Random Tusks

Sterling Webb puts the pieces together: Antarctic airbursts independently predicted

Sterling K. Webb

In  February, 2006, SWRI published this paper in Science announcing the discovery of an new recent asteroid family associated with the asteroid 1270 Datura:

http://www.boulder.swri.edu/~bottke/Reprints/Nesvorny_Vok_Bottke_Science_2006_Datura_breakup.pdf    [Tusk will drop on Scribd]

A summary of its conclusions:    “The newly identified family is a group of six 1- to 2.5-km-diameter asteroids, whose orbits are very tightly clustered near the inner main belt of the ~10-km-diameter object 1270 Datura.

The result (of hundreds of numerical simulations) shows that the Datura cluster is 450 +/- 50 thousand years (ky) old (Fig. 1), considerably younger than other known asteroid families…    We estimate that the disrupted parent body was ~15 km in diameter. Apparently, a substantial fraction of the parent body’s mass was ejected to space as fragments ranging in size down to micrometer-sized dust particles.The production of these particles implies that the Datura cluster may be a source of some of the material in the circumsolar (zodiacal) dust cloud…

We estimate that micrometer-sized Datura particles migrate by radiation effects from 2.235 AU to 1 AU in only ~2000 years. Therefore, a wave of micrometer-sized Datura particles may have reached Earth only a few thousand years after the formation of the Datura cluster.Signs of this event may be found by analyzing tracers of extraterrestrial dust in deep ocean sediments and Antarctic ice cores.”
I wonder if these researchers were following up on that last suggestion? At any rate, the minimum amount of dust found in these separated cores would require (they say) 3,000,000,000 kg of dust to reach the ice surface.The characteristics of the dust show it was not blown in, but was deposited at one time from above.

This amount of dust is 100 times greater than the “normal” annual cosmic dustfall for the entire planet.This is too much to be accounted for by dust bands drifting in from the orbits of the Datura family. No, it would require a “chunk” to be delivered all at once.

The impactor would have to be bigger than this minimum amount of 3 million tons. To account for this amount of dust would require a sphere of silicate about 130-135 meters in diameter. If the dust is found further afield in Antarctica or if not all of the body reached the ground as dust (it never does), you are probably talking about an impactor at least a quarter-kilometer in diameter.

As an illustration of this problem, everyone seems to agree that Tunguska was an airburst of an object that had to weight at least 50,000 or 100,000 tons, and yet no convincing traces of the “impactor” can be demonstrated. This suggests that if three million tons of Datura-type dust reached the ice surface, the “impactor” could have been much, much larger than three million tons.

Interestingly, ALL the news stories are ignoring the most amazing part of this story. There are TWO dust layers BOTH found 2000-3000 km apart. One is at 481,000 years ago and the other is at 434,000 years ago. Both produced big traces at both locations. So, we are talking about TWO big impactors in less than 50,000 years.

One of the questions worth asking when talking about recent big impactors is “Anymore at home like you?”If, as Bottke says, the parent body was about 15 km and we have a 10 km body and six 1- to 2.5-km-diameter asteroids surviving from its breakup, they account for only 40% of the parent body. 60% of the original body is unaccounted for.
Did all 60% get turned into dust? Or are there some undetected sub-kilometer bodies left over? 60% of that parent body is enough to make up 16,000 500-meter bodies. Even if 90% was turned to dust, that leaves 10% or enough for 1600 500-meter bodies. 1600 such bodies is more than enough potential impactors. So is 1% or 160 500-meter bodies. Of course, in the real world there is a variety of sizes created by a collision, but there seems to plenty of rocks to go around.

Two massive airbursts over Antarctica in 47,000 years is pushing at the limits of shrugging things off as random chance. Antarctica is a mere 2.5% of the Earth’s land surface. If two such bodies fell on it in a short window of time, we are dealing with a “shower.” Shouldn’t random chance suggest 40 times as many impacts for the rest of the Earth in that same 47,000 years?

Has anyone gone into the freezer and pulled the old Greenland ice cores for 400 to 500 kyears ago? Can this type of analysis be performed on Greenland cores? They appear to only go back 500,000 years, are less complete at this age, and Greenland has only 14% of the area of Antarctica. I can’t find any studies that show cosmic dust in the ice at that age (or that looked for it).

The discovery of a fresh asteroid family has generated spectral studies of their surfaces, mostly to gauge the degree of “space weathering.” Here’s the result of optical spectroscopy using the large Gemini telescopes:http://www.gemini.edu/node/11082

They say:    “Optical spectroscopy from the Gemini telescopes has revealed a relatively uncommon type of asteroid in the main-belt for the first time… They found that spectra of asteroids in the newly discovered Datura family have a deep absorption feature near 0.8 microns which classifies them as ?Q-type? asteroids. This spectral feature is produced by silicate material, in particular olivine and pyroxene. Most interestingly, the spectra of Q-type objects are well-matched to the most common type of meteorite found on Earth called an ordinary chondrite (OC).”

The full paper on the spectra is available at:http://adsabs.harvard.edu/abs/2008A%26A…486L…9M
OC’s are only “ordinary” because they’re common in this geological era. 75% of all current falls are OC’s. There is a logic to the suggestion that geologically recent falls come from the most recent asteroidal breakups. Are today’s ordinary chondrites common because they all came from the youngest families like Datura? Some think so. (The Datura spectra is an almost perfect match for the OCFAYETTEVILLE):http://www.sciencedaily.com/releases/2008/07/080710103903.htm

This odd because Fayetteville is a breccia of two very different materials, and there is lots of argument about it. Perhaps it dates from the breakup and one of the materials is that of the disrupting body and the other is from the Datura family parent body (or Datura itself).Nobody knows (yet).

It all makes me wonder if there is any other evidence of numerous impacts elsewhere on Earth between 400,000 and 500,000 years ago?

Sterling K. Webb

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