Meteorites impacts

Stishovite. Stishovite was first prepared (68) ia the laboratory ia 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequentiy discovered, along with natural coesite, ia the Ari2ona meteor crater. It has been suggested that these minerals are geological iadicators of meteorite impact stmctures. Stishovite (p = 4.35 g/cm ) is the densest known phase of silica. The stmcture, space group P42/nmn is similar to that of... 

The emphasis of the present work is science and technology in the laboratory. The natural shock-compression laboratory of meteoritic impact should not be overlooked. In these environments unique solid state materials have been synthesized for the first time. Perhaps the most common features of our Earth, Moon, and other planets and moons are the craters produced by such high velocity impacts [67C01, 87A03]. 

Now and then, projectiles from outer space cause excitement and surprises, as in January 2000, when a meteorite impacted the frozen surface of Lake Targish in Canada. It was a new type of C-chondrite with a carbon concentration of 4-5%, and probably came from a D-type asteroid (Hiroi et al., 2001). More exact analysis of the Targish meteorite showed the presence of a series of mono- and dicarboxylic acids as well as aliphatic and aromatic hydrocarbons (Pizzarello et al., 2001). Aromatic compounds and fullerenes were detected in the insoluble fraction from the extraction this contained planetary helium and argon, i.e., the 3He/36Ar ratio was... 

Shock waves can be natural in origin, for example, lightning, volcanic explosions or meteorite impacts. But human activity can also generate shock waves, for example in chemical or nuclear explosions. On the positive side, shock waves can reduce human suffering when used in shock wave lithotripsy (a method for fragmentation of kidney stones and gallstones). 

The earlier assumption of a reducing atmosphere has been modified in favour of a neutral one. It is hypothetically possible that iron vapour and reduced forms of carbon from meteorite impacts on the ocean could have led to limited regions with reducing properties. 

The hydrothermal systems, hundreds of meters under the surface of the ocean, would have protected evolving systems from the high-energy cosmic radiation as well as from meteorite impacts. Even partial evaporation of sea water, due to gigantic impacts, could have been resisted by molecular systems present at great depths (Holm and Andersson, 1995). 

The Amazonian epoch, which includes the last 1.8 billion years in this period, there were only a small number of meteorite impacts, and Mons Olympus was formed. 

Some have left behind them traces of elements rare on Earth, such as iridium. The detection of traces of iridium from a large meteorite has led to a proposal that major extinction of species on Earth, namely the dinosaurs, could be due to the meteorite impact, causing chemical, light and temperature fluctuations, i.e. global climate changes, though this proposal is not universally accepted. 

The delivery of volatiles to Earth and Mars must have been similar but where has the early Martian atmosphere gone The atmosphere of the inner planets can be seen in Table 7.3. Cometary and meteorite impacts can deliver material to a planet but are also responsible for a process called impact erosion where the atmosphere could be lost due to an impact such as the Earth-Moon capture event. Current estimates suggest that impact erosion may be responsible for the loss of 100 times the current mass of the Martian atmosphere. 

Ganapathy, R., A Major Meteorite Impact on the Earth 65 Million Years Ago, Evidence from the Cretaceous-Tertiary Boundary Clay, Science 209, 921-923 (1980). 

Tektites Glassy rocks that range up to a few centimeters in length. Formation is thought to have been a result of meteorite impacts that liquefied and ejected molten crustal rocks. As the ejecta feU back to Earth, they cooled and froze into teardrop and dumbbell shapes. Small tektites are called microtektites. 

Stishovite is the most dense phase of silica. Its density is 4.35 g/cm. It has a rutde-type crystal structure in which the sdicon atom is octahedrally surrounded by six oxygen atoms. Four Si—O bonds are 1.76A and two 1.8lA. Stishovite has been prepared similarly to coesite but at temperatures between 1,200 to 1,400°C and a pressure above 150,000 atm. Both the coesite and stishovite are found in nature in certain meteorite craters resulting from meteorite impacts. 

Taylor S.R. (1967) Composition of meteorite impact glass across the Henbury strewnfield. Geochim. Cosmochim. Acta 31, 961-968. 

Absolute time measurements using radioactive isotopes are presently limited to samples that can be carefully processed and analyzed in the laboratory. Estimates of the relative times of formation of various geologic units on other planets can be gained from spacecraft imagery, by crater counting. The older a surface is, the longer it has been exposed to meteorite impacts and thus the higher the density of craters on that surface. However, very old surfaces can become saturated with craters, when every new crater effectively obliterates an older one. 

Defects in Crushed Si02 and Those of Quartz in Meteorite Impact Crater. 

Tektites are glass spheroids, which cover parts of North America (about 35-million-years old), central Europe (about 15-million-years old), western Africa (about 1.1-million-years old) and Australia-Asia (about 780000 years old). Most tektites are less than 1 mm in diameter (microtektites), although some are centimeter-sized (Koeberl et al., 1997, 1745). Researchers generally believe that tektites represent terrestrial surface materials that were melted and ejected high into the atmosphere because of large meteorite impacts (Koeberl et al., 1997, 1746). 

Tektite Glassy spheroids that probably form from meteorite impacts. Most tektites are less than 1 mm in... 

Bloch, M. R., Fechtig, H., Gentner, W., Neukum, G., Schneider, E. (1971) Meteorite impact craters, crater simulations, and the meteoroid flux in the early solar system. Proc. Second Lunar Science Conf., 3, 2639-52. 

Shock-wave phenomena are important in meteorite impacts where high-pressure minerals are often formed. Small diamonds useful for lapping and polishing are made commercially by shocking graphite mixed with iron and copper. The metals cool the diamonds before they can transform back to graphite on pressure release. 

Fusion of soil during meteorite impact and the chemical composition of tektites. Nature [London] 195, 32—33 (1962). 

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