Follow-Up on the Dino-Killing Asteroid

Introduction

Last night, after making my post “What’s Going on with the Dinosaur-Killing Asteroid?” I contacted the lead author from the original 2007 paper, Bill Bottke (in the interest of full disclosure, I actually collaborate with him and see him about once a week, including yesterday morning).

I asked Bill if he would be willing to glance over the short post and let me know if I got any of the science outrageously wrong. His reply was a bit more than I had expected of a simple “yes” or “no,” where he instead wrote a more elaborate explanation of what was going on. I asked if I could post his response to my blog as an addendum and instead, he sent a more detailed reply for me to post. Since it was somewhat lengthier than the original post, I figured I’d just make a separate one. What follows is Dr. Bottke’s reply, slightly edited for grammar/spelling as he requested.

Note: You should read my original post before reading Dr. Bottke’s response.

Bottke’s Reaction

First of all, this is science, and not every idea is going to work. One has to do the best one can with the available data, and some models do not survive first contact with new observations.

With that said, let me try to realistically assess where we are and where we are not.

From the dynamical end of things, having a smaller parent body and smaller family members means things can get out of the main belt faster than before. If anything, this moves the impact closer to the peak of the impact spike distribution, which is good for our 2007 model. Moreover, many potential impactors can now get out by being injected directly into the “escape hatch” right on top of the family. We did not model direct injection in detail in 2007 because the K/T hit appeared to be made in the tail of the Baptistina shower — those results would not impact our work. Now that things have changed, we can examine this more closely.

Overall, I find it highly suspicious that K/T occurred in the middle of the Baptistina asteroid shower. Asteroid showers are very rare in solar system history, though coincidences do happen in nature. This makes me think the new results could potentially strengthen our story, not weaken it.

A smaller asteroid means there are fewer large projectiles in the Baptsitina population. This hurts our original model. Interestingly, though, new estimates of Ir (iridium) and Os (osmium) associated with K/T that came out after our paper suggest the impactor may have had a diameter 4-6 km, not 10 km, so this may all be a wash. Impact energy is strongly a function of velocity, and impact velocities on Earth can be very high for asteroids, so there is not necessarily a contradiction here. For those that want to know more, see recent papers by Frank Kyte and Paquay et al. (2008) (“Determining Chondritic Impactor Size from the Marine Osmium Isotope Record” in Science).

The main hit to the 2007 story from the recent WISE work is composition. If Baptistina and its family members turn out to be a different asteroid composition than we suggested in our 2007 paper, we cannot link the family to the limited compositional information we have on the K/T impactor. From Cr (chromium) studies of K/T terrains on Earth, it looks like the impactor was a particular kind of carbonaceous chondrite. A high albedo (reflectivity) for Baptistina could suggest it is not actually this composition. Preliminary spectra for Baptistina family members may also work against it being a carbonaceous chondrite, though most of the family has not been examined from a spectral standpoint. Observers have mainly looked at asteroids near Baptistina, not “in it” as defined by our paper, and interlopers in this part of the main belt are a major pain to deal with. What observers need to do is look at the prominent “clouds” of objects observed for the family, where interlopers are less of an issue. This should be dealt with in the near future.

Note that if Baptistina family members turn out to have a radically different composition than carbonaceous chondrite, it would imply we were strongly misled by the Sloan Digital Sky Survey colors for Baptistina. Nearly 300 objects have been examined, and they have been classified as C/X-types of asteroids, which link to most objects as carbonaceous chondrite-like objects (see Parker et al., 2008).

There is also the surprising and unusual possibility that some asteroids that look like carbonaceous chondrites may have higher albedos that we expect. For example, interesting work on (21) Lutetia, which was recently visited by the Rosetta spacecraft, has a high albedo and a composition that many say looks like a carbonaceous chondrite. For those that know and love asteroid taxonomy, K-types asteroids look like they may be able to produce many kinds of carbonaceous chondrites, yet they are spectrally similar in many ways to those asteroids that may produce ordinary chondrites.

Note that even if composition is knocked away, one could question whether Cr is diagnostic, or whether different parts of the asteroid could have different Cr signatures. However, this strikes me as a desperation ploy, and I will do no more than mention it until new information on Cr comes to the fore.

Final Thoughts

With that response from Bill – more technical than I normally have in my blog but I think important for those who are interested – I’ll close out by reminding readers of what he stated at the beginning and what I have stated many times on this blog: This is how science works. We make observations, gather data, create models, make predictions, and in light of the evidence revise our models or make new ones. Contrast that with the way many creationists, conspiracy theorists, UFOlogists, astrologers, etc. work.