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    Chondrules with peculiar REE patterns: implications for solar nebular condensation at high C/O (2004)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2004-02-14
    Description: Rare earth element (REE) data from two ordinary chondrite chondrules show distinct negative chondrite-normalized concentration anomalies of samarium, europium, and ytterbium. The peculiar patterns may be the result of REE gas/solid fractionation at an oxygen fugacity lower than has been assumed for the canonical solar nebula. We suggest that the two ordinary chondrite chondrules acquired the fractionated REE patterns by incorporation of highly reduced, ultrarefractory condensates in their precursors. This interpretation implies that high-temperature condensation processes occurred in nebular environments with a strong deficit in oxygen, such as regions with an enhanced carbon/oxygen ratio.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Pack, Andreas -- Shelley, J Michael G -- Palme, Herbert -- New York, N.Y. -- Science. 2004 Feb 13;303(5660):997-1000.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institut fur Mineralogie und Geochemie, Universitat Koln, Zulpicher Strasse 49b, D-50674 Koln, Germany. apack@crpg.cnrs-nancy.fr〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/14963326" target="_blank"〉PubMed〈/a〉
    Keywords: *Carbon ; Europium ; *Lanthanoid Series Elements ; *Meteoroids ; *Oxygen ; Samarium ; *Solar System ; Ytterbium
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
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    Integrating planning and reactive control (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Artificial intelligence research on planning is concerned with designing control systems that choose actions by manipulating explicit descriptions of the world state, the goal to be achieved, and the effects of elementary operations available to the system. Because planning shifts much of the burden of reasoning to the machine, it holds great appeal as a high-level programming method. Experience shows, however, that it cannot be used indiscriminately because even moderately rich languages for describing goals, states, and the elementary operators lead to computational inefficiencies that render the approach unsuitable for realistic applications. This inadequacy has spawned a recent wave of research on reactive control or situated activity in which control systems are modeled as reacting directly to the current situation rather than as reasoning about the future effects of alternative action sequences. While this research has confronted the issue of run-time tractability head on, in many cases it has done so by sacrificing the advantages of declarative planning techniques. Ways in which the two approaches can be unified are discussed. The authors begin by modeling reactive control systems as state machines that map a stream of sensory inputs to a stream of control outputs. These machines can be decomposed into two continuously active subsystems: the planner and the execution module. The planner computes a plan, which can be seen as a set of bits that control the behavior of the execution module. An important element of this work is the formulation of a precise semantic interpretation for the inputs and outputs of the planning system. They show that the distinction between planned and reactive behavior is largely in the eye of the beholder: systems that seem to compute explicit plans can be redescribed in situation-action terms and vice versa. They also discuss practical programming techniques that allow the advantages of declarative programming and guaranteed reactive response to be achieved simultaneously.
    Keywords: ADMINISTRATION AND MANAGEMENT
    Type: JPL, California Inst. of Tech., Proceedings of the NASA Conference on Space Telerobotics, Volume 2; p 359-366
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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