Radio observations of Jupiter-family comets     

Jacques Crovisier  Nicolas Biver  Pierre Colom 《Planetary and Space Science》2009,57(10):1162-1174

Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us: (i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); (ii) to investigate the chemical composition of comets; (iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei.Comets are classified from their orbital characteristics into two separate classes: (i) nearly isotropic, mainly long-period comets and (ii) ecliptic, short-period comets, the so-called Jupiter-family comets (JFCs). These two classes apparently come from two different reservoirs, respectively, the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and—possibly—their different origin, they may have different chemical and physical properties that are worth being investigated.The present article reviews the contribution of radio observations to our knowledge of the JFCs. The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, JFCs are more easily accessed by space missions. However, unique opportunities to observe JFCs are offered when these objects come by chance close to the Earth (like 73P/Schwassmann-Wachmann 3 in 2006), or when they exhibit unexpected outbursts (as did 17P/Holmes in 2007).About a dozen JFCs were successfully observed by radio techniques up to now. Four to ten molecules were detected in five of them. No obvious evidence for different properties between JFCs and other families of comets is found, as far as radio observations are concerned.  相似文献   

The Chemical Diversity of Comets: Synergies Between Space Exploration and Ground-based Radio Observations     

Jacques Crovisier  Nicolas Biver  Dominique Bockelée-Morvan  Jérémie Boissier  Pierre Colom  Dariusz C. Lis 《Earth, Moon, and Planets》2009,105(2-4):267-272

A fundamental question in cometary science is whether the different dynamical classes of comets have different chemical compositions, which would reflect different initial conditions. From the ground or Earth orbit, radio and infrared spectroscopic observations of a now significant sample of comets indeed reveal deep differences in the relative abundances of cometary ices. However, no obvious correlation with dynamical classes is found. Further results come, or are expected, from space exploration. Such investigations, by nature limited to a small number of objects, are unfortunately focussed on short-period comets (mainly Jupiter-family). But these in situ studies provide “ground truth” for remote sensing. We discuss the chemical differences in comets from our database of spectroscopic radio observations, which has been recently enriched by several Jupiter-family and Halley-type comets.  相似文献   

The mass–metallicity relation in galaxy clusters: the relative importance of cluster membership versus local environment     

Sara L. Ellison  Luc Simard  Nicolas B. Cowan  Ivan K. Baldry  David R. Patton  Alan W. McConnachie 《Monthly notices of the Royal Astronomical Society》2009,396(3):1257-1272

Using a large (14 857), homogenously selected sample of cluster galaxies identified in the Sloan Digital Sky Survey Data Release 4, we investigate the impact of cluster membership and local density on the stellar mass–gas phase metallicity relation (MZR). We show that stellar metallicities are not suitable for this work, being relatively insensitive to subtle changes in the MZR. Accurate nebular abundances can be obtained for 1318 cluster galaxies in our sample and we show that these galaxies are drawn from clusters that are fully representative of the parent sample in terms of mass, size, velocity dispersion and richness. By comparing the MZR of the cluster galaxies with a sample of control galaxies matched in mass, redshift, fibre covering fraction and rest-frame   g − r   colour cluster galaxies are found to have, on average, higher metallicities by up to 0.04 dex. The magnitude of this offset does not depend strongly on galactic half-light radius or cluster properties such as velocity dispersion or cluster mass. The effect of local density on the MZR is investigated, using the presence of a near neighbour and both two- and three-dimensional density estimators. For all three metrics, it is found that the cluster galaxies in locally rich environments have higher median metallicities by up to ∼0.05 dex than those in locally poor environments (or without a near neighbour). Control (non-cluster) galaxies at locally high densities exhibit similar metal enhancements. Taken together, these results show that galaxies in clusters are, on average, slightly more metal rich than the field, but that this effect is driven by local overdensity and not simply cluster membership.  相似文献   

Detailed study of an hydrological system of valleys, a delta and lakes in the Southwest Thaumasia region, Mars     

