The MESSENGER mission to Mercury: scientific objectives and implementation     

Sean C. Solomon  Ralph L. McNutt  Jr.  Robert E. Gold  Mario H. Acua  Daniel N. Baker  William V. Boynton  Clark R. Chapman  Andrew F. Cheng  George Gloeckler  James W. Head III  Stamatios M. Krimigis  William E. McClintock  Scott L. Murchie  Stanton J. Peale  Roger J. Phillips  Mark S. Robinson  James A. Slavin  David E. Smith  Robert G. Strom  Jacob I. Trombka  Maria T. Zuber 《Planetary and Space Science》2001,49(14-15)

Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercury's anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercury's geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercury's core, and the processes controlling volatile species in Mercury's polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercury's exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.  相似文献   

Aerogel keystones: Extraction of complete hypervelocity impact events from aerogel collectors     

Andrew J. Westphal  Christopher Snead  Anna Butterworth  Giles A. Graham  John P. Bradley  Sa&#x;a Bajt  Patrick G. Grant  Graham Bench  Sean Brennan  Piero Pianetta 《Meteoritics & planetary science》2004,39(8):1375-1386

Abstract— In January 2006, the Stardust mission will return the first samples from a solid solar system body beyond the Moon and the first samples of contemporary interstellar dust ever collected. Although sophisticated laboratory instruments exist for the analysis of Stardust samples, techniques for the recovery of particles and particle residues from aerogel collectors remain primitive. Here, we describe our recent progress in developing techniques for extracting small volumes of aerogel, which we have called “keystones,” which completely contain particle impacts but minimize the damage to the surrounding aerogel collector. These keystones can be fixed to custom‐designed micromachined silicon fixtures (so called “microforklifts”). In this configuration, the samples are self‐supporting, which can be advantageous in situations where interference from a supporting substrate is undesirable. The keystones may also be extracted and placed onto a substrate without a fixture. We have also demonstrated the capability of homologously crushing these unmounted keystones for analysis techniques that demand flat samples.  相似文献   

Density within the moon and implications for lunar composition     

Sean C. Solomon 《Earth, Moon, and Planets》1974,9(1-2):147-166

Density models for the Moon, including the effects of temperature and pressure, can satisfy the mass and moment of inertia of the Moon and the presence of a low density crust indicated by the seismic refraction results only if the lunar mantle is chemically or mineralogically inhomogeneous. IfC/MR ² exceeds 0.400, the inferred density of the upper mantle must be greater than that of the lower mantle at similar conditions by at least 0.1 g cm^–3 for any of the temperature profiles proposed for the lunar interior. The average mantle density lies between 3.4 and 3.5 g cm^–3, though the density of the upper mantle may be greater. The suggested density inversion is gravitationally unstable, but the implied deviatoric stresses in the mantle need be no larger than those associated with lunar gravity anomalies. UsingC/MR ³=0.400 and the recent seismic evidence suggesting a thin, high density zone beneath the crust and a partially molten core, successful density models can be found for a range of temperature profiles. Temperature distributions as cool as several inferred from the lunar electrical conductivity profile would be excluded. The density and probable seismic velocity for the bulk of the mantle are consistent with a pyroxenite composition and a 100 MgO/(MgO+FeO) molecular ratio of less than 80.Communication presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973.  相似文献   

Increasing Use of Human-Dominated Habitats as CO<Subscript>2</Subscript> Emissions Warm and Acidify Oceans     

Katherine A. Heldt  Sean D. Connell  Pablo Munguia 《Estuaries and Coasts》2018,41(6):1660-1666

Urban and artificial structures are increasingly added to the world’s coasts during a time in which changing climate is forecast to drive shifts in naturally occurring habitats. We ask whether the role of artificial structures as marine habitats will increase in importance relative to their natural counterparts, particularly as natural habitats are negatively affected by ocean warming and acidification. To evaluate this model, we contrasted use of natural (kelp forest and turfing algae) and artificial habitat (plastic pier-piling) by a nest-building amphipod (Cymadusa pemptos) under current and future climate conditions of CO₂ and temperature. Under future conditions, amphipod populations in mesocosms increased, but this did not lead to greater proportional colonization of kelp and turf. Instead, colonization doubled in artificial habitats, and there was increasing production and occupation of nests on artificial habitats relative to natural habitats. In an age when human modification of natural substrata is increasingly cited as an agent of wildlife decline, understanding the future role of artificial habitats as replacement dwellings for natural habitats is critical. We pioneer an understanding of the future role of natural and artificial habitats, identifying the possibility that the role of urban structures as marine habitats may only increase.  相似文献   

