What did scientists think killed the dinosaurs before the impact theory was put forward?
Before the late 1970s, there were any number of different theories to explain the mass extinction, with no real consensus. One theory put it that mammals had eaten all the dinosaurs' eggs, but this has now been largely discredited. Another, much more widely supported, theory was that massive volcanism caused environmental changes which drove the dinosaurs to extinction. Walter and Louis Alvarez first proposed the impact theory in 1979. Soon it would dominate all thinking on the subject.
What is iridium? Does it occur naturally on the Earth?
Iridium is an element which is found on Earth, mostly in the molten mantle, the layer beneath the crust. Sometimes it is ejected in volcanic eruptions, but it's still extremely rare. It is found in far higher concentrations in some kinds of asteroids. The impact theory was first put forward when scientists discovered asteroid-type concentrations of iridium in the KT boundary layers.
How do scientists explain the survival of some groups of animals during and after the impact?
It depends on who you ask. Some supporters of the impact theory suggest animals like frogs, turtles, crocodiles and fish survived by immersion in water or mud. This immersion, they argue, protected them from the initial heat wave and from the subsequent cold of an 'impact winter', when dust and atmospheric chemicals blocked out the sun.
Mammal species also survived. The theory has it that because most of them were small they could burrow to escape the heat and the cold.
Those who oppose the impact theory argue that the very survival of these kinds of animals, some very sensitive to cold and pollution, undermines the idea that there was a global environmental catastrophe at all.
How likely is that two massive asteroids could have hit the earth within 300,000 years of each other as Prof Keller suggests?
There is evidence that impacts might come in clusters, caused by a lot of debris flying around the solar system at particular times. For example, two huge craters from about 35 million years ago were created about 200,000 years apart: Popigai crater in Russia and Chesapeake Bay crater in the USA. So although it would be unusual, it's certainly not impossible that there were two big impacts at the KT boundary.
How likely is that Prof Keller's team will find a crater from the second impact that they say coincided with the end of the dinosaurs?
It is possible but perhaps not likely. 70% of the Earth's crust has been 'subducted' - pushed back into the mantle by another tectonic plate - over the last 65 million years. Of course, supporters of the single impact theory argue it will never be found, since it doesn't exist!
Why do Keller's team believe that another impact caused the extinction? Why don't they think it could have been something else?
They believe that there were two impacts, one at Chicxulub 300,000 years before the final disappearance of the dinosaurs, and another which coincides with the 'KT boundary', the rock layer which most believe marks the dinosaurs' demise.
But they do not believe either impact was primarily responsible for the mass extinction. Instead, they argue that a combination of factors such as massive volcanic activity in western India and global warming were also responsible.
What other reasons are there for doubting the impact theory?
One addition to the impact theory was that returning molten debris set forests ablaze all over the planet. But Claire Belcher of Royal Holloway, University of London has shown that it is very unlikely that there were such wildfires beyond about 2,000 kilometres from Chicxulub. This does not suggest that there was no impact, only that its effects were not as calamitous as once assumed.
Where does the debate stand now?
There are still several broad schools of thought. The dominant school is still without doubt the impact theory, which the majority of earth scientists support.
There is another long standing tradition which stresses the importance of the volcanic Deccan Traps in western India. This had been active for several hundred thousand years before the impact(s) and, it is argued, caused a variety of dramatic climatic changes which caused the mass extinction.
The third school - which many palaeontologists have always favoured - is that global warming and the disappearance of habitat were responsible. In the half million years before the impact a massive inland sea which had covered much of North America disappeared, taking with it the coastal plain habitat on which it is thought the dinosaurs depended.
Others still believe that all of these schools are valid, and that the dinosaurs were killed off by a lethal combination of circumstances.
But isn't it obvious that the dinosaurs were killed by the asteroid, because they died out just when the asteroid hit?
This again is a matter of controversy, and scientists cannot agree on how to interpret the fossil record.
Dinosaur fossils are relatively rare, and complete specimens rarer still. This means that in comparison with other more populous animals it is very hard to tell exactly when the dinosaurs became extinct.
Some palaeontologists however do argue that the record shows they were annihilated by a sudden event. Others maintain that the dinosaur fossil record cannot support this idea, and that it is likely they were dying out (along with many other creatures with better fossil records) for millions of years before the impact(s). We know that after the asteroid hit there weren't any dinosaurs, but it's possible a lot of them died out earlier than that.
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