Asteroid impacts

However, the question must always be asked as to whether these processes could have taken place on the primordial Earth in its archaic state. The answer requires considerable fundamental consideration. Strictly speaking, most of the experiments carried out on prebiotic chemistry cannot be carried out under prebiotic conditions , since we do not know exactly what these were. In spite of the large amount of work done, physical parameters such as temperature, composition and pressure of the primeval atmosphere, extent and results of asteroid impacts, the nature of the Earth s surface, the state of the primeval ocean etc. have not so far been established or even extrapolated. It is not even sure that this will be possible in the future. In spite of these difficulties, attempts are being made to define and study the synthetic possibilities, on the basis of the assumed scenario on the primeval Earth. Thus, for example, in the case of the SPREAD process, we can assume that the surface at which the reactions occur could not have been an SH-containing thiosepharose, but a mineral structure of similar activity which could have carried out the necessary functions just as well. The separation of the copy of the matrix could have been driven by a periodic temperature change (e.g., diurnal variation). For his models, H. Kuhn has assumed that similar periodic processes are the driving force for some prebiotic reactions (see Sect. 8.3). 

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

We have developed an asteroid-impact theory that satisfied these conditions and many others [1]. The theory assumes the... 

Four different ways of calculating the asteroid diameter all give a value of 10 km and this consistency lends confidence to the asteroid-impact theory. The Ir anomaly was first observed by us in Italian rock. Our theory predicted that the unusually abundant Ir should appear all over the world where the C-T boundary is exposed (intact). Part of the hypothesis was confirmed when the anomalously high Ir abundance was found in the C-T boundary layers in Denmark, northern and south-east Spain, and half-way round the world in New Zealand. Another prediction of the theory is that a component of the clay layer at the C-T boundary would be different in composition from other clays in the same section because it contained a component from the impact site. This prediction was confirmed in measurements of the Italian and Danish sections [1]. 

The Ir anomaly has been observed in uplifted marine sediments in four locations in Italy, two in Spain, and one in New Zealand. It has not been found yet in continental sediments or deep-sea cores. If the asteroid-impact theory is valid, the Ir anomaly should be found wherever the C-T boundary is intact. 

If the asteroid-impact theory is correct, the extinctions should be repetitive and the Ir anomaly should be observed in other geological stratigraphic levels corresponding to known extinctions. About five other massive extinctions (besides the one at the end of the Cretaceous Period) have been noted [25]. These come at the end of the Cambrian ( 500 MY ago), the Ordovician (M35 MY age), the Devonian ( 345 MY ago), the Permian ( 230 MY ago) and the Triassic ( 195 MY ago) Periods. 

Glikson A. and Allen C. (2004) Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia evidence for a major asteroid impact in simatic crustal regions of the early pro-terozoic Earth. Earth Planet. Sci. Lett. 220, 247-264. 

Yet it is not impossible to gather enough clues to find out what happened long ago. Just as paleobiologists—scientists who study ancient life—have theorized about a comet or asteroid impact causing the dinosaur extinction, archaeological chemists have studied artifacts to learn something about the life and death of Otzi, as well as the rise and fall of Roman civilization. 

Many believe that the Earth is like an inmate waiting on death row. Even if we do not die by a comet or asteroid impact, we know the Earth s days are numbered. The Earth s rotation is slowing down. Far in the future, day lengths will be longer than today s months. The Moon will hang in the same place in the sky, and the lunar tides will stop. 

Fullerenes are rather easily oxidized, which explains the fact that, despite their commonplace occurrence in soots, they had escaped detection for so long. Fullerenes have now been found in Precambrian carbonaceous rocks from Karelia, Russia in breccias associated with the 1.85-billion-year-old Sudbury impact structure in Canada and in a sooty layer (believed to be due to fires from the asteroid impact that is thought to have killed off the dinosaurs) marking the Cretaceous-Tertiary boundary in New Zealand.8... 

In recent years. Znller and associates University of Maryland) have studied six active volcanoes iAugustine, Mount St. Helens. El Chiehdn. Arcnal. Poas. and Colima) and have found no evidence of lr enrichment. The new Kilauea evidence of volcanic action as an Ir source tends to conflict with that of other researchers who have generally attributed the Ir anomaly to an extraterrestrial source, such as resulting from a cataclysmic meteorite or asteroid impact, notably in connection with (he Cretaceous-Tertiary [K i t boundary layer. 

Like iridium, arsenic is enriched in Cretaceous-Tertiary boundary shales from New Zealand (Brooks et al., 1984 Strong et al., 1987). The iridium is believed to have originated from an asteroid impact that caused the massive extinction at the end of the Cretaceous period about 65 million years ago. In contrast, most of the arsenic in the boundary shales probably had a terrestrial origin (Strong et al., 1987). The extinction of marine organisms, especially plankton, from the impact may have been responsible for increased anoxic conditions in the oceans, which led to the precipitation of arsenic in the marine deposits (Brooks et al., 1984), 541. 

A semi-quantitative asteroid impact hypothesis of tektite origin. J. Geophys. Res. 66, 2521 (1961). 

Finally, much has been written in recent years about the hazard presented to mankind by asteroid impact (Gehrels, 1994). While considerable effort has been put into understanding possible mitigation schemes, it is important to remember that energy scales linearly with mass. Thus, a metallic... 

Sleep N. H., Zahnle K. J., Kasting J. F., and Morowitz H. J. (1989) Annihilation of ecosystems by large asteroid impacts on the early Earth. Nature 342, 139-142. 

Kerr A. C. (1997) Asteroid impact and mass extinction at the K-T boundary an extinct red herring Geol. Today 13, 157-159. 

Sleep N. H. and Zahnle K. (1998) Refugia from asteroid impacts on early Mars and the early Earth. J. Geophys. Res. E—Planets. 103, 28529-28544. 

In 2003, astronomers announced that data and observations indicated one particular crater might well be classified as the youngest crater thus far discovered. The crater, formed in 1953 by an asteroid impact, is the only known lunar crater to have been formed during recorded human history. New and more powerful telescopes, along with orbiting satellite photos now allow examination of the impact site and astronomers observed indications of a fresh crater in the impact zone (i.e., the area corresponding to the impact flash observed in 1953). 

Lewis, John S. Comet and Asteroid Impact Hazards on a Populated Earth Computer Modeling. Academic Press, 1999. Remo, John L., ed. Near-Earth Objects The United Nations Conference on Near-Earth Objects (Annals of the New York Academy of Sciences V. 822). New York New York Academy of Sciences, 1997. 

K.O. Pope, K.H. Baines, A.C. Ocampo, and B.A. Ivanov, hnpact winter and the Cretaceous/Tertiary extinctions results of a Chiexulub asteroid impact model. 

---