共查询到20条相似文献，搜索用时 31 毫秒
1.
Stuart J. Robbins Wesley A. Watters John E. Chappelow Veronica J. Bray Ingrid J. Daubar Robert A. Craddock Ross A. Beyer Margaret Landis Lillian R. Ostrach Livio Tornabene Jamie D. Riggs Brian P. Weaver 《Meteoritics & planetary science》2018,53(4):583-637
One important, almost ubiquitous, tool for understanding the surfaces of solid bodies throughout the solar system is the study of impact craters. While measuring a distribution of crater diameters and locations is an important tool for a wide variety of studies, so too is measuring a crater's “depth.” Depth can inform numerous studies including the strength of a surface and modification rates in the local environment. There is, however, no standard data set, definition, or technique to perform this data-gathering task, and the abundance of different definitions of “depth” and methods for estimating that quantity can lead to misunderstandings in and of the literature. In this review, we describe a wide variety of data sets and methods to analyze those data sets that have been, are currently, or could be used to derive different types of crater depth measurements. We also recommend certain nomenclature in doing so to help standardize practice in the field. We present a review section of all crater depths that have been published on different solar system bodies which shows how the field has evolved through time and how some common assumptions might not be wholly accurate. We conclude with several recommendations for researchers which could help different data sets to be more easily understood and compared. 相似文献
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Astrobiology, like many (but not all) sciences, must take into account questions of the “Why?”, “Where?”, “How?” and “When?” type. In this introductory chapter, we explain why, in this book, we will only consider two of these questions that are, moreover, deeply interrelated. Chronology is by definition related to the “when?” question but as soon as we are interested in the history of Earth or the history of life, it is impossible to treat these questions and their answers without explicit references to the “how?” questions. We also present in this chapter the genesis and the aim of the book. 相似文献
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《New Astronomy Reviews》1999,43(2-4):83-109
This is a course on cosmic microwave background (CMB) anisotropies in the standard cosmological model, designed for beginning graduate students and advanced undergraduates. “Standard cosmological model” in this context means a Universe dominated by some form of cold dark matter (CDM) with adiabatic perturbations generated at some initial epoch, e.g., Inflation, and left to evolve under gravity alone (which distinguishes it from defect models). The course is primarily theoretical and concerned with the physics of CMB anisotropies in this context and their relation to structure formation. Brief presentations of the uniform Big Bang model and of the observed large-scale structure of the Universe are given. The bulk of the course then focuses on the evolution of small perturbations to the uniform model and on the generation of temperature anisotropies in the CMB. The theoretical development is performed in the (pseudo-)Newtonian gauge because it aids intuitive understanding by providing a quick reference to classical (Newtonian) concepts. The fundamental goal of the course is not to arrive at a highly exact nor exhaustive calculation of the anisotropies, but rather to a good understanding of the basic physics that goes into such calculations. 相似文献
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A new conceptual framework for the foundations of statistical mechanics starting from dynamics is presented. It is based on the classification and the study of invariants in terms of the concepts of our formulation of non-equilibrium statistical mechanics. A central role is played by thecollision operator. The asymptotic behaviour of a class of states is determined by the collisional invariants independently of the ergodicity of the system. For this class of states we have an approach to thermodynamical equilibrium. We discuss the existence of classes of states which approach equilibrium. The complex microstructure of the phase space, as expressed by the weak stability concept which was introduced by Moser and others, plays here an essential role. The formalism that we develop is meaningful whenever the “dissipativity condition” for the collision operator is satisfied. Assuming the possibility of a weak coupling approximation, this is in fact true whenever Poincaré's theorem on the nonexistence of uniform invariants holds. In this respect, our formalism applies to few body problems and no transition to the thermodynamic limit is required. Our approach leads naturally to a ‘classical theory of measurement’. In particular a precise meaning can now be given to ‘thermodynamic variables’ or to ‘macrovariables’ corresponding to a measurement in classical dynamics. 相似文献
