Numerical simulations of impacts involving porous bodies: II. Comparison with laboratory experiments     

Martin Jutzi  Patrick Michel  Akiko M. Nakamura 《Icarus》2009,201(2):802-813

In this paper, we compare the outcome of high-velocity impact experiments on porous targets, composed of pumice, with the results of simulations by a 3D SPH hydrocode in which a porosity model has been implemented. The different populations of small bodies of our Solar System are believed to be composed, at least partially, of objects with a high degree of porosity. To describe the fragmentation of such porous objects, a different model is needed than that used for non-porous bodies. In the case of porous bodies, the impact process is not only driven by the presence of cracks which propagate when a stress threshold is reached, it is also influenced by the crushing of pores and compaction. Such processes can greatly affect the whole body's response to an impact. Therefore, another physical model is necessary to improve our understanding of the collisional process involving porous bodies. Such a model has been developed recently and introduced successfully in a 3D SPH hydrocode [Jutzi, M., Benz, W., Michel, P., 2008. Icarus 198, 242-255]. Basic tests have been performed which already showed that it is implemented in a consistent way and that theoretical solutions are well reproduced. However, its full validation requires that it is also capable of reproducing the results of real laboratory impact experiments. Here we present simulations of laboratory experiments on pumice targets for which several of the main material properties have been measured. We show that using the measured material properties and keeping the remaining free parameters fixed, our numerical model is able to reproduce the outcome of these experiments carried out under different impact conditions. This first complete validation of our model, which will be tested for other porous materials in the future, allows us to start addressing problems at larger scale related to small bodies of our Solar System, such as collisions in the Kuiper Belt or the formation of a family by the disruption of a porous parent body in the main asteroid belt.  相似文献   

On measuring the gravitational-wave background using Pulsar Timing Arrays     

Rutger van Haasteren  Yuri Levin  Patrick McDonald  Tingting Lu 《Monthly notices of the Royal Astronomical Society》2009,395(2):1005-1014

The long-term precise timing of Galactic millisecond pulsars holds great promise for measuring the long-period (months to years) astrophysical gravitational waves. Several gravitational-wave observational programs, called Pulsar Timing Arrays (PTA), are being pursued around the world.
Here, we develop a Bayesian algorithm for measuring the stochastic gravitational-wave background (GWB) from the PTA data. Our algorithm has several strengths: (i) it analyses the data without any loss of information; (ii) it trivially removes systematic errors of known functional form, including quadratic pulsar spin-down, annual modulations and jumps due to a change of equipment; (iii) it measures simultaneously both the amplitude and the slope of the GWB spectrum and (iv) it can deal with unevenly sampled data and coloured pulsar noise spectra. We sample the likelihood function using Markov Chain Monte Carlo simulations. We extensively test our approach on mock PTA data sets and find that the algorithm has significant benefits over currently proposed counterparts. We show the importance of characterizing all red noise components in pulsar timing noise by demonstrating that the presence of a red component would significantly hinder the detection of the GWB.
Lastly, we explore the dependence of the signal-to-noise ratio on the duration of the experiment, number of monitored pulsars and the magnitude of the pulsar timing noise. These parameter studies will help formulate observing strategies for the PTA experiments.  相似文献   

The Circum-Hellas Volcanic Province,Mars: Overview     

David A. Williams  Ronald Greeley  Robin L. Fergason  Ruslan Kuzmin  Thomas B. McCord  Jean-Phillipe Combe  James W. Head  Long Xiao  Leon Manfredi  François Poulet  Patrick Pinet  David Baratoux  Jeffrey J. Plaut  Jouko Raitala  Gerhard Neukum 《Planetary and Space Science》2009,57(8-9):895-916

