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  • Astrophysics  (15)
  • Lunar and Planetary Science and Exploration  (8)
  • 1
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    The Infrared Spectrum of Matrix-Isolated Hexamethylenetetramine in Argon at 12 K (1994)
    In:  Other Sources
    Details
    Publication Date: 2019-07-17
    Description: It is commonly accepted that constant exposure of comets and ice grains to ultraviolet (UV) radiation causes an organic crust to form on their surface, but the exact molecules that comprise the crust remains a mystery. Based on our lab simulations of interstellar and cometary ices (a paper to be submitted this spring) we believe that Hexamethylenetetramine (HMT) may be a major component of the organic crust on the surface of comets and ice grains. The enclosed paper presents the laboratory infrared spectra and UV photochemistry of HMT under conditions akin to that of the interstellar medium.
    Keywords: Astrophysics
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  • 2
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    Exobiological Exploration of Mars (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Of all the other planets in the solar system, Mars remains the most promising for studying concepts about chemical evolution and the origin of life. Strategies were developed to pursue three exobiological objectives for Mars: determine abundance and distribution of biogenic elements and organic compounds, detect evidence of an ancient biota, and determine if organisms exist anywhere on the planet. The three strategies share the same sequence of phases. In the first phase, each requires global reconnaissance and remote sensing by orbiters to select sites of interest for detailed in situ analyses. In the second phase, lander missions are conducted to characterize the chemical and physical properties of the selected sites. The third phase involves conducting "critical" experiments at sites whose properties make them particularly attractive for exobiology. These critical experiments would include, for example, identification of organics, detection of fossils, and detection of extant life. The fourth phase is the detailed analysis of samples returned from these sites in Earth-based laboratories to confirm and extend previous discoveries. Finally, in the fifth phase, human exploration is needed to establish the geological context or to discover and explore sites that are not accessible to robotic spacecraft.
    Keywords: Lunar and Planetary Science and Exploration
    Type: NSCORT Symposium, ACS Spring 1994 Meeting; Mar 21, 1994; San Diego, CA; United States
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  • 3
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    Organic Chemistry of Meteorites (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Studies of the molecular structures and C,N,H-isotopic compositions of organic matter in meteorites reveal a complex history beginning in the parent interstellar cloud which spawned the solar system. Incorporation of interstellar dust and gas in the protosolar nebula followed by further thermal and aqueous processing on primordial parent bodies of carbonaceous, meteorites have produced an inventory of diverse organic compounds including classes now utilized in biochemistry. This inventory represents one possible set of reactants for chemical models for the origin of living systems on the early Earth. Evidence bearing on the history of meteoritic organic matter from astronomical observations and laboratory investigations will be reviewed and future research directions discussed.
    Keywords: Astrophysics
    Type: 207th ACS National Meeting; Mar 13, 1994 - Mar 18, 1994; San Diego, CA; United States
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  • 4
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    Photoevaporation of Disks Around Young Stars (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Young massive stars produce sufficient Lyman continuum luminosity phi to have a significant effect on the structure and evolution of the accretion disks surrounding diem. We show that inside a critical disk radius r(sub g), an isothermal 10(exp 4) K atmosphere forms with a scale height that increases with r(sup3/2) for r less than or equal to r(sub g). For r less than or equal to r(sub g), the diffuse field caused by hydrogen recombinations to the ground state in the atmosphere produces a steadily evaporating disk. The mass loss from this outer region of the disk is of order 10(exp -5) Mo/yr phi(sub 49)1/2, where phi(sub 49) is defined as phi/10(exp 49) photons/s. The mass loss has two important consequences. First, the slow (10-50 km/s) wind that results may explain the long life of unresolved ultracompact HII regions. Secondly, the dependence on phi implies that accretion through the disk onto the star will be quenched once the photoevaporation rate exceeds the accretion rate. This may act to limit the mass of the forming star.
