DEATH OF THE DINOSAURS: It may have been a speeding comet, US scientists say

Sixty-five million years ago, a massive object from outer space slammed into the Earth near what is now the Yucatan peninsula in modern-day Mexico. It released thousand of times the energy of an atomic bomb, leaving behind a crater more than 180 kilometres wide. The collision rained molten and pulverised rock over an area of more than 2,500 kilometres in diameter, creating devastating firestorms. It also caused worldwide temperatures to plummet as millions of tonnes of dust was thrown up into the atmosphere, blocking out the Sun for decades. As plants died off, the food chains that supported land dinosaurs and their marine reptile relatives collapsed. Dinosaurs soon became extinct, enabling mammals and later, humans, to take the dinosaurs’ place as dominant animals.

English: This shaded relief image of Mexico's ...
English: This shaded relief image of Mexico’s Yucatan Peninsula show a subtle, but unmistakable, indication of the Chicxulub impact crater. Most scientists now agree that this impact was the cause of the Cretatious-Tertiary Extinction, the event 65 million years ago that marked the sudden extinction of the dinosaurs as well as the majority of life then on Earth. (Photo credit: Wikipedia)

It has been heavily debated about what exactly gorged out the infamous Chicxulub crater, with the guilty party either assumed to be a meteor or comet. Now a group of researchers in New Hampshire state, America, say they are almost certain it was a comet pulled in off its course by Earth’s powerful gravitational field. In findings shown to observers at the 44th Lunar and Planetary Science Conference, the researchers used complex chemical analyses to determine that Chicxulub was impacted by an object smaller than previously realised.

Until recently, most scientists believed that the space object that wiped out the dinosaurs as well most other life on Earth was a predominately metallic asteroid containing the rare metal iridium. It was presumed to be relatively large, slow-moving and heavy in metal content, based on traces of iridium found in rock layers worldwide dating from the impact time. Iridium does not occur naturally on our planet in any large quantities and such a spike in the metal’s levels was attributed to fallout from the gigantic iridium-rich meteor settling over the Earth’s land masses.

Chicxulub impact - artist impression
Chicxulub impact – artist impression (Photo credit: Wikipedia)

However the U.S. researchers says that established iridium counts are inaccurate. By comparing iridium counts with that of another rare element, osmium, found in asteroids, they discovered that far less iridium has been deposited than would have been expected for such a massive extraterrestrial object. According to their studies, the lethal asteroid would have needed to be 5km wide to have deposited the two metals in such quantities, but that was inconsistent with the size of the Mexican crater. They believe that the real culprit behind the extinction of the dinosaurs was a very fast and small long-tailed comet.

Long-tailed comets are balls of ice, water and frozen gases that typically travel through space and occasionally pass by the Sun. Their trajectories can take many thousands or even millions of years to complete as they move around our solar system. If such a comet strayed too close to the Earth, it would have been snagged easily and its high speed would have created enough force to generate a wide crater, holding sufficient power to have been able to wipe 70% of all species, as has happened in the great Cretaceous-Tertiary extinction event around 65 millions years ago.

However the findings have been questioned by other scientists. Dr Gareth Collins of the Imperial College in London, who specialises in studying the nature of impact craters, said “I don’t think it is possible to accurately determine the impactor size from geochemistry”.

Geochemistry tells you – quite accurately – only the mass of meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater.

Even the researchers in New Hampshire, working at the Dartmouth College, admit that they are not completely certain if it was really a stray comet that killed off the dinosaurs, but one, Jason Moore said that ultimately “the overall aim of our project is to better characterise the impactor that produced the crater in the Yucatan peninsula [in Mexico]

Chicxulub (Photo credit: Wikipedia)

A geophysical map of the Chicxulub crater. Most scientists are certain that this was the aftermath of the comet that ended the reign of dinosaurs.

Understanding what exactly wiped out three-quarters of the planet’s living things 65 million years ago is vital for when we are faced with such a situation in the distant future, which scientists unanimously agree is more likely than not. About 95% of all near-Earth objects in space with a diameter of one kilometre or more have been discovered by astronomers. However, only about 10% of the 13,000 – 20,000 asteroids above the size of 140m have been detected and tracked by scientists at organisations like NASA, the world leader in asteroid astronomy.

There are believed to be far more comets in our solar system than near-Earth asteroids but NASA scientists say that the likelihood of the Earth being pummelled by another comet is small. Most comets travel far beyond the Earth and Sun with many not being sighted for millennia. A NASA census of historical collisions between our world and so-called near-Earth objects shows that only 10% were comets.

Many thanks to Sunny Atwal for suggesting today’s article.

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“Dinosaur-killing space rock ‘was a comet’ ” – Paul Rincon, BBC News Science and Environment LINK

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