65 million years ago, at the end of the Cretaceous, the Earth underwent the last of the five great mass extinctions recorded in the fossil record. While this is by no means the largest of these events, it is the most familiar to the general public, as it was responsible for the extinction of, amongst other things, the non-Avian Dinosaurs and the large marine Reptiles of the Mesozoic Era. For many years the exact nature of this event was a mystery to scientists, and while many theories were proposed, prior to the 1980s few of these were grounded in any actual data.
In 1980, a team of scientists led by Luis Alvarez of the Lawrence Berkeley Laboratory at the
University of California, Berkeley proposed in a
paper in the journal
Science that the extinction might have been brought about by the extinction could have been caused by the impact of a large extra-terrestrial body with the Earth, based upon the discovery of a distinct layer of iridium-rich sediments at the top of Cretaceous strata in several parts of the world (iridium is rare in terrestrial rocks, but present at much higher levels in many meteorites). This is a dramatic theory, and quickly caught the imagination of the world’s media and the non-scientific public. What is more, unlike many other theories proposed for the end-Cretaceous extinction, it was possible to look for evidence to either support or undermine the theory, an important test in the eyes of the scientific community. Since this time the impactor theory has become one of two main rival explanations for the end-Cretaceous mass extinction (the other being flood-volcanism in the Deccan Traps in India).
An artists impression of the theoretical end-Cretaceous impact event. Don Davis.
In order to make calculations about the energy released by a collision with an extra-terrestrial object, the size and nature of this object need to be estimated with some degree of accuracy (exact details about an object destroyed 65 million years ago in a huge explosion are unlikely to be forthcoming), something which was not possible in the 1980s, though a number of theories were put forward. In 1983 Alvarez proposed in a
paper in the
Proceedings of the National Academy of Sciences of the United States if America that this object was a large asteroid, while in 1984 David Raup and John Sepkoski of the
Department of Geophysical Sciences at the
University of Chicago proposed in a
paper in the Proceedings of the National Academy of Sciences of the United States of America that the repeated nature of mass extinctions in the fossil record might periodic in nature, and that this periodicity might have an extra-terrestrial cause and in 1987 a team of scientists led by
Piet Hut of the
The Institute for Advanced Study in Princeton, New Jersey proposed in a
paper in the journal
Nature that this repeated nature of mass extinctions in the fossil record might be due to repeated encounters with a cometary cloud. While the idea that the Earth’s mass extinctions have a regular and predictable nature with an extra-terrestrial cause is no longer taken seriously, the question of whether such an impact could have been caused by an asteroid or a comet is still debatable.
In 1991 a team of scientists led by Alan Hildebrand of the
Department of Planetary Sciences at the
University of Arizona published a
paper in the journal
Geology in which they announced the discovery of a large impact crater, between 180 and 200 km in diameter, buried beneath Tertiary deposits near Chicxulub on the Yutican Peninsula in Mexico, which they proposed might be direct evidence of an Alvarez-type impact at the end of the Cretaceous (though some geologists still dispute that this crater does actually date from the end of the Cretaceous; if it is simply of Late Cretaceous origin, pre-dating the end of the period by hundreds of thousands of years, then it is irrelevant).
Using four different methods to calculate the mass of the object which caused the Chicxulub crater, all of which rely on scaling up the levels of energy known to have been released from nuclear explosions which caused craters of known sizes, Durand-Manterola and Cordero-Tercero calculate that the object must have had a mass of between 5 700 000 and 460 000 000 megatons, and a diameter of between 5.1 and 80.9 km.
Within this range exactly how much energy would have been needed to cause the crater depends upon the nature of the object involved in the impact; Durand-Manterola and Cordero-Tercero considered three possibilities, an iron asteroid, a stony asteroid and an (icy) comet, with the most energy being needed to cause the crater with a comet and the least with an iron asteroid. This is because an iron meteorite would be more than three times as dense as the limestone which the object is thought to have impacted, a stony asteroid slightly denser than the limestone and an icy comet considerably less dense (it would take more energy to break a window with a snowball than with a stone of equal mass). Thus if the object was an icy comet it would need to have been either considerably larger or considerably faster than if it was a stony or iron asteroid.
Durand-Manterola and Cordero-Tercero consider that the ratio of iridium to dust in the terminal Cretaceous boundary layer is closer to that found in comets than in either iron or stony asteroids, and therefore propose that the object was a comet (comets are thought to contain considerably less iridium by mass than either type of asteroid), moreover they suggest that the overall levels of iridium are low enough to suggest the object was towards the smaller end of the calculated possible range for the size of the original object, suggesting that the impact must have occurred at an exceptionally high velocity. For this reason they suggest that the impact may have been caused by a long period comet originating in the Öpik-Oort cloud.
This last set of calculations seem slightly optimistic from a geological point of view; it is highly unlikely that the iridium:dust ratio in the terminal Cretaceous layer would reflect that seen in the original impactor, and quite possible that the level of iridium in the layer could have been either concentrated or diluted by sedimentary processes; the iridium and dust are both thought to have passed from the impact site into the atmosphere, then the hydrosphere, then the sedimentary record. Passage through the atmosphere and hydrosphere are both known to sort particles my mass and surface area (feathers fall through the atmosphere more slowly than stones, and more massive particles sink more rapidly through water than less massive ones). Furthermore a considerable amount of debris is likely to have originated at the impact site, rather than in the impactor.
It should also be noted that volcanism is also known to produce iridium, although at rather lower concentrations than would be predicted from an asteroid impact. While the iridium layer at the end of the Cretaceous is rather more concentrated than would be expected from a volcanic eruption, which is generally considered to be evidence for an extra-terrestrial origin for the layer, Durand-Manterola and Cordero-Tercero’s calculations suggest that the layer is considerably less dense than would be predicted from an asteroid impact, and therefore, if accurate, would tend to suggest that the iridium layer should be seen as rather less conclusive evidence.
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