An asteroid impact is the most commonly accepted scientific explanation for why the dinosaurs died out.  Scientists point to high concentrations of iridium in the layer of rock formed at the time of the extinction as evidence. However, Jason Moore and Mukul Sharma of the Department of Earth Sciences at Dartmouth claim that iridium measurements have been inaccurate and that an asteroid probably did not cause the extinction.  A cometary impact is the most likely explanation for the death of the dinosaurs.

Non-avian dinosaurs became extinct about 66 million years ago, during the Cretaceous-Paleogene (K-Pg), or Cretaceous-Tertiary (K-T), extinction event, in which 75% of  all plant and animal species died.

Scientists developed a number of  explanations for why this mass extinction event occurred.  Some hypotheses blame  gradual changes in climate or in sea level.

However, other explanations point to  a sudden catastrophe that severely reduced the amount of sunlight reaching the Earth. This would have made it difficult for photosynthesis to occur. Many plant species, as well as the animals that depended upon them for existence, would have become extinct.

One such hypothesis states that increased volcanic activity caused the K-Pg extinction. . The Deccan Traps, a large volcanic rock formation in India, formed at about the time extinction event.  Volcanic activity would have blocked sunlight and emitted an enormous amount of greenhouse gases into the atmosphere.

However, the most widely accepted explanation is that an asteroid crashed into the Earth.

In 1980, Luis and Walter Alvarez found a high concentration of iridium at the K-Pg boundary. Because iridium is often found in asteroids,  the Alvarezes concluded that the iridium found at the boundary came from a large asteroid.

A panel of 41 scientists published a report supporting the asteroid impact theory in 2010.

Scientists think the asteroid that killed the dinosaurs created Chicxulub Crater in Mexico’s Yucatán peninsula. This crater, 110 miles in diameter, was formed around the time of the extinction.

Although Moore and Sharma agree that an extraterrestrial object smashed into the Earth, , they do not think that this object was an asteroid.

In a paper presented at the 44th Lunar and Planetary Science Conference in March 2013, the researchers revealed that after examining the data, they found large variations in iridium levels at the K-Pg boundary. They claimed that some of the iridium would have come from terrestrial sources. High concentrations of iridium found in deep ocean cores, for example, were probably derived from iridium-containing minerals in marine sediments.

After correcting the data to account for these irregularities, the Dartmouth researchers discovered that levels of iridium at the K-Pg boundary were much lower than scientists had previously thought.

The researchers also analyzed data on  levels of osmium, another element found in asteroids, and found that osmium levels at the boundary were also much lower than previous measurements indicated.

These findings show that the probability that an asteroid crashed into the Earth 66 million years ago is less likely than scientists had believed.

Something must have created Chicxulub Crater, however . Moore and Sharma argue that a comet could have done it.  A comet would have less iridium and osmium than an asteroid, and because a comet moves so quickly, it would have enough energy to create the enormous crater.

A cometary impact would have thrown debris into the air, darkening the skies and halting photosynthesis.

Moore says that until recently,  scientists did not know much about the structure of comets,  and this made them less likely to accept that a comet could have caused the mass extinction.  However cometary exploration missions, such as NASA’s deep impact, have provided a wealth of data about comet composition.  We now know that a comet  could have formed Chicxulub Crater and caused the death of the dinosaurs.