Long-Period Comets and the Oort Cloud     

Fernández  Julio A. 《Earth, Moon, and Planets》2000,89(1-4):325-343

Long-period (LP) comets, Halley-type (HT) comets, and even some comets of the Jupiter family, probably come from the Oort cloud, a huge reservoir of icy bodies that surrounds the solar system. Therefore, these comets become important probes to learn about the distant Oort cloud population. We review the fundamental dynamical properties of LP comets, and what is our current understanding of the dynamical mechanisms that bring these bodies from the distant Oort cloud region to the inner planetary region. Most new comets have original reciprocal semimajor axes in the range2 × 10^-5 < 1/a_orig < 5 × 10^-5AU^-1. Yet, this cannot be taken to represent the actual space distribution of Oort cloud comets, but only the region in the energy space in which external perturbers have the greatest efficiency in bringing comets to the inner planetary region. The flux of Oort cloud comets in the outer planetary region is found to be at least several tens times greater than the flux in the inner planetary region. The sharp decrease closer to the Sun is due to the powerful gravitational fields of Jupiter and Saturn that prevent most Oort cloud comets from reaching the Earth’s neighborhood (they act as a dynamical barrier). A small fraction of ～10^-2 Oort cloud comets become Halley type (orbital periods P < 200 yr), and some of them can reach short-period orbits with P < 20 yr. We analyze whether we can distinguish the latter, very ‘old” LP comets, from comets of the Jupier family coming from the Edgeworth-Kuiper belt.  相似文献   

Self-similar shocked accretion of collisional gas with radiative cooling     

Mario G. Abadi  Richard G. Bower  Julio F. Navarro 《Monthly notices of the Royal Astronomical Society》2000,314(4):759-767

We describe similarity solutions that characterize the collapse of collisional gas on to scale-free perturbations in an Einstein–de Sitter universe. We consider the effects of radiative cooling and derive self-similar solutions under the assumption that the cooling function is a power law of density and temperature, Λ( T , ρ )∝ ρ ^3/2 T . We use these results to test the ability of smooth particle hydrodynamics (SPH) techniques to follow the collapse and accretion of shocked, rapidly cooling gas in a cosmological context. Our SPH code reproduces the analytical results very well in cases that include or exclude radiative cooling. No substantial deviations from the predicted central mass accretion rates or from the temperature, density and velocity profiles are observed in well-resolved regions inside the shock radius. This test problem lends support to the reliability of SPH techniques to model the complex process of galaxy formation.  相似文献   

The mesoscale variability in the Caribbean Sea. Part I: Simulations and characteristics with an embedded model     

Julien Jouanno  Julio Sheinbaum  Bernard Barnier  Jean-Marc Molines  Laurent Debreu  Florian Lemari 《Ocean Modelling》2008,23(3-4):82-101

The variability in the Caribbean Sea is investigated using high resolution (1/15°) general circulation model experiments. For the first time in this region, simulations were carried out with a 2-way nested configuration of the NEMO primitive equation model. A coarse North Atlantic grid (1/3°) reproduces the main features of the North Atlantic and Equatorial circulation capable of influencing ocean dynamics in the Caribbean Sea. This numerical study highlights strong dynamical differences among basins and modifies the view that dynamics are homogeneous over the whole Caribbean Basin. The Caribbean mean flow is shown to organize in two intense jets flowing westward along the northern and southern boundaries of the Venezuela Basin, which merge in the center of the Colombia Basin. Diagnostics of model outputs show that width, depth and strength of baroclinic eddies increase westward from the Lesser Antilles to the Colombia Basin. The widening and strengthening to the west is consistent with altimetry data and drifter observations. Although influenced by the circulation in the Colombia Basin, the variability in the Cayman Basin (which also presents a westward growth from the Chibcha Channel) is deeper and less energetic than the variability in the Colombia/Venezuela Basins. Main frequency peaks for the mesoscale variability present a westward shift, from roughly 50 days near the Lesser Antilles to 100 days in the Cayman Basin, which is associated with growth and merging of eddies.  相似文献   