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D.D. Barbosa 《Planetary and Space Science》1977,25(10):981-984
Stability criteria for parallel propagating plasma waves driven unstable by a nearly isotropic bump-in-energy “half-shell” beam of electrons along a magnetic field are investigated. Comparison with the drifting Maxwellian at equal densities reveals smaller growth rates for the half-shell and a shift in the unstable wavenumbers towards smaller phase velocities. The limit of complete isotropy is stable to the waves under consideration. 相似文献
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Stability of the planar full 2-body problem 总被引：1，自引：0，他引：1
D. J. Scheeres 《Celestial Mechanics and Dynamical Astronomy》2009,104(1-2):103-128
The stability of the Full Two-Body Problem is studied in the case where both bodies are non-spherical, but are restricted to planar motion. The mutual potential is expanded up to second order in the mass moments, yielding a highly symmetric yet non-trivial dynamical system. For this system we identify all relative equilibria and determine their stability properties, with an emphasis on finding the energetically stable relative equilibria and conditions for Hill stability of the system. The energetically stable relative equilibria always correspond to the classical “gravity gradient” configuration with the long ends of the two bodies pointed at each other, however there always exists a second equilibrium in this configuration at a closer separation that is unstable. For our model system we precisely map out the relations between these different configurations at a given value of angular momentum. This analysis identifies the fundamental physical constraints and limitations that exist on such systems, and has immediate applications to the stability of asteroid systems that are fissioned due to a rapid spin rate. Specifically, we find that all contact binary asteroids which are spun to fission will initially lie in an unstable dynamical state and can always re-impact. If the total system energy is positive, the fissioned system can disrupt directly from this relative equilibrium, while if it is negative the system is bound together. 相似文献
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X-radiographs of eight slabs of the Allende meteorite, each about 8 mm thick, reveal significant chemical and textural heterogeneities of our Allende fragment at the 100–1000 g level. They also reveal systematic arrangements of chondrules (“strings”). Our preliminary interpretation is that the observed features are probably all due to regolith processes on Allende's parent body, and that these processes included “mini” slope slides along dry, “sandy” slopes. 相似文献
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《Chinese Astronomy and Astrophysics》1999,23(2):249-257
Part of the “Xia-Shang-Zhou Chronology Project” is the study of a historical record of “double dawn” and its astronomical interpretation. We used the light meter on ordinary cameras to determine the sky variation during normal sunrises and sunsets, set up a way of calculating the variation when the rising sun is in eclipse, and identified the range and intensity of the double dawn phenomenon. For this, we organized a mass participation of the observation of the 1997-03-09 eclipse in Xinjiang Province. The observations are in good agreement with our model calculation and prove that an eclipsed sunrise could indeed give rise to the phenomenon of “double dawn” 相似文献
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Abstract— High-purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3 , released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL , consisting of isotopically anomalous Xe-HL and Kr-H, Ar with high 38Ar/36Ar, and most of the gas making up Ne-A2 and He-A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble-gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe-H and Xe-L or other isotopic variations in the HL component. However, because ～1010 diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble-gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble-gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud. 相似文献
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I. B. Vavilova T. G. Artemenko L. K. Pakuliak 《Kinematics and Physics of Celestial Bodies》2014,30(1):46-52
The new electronic database developed by the authors and titled “Astronomers of Ukraine” is described as a source of the main biographical data on astronomers of Ukraine from the 15th century until the beginning of the 21st century. The database is an upgrading component of the Ukrainian Virtual Observatory portal and contains the main biobibliographical data and papers concerning astronomers of Ukraine, as well as links to their publications. The existing biographical sources about astronomers in the world are discussed briefly. A list of the principal publications about astronomers of Ukraine is given. 相似文献