    Keywords: Astrophysics
    Type: Jan 11, 1994 - Jan 14, 1994; Arlington, VA; United States
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  • 5
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    Tunguska Workshop: Applying Modern Tools to Understand the 1908 Tunguska Impact (2018)
    In:  CASI
    Details
    Publication Date: 2019-07-20
    Description: A one-day workshop was held at NASA Ames Research Center, January 16, 2018, to re-examine the 1908 Tunguska impact using modern computational tools, many of them developed in response to the 2013 Chelyabinsk airburst. Twelve international experts gave presentations, with another 40 attending in-person or remotely. The most likely models for Tunguska converged on an energy of 10-20 Megatons, released in an airburst at a height of about 10 km. If the Tunguska impactor was a stony asteroid similar to Chelyabinsk, the diameter was roughly 50-80m. A comparison with current understanding of the population of asteroids in this size range indicates that the interval between such events is millennia, not centuries as had been concluded previously. The primary constraints on our understanding of Tunguska are the dearth of quantitative data, not weakness of the computational models. The workshop was sponsored by the NASA Ames Asteroid Threat Assessment Project and supported the NASA Planetary Defense Coordination Office.
    Keywords: Lunar and Planetary Science and Exploration
    Type: NASA/TM-2018-220174 , ARC-E-DAA-TN64323
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  • 6
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    Asteroid Characterization Priorities for Planetary Defense (2015)
    In:  CASI
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    Publication Date: 2019-07-19
    Description: We propose a prioritized list of asteroid characterization needs for planetary defense. In particular, we consider the properties of asteroids that are of greatest interest for assessment of planetary defense options, including gravity tractors, kinetic impactors, and nuclear explosives. In addition, much of our discussion is relevant for impact assessments and subsequent emergency-response planning. Rather than intending this as a definitive answer, however, our purpose is to stimulate and focus disscussion regarding characterization needs for planetary defense, with a specific list as a starting point. A key theme is understanding the sensitivity of the outcome of an asteroid deflection or disruption effort to the asteroids physical properties.There is a range of previous work relevant to our topic, including some explicit discussions as well as many more that are implicitly relevant. We incorporate elements from such reports while extending them using our own experience and perspectives. After introducing our prioritized list, we provide further discussion on each element, with details on the relevance of each characteristic for modeling purposes, our rationale for the assigned priority, and examples of analyses that require improved characterization information. Our goal is to establish a framework that can be modified and adapted by the community for a variety of purposes, such as mission design and optimization, development of new measurement techniques, or prioritization of research efforts. The objective is to increase understanding and reduce uncertainties in the specific aspects of characterization that most benefit accurate assessments of practical techniques for planetary defense.
    Keywords: Astrophysics
    Type: ARC-E-DAA-TN21736 , IAA Planetary Defense Conference; Apr 13, 2015 - Apr 17, 2015; Frascati; Italy
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  • 7
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    The Mineralogy of Martian Dust: Design and Analysis Considerations for an X-Ray Diffraction/X-Ray Fluorescence (XRD/XRF) Instrument for Exobiological Studies (1994)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: A principal objective of Mars exploration is the search for evidence of past life which may have existed during an earlier clement period of Mars history. We would like to investigate the history of surface water activity (which is a requirement for all known forms of life) by identifying and documenting the distribution of minerals which require water for their formation or distribution. A knowledge of the mineralogy of the present Martian surface would help to identify areas which, due to the early activity of water, might have harbored ancient life. It would be desirable to establish the presence and characterize the distribution of hydrated minerals such as clays, and of minerals which are primarily of sedimentary origin such as carbonates, silica and evaporites. Mineralogy, which is more critical to exobiological exploration than is simple chemical analysis (absent the detection of organics), will remain unknown or will at best be imprecisely constrained unless a technique sensitive to mineral structure such as powder X-ray diffraction (XRD) is employed. Additional information is contained in the original extended abstract.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Mars Surveyor Science Objectives/Measurement Requirements Workshop; May 10, 1994 - May 12, 1994; Pasadena, CA; United States
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  • 8
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    Organic Material in the ISM (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Spectra of objects which lie along several lines of sight through the diffuse interstellar medium (ISM) all contain an absorption feature near 3.4 micrometers which has been attributed to saturated aliphatic hydrocarbons on interstellar grains. The similarity of the absorption bands near 3.4 micrometers along different lines of sight reveal that the carrier of this band lies in the diffuse dust. Several materials have been proposed as "fits" to the 3.4 micrometers feature over the years. A comparison of these identifications is presented. A remarkable similarity between the spectrum of the diffuse dust and an organic extract from the Murchison meteorite suggests that some of the interstellar organic material may be preserved in primitive solar system bodies. The optical depth/extinction tau /A(sub v) ratio for the 3.4 micrometers band is higher toward the Galactic center than toward sources which sample the interstellar medium in the local neighborhood. A similar trend has been observed previously for silicates, indicating that the two materials may be simultaneously enhanced in the Galactic center.