Dynamical capture and physical decay of short-period comets     

Julio A. Fernández 《Icarus》1985,64(2):308-319

The brightness evolution of short-period comets is discussed in connection with their physical lifetimes. It is shown that changes in the fraction of the free-subliming area of the nuclear surface may be more important than mass decrease in determining brightness variations. The decrease in the activity of short-period comets caused by the buildup of a dust mantle may be interrupted—and partially reversed—by dust blowoffs that leave exposed areas of fresh ices. Short-period comets may thus be subject to random brightness fluctuations that make quite uncertain any derivation of their physical lifetime based on comparisons of their absolute brightness at different apparitions. As an alternate procedure, the numerical integration of the whole sample of short-period comet orbits carried out by A. Carusi, L.Kresák, E. Perozzi and G. B. Valsecchi (1984, Long-Term Evolution of Short-Period Comets. Istituto Astrofisica Spaziale Internal Report 12, Rome) is used to draw conclusions about the transfer rate of their perihelia from Jupiter's region to the region of the terrestrial planets (heliocentric distances<1.5 AU). It is found that about one short-period comet per century reaches the region of the terrestrial planets. From this result and under the assumption of a steady-state comet population, an average lifetime of the order of 6 × 10³ years (～10³ revolutions) is derived for a typical kilometer-sized short-period comet of perihelion distance q ～ 1 AU. Such a rather long comet lifetime, as compared to some previous derivations, is consistent with the survival of some periodic comets on small-q orbits of long dynamical time scales.  相似文献   

Time relationships between volcanism–plutonism–alteration–mineralization in Cu-stratabound ore deposits from the Michilla mining district,northern Chile: a <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronological approach     

Verónica Oliveros  Dania Tristá-Aguilera  Gilbert Féraud  Diego Morata  Luis Aguirre  Shoji Kojima  Fernando Ferraris 《Mineralium Deposita》2008,43(1):61-78

The Michilla mining district comprises one of the most important stratabound and breccia-style copper deposits of the Coastal Cordillera of northern Chile, hosted by the Middle Jurassic volcanic rocks of the La Negra Formation. ⁴⁰Ar/³⁹Ar analyses carried out on igneous and alteration minerals from volcanic and plutonic rocks in the district allow a chronological sequence of several magmatic and alteration events of the district to be established. The first event was the extrusion of a thick lava series of the La Negra Formation, dated at 159.9 ± 1.0 Ma (2σ) from the upper part of the series. A contemporaneous intrusion is dated at 159.6 ± 1.1 Ma, and later intrusive events are dated at 145.5 ± 2.8 and 137.4 ± 1.1 Ma, respectively. Analyzed alteration minerals such as adularia, sericite, and actinolite apparently give valid ⁴⁰Ar/³⁹Ar plateau and miniplateau ages. They indicate the occurrence of several alteration events at ca. 160–163, 154–157, 143–148, and 135–137 Ma. The first alteration event, being partly contemporaneous with volcanic and plutonic rocks, was probably produced in a high thermal gradient environment. The later events may be related either to a regional low-grade hydrothermal alteration/metamorphism process or to plutonic intrusions. The Cu mineralization of the Michilla district is robustly bracketed between 163.6 ± 1.9 and 137.4 ± 1.1 Ma, corresponding to dating of actinolite coexisting with early-stage chalcocite and a postmineralization barren dyke, respectively. More precisely, the association of small intrusives (a dated stock from the Michilla district) with Cu mineralization in the region strongly suggests that the main Michilla ore deposit is related to a magmatic/hydrothermal event that occurred between 157.4 ± 3.6 and 163.5 ± 1.9 Ma, contemporaneous or shortly after the extrusion of the volcanic sequence. This age is in agreement with the Re–Os age of 159 ± 16 Ma obtained from the mineralization itself (Tristá-Aguilera et al., Miner Depos, 41:99–105,2006).  相似文献   

Architecture and recognition criteria of ancient shelf ridges; an example from Campanian Almond Formation in Hanna Basin,USA          下载免费PDF全文

