Most of these points have severe problems. I will be addressing them soon, on the previous linked page.
1. The Chicxulub impactor: comet or asteroid ?
- Steve Desch, Alan Jackson, Jessica Noviello & Ariel Anbar, June 2021
To consider a comet more likely than an asteroid requires extreme assumptions about how comets fragment, conflation of carbonaceous chondrites with specific types of carbonaceous chondrites, and a blind eye to the evidence of the iridium (Ir) layer. Extrapolating from their Ir data, they inferred a 6.6 km diameter carbonaceous chondrite-like asteroid as an impactor. Since the identification of the Chicxulub crater, the diameter of the impactor is more commonly accepted to be 10 km.
The main constraint discriminating between a comet and an asteroid has long been the same Ir content of the global clay layer at the K–Pg boundary from which Alvarez et al. (1980) first inferred an extraterrestrial impactor.
The impactor diameter D consistent with the Chicxulub crater is either D = 10 km for an asteroid, or D = 7 km for a comet, which generally collides at higher velocity. Brittan (1997) demonstrated that a D = 10 km carbonaceous chondrite-like asteroid would deliver 2.3 × 10^11 g of Ir, very satisfactorily matching the requirement. In contrast, a 7 km comet is estimated to deliver only 0.1×10^11 g of Ir (23 times less iridium), because it is smaller and half ice (50%). On this basis alone, an asteroid is strongly favoured as the impactor, and a comet is practically ruled out.
Chicxulub impactor was a particular type of carbonaceous chondrite (CM or CR) not matched by comets at all.
* Wikipedia quotes the above paper for the impactor having a 10 km diameter. I wonder how a consensus was reached. There are other papers, with different diameters, impactor sizes & velocities.
2. The breakup of a long-period comet is not a likely match to the Chicxulub impactor
- Steve Desch, Alan Jackson, Jessica Noviello, Ariel Anbar, June 2022
Since the discovery of Ir (iridium) in the clay layer at the K-Pg boundary ...While the first proposal was for an asteroid, for a while some theories invoked a cometary impactor ... Such models have long been disfavored by the mass of Ir in the clay layer, inferred to be 2 to 2.8 × 10^11 g. The size of the Chicxulub crater (180 km) leads to an estimated asteroid impactor diameter of 10 km (crater is 18X bigger than impactor, usually 10X to 20X as per crater scaling laws).
Comets typically impact at higher speeds, reducing the impactor mass for the same impact energy. Although it is increasingly recognized that a continuum exists between comets and asteroids, ’comets’ are considered to be more ice-rich (estimates for 67P are about 20% ice), implying lower Ir contents per impactor mass. A carbonaceous chondrite-like asteroid of the appropriate size would likely deliver 2.3×10^11 g of Ir, in the center of the estimated mass range of the global Ir layer; but a comet would only deliver 0.1×10^11 g (23 times less iridium), because it would be less massive.
Although these conclusions are long standing, Siraj and Loeb have recently argued anew in favor of a comet over an asteroid, based on dynamical and geochemical evidence. Here we demonstrate that their arguments are based on misinterpretations of the literature, and that an asteroid is in fact still highly favored over a comet.
10. The Science Of Why An Asteroid, Not A Comet, Wiped Out The Dinosaurs
- Astrophysicist Ethan Siegel
There are 4 main pieces of evidence that one has to account for when it comes to the mass extinction event from 66 million years ago :
- The extinction of well over 50% of marine and land species of plants and animals all in a very short time window.
- The size, magnitude, and distribution of the layer of clay and ash found around the globe, including the abundance of the various rare elements discovered.
- The energy that must have been deposited by an impactor to cause the formation of Chicxulub crater.
- And the frequency of how often asteroids vs. comets are expected to fulfill these three earlier criteria, to help compute which one is more likely than the other.
That clay layer contains rare elements and rare isotopes of elements in great concentrations, as well as amino acids not used in life processes on Earth: consistent with what we find in meteorites.
Because comets typically originate from farther out than asteroids do, they move at faster speeds when they cross Earth’s orbit: a comet would only need to be about 7 kilometers in diameter to impact Earth with enough energy to create Chicxulub crater, while an asteroid would need to be a larger 10 kilometers across.
A 10 kilometer impact from a specific type of chondrite — the carbonaceous chondrites, making up about 5% of all intact meteorites — would deliver about 230,000 tonnes of iridium.
An impact from a 7 kilometer comet, based on the comets we’ve examined, could deliver no more than 10,000 tonnes of iridium (23 times less iridium), as it’s only about one-third the volume, made out of lighter elements overall, and mostly composed of ice.
While there are carbonaceous chondrites found among both asteroids and comets, the specific sub-types of carbonaceous chondrites that fit the observed evidence from the composition of the boundary layer — either CM or CR carbonaceous chondrites — are exclusive to asteroids, and not matched by comets at all.
Including the constraints that the impactor must match a CM or CR carbonaceous chondrite types and supply the [iridium] in the global clay layer, the probability of a comet is 0%.
The type of carbonaceous chondrite that overwhelmingly corresponds to comets is known as a CI chondrite, which is incompatible with the asteroid-based CM or CR chondrites that fit the observed amino acid, Chromium-54, fossil meteorite, and platinum-group element abundances of the clay boundary layer.
The asteroidal nature of the Chicxulub impactor is not in doubt.
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