    Keywords: Astrophysics
    Type: Jun 06, 1994 - Jun 17, 1994; Erice, Sicily; Italy
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  • 9
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    Encounter with Asteroids Gaspra and Ida: Imaging Results (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: A progress report is given on imaging science results from the Galileo spacecraft flybys of the S-type main-belt asteroids Gaspra and Ida. Images were obtained in six colors of both asteroids, a few at resolutions better than 100 m. The overall sizes, shapes, and spin vectors of both objects are defined, and more detailed geological studies are possible for approximately half the surface of each (the hemisphere facing the spacecraft at closest approach). Each is elongated and highly irregular in profile, with heavily cratered surfaces. A number of grooves, other linear features, and surface blocks may also be associated with a recent history of impacts. Modest variations in surface albedo and color are probably due to differences in regolith development rather than compositional differences across the surfaces of each. Both the density and the size distributions of craters differ from Gaspra to Ida, in spite of its younger anticipated dynamical age, Ida is more density cratered. Ida also has a small satellite in a roughly 100-km orbit, allowing the possibility of determining the density of Ida with sufficient precision to constrain alternative models of its bulk composition.
    Keywords: Astrophysics
    Type: IAU 22nd General Assembly; Aug 13, 1994 - Aug 27, 1994; The Hague; Netherlands
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  • 10
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    Multiple Core Galaxies (1994)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Nuclei of galaxies often show complicated density structures and perplexing kinematic signatures. In the past we have reported numerical experiments indicating a natural tendency for galaxies to show nuclei offset with respect to nearby isophotes and for the nucleus to have a radial velocity different from the galaxy's systemic velocity. Other experiments show normal mode oscillations in galaxies with large amplitudes. These oscillations do not damp appreciably over a Hubble time. The common thread running through all these is that galaxies often show evidence of ringing, bouncing, or sloshing around in unexpected ways, even though they have not been disturbed by any external event. Recent observational evidence shows yet another phenomenon indicating the dynamical complexity of central regions of galaxies: multiple cores (M31, Markarian 315 and 463 for example). These systems can hardly be static. We noted long-lived multiple core systems in galaxies in numerical experiments some years ago, and we have more recently followed up with a series of experiments on multiple core galaxies, starting with two cores. The relevant parameters are the energy in the orbiting clumps, their relative.masses, the (local) strength of the potential well representing the parent galaxy, and the number of cores. We have studied the dependence of the merger rates and the nature of the final merger product on these parameters. Individual cores survive much longer in stronger background potentials. Cores can survive for a substantial fraction of a Hubble time if they travel on reasonable orbits.
    Keywords: Astrophysics
    Type: Dynamical Astronomy/DDA Committee; Mar 27, 1994 - Mar 29, 1994; Kingsville, TX; United States
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