Julio Leva López  Valentina Marzia Rossi  Cornel Olariu  Ronald J. Steel 《Sedimentology》2016,63(6):1651-1676

Shelf ridges are sedimentary bodies formed on the continental shelf due to transgressive reworking (tidal or storm) of lowstand deposits. Common on modern shelves, they are under‐represented in the geological record due to a lack of recognition criteria and facies model. This article proposes a new facies and architectural model for shelf ridges, linked to their inception–evolution–abandonment cycle and the process regime of the basin. The model is mainly based on new outcrop data and interpretations from three sandstone bodies of the Almond Formation, an overall transgressive interval during the infill of the Campanian Western Interior Seaway. Building from the case study, and ancient and modern examples, six characteristics are proposed for the recognition of ancient shelf ridges. Shelf ridges: (i) are encased between thick marine mudstone intervals; (ii) have a basal unconformity that erodes into marine muds or into the remnants of a previous shoreline; (iii) have a non‐erosional upper boundary that transitions into marine muds; (iv) are characterized by clean and well‐sorted sandstones, often cross‐bedded; (v) contain fully marine ichnofauna; and (vi) present compound architectures with large accretion surfaces and lower order structures. Although shelf ridges have been described in previous studies as generated exclusively by either tidal or storm currents, it is clear, from modern examples and the case study, that these two processes can be recorded and preserved in a single shelf ridge. The stratigraphy of these sandstone bodies is therefore much more complex than previously recognized, bearing the signature of changing tidal and storm intensity through time. Because they are developed during transgressions, shelf ridges are commonly subject to strong changes in process regime as sea‐level changes can easily affect the oceanographic conditions and the morphology of the basin. For this reason, shelf ridges can provide the best record of shelf process variability during transgressions.  相似文献   

The Capilla del Monte pluton,Sierras de Córdoba,Argentina: the easternmost Early Carboniferous magmatism in the pre-Andean SW Gondwana margin     

Juan A. Dahlquist  Robert J. Pankhurst  Carlos W. Rapela  Miguel A. S. Basei  Pablo H. Alasino  Julio Saavedra  Edgardo G. Baldo  Juan A. Murra  Mario da Costa Campos Neto 《International Journal of Earth Sciences》2016,105(5):1287-1305

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Isotopic and Elemental Composition of Marine Macrophytes as Biotracers of Nutrient Recycling Within a Coastal Lagoon in Baja California,Mexico     

José?D.?CarriquiryEmail author  Pablo?Jorgensen  Julio?A.?Villaescusa  Silvia?E.?Ibarra-Obando 《Estuaries and Coasts》2016,39(2):451-461

Nutrient sources of San Quintin Bay, a coastal lagoon affected by coastal upwelling off Baja California (Mexico), were traced using generalized additive (mixed) models (GAMM) to the stable nitrogen isotopic composition, C:N and N content of two co-occurring macrophytes (the macroalgae Ulva spp. and the seagrass Zostera marina). The geochemical tracers followed a spatial trend that partly responded to the long-term nutrient gradient from the ocean towards the interior of the bay. N content in Z. marina and Ulva spp. decreased linearly (while C:N increased) towards the middle section of the bay to concentration levels that indicate potential N limitation for growth. Concurrently midway into the bay (6–9 km), the δ¹⁵N of both macrophytes showed a gradual enrichment in ¹⁵N reflecting progressive denitrification. The spatial pattern of δ¹⁵N and the decrease in C:N of the macrophytes towards the innermost section of the bay indicated an additional nonoceanic source of dissolved nitrogen in this zone. The similarity of the δ¹⁵N pattern of Z. marina and Ulva spp. implies that their δ¹⁵N composition is mainly controlled by the availability of N, in spite of the physiological differences between taxa. A better fit of GAMM to N content and C:N was obtained for Z. marina than for Ulva spp. indicating that the former delineate more steadily and smoothly the influence of upwelling along the spatial gradient. Nonetheless, Ulva spp. may be analyzed in combination with Z. marina to characterize the environmental conditions at the time of sampling.  相似文献   

