Records of shooting stars and fast moving bright objects in the sky go back to the Egyptian papyrus writings of c 2000 BC (Nininger 1959). Records of meteorites, the stone and metal objects themselves, go back nearly as far. Falls are well documented in China from around 89 BC. The sacred stone of the Kaaba, in Mecca, Arabia, is reputed to have fallen from the sky before the birth of Islam - but this cannot be confirmed because religious interdict forbids scientists from examining the stone.

Krinov (1960) quotes a description of a fall at Velikii Ustig, Russia, in 1296.

Despite the records scientists were reluctant to acknowledge to the fall of objects from space because until the 18^th Century science was very much a part of religion and folk-lore, and vice versa. It was not until Chladni published papers in 1794 and 1819 that the existence of these objects was considered in the scientific community.

The theory that objects from space collide with the Moon predates Chladni’s papers by over 100 years. In 1609, Galileo concluded that the spots he observed on the moon were circular depressions but made no speculation as to the origin of these craters. In 1665, Robert Hooke suggested internal volcanic activity as the origin. He also considered an impact origin, but dismissed this idea because at that time interplanetary space was thought to be empty.

The proposal of a volcanic origin for the craters presided until the early 20^th Century. The main argument that astronomers had against an impact origin was that all the craters observed were circular. They believed that this would mean that all of the objects would have hit the Moon head-on - not a plausible explanation. The break-through came when impact craters were compared to those made by high explosives. It was then shown that at such high velocities, even objects hitting the surface at fairly oblique angles would create a circular crater similar to that caused by an explosion.

The first possible link between an object from space and a crater on Earth was made by D.M. Barringer in 1906. Barringer did extensive studies on Meteor Crater, Arizona - a 1 km crater in the Arizona desert. He found an abundance of meteoritic iron in the vicinity of the crater, and also deposits of pulverised rock that he believed were produced by the impact. He spent years trying to find a large meteorite mass buried in the crater but was unsuccessful. Barringer did not realise that the meteorite could have been almost entirely vaporised during the impact (for an explanation of this process see Chapter 4 - The Physics of Cratering).

This a photograph of Meteor Crater, Arizona

(http://www.hawastsoc.org/solar/eng/tercrate.htm).

Up until the 1950s, the absence of large quantities of meteoritic material in craters led scientists to believe that they had been formed by an unknown volcanic process - named cryptovolcanism. Geological activity on Earth degrades impact craters, and sometimes completely obscures them. It was not until more craters were discovered, and the processes of cratering mechanics were better understood that impact cratering was recognised as a geological process.

When an object strikes a rock at high speed the rock will exhibit some shock metamorphic effects. These are due to the high pressures caused by the impacting body. Calvin Hamilton (1997) lists shock metamorphic effects which he believes are: "uniquely associated with meteorite impact craters; no other earthly mechanism, including volcanism, produces the extremely high pressures that cause them. They include shatter cones, multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass, and high-pressure mineral phases such as stishovite."

A shatter cone is a striated cone, with its apex generally pointing toward the centre of the crater, indicating the source of the shock that fractured the rock. In 1947, Dietz proposed that these features were peculiar to impact structures. This hypothesis was supported by an experiment performed by Shoemaker (1961) where shatter cones were produced by small-scale impacts in dolomite.

Longitudinal striae with horsetail patterns, decoratethe fracture surfaces. Complete cones are rare; segments of cones are commonand frequently intersect each other. The angle of the apex of the coneis typically about 90 degrees, and they are 1 centimetre to 5 m high. (http://dsaing.uqac.uquebec.ca/~mhiggins/MIAC/impact.htm)

The most famous shock indicators are two forms of high pressure quartz - Coesite and Stishovite. Volcanic activity cannot approach the pressures necessary to produce these dense phases of silica. Samples of this shocked quartz were found in Meteor Crater, Arizona, and provided evidence to support the impact hypothesis proposed by Barringer. Coesite and Stishovite were also found at the Ries crater, Germany, and in the K/T clay boundary layer in Montana.

Glass spherules, called tektites, have also been found around impact sites. These glassy blobs of once molten rock are ejected during the impact. The incredible pressures at the impact sites cause tektites to be thrown out at high speed. They can, therefore, be found hundreds of kilometres away from the impact site.

The scale bar shows divisions of millimetres. At this locality the ejecta layer from the Chicxulub crater is ~2 cm thick and at some places preserves the shapes of the individual

tektites that compose the layer.

(http://dsaing.uqac.uquebec.ca/~mhiggins/MIAC/chicxulub.htm)

Table 2.1 below shows approximate the pressures, in Giga Pascals, at which the phase transitions or other shock-induced features occur in rocks and minerals, during shock compression.

Table 2.1 Petrographic shock indicators (from Melosh 1989, p.41)

65 million years ago about 70 % of the species living on Earth became extinct. This mass extinction marks the divide between the Cretaceous and Tertiary periods, and is known as the K/T boundary. This is probably the most famous extinction in history because at this time the last dinosaurs were wiped out. There has always been much speculation as to the cause of the mass extinction. Hypotheses include changes in oceanographic, atmospheric, or climatic conditions; a magnetic reversal; and a nearby supernova. In 1980 Alvarez and his team believed they had found the answer.

The Alvarez team found an unusual sedimentary clay layer in sedimentary rocks, that had been laid down at the time of the K/T extinction. This layer contained a large amount of iridium. The clay layer was first found in Italy, then in Denmark and finally the Alverez team found the same 1 cm layer in New Zealand. The evidence suggested that the clay layer covered the entire surface of the globe (Alvarez et al. 1980).

Iridium, and other platinum group elements, are only found in very small abundance in the Earth’s crust and the upper mantle. It is believed that these rare elements were carried down into the Earth’s core when it was still largely molten. However, primitive chondritic meteorites still have their primordial solar system abundance of platinum elements - higher than we find in terrestrial rock. The Alvarez team believe that the anomalous quantity of iridium could be deposited from the impact of a 10 km diameter chondritic asteroid colliding with the Earth 65 million years ago.

The asteroid would have injected large quantities of dust containing the iridium into the atmosphere which would have slowly fallen out across the entire globe. The Alvarez team also found soot in the clay layer which could have been generated by the global fires caused by the high temperatures involved in the impact. Shocked quartz found in the layer gave Alvarez more evidence to suggest a huge impact had occurred.

When the Alvarez team published their paper, no suitable crater had been found that would have fitted with both the date and size of the impact. However, in 1990, a cosmochemist named Alan Hildebrand noticed that gravity measurements made by geophysicists looking for oil in the Yucatan peninsula, Mexico, showed a ring structure corresponding to a 180 km crater (Hildebrand et al. 1995). New evidence suggests that the structure could be 310 km in diameter (Cygan et al. 1996). When dated (using the 40 Ar/39 Ar method) it was found to be 65 million years old (Swisher et al. 1992). The Chicxulub Crater, as it was named, was found to have the right age and size to be site where the 10 km object hit. When the area around the crater was further explored, tektites were found - providing more evidence for the impact hypothesis (Swisher et al. 1992).