Asteroids collisions

We were not able to determine exactly the length of time associated with the deposition of the clay layer. Instead the laboratory studies on the chemical and physical nature of the Cretaceous-Tertiary boundary led to the theory that an asteroid collision with the earth was responsible for the extinction of many forms of life including the dinosaurs. 

When asteroid collisions are especially violent, sufficient kinetic energy may be imparted to launch fragments at greater than escape velocities. In that case, separate asteroids are formed. These fragments share similar orbital characteristics and are referred to as families. The members of most asteroid families share the same spectral characteristics, further linking them together. Families composed of fragments of differentiated asteroids can potentially provide important information on their internal compositions. 

Cosmogenic radioactivity Be is created as a collision fragment in meteorites when cosmic rays strike the meteorite during its journey to the Earth. This radioactivity is counted in the lab, alive Be today in the meteorite, and gives information on its history in space just before its fall. Most meteorites can be concluded to have been exposed to cosmic rays for only about a million years during travel to Earth. Prior to that time they were shielded by deep burial within an asteroid, from which they were liberated by an asteroidal collision about a million years ago. 

One factor that had previously cast doubt on an asteroid collision as the cause of the P-T catastrophe was the lack of iridium found in sediments from this period. However Becker and other scientists argue that this absence probably means the impacting object may have been a comet rather than an asteroid. It is also possible that such a blow could have intensified the volcanism already under way on earth at that time, delivering a one-two punch that almost obliterated life on earth, according to Becker. 

Without iron and stony-iron meteorites, our chances of ever sampling the deep interior of a differentiated planetary object would be next to nil. Although we live on a planet with a very substantial core, we will never be able to sample it. Fortunately, asteroid collisions provide us with a rich sampling of the deep interiors of differentiated asteroids. 

Suggest two ways that would enable you to test the asteroid collision hypothesis. 

NAA has been used in many applications. One very special one is the determination of the iridium content in the earth s cmst and in a 1-cm thick clay layer round the earth. This day layer was formed 65 million years ago and defines the boundary between the Cretaceous and the Tertiary pericxls of geological history. Shells offoraminifera are found in all marine environments and in abundance in the limestone above and below the spedal day layer. But not in the layer itself. In addition it was shown by NAA that the iridium content of the limestone layers above and below is 0.3 ppb while the concentration in the layer is 10-40 pbb. This is the source of the theory of a giant asteroid collision with the earth at this time, a collision that brought the end of the dinosaur era. 

An internal one by gas emission after accretion of the Earth, and An external one, via collisions with comets and asteroids which contained... 

Chondrules from metal-rich (CH, CB) chondrites formed significantly later than those in other classes (Fig. 9.9), with Pb-Pb measurements giving ages of -4562.7 Ma, 5.5 Myr after CAIs (Krot et al., 2005). These chondrules likely formed by a different mechanism than the chondrules in other classes. One model of their formation suggests they are products of a collision between two asteroid-sized bodies and that they formed by recondensation of a vapor plume generated in the collision. 

Collisions among asteroids are commonplace, and impact craters occur on all asteroid surfaces (Fig. 11.1) (Chapman, 2002). Cratering rates within the main asteroid belt are a... 

Evidence for rubble pile asteroids comes from a variety of observations. The low densities of many asteroids imply that they have high porosities, presumably resulting from the assembly of loose fragments. Spectral variations seen in some S-class asteroids as they rotate also support rubble piles. The variations suggest that portions of the surface have experienced different degrees of thermal metamorphism. Catastrophic collision and reassembly has transformed bodies that formerly had onion shell structures into rubble piles. 

It is tempting to describe the parent bodies of the future carbonaceous chondrites, once accretion was completed, as cold objects in motion around the sun. Such a description was broadly accepted until recently. The situation has now changed drastically, and the (unknown) parent bodies of Cl 1 and CM 2 carbonaceous chondrites are described as objects that have experienced hydrothermal metamorphism and which therefore were not cold and chemically dead during all their long lives. Collisions between asteroids seem not to be exceptional. Some of these collisions lead to the rupture of the asteroids which then become potential parent bodies. Other less dramatic collisions only produce heating. This heating may be an initial step towards hydrothermal metamorphism. 

Bottke et al (2005a,b) found that the current asteroid size distribution arose early in its history, when the total mass and collision rate were much higher than today. Once the Asteroid Belt was dynamically depleted and reached roughly its current mass (via the processes described above), there was little further evolution of the size distribution, and hence it has been referred to as a fossil size distribution. Collisions still occur, albeit at a reduced rate, and large collisions lead to the formation of asteroid families, which are groups of asteroids that are clustered in orbital-element (a, e, i) space. Numerous asteroid families can be seen in Fig. 10.6. 

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