Building on previous studies of volcanoes around the Hellas basin with new studies of imaging (High-Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), Mars Orbiter Camera (MOC), High-Resolution Imaging Science Experiment (HiRISE), Context Imager (CTX)), multispectral (HRSC, Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA)), topographic (Mars Orbiter Laser Altimeter (MOLA)) and gravity data, we define a new Martian volcanic province as the Circum-Hellas Volcanic Province (CHVP). With an area of >2.1 million km², it contains the six oldest central vent volcanoes on Mars, which formed after the Hellas impact basin, between 4.0 and 3.6 Ga. These volcanoes mark a transition from the flood volcanism that formed Malea Planum ～3.8 Ga, to localized edifice-building eruptions. The CHVP volcanoes have two general morphologies: (1) shield-like edifices (Tyrrhena, Hadriaca, and Amphitrites Paterae), and (2) caldera-like depressions surrounded by ridged plains (Peneus, Malea, and Pityusa Paterae). Positive gravity anomalies are found at Tyrrhena, Hadriaca, and Amphitrites, perhaps indicative of dense magma bodies below the surface. The lack of positive-relief edifices and weak gravity anomalies at Peneus, Malea, and Pityusa suggest a fundamental difference in their formation, styles of eruption, and/or compositions. The northernmost volcanoes, the ～3.7–3.9 Ga Tyrrhena and Hadriaca Paterae, have low slopes, well-channeled flanks, and smooth caldera floors (at tens of meters/pixel scale), indicative of volcanoes formed from poorly consolidated pyroclastic deposits that have been modified by fluvial and aeolian erosion and deposition. The ～3.6 Ga Amphitrites Patera also has a well-channeled flank, but it and the ～3.8 Ga Peneus Patera are dominated by scalloped and pitted terrain, pedestal and ejecta flow craters, and a general ‘softened’ appearance. This morphology is indicative not only of surface materials subjected to periglacial processes involving water ice, but also of a surface composed of easily eroded materials such as ash and dust. The southernmost volcanoes, the ～3.8 Ga Malea and Pityusa Paterae, have no channeled flanks, no scalloped and pitted terrain, and lack the ‘softened’ appearance of their surfaces, but they do contain pedestal and ejecta flow craters and large, smooth, bright plateaus in their central depressions. This morphology is indicative of a surface with not only a high water ice content, but also a more consolidated material that is less susceptible to degradation (relative to the other four volcanoes). We suggest that Malea and Pityusa (and possibly Peneus) Paterae are Martian equivalents to Earth's giant calderas (e.g., Yellowstone, Long Valley) that erupted large volumes of volcanic materials, and that Malea and Pityusa are probably composed of either lava flows or ignimbrites. HRSC and OMEGA spectral data indicate that dark gray to slightly red materials (often represented as blue or black pixels in HRSC color images), found in the patera floors and topographic lows throughout the CHVP, have a basaltic composition. A key issue is whether this dark material represents concentrations of underlying basaltic material eroded by various processes and exposed by aeolian winnowing, or if the material was transported from elsewhere on Mars by regional winds. Understanding the provenance of these dark materials may be the key to understanding the volcanic diversity of the Circum-Hellas Volcanic Province.  相似文献   

Cross-scale: multi-scale coupling in space plasmas     

Steven J. Schwartz  Timothy Horbury  Christopher Owen  Wolfgang Baumjohann  Rumi Nakamura  Patrick Canu  Alain Roux  Fouad Sahraoui  Philippe Louarn  Jean-André Sauvaud  Jean-Louis Pinçon  Andris Vaivads  Maria Federica Marcucci  Anastasios Anastasiadis  Masaki Fujimoto  Philippe Escoubet  Matt Taylor  Steven Eckersley  Elie Allouis  Marie-Claire Perkinson 《Experimental Astronomy》2009,23(3):1001-1015

Most of the visible universe is in the highly ionised plasma state, and most of that plasma is collision-free. Three physical phenomena are responsible for nearly all of the processes that accelerate particles, transport material and energy, and mediate flows in systems as diverse as radio galaxy jets and supernovae explosions through to solar flares and planetary magnetospheres. These processes in turn result from the coupling amongst phenomena at macroscopic fluid scales, smaller ion scales, and down to electron scales. Cross-Scale, in concert with its sister mission SCOPE (to be provided by the Japan Aerospace Exploration Agency—JAXA), is dedicated to quantifying that nonlinear, time-varying coupling via the simultaneous in-situ observations of space plasmas performed by a fleet of 12 spacecraft in near-Earth orbit. Cross-Scale has been selected for the Assessment Phase of Cosmic Vision by the European Space Agency.   相似文献   

The formation of the Baptistina family by catastrophic disruption: Porous versus non‐porous parent body     

Martin Jutzi  Patrick Michel  Willy Benz  Derek C. Richardson 《Meteoritics & planetary science》2009,44(12):1877-1887

Abstract— In this paper, we present numerical simulations aimed at reproducing the Baptistina family based on its properties estimated by observations. A previous study by Bottke et al. (2007) indicated that this family is probably at the origin of the K/T impactor, is linked to the CM meteorites and was produced by the disruption of a parent body 170 km in size due to the head‐on impact of a projectile 60 km in size at 3 km s^?1. This estimate was based on simulations of fragmentation of non‐porous materials, while the family was assumed to be of C taxonomic type, which is generally interpreted as being formed from a porous body. Using both a model of fragmentation of non‐porous materials, and a model that we developed recently for porous ones, we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models. Our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head‐on impact of a projectile 54 km in size at 3 km s^?1. Thus, our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated, but the difference remains small enough to not affect the proposed scenario of this family history. We then find that the break‐up of a porous body leads to different outcomes than the disruption of a non‐porous one. The real properties of the Baptistina family still contain large uncertainties, and it remains possible that its formation did not involve the proposed impact conditions. However, the simulations presented here already show some range of outcomes and once the real properties are better constrained, it will be easy to check whether one of them provides a good match.  相似文献   

