Catastrophic disruption of asteroids and family formation: a review of numerical simulations including both fragmentation and gravitational reaccumulations

In the last few years, thanks to the development of sophisticated numerical codes, a major breakthrough has been achieved in our understanding of the processes involved in small body collisions. In this review, we summarize the most recent results provided by numerical simulations, accounting for both the fragmentation of an asteroid and the gravitational interactions of the generated fragments. These studies have greatly improved our knowledge of the mechanisms that are at the origin of some observed features in the asteroid belt. In particular, the simulations have demonstrated that, for bodies larger than several kilometers, the collisional process not only involves the fragmentation of the asteroid but also the gravitational interactions between the ejected fragments. This latter mechanism can lead to the formation of large aggregates by gravitational reaccumulation of smaller fragments, and helps explain the presence of large members within asteroid families. Numerical simulations of the complete process have thus reproduced successfully for the first time the main properties of asteroid families, each formed by the disruption of a large parent body, and provided information on the possible internal structure of the parent bodies. A large amount of work remains necessary, however, to understand in deeper detail the physical process as a function of material properties and internal structures that are relevant to asteroids, and to determine in a more quantitative way the outcome properties such as fragment shapes and rotational states.     

From strike-slip faulting to oblique subduction: A survey of the Alpine Fault-Puysegur Trench transition,New Zealand,results of cruise Geodynz-sud leg 2

The Geodynz-sud cruise on board the R/V l'Atalante collected bathymetric, side-scan sonar and seismic reflection data along the obliquely convergent boundary between the Australian and Pacific plates southwest of the South Island, New Zealand. The survey area extended from 44°05 S to 49°40 S, covering the transition zone between the offshore extension of the Alpine Fault and the Puysegur Trench and Puysegur Ridge. Based on variations in the nature and structure of the crust on either side of the margin, the plate boundary zone can be divided into three domains with distinctive structural and sedimentary characteristics. The northern domain involves subduction of probably thinned continental crust of the southern Challenger Plateau beneath the continental crust of Fiordland. It is characterized by thick sediments on the downgoing slab and a steep continental slope disrupted by fault scarps and canyons. The middle domain marks the transition between subduction of likely continental and oceanic crust defined by a series of en echelon ridges on the downgoing slab. This domain is characterized by a large collapse terrace on the continental slope which appears to be due to the collision of the en echelon ridges with the plate margin. The southern domain involves subduction of oceanic crust beneath continental and oceanic crust. This domain is characterized by exposed fabric of seafloor spreading on the downgoing slab, a steep inner trench wall and linear ridges and valleys on the Puysegur ridge crest. The data collected on this cruise provide insights into the nature and history of both plates, and factors influencing the distribution of strike-slip and compressive strain and the evolution of subduction processes along a highly oblique convergent margin.     

Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions,Western Mediterranean): Linking the outer shelf and the basin sedimentary records

Emplacement of post-glacial turbidites is commonly controlled by rapid changes in sea level or by seismicity. On the continental rise of the Gulf of Lions (Western Mediterranean), an aseismic area, we identified turbiditic beds deposited during the rising stage and highstand of sea level. Swath bathymetry, sediment cores, in situ Cone Penetrating Tests (CPTU), heavy mineral associations and radiocarbon dating determined the source, composition, distribution and age of the turbiditic beds. Turbidites are composed of homogeneous to positively graded silts to medium sand with quartz (up to 90%), shell debris and shelfal benthic faunas. Their distribution on the sea floor is very patchy and controlled by abundant inherited erosional bedforms. Their source is found in relict regressive sands at the outershelf. Their deposition occurred just after the onset of the post-glacial sea level rise and the concomitant sediment starvation of the Rhône deep sea turbiditic system until recently. Whilst canyons are fed with sand by strong seasonal hydro-sedimentary dynamics on the outershelf, the emplacement of post-glacial turbidites is not controlled by sea level changes but probably by the periodic flushing of the canyons. Our study revealed that this low energy aseismic margin undergoes significant transport of sand, down to the base of slope, during the sea-level rise and the Holocene highstand.     

