# Could this estimate of the size and mass of the Chicxulub Impactor be accurate?

My understanding has always been that the Chicxulub Impactor was in all likelihood an asteroid with a diameter of about 8 - 12 km, but then, the other day, I came across this paper. In it, they say that the diameter of the impactor could have been as large as 81(!) km, and that it was probably a comet and not an asteroid, a claim they base on the amount of iridium found in the layer associated with the impact. I'm not an astronomer so I'm not really in a position to evaluate the veracity of their claims and how they arrived at them, which is why I'm wondering if what they say could plausibly have been the case?

• An 81 km comet would have much more mass than an 8-12 km rocky meteor, but not necessarily more iridium, though I don't think precise estimates are possible given the limited samples taken from comets. I will add that the amount of Iridium might be a method for calculating impacting object size and velocity, but only when we have enough reliable measurements of comet and asteroid composition. I like their approach even if I'm skeptical of their conclusions. Sep 5 '19 at 13:22
• I'm guessing there was a misplaced hyphen at some point - changing "8-12" to "81-2". Printing error maybe? Bad OCR picked up a smudge on the page? That kind of thing. Sep 5 '19 at 20:51
• @DarrelHoffman: Did you look at the paper? It repeatedly uses "80.9 km" - which the OP rounded to "81". Sep 5 '19 at 21:08
• Also note that 80.9 is the maximum of the range they give. The lower end overlaps the smaller range in your question. Sep 5 '19 at 21:12
• And I agree with userLTK when he points out that while it's a novel method they have drawn on in theory, the results it produces do not hold up without reliable data on the amount of iridium you find in different types of celestial objects. Sep 6 '19 at 7:55

One may also estimate the impact energy by using the size of the crater. Since that is related to kinetic energy it is a way to estimate the mass and velocity of the impactor, but this are coupled. Not only that, the kinetic energy depends on $$m$$ and on $$v^2$$ thus is very sensitive to the velocity of the object. The impactor might have had $$m = 1$$ and $$v = 1$$, or $$m = 0.25$$ and $$v = 2$$ (in arbitrary units) and the kinetic energy would have been the same. As you can see this dependence is a source of high uncertainty in the mass. The assumption of the impact velocity is needed and the precission of this assumption is very important if you don't want a wide variety of masses for the impactor. Once you got the mass of the impactor you need to know the density of it to estimate its size (another assumption). If it was a comet then the density is as low that for that mass the object would have been gigantic (but then one have to explain the low impact speeds since a comet would have been traveling faster in general). A purely rocky body is denser and thus for the same mass you need a smaller diameter. We in fact have done geochemical measurements on the impact site and there is strong evidence that the object was an asteroid and in particular a carbonaceous chondrite, which means that we have a good estimate of the density of the object. Detailed analysis of the structure of the impact crater can also decouple the mass-velocity constraint whithin the $$E_{kinetic} = \frac{1}{2}mv^2$$ relationship. Overall there best estimate is still around 12 km in diameter (+/- 3 km).