A comet?

Posted here last year was that Chicxulub killed the dinos and that it came in at a 60 degree angle. Avi Loeb, Frank B. Baird Jr., and Amir Siraj from Harvard are looking into what this meteor was and whence it came. These are related questions.

I've been saying "meteor" (maybe "bolide") and not "asteroid" because I've been trying to be neutral, although I do have a bias (see below). A meteor is an invader into our atmosphere, when it becomes visible. Asteroid implies we know its origin - from the asteroid belt, as opposed to the Oort Cloud where live comets. (I understand that the Kuiper Belt does not produce Earth-crossing rocks or snowballs, so is deemed moot.)

The latest study claims that Chicxulub was carbonaceous. The type isn't common in the nearer asteroids, which mainly come from Vesta inbound, and are stony or even iron (also uncommon). They are more common from Ceres on out but - these generally stay 3 AU away and out. [UPDATE MAY 2022: As witness Ceres herself.] You know what does come into Earth-crossing orbits? Comets do, that's what.

For that we need a theory on how many comets have been coming this way over the hundred-million-year timescale. Loeb's team thinks they've found such a theory: that Jupiter has pulled in a lot of Oort, sunward. These then break up close to the sun and, presumably, a lot of those fragments just burn or boil. But even these fragments are massive, and the Sun doesn't get them all; such that when one doesn't burn up but hits Earth instead, it goes boom.

All this said, I do have a complaint. Less to Loeb, perhaps, than to his interlocutors. We have the full study. "Iridium" isn't mentioned in it.

The whole Alvarez impactor theory came about in the 1980s because the K/T layer had iridium. That is a metal, "siderophile" so-called; bound to iron. I was until today unaware that carbonaceous rocks are enriched in this, much less snowballs from beyond Yuggoth. (And there exists no natural iridium deposit in the shallow coasts of the Gulf.) Some studies (Loeb tells us) have accounted for that. Loeb et al. point to Trinquier, A., Birck, J. .-L. & Jean Allègre, C. "The nature of the KT impactor. A 54 Cr reappraisal. Earth Planet", Sci. Lett. 241 (2006), 780-8. As I read Anne Trinquier, doi 10.1016/j.epsl.2005.11.006: yeah, the iridium (and chromium-54) is accounted.

Best I can tell Loeb is letting Trinquier speak for him; Loeb is, there, talking to his peers and not to us midwits. So the reporter - who is talking to us midwits - hasn't let Loeb down; the reporter has let us down, for not asking "durr what about the iridium".

One thread Trinquier left open which Loeb didn't close - what's the chemical composition of an Oort comet, against a trans-Ceres clayball. We may have to await another comet mission like the missions recently sent to Bennu.

UPDATE 2/25: Globally distributed iridium layer preserved within the Chicxulub impact structure. Also talking carbonaceous chondrite. The word "comet" is absent... as is "Trinquier". Coïncidentally Jupiter does snag visiting comets here a lot.