Are American Interlocking Directorates Associated with Brain Circulation and do They Translate into Higher Corporate Performance?          下载免费PDF全文

Sean O'Hagan 《Geographical review》2015,105(3):344-359

This study explores interlocking directorates, with specific focus on those connections when a director sits as an inside director for an American corporation and an outside director for firm located outside of North America. The goal of this study is to explore the spatial relationship between the personal histories of directors and the headquarter's location of the corporate boards on which they sit. First, it was found that a geographical relationship in the interlocking network of American companies exists. Second, it was determined that domestic personal histories played a significant role as 96.5 percent of directors attended a university in the United States and 93.4 percent directors were born there. When North American universities and North American births were deleted from the sample, a geographical pattern also emerges for Asian firms. Finally, the study found that the lone region that consistently displayed greater firm performance where a director's personal history and the interlocked company were the same as Asia.  相似文献   

The geography of Canadian interlocking directorates: how do they relate to brain circulation?     

Sean B. O’Hagan  Murray D. Rice 《Urban geography》2015,36(6):823-843

This study examines the spatiality of interlocking directorates of Canadian companies, with particular emphasis given to international interlocks. From a geographical standpoint, do these connections to foreign firms reinforce the importance of Canada’s top corporate cities as ‘gateways’ to international business? Is there a geography associated with the interlocking to these gateway cities? Findings build on James O. Wheeler’s body of research focused on corporate headquarters. This study is placed within the context of the current economic geography dialogue. Particular focus is placed on Saxenian’s theory of brain circulation, wherein she argues that individuals with international experience serve as brokers linking domestic businesses with those in distant regions. A further purpose of this study then is to examine Saxenian’s argument in the context of Canadian corporations. First, is there a geographical association between directors’ foreign educations and where Canadian firms are interlocked to internationally? Second, the corporate performance of Canadian firms interlocked internationally will be measured against those firms that are not. It seems logical to assume that increased connections would yield more diverse and rich knowledge, which could lead to better decisions, and thus corporate performance, in today’s global world. But does it? This study finds that being linked to an international company through a common director did translate into more profitability for Canadian firms. Findings also revealed a geographical link between international interlocks and a director’s foreign education. However, when Canadian firms were interlocked with an international company with a director who had international education, greater profits did not necessarily result.  相似文献   

The formation and habitability of terrestrial planets in the presence of close-in giant planets     

Sean N. Raymond  Thomas Quinn 《Icarus》2005,177(1):256-263

‘Hot jupiters,’ giant planets with orbits very close to their parent stars, are thought to form farther away and migrate inward via interactions with a massive gas disk. If a giant planet forms and migrates quickly, the planetesimal population has time to re-generate in the lifetime of the disk and terrestrial planets may form [P.J. Armitage, A reduced efficiency of terrestrial planet formation following giant planet migration, Astrophys. J. 582 (2003) L47-L50]. We present results of simulations of terrestrial planet formation in the presence of hot/warm jupiters, broadly defined as having orbital radii ?0.5 AU. We show that terrestrial planets similar to those in the Solar System can form around stars with hot/warm jupiters, and can have water contents equal to or higher than the Earth's. For small orbital radii of hot jupiters (e.g., 0.15, 0.25 AU) potentially habitable planets can form, but for semi-major axes of 0.5 AU or greater their formation is suppressed. We show that the presence of an outer giant planet such as Jupiter does not enhance the water content of the terrestrial planets, but rather decreases their formation and water delivery timescales. We speculate that asteroid belts may exist interior to the terrestrial planets in systems with close-in giant planets.  相似文献   

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments     

Doerthe Tetzlaff  James Buttle  Sean K. Carey  Matthew J. Kohn  Hjalmar Laudon  James P. McNamara  Aaron Smith  Matthias Sprenger  Chris Soulsby 《水文研究》2021,35(1):e14023

We compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well-known long-term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil-root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.  相似文献   

Melting phase relations of a mica–clinopyroxenite from the Milk River area, southern Alberta, Canada     

Sean P. Funk  Robert W. Luth 《Contributions to Mineralogy and Petrology》2013,166(2):393-409

Melting experiments were conducted on a mica–clinopyroxenite xenolith brought up in a minette dyke in southern Alberta, Canada, near Milk River. Both the minettes and mica–clinopyroxenite xenoliths were studied by Buhlmann et al. (Can J Earth Sci 37:1629–1650, 2000), who hypothesized that the minettes formed by partial melting of a mantle source containing clinopyroxene + phlogopite ± olivine, at pressures ≥1.7 GPa. In liquidus experiments performed on the most primitive minette in our previous study (Funk and Luth in Contrib Mineral Petrol 164:999–1009, 2012), we found a multiple saturation point where olivine and orthopyroxene coexisted with liquid at 1.77 GPa and 1,350 °C. We argued that the minette originally formed by partial melting of clinopyroxene + phlogopite, but had re-equilibrated with a harzburgite during ascent. In the current study, we wanted to test both the source region hypothesis of Buhlmann et al. and our re-equilibration hypothesis by studying the near-solidus phase equilibria of a mica + clinopyroxene assemblage. We found the solidus for our xenolith has a steep slope in P–T space and lies at temperatures above those of a normal cratonic geotherm, implying that this mica–clinopyroxenite is stable in the cratonic mantle. Melting could occur at greater depths, where the solidus is extrapolated to cross the geotherm or must be induced either by raising the temperatures of the surrounding rocks or by introducing hydrous fluids into the source. Our melts are in equilibrium with clinopyroxene and olivine. The compositions of the liquids derived from melting this xenolith are similar to madupitic lamproites from the Leucite Hills, Wyoming, studied by Carmichael (Contrib Mineral Petrol 15:24–66, 1967) and Barton and Hamilton (Contrib Mineral Petrol 66:41–49, 1978; Contrib Mineral Petrol 69:133–142, 1979). Barton and Hamilton (Contrib Mineral Petrol 69:133–142, 1979) proposed that the madupitic lamproites may have come from a source containing mica and pyroxene. This study supports their hypothesis. The composition of the most primitive minette from southern Alberta lies between our experimental melt and a population of representative mantle orthopyroxenes. We conclude from our study that the Milk River minettes were likely derived from a source containing phlogopite, clinopyroxene and trace amounts of apatite, which formed olivine upon melting. During ascent, the melts changed composition by reacting with orthopyroxene.  相似文献   

Regional Correspondence in Habitat Occupancy by Gray Snapper (Lutjanus griseus) in Estuaries of the Southeastern United States     

Kerry E. Flaherty  Theodore S. Switzer  Brent L. Winner  Sean F. Keenan 《Estuaries and Coasts》2014,37(1):206-228

Reef fishes, such as gray snapper, support important recreational and commercial fisheries and use a variety of habitats throughout ontogeny. Gray snapper juveniles may be found in estuarine nursery areas, such as seagrass beds, or mangrove shorelines, while adults are most often found in deep channels and farther offshore, associated with hard-bottom habitats. Juvenile and subadult gray snapper were collected from 1996 through 2009 during long-term fishery-independent monitoring of several estuarine systems along the Gulf and Atlantic coasts of Florida. Indices of abundance and habitat suitability were constructed for gray snapper to determine size-specific relationships between abundance, habitat, and environmental conditions. Juvenile and subadult gray snapper were collected year-round only in the southernmost estuaries but were most common from July through December in all estuaries sampled. In addition to timing of estuarine occupancy, abundance varied with latitude; gray snapper were more frequently collected in warmer, southern estuaries. In general, gray snapper were most abundant in euhaline areas with a high percentage of submerged aquatic vegetation (SAV) and, in most cases, where overhanging shoreline vegetation was also present. Annual abundance varied over the sampling period, with some juvenile peaks in abundance translating to subadult peaks in subsequent years. Although strong correspondence between juvenile and subadult populations was not observed in all systems, long-term, broad-scale habitat selection patterns as described in this study are critical to more effectively assess populations of estuarine-dependent species.  相似文献