Internal vein texture and vein evolution of the epithermal Shila-Paula district,southern Peru     

Alain?ChauvetEmail author  Laurent?Bailly  Anne-Sylvie?André  Patrick?Monié  Daniel?Cassard  Fernando?Llosa?Tajada  Juan?Rosas?Vargas  Johann?Tuduri 《Mineralium Deposita》2006,41(4):387-410

The epithermal Shila-Paula Au–Ag district is characterized by numerous veins hosted in Tertiary volcanic rocks of the Western Cordillera (southern Peru). Field studies of the ore bodies reveal a systematic association of a main E–W vein with secondary N55–60°W veins—two directions that are also reflected by the orientation of fluid-inclusion planes in quartz crystals of the host rock. In areas where this pattern is not recognized, such as the Apacheta sector, vein emplacement seems to have been guided by regional N40°E and N40°W fractures. Two main vein-filling stages are identified. stage 1 is a quartz–adularia–pyrite–galena–sphalerite–chalcopyrite–electrum–Mn silicate–carbonate assemblage that fills the main E–W veins. stage 2, which contains most of the precious-metal mineralization, is divided into pre-bonanza and bonanza substages. The pre-bonanza substage consists of a quartz–adularia–carbonate assemblage that is observed within the secondary N45–60°W veins, in veinlets that cut the stage 1 assemblage, and in final open-space fillings. The two latter structures are finally filled by the bonanza substage characterized by a Fe-poor sphalerite–chalcopyrite–pyrite–galena–tennantite–tetrahedrite–polybasite–pearceite–electrum assemblage. The ore in the main veins is systematically brecciated, whereas the ore in the secondary veins and geodes is characteristic of open-space crystallization. Microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15.5 wt% NaCl equivalent and homogenization temperatures bracketed between 200 and 330°C. Secondary CO₂-, N₂- and H₂S-bearing fluid inclusions are also identified. The age of vein emplacement, based on ⁴⁰Ar/³⁹Ar ages obtained on adularia of different veins, is estimated at around 11 Ma, with some overlap between adularia of stage 1 (11.4±0.4 Ma) and of stage 2 (10.8±0.3 Ma). A three-phase tectonic model has been constructed to explain the vein formation. Phase 1 corresponds to the assumed development of E–W sinistral shear zones and associated N60°W cleavages under the effects of a NE–SW shortening direction that is recognized at Andean scale. These structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation. Phase 2 is a reactivation of earlier structures under a NW–SE shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce N50°E-striking S2-cleavage planes filled by the stage 2 pre-bonanza minerals. Phase 3 coincides with the bonanza ore emplacement in the secondary N45–60°W veins and also in open-space in the core of the main E–W veins. Our combined tectonic, textural, mineralogical, fluid-inclusion, and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation.  相似文献   

Misperceptions of climate-change risk as barriers to climate-change adaptation: a case study from the Rewa Delta, Fiji   总被引：2，自引：0，他引：2  

Shalini Lata  Patrick Nunn 《Climatic change》2012,110(1-2):169-186

While increasing research is focusing on the effective adaptation to climate change in richer (developed) countries, comparatively little has focused specifically on this subject in poorer (developing) countries such as most in the Pacific Islands region. A significant barrier to the development of effective and sustainable adaptive strategies for climate change in such places is the gap between risk and perceived risk. This study looks at a vulnerable location in Fiji—the densely populated Rewa River Delta where environmental changes resulting from shoreline retreat and floods are expected to increase over the next few decades and entail profound societal disruption. The numbers of people living in the Rewa Delta who know of climate change and could correctly identify its contributory causes are few although many rank its current manifestations (floods, riverbank erosion, groundwater salinization) as among their most serious environmental challenges. While lack of awareness is a barrier to adaptation, there are also cultural impediments to this such as short-term planning perspectives, spiritual beliefs, traditional governance structures. One way forward is to empower community leaders in places like the Rewa Delta to make appropriate decisions and for regional governments to continue working together to find solutions that acknowledge the variation in sub-regional trans-national vulnerability to climate change.  相似文献   

Dextral-Slip Thrust Faulting and Seismic Events of the Ms 8.0 Wenchuan Earthquake,Longmenshan Mountains,Eastern Margin of the Tibetan Plateau     

Patrick J.BAROSH 《《地质学报》英文版》2009,83(4)