The coral communities of the Islas Marias archipelago,Mexico: structure and biogeographic relevance to the Eastern Pacific          下载免费PDF全文

Andrés López‐Pérez  Amilcar Cupul‐Magaña  Miguel Angel Ahumada‐Sempoal  Pedro Medina‐Rosas  Héctor Reyes‐Bonilla  María Dinorah Herrero‐Pérezrul  Cristóbal Reyes‐Hernández  Julio Lara‐Hernández 《Marine Ecology》2016,37(3):679-690

Various authors have suggested that the Islas Marias archipelago, Mexico, may play a significant biogeographic role in the dispersal of Indo Pacific coral species into the Eastern Pacific; however, the coral communities of this archipelago have received scarce attention to date. Here, we first addressed coral community structure across the islands and, by employing ordination analysis, minimum spanning tree and particle‐tracking experiments, used this information to evaluate the relevance of the archipelago for coral dispersal. Twenty‐four coral communities were recorded in the archipelago. Coral cover varied significantly among islands: Maria Cleofas had large values (38.5%), intermediate values were observed for Maria Madre (26.5%) and Maria Magdalena (22.84%), and relatively low values were recorded for San Juanito (18.5%). Coral communities mainly consisted of Pocillopora (57.3%) and Porites (25.5%) species, while species of Pavona (16%) and Psammocora (0.6%) made relatively minor contributions. Thirteen stony coral species were identified in the archipelago; of these Psammocora profundacella and Pavona duerdeni represent new records. Ordination analysis, minimum spanning tree and particle‐tracking experiments suggested similar connecting paths in the studied area; in general, the Islas Marias stands as a route for coral dispersal of Indo Pacific species into the Northeastern Pacific. In a regional context, the Islas Marias has three major biogeographic implications to coral dispersal: (i) the archipelago stands as a major stepping stone for the transport of species and individuals among the Revillagigedo archipelago, the Gulf of California and the tropical Mexican Pacific; (ii) the Islas Marias may play a seminal role in maintaining the genetic connectivity between southern and northern coral populations along the Mexican Pacific and (iii) because of its relatively pristine status and low levels of human impact, the archipelago may potentially serve as a source of coral propagules for ecosystem recovery in the Gulf of California and along the Pacific coast of the Mexican mainland following natural and/or human induced perturbations.  相似文献   

Evolution of comet orbits under the perturbing influence of the giant planets and nearby stars     

Julio A. Fernández 《Icarus》1980,42(3):406-421

The orbital evolution of 500 hypothetical comets during 10⁹ years is studied numerically. It is assumed that the birthplace of such comets was the region of Uranus and Neptune from where they were deflected into very elongated orbits by perturbations of these planets. Then, we adopted the following initial orbital elements: perihelion distances between 20 and 30 AU, inclinations to the ecliptic plane smaller than 20°, and semimajor axes from 5 × 10³ to 5 × 10⁴ AU. Gravitational perturbations by the four giant planets and by hypothetical stars passing at distances from the Sun smaller than 5 × 10⁵ AU are considered. During the simulation, somewhat more than 50% of the comets were lost from the solar system due to planetary or stellar perturbations. The survivors were removed from the planetary region and left as members of what is generally known as the cometary cloud. At the end of the studied period, the semimajor axes of the surviving comets tend to be concentrated in the interval 2 × 10⁴ < a < 3 × 10⁴ AU. The orbital planes of the comets with initial $\text{a ≧ 3 × 10}{}^4\text{AU}$ acquired a complete randomization while the others still maintain a slight predominance of direct orbits. In addition, comet orbits with final a < 6 × 10⁴AU preserve high eccentricities with an average value greater than 0.8 Most “new” comets from the sample entering the region interior to Jupiter's orbit had already registered earlier passages through the planetary region. By scaling up the rate of paritions of hypothetical new comets with the observed one, the number of members of the cometary cloud is estimated to be about 7 × 10¹⁰ and the conclusion is drawn that Uranus and Neptune had to remove a number of comets ten times greater.  相似文献