## Collisions in the Solar System 531 Case Study Meteor Crater Arizona

On Earth, a meteor impact occurred 50,000 years ago, a crater 180 m deep with a rim rising 30-60 m above the surrounding plain and a diameter of 1,200 m was formed. This crater is known as the Meteor crater or as Barringer Crater (see Fig. 5.6). Kring [130] calculated magnitudes of pressures and wind velocities as a function of distance for that event. He assumed two cases for the energy that was set free: 20 megaton and 40 megaton. For comparison, the Hiroshima bomb had only 15 kt equivalent energy, thus, the energy released through the impact corresponded to at least 1,000 Hiroshima bombs. The results are given in Table 5.2.

It can be roughly estimated that the devastated area around Meteor Crater was about 800-1500 km2. Let us take as a mean value 1,000 km2 and estimate the chances of it being in these 1,000 km2 compared to the total surface of the Earth which is 510 x106 km2. This is 1:500,000. It also can be estimated that the probability of such an impact is 1:1,500, which means that it can occur on an average every 1,500 years. Then, the combined probability for a person standing in the right area at the right time, that is, being killed by an impact that lead to the Meteor Crater is 1 in 7.5x 108.

In Fig. 5.7, the approximate frequency of impacts vs. megatons TNT equivalent energy is given: for example, in every decade, an event with an approximate equivalent energy of ~ 1 megatons has to be expected. The impact effects of the Meteor Crater event affected only about 1,000 km2 and no global extinction resulted. From this Figure, it can be deduced that an impact of 104 megatons TNT equivalent would

Fig. 5.6 The 50 000 year old Meteor Crater in Arizona
 20 megatons 40 megatons Peak Peak Max. Distance Distance Overpressure Dynamic Wind (km) (km) (psi) Pressure (psi) Velocity (km/h) 100 120 2,300 2.8 3.6 50 40 1,500 3.8 4.8 30 17 1,100 4.9 6.2 20 8.1 800 5.9 7.4 10 2.2 470 8.5 11 5 0.6 260 12 16 2 0.1 110 21 27 1 0.02 60 32 40

lead to a global catastrophe. The marked events (Tunguska and K-T impact) will be discussed below.

Let us do another example for the calculation of the probability that a person could be killed due to an impact event.5 A 105 megaton impact certainly would lead to a global catastrophe and such an event could be expected every 5 x 105 years. Let us assume that one in four people would be killed in such a global catastrophe. Then, the chances for any person dying in such an event during the next year is one in two million.6 The diameter of a crater that such an object would cause is between 10 and 20 km.

5 After Lind M.V. Martel http://solarsystem.nasa.gov

6 The chances of being killed in a car accident is about 1 in 5,000.

Megatons TNT Equivalent Energy

Fig. 5.7 Approximate frequency of impacts vs. megatons TNT equivalent energy (adapted from [130])

Megatons TNT Equivalent Energy

Fig. 5.7 Approximate frequency of impacts vs. megatons TNT equivalent energy (adapted from [130])

5.3.2.1 Evidence for Impact Theory

Sixty-Five million years ago, about 70% of all species on Earth disappeared within a very short time. This mass extinction is known as the K-T event because it occurred at the Cretaceous-Tertiary border in the Earth's history. In a layer near Gubbio, Italy, Alvarez, Asaro, and Michel [1] found a peculiar sedimentary clay layer (only 1 cm thick) that was deposited at the time of a mass extinction event; moreover, this layer contained anomalous amounts of the rare element iridium. It can be easily explained why that element is rare on Earth. At the early stage of Earth formation, heavy elements like iridium, platinum, or iron sank down to the core when the Earth was largely molten (differentiation process, see planetary formation). Such a differentiation process did not occur on the small bodies of the solar system such as meteoroids or asteroids. These objects still have the primordial solar system composition. To explain the Ir abundance and other element anomalies found in that layer, the impact of a 10 km chondritic asteroid would have been sufficient.7 Such a large impact would have had approximately the force of 100 trillion tons of TNT, i.e., about 2 million times as great as the most powerful thermonuclear bomb ever tested.

7 Under the assumption that it contained the normal percentage of iridium found in chondrites.

The impact theory can also be traced back to M. W. DeLaubenfels' hypothesis [50].

Summarizing, the Alvarez impact theory is supported by several observational facts:

• Chondritic meteorites and asteroids contain a much higher iridium concentration than the earth's crust because they have about the same concentration of iridium as the whole earth and were not differentiated.

• The isotopic composition of iridium in asteroids is similar to that of the K-T boundary layer but differs from that of iridium in the earth's crust.

• Chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments also strongly support the impact theory and suggest that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.

• Shocked quartz granules, glass spherules, and tektites are common, especially in deposits found in the Caribbean area.

• All of these constituents are embedded in a layer of clay, which can be interpreted as the debris spread all over the Earth's surface by the impact.

While the element anomaly was the first evidence of an impact event that led to the mass extinction, the question remained as to where on Earth this impact did occur. In 1990, Hildebrand and Boyton [102] became aware of data by geophysicists that were searching for oil in the Yucatan region of Mexico. They found a 180 km diameter ring like structure named Chicxutub crater. Using the 40Ar/39Ar method, the age of the crater was determined to be 65 million years.

Schuraytz et al. [220] found Ir anomalies even in subsplits of melt rock and melt breccia from the Chicxulub impact basin.

