Impact! Transcript Date: Thursday, 17 December 2009 - 12:00AM Location: Museum of London Impact! Professor Ian Morison 17/12/2009 An image of the Moon is a salutary reminder that bodies in the solar system, including the Earth, have suffered millions of impacts in the past - and will continue to do so, but happily at a far reduced rate. If anything, due to its greater mass, the Earth will have suffered more impacts that the Moon but erosion has remove the evidence of all but a few from its surface. The impacts of solar system debris, such as asteroids and comets, give rise to what are termed Impact Craters, a name that can be applied to any depression resulting from the impact at very high velocity of an object into a larger body. Impact craters are approximately circular depressions that usually have raised rims, and range from small, smooth, bowl-shaped depressions to large craters having terraced rims often with a central peak or peaks. Massive impacts give rise to giant impact basins such as the Mare on the Moon where the lunar crust was breached and so lava was able to well up from below and fill the depression caused by the impact. These 'mare' depressions, also seen on Mercury, were the result of a period of intense bombardment in the inner Solar System that ended about 3.8 billion years ago. Since then, though still appreciable, the rate of crater production within the inner solar system has been considerably lower but about every million years the Earth experiences a few impacts large enough to produce a 20 km diameter crater. Erosion on Earth quickly destroys them, but about 170 terrestrial impact craters have been identified - the largest of which are listed in the table at the end of this section. Their size ranges from a few tens of metres up to about 300 km in diameter. Most are less than 200 million years old. Until the 1930's it was widely believed that the craters found on the Earth were volcanic in origin rather than the result of impacts (Patrick Moore was quite convinced until relatively recently that the Lunar craters were volcanic in origin!) and it was not until the 1960's that researchers, notably Eugene M. Shoemaker, found clear evidence that they had been created by impacts, identifying, for example, shocked quartz which could only be formed in an impact event. By 1970, more that 50 impact craters had been found on the Earth. Crater Formation The speed at which an object hits the Earth can range from about 11km/s up to 70 km/s with a typical impact speed of ~ 25 km/s - these speeds are derived from the orbital speeds of objects within the solar system. It is now possible to simulate such events and it appears that the impacting object will normally penetrate the ground and then cause a below ground explosion that melts and vaporises material. In this case, it does not normally matter at what angle the object has hit the Earth and the resulting craters are nearly always circular with only very low angle impacts giving rise to elliptical craters. In large impacts, much material will be ejected and mostly fall within a few crater radii, but some may travel significant distances and may form 'rays' as seen centred on the lunar crater, Tycho. Some ejected material may even exceed the planet or satellite's escape velocity and then travel within the solar system to perhaps fall as meteorites on the Earth. This is how we have some Martian rock samples to investigate for signs of life. [The team who have investigated the Martian meteorite ALH 84001 believe that new investigations show evidence of simple life forms within it.] Craters on the Earth Perhaps the best known Impact crater on Earth is Meteor Crater, some 43 miles east of Flagstaff in northern Arizona, USA, which was formed about 50,000 years ago when the area was open grassland inhabited by woolly mammoths! It is often referred to as the 'Barringer Crater' in honour of Daniel Barringer who was first suggested that it was produced by meteorite impact. It is about 1.2 km in diameter and 170 m deep, surrounded by a rim that rises 45 m above the surrounding plains. At its centre is a ~220 m pile of rubble. The remains of the meteorite are believed to be embedded under the rim at the south side of the crater. It was a nickel-iron meteorite about 50 meters across, which, it is thought, impacted the plain at a speed of 12.8 km/s (28,600 mph). The meteorite would have initially weighed ~600,000 tons but it is suspected that half may have vaporised as it passed through the Earth's atmosphere. The meteorite that formed the crater is officially called the Canyon Diablo Meteorite - it is named after the town of Canyon Diablo, Arizona, which, now a ghost town, was 12 miles to the north. In 1960, research by Eugene M. Shoemaker confirmed Barringer's hypothesis with the discovery of the presence in the crater of the mineral stishovite. This is a rare form of silica found only where quartz-bearing rocks have been severely shocked by through an impact event or nuclear explosion. It cannot be created by volcanic action. Since Shoemaker produced the first definitive proof of an extraterrestrial impact on the Earth's surface many impact craters have been identified around the world but Meteor Crater is still the most visually impressive. It was used by Shoemaker to train the Astronauts in crater geology prior to the Apollo missions in the 1960's. The Nördlinger-Ries and Steinheim Craters in Germany Two large impact craters in the south of Germany are the nearest significant impact craters to the United Kingdom and are thought to result from the impact of a binary asteroid (two co-rotating asteroids) about 14.4 million years ago. The larger, the Ries crater, is 24 km across and its floor is about 120 m below the eroded remains of the rim. On a very clear day it is possible to see from one side across to the far rim (as has the author), but the size is such that the crater does not appear too obvious. As with the Barringer crater, Eugene Shoemaker showed that it was caused by meteorite impact as, with Edward Chao, they found shocked quartz (coesite) in the stone that had been used to build Nördlingen town church which lies within the crater. Interestingly, stone buildings in Nördlingen contain millions of tiny diamonds, all less than 0.2 millimeter across - formed when part of the asteroid impacted a local graphite deposit from which stone has been quarried over the centuries. The smaller,Steinheim, crater is 42 km west-southwest from the centre of the Ries crater and is 3.8 km across. Viewing from the crater wall at one side, its crater form is quite obvious and there is a low outcrop of material on the crater floor. It is thought that the Reis crater was formed by an asteroid of 1.5 km diameter and the Steinheim crater one of 150 m diameter. The pair impacted the region at an angle around 40 degrees from the surface from a west-south-westerly direction. They impacted at about 20 km/s and had an explosive power of 1.8 million Hiroshima bombs. Ejecta from the Reis crater has been found up to 450 km to the north-east and is believed to be the source of moldavite tektites found in Bohemia and Moravia. The largest known impact craters on Earth: The Chixilub Event The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico with its centre located near the town of Chicxulub. The crater is more than 180 km in diameter, making it the third largest confirmed impact structures in the world - formed by the impact of an asteroid at least 10 km across. The crater was discovered by a geophysicist, Glen Penfield, who had been prospecting for oil during the late 1970s. Within his data, Penfield found a huge underwater arc 70 km in length which agreed with a gravitational anomaly shown on map of the Yucatán made in the 1960s. This also suggested a giant crater but was not publicised due to commercial considerations. Penfield found another arc on land which, together with offshore arc, formed a circle, 180 km wide. As at the sites of other impact craters, shocked quartz was found confirming the structural and gravitational evidence of a massive impact. The age of the rocks and isotope analysis show that this impact structure dates from the end of the Cretaceous Period (called the K-T boundary), roughly 65 million years ago and so it is thus implicated in causing the extinction of the dinosaurs. However this may not have been the sole reason for their demise. The impact released the equivalent of 100,000,000 megatons of TNT - 2 million times greater than the most powerful man-made explosion! In 2007, a paper published in 'Nature' proposed that the 'Chicxulub asteroid' resulted from a collision in the asteroid belt 160 million years ago that gave rise to the creation of the Baptistina family of asteroids. There is evidence that the impactor was a member of a rare class of asteroids called carbonaceous chondrites, like the Baptistina family of which the largest surviving member of is 298 Baptistina. The impact would have caused one of the largest tsunamis in the Earth's history, reaching several thousand feet high whilst a cloud of super-heated dust, ash and steam would have spread from the crater.