Nicolas Mangold  Véronique Ansan 《Icarus》2006,180(1):75-87

The occurrence of fluvial activity and standing bodies of water on early Mars is the subject of debate. Using MOC, MOLA, and THEMIS data, we identify a whole set of landforms in the Thaumasia region which attest to water flows during geologically long periods of more than thousand years. A thick fan-delta is identified within an impact crater at the outlet of a deep valley. Ponded water filled and overflowed this crater's rim, creating entrance and exit breaches and an outlet valley. These landforms show that the 25-km diameter impact crater contained a lake up to 600 m deep. At the head of this crater's deep contributing valley, a closed depression may have contained another lake, but depositional landforms are not evident in this headward basin. Alternatively, groundwater discharge may have supplied the valley, but the observed landforms are not consistent with a sudden release of water, as is usually invoked for the large martian outflows channels. Stratigraphic relationships show that this hydrological activity occurred during the Hesperian period, thus relatively late in the history of martian valley network development.  相似文献   

Centimeter Accuracy for the French Transportable Laser Ranging Station (FTLRS) through Sub-System Controls     

J. Nicolas  F. Pierron  E. Samain  F. Barlier 《Surveys in Geophysics》2001,22(5-6):449-464

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Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron     

Endale Tamrat  Nicolas Thouveny  Maurice Taieb 《Studia Geophysica et Geodaetica》1996,40(3):313-335

Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α₉₅=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing⁴⁰Ar/³⁹Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.  相似文献   

The whaling issue: Conservation,confusion, and casuistry     

Phillip J. Clapham  Simon Childerhouse  Nicolas J. Gales  Lorenzo Rojas-Bracho  Michael F. Tillman  Robert L. Brownell Jr. 《Marine Policy》2007

Morishita's “multiple analysis” of the whaling issue [Morishita J. Multiple analysis of the whaling issue: Understanding the dispute by a matrix. Marine Policy 2006;30:802–8] is essentially a restatement of the Government of Japan's whaling policy, which confuses the issue through selective use of data, unsubstantiated facts, and the vilification of opposing perspectives. Here, we deconstruct the major problems with Morishita's article and provide an alternative view of the whaling dispute. For many people in this debate, the issue is not that some whales are not abundant, but that the whaling industry cannot be trusted to regulate itself or to honestly assess the status of potentially exploitable populations. This suspicion has its origin in Japan's poor use of science, its often implausible stock assessments, its insistence that culling is an appropriate way to manage marine mammal populations, and its relatively recent falsification of whaling and fisheries catch data combined with a refusal to accept true transparency in catch and market monitoring. Japanese policy on whaling cannot be viewed in isolation, but is part of a larger framework involving a perceived right to secure unlimited access to global marine resources. Whaling is inextricably tied to the international fisheries agreements on which Japan is strongly dependent; thus, concessions made at the IWC would have potentially serious ramifications in other fora.  相似文献   

A review of present techniques and methodological advances in analyzing Th in aquatic systems     

Michiel Rutgers van der Loeff  Manmohan M. Sarin  Mark Baskaran  Claudia Benitez-Nelson  Ken O. Buesseler  Matt Charette  Minhan Dai   rjan Gustafsson  Pere Masque  Paul J. Morris  Kent Orlandini  Alessia Rodriguez y Baena  Nicolas Savoye  Sabine Schmidt  Robert Turnewitsch  Ingrid Vge  James T. Waples 《Marine Chemistry》2006,100(3-4):190

The short-lived thorium isotope ²³⁴Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of ²³⁴Th in both marine and freshwater systems. The original ²³⁴Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of ²³⁴Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate ²³⁴Th analytical technique for a specific aquatic regime and known sampling constraints.  相似文献   

Modelling bathymetry by inverting satellite altimetry data: A review   总被引：3，自引：0，他引：3  

Stephane Calmant  Nicolas Baudry 《Marine Geophysical Researches》1996,18(2-4):123-134

With the advent of satellite altimetry it has become possible to determine the gravity field of the oceans on a global scale. This set of data can be used to predict the bathymetry of deep-seafloor features such as seamounts and ridges. During the last two decades, several algorithms which can be used to develop bathymetric predictions from satellite altimeter data have been published. The characteristics and quality of these algorithms are reviewed in this study. Based on this analysis, we suggest some guidelines for processing data towards the production of maps showing predicted bathymetry for economical purposes.  相似文献   

Spatial considerations for the Dakhla stock of Octopus vulgaris: indicators, patterns, and fisheries interactions     

Faraj  Abdelmalek; Bez  Nicolas 《ICES Journal of Marine Science》2007,64(9):1820-1828

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