I have Googled "iridium content of comets" and can't find anything that suggests comets contain iridium in any detectable quantities. Yet, whenever I read an article about the Chicxulub impactor, the article invariably says something to the effect that it may have been created by an asteroid or a comet. But Alvarez and company first offered the impact hypothesis for the K-T extinction by observing an iridium layer in global sedimentary deposits, which could only have come from an asteroid impact, since iridium is extremely rare in the Earth's crust. Later the Chicxulub structure was identified as the probable site of the asteroid impact.

While a comet could have caused the Chicxulub crater, it could not have been responsible for the global iridium layer. So why do science writers offer the comet alternative? If Chicxulub was created by a comet, then another site must have been the source of the iridium layer.

  • $\begingroup$ Can you provide an example? $\endgroup$ – Rob Jeffries May 8 '17 at 21:16

I also googled "iridium content of comets", and the first result was https://news.dartmouth.edu/news/2013/04/dartmouth-researchers-say-comet-killed-dinosaurs

Now there is currently no consensus on the nature of the Chicxulub impactor. The observations of iridium and osmium suggest an asteroid. A minority opinion is that a comet may be responsible. The authors of the article in the linked news report suggest that measurements of iridium concentrations from ocean cores may be erroneously high, due to sorting in marine sediments. If these cores are excluded, the amount of iridium measured is lower, and consistent with some models of the composition of comets.

So, comets do contain some iridium. The amount of iridium is not certain. Some models of comet nuclei contain more dust and less ice, and so have more iridium. The exact amount of iridium in the KT boundary layer is also uncertain. The low end of estimates would be consistent with a cometary impact. The nature of the impactor is not yet settled science, but the balance of opinion currently favours a stony asteroid.

  • $\begingroup$ I too saw that link but never followed it, as my mind was fixed on articles that explicitly were about the chemical makeup of comets (though frankly, I meant to read it but forgot by the time I had read some other articles). I'm not sure I agree with the researchers' practice of discarding oceanic iridium deposits, as much of that could (probably?) have been iridium laid down from the impact and washed into the oceans from the continents. $\endgroup$ – BillDOe May 9 '17 at 19:29
  • $\begingroup$ Marking as answered. Thanks for taking the time. While I do not agree with the researchers' excluding oceanic sediments in their analysis, it still indicates that comets can contain appreciable amount of iridium, and that was the original question. $\endgroup$ – BillDOe May 10 '17 at 23:47

It occurs to me that part of your confusion may be in defining the difference between a comet and an asteroid in the first place. Here are a couple claims.

From UniverseToday,

Asteroids and comets have a few things in common. They are both celestial bodies orbiting our Sun, and they both can have unusual orbits, sometimes straying close to Earth or the other planets. They are both “leftovers” — made from materials from the formation of our Solar System 4.5 billion years ago. But there are a few notable differences between these two objects, as well. The biggest difference between comets and asteroids, however, is what they are made of.

While asteroids consist of metals and rocky material, comets are made up of ice, dust, rocky materials and organic compounds. When comets get closer to the Sun, they lose material with each orbit because some of their ice melts and vaporizes. Asteroids typically remain solid, even when near the Sun.

From CalTech,

The main difference between asteroids and comets is their composition, as in, what they are made of. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago. Asteroids formed much closer to the Sun, where it was too warm for ices to remain solid. Comets formed farther from the Sun where ices would not melt. Comets which approach the Sun lose material with each orbit because some of their ice melts and vaporizes to form a tail.

As JamesK noted, it is possible for "rocky material" in either a comet or an asteroid to contain iridium -- or not contain iridium. I think the reason iridium is tied to the KT event is that there appears to be a world-wide (ish) layer with iridium, and that layer consistently dates to approximately the same epoch.

P.S. note also Physics.SE

  • $\begingroup$ Sorry, but I do know the difference. $\endgroup$ – BillDOe May 9 '17 at 19:31

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