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Seven 1.5° × 1.5° fields in which “guest stars” had been reported by ancient oriental astronomers have been surveyed at 610 MHz for radio remnants with the Westerbork Telescope. No diffuse radio emission was detected. A list of discrete radio sources in the fields is given together with suggested optical identifications for those sources with |b| < 15°. 相似文献
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V. Trimble 《Astronomische Nachrichten》2009,330(7):761-769
More than 2000 years ago, Epicurus taught that there are an infinite number of other worlds, both like and unlike ours, and Aristotle taught that there are none. Neither hypothesis can currently be falsified, and some versions of current multiverses perhaps never can be, which has contributed to occasional claims that “this isn't science!” (a common complaint about cosmology for centuries). Define “cosmos”, or “world”, or “universe” to mean the largest structure of which you and the majority of knowledgeable contemporaries will admit to being a part. This begins with the small, earth‐centered worlds of ancient Egyptian paintings, Greek mythology, and Genesis, which a god could circumnavigate in a day and humans in a generation. These tend to expand and become helio‐rather than geo‐centric (not quite monotonically in time) and are succeeded by various assemblages of sun‐like stars with planets of their own. Finite vs. infinite assemblages are debated and then the issue of whether the Milky Way is unique (so that “island universes” made sense, even if you were against the idea) for a couple of centuries. Today one thinks as a rule of the entire 4‐dimensional space‐time we might in principle communicate with and all its contents. Beyond are the modern multi‐verses, sequential (cyclic or oscillating), hierarchical, or non‐communicating entities in more than four dimensions. Each of these has older analogues, and, in every milieu where the ideas have been discussed, there have been firm supporters and firm opponents, some of whose ideas are explored here. Because astronomical observations have firmly settled some earlier disputes in favor of very many galaxies and very many stars with planets, “other worlds” can now refer only to other planets like Earth or to other universes. The focus is on the latter (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Meteorite and meteoroid: New comprehensive definitions 总被引：1，自引：0，他引：1
Abstract– Meteorites have traditionally been defined as solid objects that have fallen to Earth from space. This definition, however, is no longer adequate. In recent decades, man‐made objects have fallen to Earth from space, meteorites have been identified on the Moon and Mars, and small interplanetary objects have impacted orbiting spacecraft. Taking these facts and other potential complications into consideration, we offer new comprehensive definitions of the terms “meteorite,”“meteoroid,” and their smaller counterparts: A meteoroid is a 10‐μm to 1‐m‐size natural solid object moving in interplanetary space. A micrometeoroid is a meteoroid 10 μm to 2 mm in size. A meteorite is a natural, solid object larger than 10 μm in size, derived from a celestial body, that was transported by natural means from the body on which it formed to a region outside the dominant gravitational influence of that body and that later collided with a natural or artificial body larger than itself (even if it is the same body from which it was launched). Weathering and other secondary processes do not affect an object’s status as a meteorite as long as something recognizable remains of its original minerals or structure. An object loses its status as a meteorite if it is incorporated into a larger rock that becomes a meteorite itself. A micrometeorite is a meteorite between 10 μm and 2 mm in size. Meteorite– “a solid substance or body falling from the high regions of the atmosphere” ( Craig 1849 ); “[a] mass of stone and iron that ha[s] been directly observed to have fallen down to the Earth’s surface” (translated from Cohen 1894 ); “[a] solid bod[y] which came to the earth from space” ( Farrington 1915 ); “A mass of solid matter, too small to be considered an asteroid; either traveling through space as an unattached unit, or having landed on the earth and still retaining its identity” ( Nininger 1933 ); “[a meteoroid] which has reached the surface of the Earth without being vaporized” (1958 International Astronomical Union (IAU) definition, quoted by Millman 1961 ); “a solid body which has arrived on the Earth from outer space” ( Mason 1962 ); “[a] solid bod[y] which reach[es] the Earth (or the Moon, Mars, etc.) from interplanetary space and [is] large enough to survive passage through the Earth’s (or Mars’, etc.) atmosphere” ( Gomes and Keil 1980 ); “[a meteoroid] that survive[s] passage through the atmosphere and fall[s] to earth” ( Burke 1986 ); “a recovered fragment of a meteoroid that has survived transit through the earth’s atmosphere” ( McSween 1987 ); “[a] solid bod[y] of extraterrestrial material that penetrate[s] the atmosphere and reach[es] the Earth’s surface” ( Krot et al. 2003 ). 相似文献