Several other craters appear to have been formed at about the time of the K-T boundary. As it has been observed during the Shoemaker-Levy 9 impact on Jupiter, an asteroid could fragment before its collision, therefore, the craters found were impacts of a larger body that fragmented before collision with the Earth. The craters (apart from the Chicxulub crater) that have been found are

1. Boltysh crater (24 km diameter, 65.17 ± 0.64 Ma old), Ukraine.

2. Silverpit crater (20 km diameter, 60-65 Ma old) in the North Sea.

3. Eagle Butte crater (10 km diameter, <65 Ma old) in Alberta, Canada.

4. Vista Alegre crater (9.5 km diameter, <65 Ma old) in Parana State, Brazil.

5.3.2.2 Mass Extinction During the KT-Event

The biological system is quite complex and the extinction of one group inevitably leads to extinction of other groups.

The K-T impact caused a major change in both marine and land ecosystems. Before the K-T extinction, about 50% of known marine species were sessile, and after it, only about 33% were sessile. On land, the dinosaurs became extinct, therefore, mammals were able to become the dominant land vertebrates; this seems important, also, for human evolution.

In North America, as many as 57% of plant species may have become extinct. The Paleocene recovery of plants began with a "fern spike" like that which signals the recovery from natural disasters (e.g., the 1980 Mount St. Helens eruption). The effects were quite different for different organisms. Some trends can be stated:

• Organisms that depended on photosynthesis became extinct or suffered heavy losses - from photosynthesizing plankton (e.g., coccolithophorids) to land plants. And so did organisms whose food chain depended on photosynthesizing organisms, e.g., tyrannosaurs (which ate vegetarian dinosaurs, which ate plants).

• Organisms which built calcium carbonate shells became extinct or suffered heavy losses (coccolithophorids; many groups of molluscs, including ammonites, rud-ists, freshwater snails, and mussels). And so did organisms whose food chain depended on these calcium carbonate shell builders. For example, it is thought that ammonites were the principal food of mosasaurs.

• Omnivores, insectivores, and carrion-eaters appear to have survived quite well. At the end of the Cretaceous, there seem to have been no purely vegetarian or carnivorius mammals. Many mammals, and the birds which survived the extinction, fed on insects, larvae, worms, snails, etc., which in turn fed on dead plant matter. So, they survived the collapse of plant-based food chains because they lived in "detritus-based" food chains.

• In stream communities few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains [225].

• Similar but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor. No land animal larger than a cat survived.

• The largest air-breathing survivors, crocodilians and champsosaurs, were semi-aquatic. Modern crocodilians can live as scavengers and can survive for as long as a year without a meal. And modern crocodilians' young are small, grow slowly, and feed largely on invertebrates for their first few years - so they rely on a detritus-based food chain.

It is not clear how long the K-T extinction took. Some theories require a rapid extinction (few years to a few 103 years), others require longer periods. It has also been argued that some dinosaurs survived into the Paleocene. This favors a gradual extinction of the dinosaurs. But this seems now very unlikely because all the remains found are fragments which could have been reworked.

Pope, D'Hondt, and Marshall [191] claimed that mass extinction of marine plankton appeared abrupt and right at the K/T boundary. Marshall and Ward (1996) found a major extinction of ammonites at or near the K-T boundary, a smaller and slower extinction of ammonites associated with a marine regression shortly before that, gradual extinction of most inoceramid bivalves well before the K-T boundary, and a small, gradual reduction in ammonite diversity throughout the very late Cretaceous. This analysis may favor the idea that several processes contributed to the mass extinction in the late Cretaceous seas.

### 5.3.2.3 The Impact and its Consequences

The asteroid that has impacted near the coast must have caused gigantic tsunamis. Evidence for such a scenario has been found all round the coast of the Caribbean and eastern USA - marine sand in locations that were then inland, and on the other hand, vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact.

The crater's shape suggests that the asteroid landed at an angle of 20° to 30° from horizontal travelling north-west. This would have directed most of the blast and solid debris into the central part of what is now the USA. Some of the most severe consequences were as follows:

• Global dust cloud: this blocked sunlight and photosynthesis became reduced for years. The extinction of plants and phytoplankton as well as of all organisms dependent on them was a consequence. This includes also the predatory dinosaurs and herbivores. However, it is clear that organisms whose food chain were based on detritus could have survived. Moreover, the asteroid landed in a bed of gypsum (calcium sulphate), which would have produced a vast sulphur dioxide aerosol. This would have further reduced the sunlight.

• Global firestorms: after the impact, when fragments fell back to the Earth, global firestorms resulted. Fluid inclusion in amber suggest a higher oxygen content, thus, combustion would have been supported. The widespread fires would have increased the CO2 content in the atmosphere (also because of lack of oxygen production). Therefore, after the long winter due to sunlight blocking, a hot atmosphere due to increased greenhouse gas concentration would have contributed to extinction of many species later than at impact time. This was studied, e.g., in the paper of Lyons et al. [151]. Their calculations also show that an object of 10 km size with impact velocity 20 km/s would produce ejecta mass of 2.0 x 1014 kg and cause a global layer of 0.17 mm thickness.8 Their conclusion was that the re-entry of 1 mm sized ejecta did not ignite fires globally.

• Acid rain: this is now believed to have been of minor importance because animals that are vulnerable to acid rain (e.g., frogs) survived. Dust cloud and aerosols (sulphuric) would wash out of the atmosphere within 10 years.

8 This does not include the mass of the impacting object.