent with the data, and that line-of-sight effects serve to lower the effective temperature derived from the filter ratio. page

Reeves, K. K.↗
NASA NTRS1981

The evolution of the lunar Nectaris multiring basin

The paper begins by discussing the pre-Nectaris crust, the formation of the basin, its age and its geophysics. The pre-mare basalt deposits are categorized into smooth plains deposits, cratered and patterned plains, knobby terrain, and massifs, and the structure, chemical composition, and probable origin of these are discussed. Alternative scenarios for the formation of the plains are that they consist of impact melt ejecta, ejecta overlying smoothed-out preexisting topographic material, or ejecta overlying extrusive deposits. The mare deposits are assessed in terms of their composition, basalt ages and thickness, and eruption styles. A probable age of 3.6 billion years for the basalts is given. The tectonics of mare ridges and fractures and floor-fractured craters are gone into. Post-mare cratering is also discussed. It is concluded that the morphology of multiring basins is strongly dependent on lithosphere thickness.

Whitford-Stark, J. L.↗