Matters Arising on "Breakup of a long-period comet as the origin of the dinosaur extinction" by Siraj & Loeb

TL;DR

This paper critiques Siraj & Loeb's claim that long-period comets are the likely cause of the dinosaur extinction, arguing that their assumptions about comet fragmentation and composition are flawed, and that asteroids are more probable impactors.

Contribution

It provides a critical analysis of previous comet impact hypotheses, clarifying misconceptions about comet fragmentation and composition, and supports the asteroid impact hypothesis.

Findings

Comet fragmentation during tidal disruption is overestimated in the criticized paper.

The impactor's composition is more consistent with asteroids than comets.

Asteroids similar to CM or CR chondrites are the most likely impactors.

Abstract

The recent publication by Siraj & Loeb (2021; Nature Scientific Reports 11, 3803) attempts to revive the debate over whether the Chicxulub impactor was a comet or an asteroid. They calculate that ~20% of long-period comets impacting Earth will have first been disrupted by passage inside the Sun's Roche limit, generating thousands of fragments, each the needed size of the Chicxulub impactor. This would increase the impact rate of comets by a factor ~15, making them as likely to hit the Earth as an asteroid. They also argue that a comet would be a factor of 10 more likely to match the geochemical constraints, which indicate the Chicxulub impactor was carbonaceous chondrite-like. These conclusions are based on misinterpretations of the literature. Siraj & Loeb [1] overestimate the number of fragments produced during tidal disruption of a comet: tens of fragments are produced, not thousands. They also conflate 'carbonaceous chondrite' with specific types of carbonaceous chondrite, and ignore the evidence of iridium, making comets seem more likely than asteroids to match the Chicxulub impactor, when in fact they likely can be ruled out. Rather than a comet, an asteroidal impactor similar to CM or CR carbonaceous chondrites is strongly favored.