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.

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What’s Going on with the Dinosaur-Killing Asteroid?

Introduction

A NASA press release spawned many, many headlines this week in pretty much all newspapers and online news sites, both major and minor, with variations on the theme of, “The origin of dino-killing rock is back to being a mystery.” If somehow you managed to miss the news, I invite you to read the BBC version, though you could also read the original NASA press release.

So, what’s going on here? Is this another case of the media blowing headlines out their collective … well, mouths? Or is there something to this where we’re no longer sure what may have ended the dinosaurs?

2007 Paper

As may have been obvious from the way I worded the question, the answer is very much the former: Headlines are meant to grab you and to be sensationalist so you’ll read the story. Stretching the story or even missing the point are generally irrelevant.

The actual story goes this way: In 2007, Bill Bottke and two co-authors had a paper in the journal Nature entitled, “An asteroid breakup 160 Myr ago as the probable source of the K/T impactor.” The article was actually much more than that, and an extension from their conclusions was that based upon their modeling, the timing for an asteroid breakup would work dynamically for spawning the asteroid that killed the dinosaurs.

Yes, the asteroid theory for the destruction of the dinosaurs is still valid, it is still by far the best one out there and the one that fits the most data the best. So let’s get that sensationalist headline grabber out of the way.

Again, the crux of the Bottke et al. article was that they dynamically modeled an asteroid family (a group of asteroids that travel together and likely formed from an original, larger asteroid, that broke up). This particular family is called the Baptistina family. Based on an assumed brightness for the asteroids, they can estimate their mass and subsequent sizes. Based on how spread out the family is, they can model when it broke up, and the mass also factors into this calculations. Based on all these, their results show that the Baptistina family formation event could have spawned the asteroid that we pretty much “know” resulted in the destruction of the dinosaurs.

Edited to Add: I have a follow-up post from the author of the 2007 paper, Bill Bottke.

2011 Paper

This brings us to the press release and subsequent hubbub this week. The new paper talks about a lot of things, but what’s most applicable is that they have a better measurement of the brightness of these asteroids, and it’s around a factor of 5 times brighter than what Bottke et al. assumed (based on some of this paper’s teams own assumptions, though it’s likely a more accurate result).

The result is that there is much less mass and smaller sizes of the asteroids in the Baptistina family. Meaning that it disperses more quickly (so would have broken up more recently). And finally meaning that the likelihood that it was the source of the asteroid that led to the death of the dinosaurs is much smaller.

To Repeat

No, this does not mean that an asteroid didn’t wipe them out. It does not mean it was a comet. It does not mean it was aliens. It does not mean it was volcanoes. It does not mean it was the Earth growing in size so the dinosaurs got crushed under their own weight. It does not mean it was [insert: whatever the person you read before coming here invented].

It means that the likelihood that the impactor is from the formation event of this family of asteroids is now significantly smaller.

Final Thoughts

Really, that’s about it. But “Higher reflectivity of asteroid family means scientists less certain of the source of the asteroid that likely killed the dinosaurs” does not a good headline make. Oh well.

Edited to Add: I have a follow-up post from the author of the 2007 paper, Bill Bottke.