Dextral-slip thrust movement of the Songpan-Garze terrain over the Sichuan block caused the Ms 8.0 Wenchuan earthquake of May 12, 2008 and offset the Central Longmenshan Fault (CLF) along a distance of-250 km. Displacement along the CLF changes from Yingxiu to Qingchuan. The total oblique slip of up to 7.6 m in Yingxiu near the epicenter of the earthquake, decreases northeastward to 5.3 m, 6.6 m, 4.4 m, 2.5 m and 1.1 m in Hongkou, Beichuan, Pingtong, Nanba and Qingchuan, respectively. This offset apparently occurred during a sequence of four reported seismic events, EQ1-EQ4, which were identified by seismic inversion of the source mechanism. These events occurred in rapid succession as the fault break propagated northeastward during the earthquake. Variations in the plunge of slickensides along the CLF appear to match these events. The Mw 7.5 EQ1 event occurred during the first 0-10 s along the Yingxiu-Hongkou section of the CLF and is characterized by 1.7 m vertical slip and vertical slickensides. The Mw 8.0 EQ2 event, which occurred during the next 10-42 s along the Yingxiu-Yanziyan section of the CLF, is marked by major dextral-slip with minor thrust and slickensides plunging 25°-35° southwestward. The Mw 7.5 EQ3 event occurred during the following 42-60 s and resulted in dextral-slip and slickensides plunging 10° southwestward in Beichuan and plunging 73° southwestward in Hongkou. The Mw 7.7 EQ4 event, which occurred during the final 60-95 s along the Beichuan-Qingchuan section of the CLF, is characterized by nearly equal values of dextral and vertical slips with slickensides plunging 45°-50° southwestward. These seismic events match and evidently controlled the concentrations of landslide dams caused by the Wenchuan earthquake in Longmenshan Mountains.  相似文献   

Structural reworking and heat transfer related to the late-Panafrican Angavo shear zone of Madagascar     

Vincent Grgoire  Anne Ndlec  Patrick Moni  Jean-Marc Montel  Jrme Ganne  Bruno Ralison 《Tectonophysics》2009,477(3-4):197

The basement of central Madagascar displays two contrasted structural patterns. The first one (D1) is characterized by north-striking foliations that are gently dipping to the west and carry W- to WSW-plunging lineations, whereas the second one (D2) is characterized by steeper foliations that are striking to the NNE and lineations that are either subhorizontal or gently plunging to the SSW. The younger pattern is related to late-Panafrican tectonics along the major Angavo shear zone that is about 1000 km in length and 40 km in width with apparently little offset. Deformation in the Angavo zone induced interference folding on both sides. The D2 event is characterized by low pressures (ca 400 MPa) and high temperatures (up to 790 °C) responsible for prograde granulite facies conditions, that resulted from heat transfer due to magma and fluid advection in the Angavo shear zone. The D2 event is pinned at 550 ± 11 Ma by a new monazite age from a reoriented Andringitrean granite near Ankaramena. A new suite of amphibole and biotite Ar–Ar geochronological data enables to retrace the thermochronogical evolution inside and outside the Angavo shear zone. Combined with new structural results from the western interference zone, these ages yield a better understanding of the late-Panafrican history of central Madagascar. No diachronism is observed along the strike of the Angavo shear zone. Conversely, amphibole and especially biotite ages decrease from West to East, i.e. towards the shear zone. These new ages range from 511 to 469 Ma. A 1-D conductive model constrains the thermal effect in relation with the Angavo shear zone to be restricted to a lateral distance of ca 60 km (in map view) for a maximum heating duration of 20 Myr. This is in agreement with the Ar–Ar data and with the width of the observed interference zone. Following this episode of deformation and heat transfer, the estimated cooling rates of the Angavo shear zone range from to 15 to 6 °C/Myr, respectively before and after 515 Ma. A post-collision intracontinental setting is suggested for the Angavo shear zone, which is regarded as a remote effect of the Kuunga Orogeny between India and Antarctica.  相似文献   

Direct Fourier transform mass spectral analysis of natural waters with low dissolved organic matter     

Rachel L. Sleighter  Georgina A. McKee  Patrick G. Hatcher 《Organic Geochemistry》2009,40(1):119-125

A major obstacle for characterizing dissolved organic matter (DOM) with ultrahigh resolution mass spectrometry has been its low concentrations in natural waters. Many previous mass spectrometric studies of both terrestrial and marine DOM typically have isolated and concentrated the DOM using solid phase C₁₈ extraction disks, ultrafiltration, or XAD resins, all of which are known to discriminate against many different classes of compounds. We have, for the first time, developed an approach to directly analyze natural water samples with less than 6 mg/l DOC (dissolved organic carbon), using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). We demonstrate the sensitivity and ability of sequential selective ion accumulation (SSIA) to detect the thousands of components in a single freshwater DOM sample without any significant pretreatment. By utilizing SSIA, the baseline noise decreases while signal to noise ratios of the peaks increase, allowing for approximately 40% more formulas to be assigned to peaks in the mass spectra.  相似文献