Karst groundwater resources in the countries of eastern Mediterranean: the example of Lebanon

The hydrogeological study of some karst systems in Lebanon shows important storage capacity, up to 27 billions m³ for Zarka system, the spring of Orontes River. Their geological and morphological settings, as well as their hydrodynamic function, show structures developed below the level of present springs, probably up to several hundreds meters at depth. Since Miocene, those carbonate formations were subject to a remarkable instability of their base levels. Variations of sea level, of which the Messinian salinity crisis is the major phase, and effects of the alpine orogenesis, combined with climate variations to develop complex, multiphased karst systems. Karstification occurs frequently in the whole formation, often deeply below sea level or underneath sediment filling of continental basins. The surface karst landscape is often intensely eroded. Those multiple, superimposed karst phases give the carbonate aquifers their large extension and storage capacity. These karst aquifers show the typical easy restoration of storage. They have the ability to bear large interannual recharge variations and support exploitation under high pumping rates. However, these aquifers have some disadvantages for sustainable management. In some regions, successful boreholes, which allow high pumping rates also induce uncontrolled exploitation. Another consequence is natural seawater intrusion, as well as the submarine discharge of fresh groundwater, in coastal aquifers. Those carbonate aquifers are subject to important economical stress which jeopardizes their durability. The systematic study and understanding of carbonate aquifers in Eastern Mediterranean is a preliminary condition to any integrated and sustainable management of water resources. Studies in progress in Lebanon may serve as examples to the whole region.     

Galactic evolution and star counts in the milky way

A model of population synthesis for our Galaxy is used to estimate the constraints imposed by photometric and astrometric star counts. We construct a statistical method to compute the degree of compatibility of the model with the data in the form of a multidimensional functionf(V, B-V, U-B, ). We apply the method to the determination of the star formation history in the Galaxy from Schmidt plates towards the anticentre. Although the presently available data do not allow us to draw firm conclusions, we show that the same kind of data with a slightly better photometric accuracy and with proper motions would give new constraints on the past star formation rate of the Galaxy.     

Cosmo-SkyMed X-band SAR data for classification of ice-free areas and glacier facies on Potter Peninsula,King George Island Antarctica

The climate change phenomena have been influencing terrestrial and glacial ecosystems around the planet. Maritime Antarctica is especially sensitive to these climate variations and over the last 50 years increasing global air temperatures have caused extensive glacial retreat. The objective of this study is to evaluate the potential use of the SAR technology in monitoring the surface dynamics of the Potter Peninsula, King George Island, maritime Antarctica. An image generated by the SAR satellite COSMO-SkyMed, obtained on 2 February 2011, was used to extract the backscattering values of targets on the surface for further processing and classification, using a supervised statistic classifier of maximum likelihood for the determination of the surface classes. The average backscattering of water bodies presented high similarity, which made its separation unattainable. On the other hand, the surface classes’ bare ice and wet snow over the glacier presented distinct average backscattering values, which allowed an efficient and precise classification using only this parameter. The classification process showed satisfactory results for periglacial environments, presenting high fidelity to the field data.     

Silicon carbide in circumstellar environment

The presence of small clusters of silicon carbide (SiC) in circumstellar dust shells surrounding late-type stars is inferred from a broad emission feature peaking at around 11µm in infrared spectra [1]. These clusters are expected to condense from molecular arrangements composed of a few carbon and silicium atoms which are present in stellar winds surrounding carbon-rich late-type stars. We have searched for the possible geometric structures of SiC _n ⁺ radicals (n 5) on the basis ofab initio calculations. It is predicted that the linear structures are energetically favored compared to the other planar and 3D ones, except for SiC ₂ ⁺ where a doubt subsists and for SiC ₃ ⁺ where the rhombic structure is clearly more stable than the linear one. In the same way, a conspicuous even-odd alternation of the stability with the number of carbon atoms along the subset of linear species is shown. Vibrational frequencies of all the structures under consideration have then been determined. The possible destabilizing influence of a finite temperature effect on these structures has also been analyzed by using general considerations of thermal statistics.