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I. L. Andronov 《Astronomische Nachrichten》1987,308(4):229-234
The mechanisms of “Noisar” phenomenon in AM Herculis-type stars are discussed. In an accretion column above the surface of the magnetized degenerate star the instability of some types may be excited, such as axi-symmetrical quasi-periodical penetration of the low-density “bulks” from the column axis to the outer parts; “boiling” with “bulks” moving inside or outside the column; “tornado” with low-density region rapidly rotating around the column axis; “switchings” of the accretion from one half of the “polar cap” to another and vice versa. The oscillations of different plasma clots (“spaghetti”) may interfer causing flux changes as well. Such “Noisar” oscillations appear in different regions of the accretion column (at the upper shock and near the column base, respectively), so hard and soft X-ray fluxes might not have correlation in their variability. The observations are in qualitative agreement with the models. 相似文献
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M. Ternullo 《Astrophysics and Space Science》2010,328(1-2):301-305
This work originates from the need of getting a picture of the spot zone that is sharp enough to efficiently help us place tighter and more realistic constraints than we would usually do on dynamo models, in order to improve their predictive performance. This paper questions the confidence in Maunder’s Butterfly Diagram (BD) as the fundamental tool for describing the magnetic flux large-scale distribution and presents a new version of the time-latitude diagram for cycles 21 through 23, where spot groups are given proportional relevance to their area. The diagram presented here confirms the active regions’ well-known tendency to repeatedly appear in a few photospheric regions (“activity nests”) tightly limited in latitude, active for a short time. Activity nests leave their signature in the BD, in the form of small portions (“knots”) characterized by the spotted area high density. The BD may be described as a cluster of knots. A knot may appear at either lower or higher latitudes than previous ones; accordingly, the spot mean latitude abruptly drifts equatorward or even poleward, even though the knot’s prevalent tendency is to appear at lower and lower latitudes. A careful inspection of the BD suggests that its intricate fine structure may be (partially) disentangled by recognizing that, in any hemisphere, the activity is split into two or more distinct “activity waves” (out of phase compared to each other), drifting equatorward at a rate higher than the spot zone as a whole. Preliminary computations confirm this suggestion. 相似文献
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Accretional and alterational differences in a carbonaceous chondrite parent body: Evidence from the NWA 5491 CV3 meteorite 
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下载免费PDF全文 The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula's composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact‐driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body. 相似文献
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Abstract— Discussions of meteorite properties often concern whether they are the result of “nebular” or “parent body” processes, but several decades of research have not resolved the issue. Part of the problem is that any gas-phase reaction involving meteoritic materials thought to have occurred is automatically assumed to be nebular, even though the most primitive solar system objects are water- and volatile-rich and could easily generate vapors. Reactions important in meteorite genesis are those involving (1) oxidation of Fe, (2) oxidation of other cations, (3) reduction, (4) olivine-pyroxene equilibria, and (5) hydration of silicates. The P-T-x conditions required are almost invariably incompatible with standard models for the early solar nebula, but clearly many of these reactions occurred prior to final agglomeration. Most of the reactions require high pressures (>10?3 atm) or, more importantly, major departures from cosmic composition, even though the final rocks are remarkably cosmic in elemental proportions. We suggest that most of these reactions occurred in a regolith rendered “dynamic” by the flow of copious volatiles. In such a scenario, liquid and gas phase reactions can occur at elevated temperatures and pressures relative to the nebula and with noncosmic gas phase compositions; but the system is “closed” to most components, so that cosmic proportions are essentially preserved. 相似文献