Prehistoric meteor shower? Options

[tty]

A real weird news story from Nature about meteor damage to pleistocene fossils:

http://www.nature.com/news/2007/071212/ful...s.2007.372.html

If traces of this meteor shower has been found in both Siberia and Alaska as the story implies, then multiple impactors must have been involved. Such small meterites would lose speed quickly so the airburst must have occurred at fairly low altitude.

[ngunn]

I really think we need Don Burt here, but I think he would say that the 'air' through which the particles were falling would also have been pretty hot, like the cloud from an explosive volcanic eruption. The animals would have suffered burns from that alone, and the spherules would have experienced less cooling (and less deceleration) than if they had been falling through ordinary cool air. The whole collapsing column of heated gas and solids would have been descending on them. I imagine them pinned to the ground and in considerable distress.

[dburt]

I really think we need Don Burt here, but I think he would say that the 'air' through which the particles were falling would also have been pretty hot, like the cloud from an explosive volcanic eruption. The animals would have suffered burns from that alone, and the spherules would have experienced less cooling (and less deceleration) than if they had been falling through ordinary cool air. The whole collapsing column of heated gas and solids would have been descending on them. I imagine them pinned to the ground and in considerable distress.

Well, this is the 3rd time my name has been mentioned here, and two people sent me personal messages, so I guess it's time to share my utter ignorance. I wasn't at AGU and haven't even downloaded the poster, so I only know what I've read here and in the news story. First, other than having been caused by impact, these observations probably have little direct relation to the slightly Ni-enriched hematitic spherules observed on Mars, particularly at Meridiani (with some spherules also spotted at Gusev). From their uniform size and sphericity, and internal granularity, we presume that the Mars spherules could have formed by particle accretion in a condensing sticky, steamy surge cloud (i.e., that they are "accretionary lapilli" or a related species), with turbulence counteracting the force of gravity as they got larger. When the spherules hit the ground rolling and bouncing, they were probably travelling no faster than and were not much hotter than the turbulent particle-rich ground-hugging cloud as a whole. If you were a mammoth standing in the way, you probably would have been knocked over and possibly shredded and parbroiled, but the spherules probably wouldn't be embedded in your tusks, locally burning them, and you probably wouldn't survive.

The tiny embedded hot Fe,Ni metal particles discussed here seem to have been travelling somewhat faster than their medium, and a "blunderbuss" or shot-gun like effect has been suggested. That might not be too far off the mark, according to this second AGU-related story about a small directed blast for the 1908 Tunguska event which came out at the same time (and consistent with ngunn's hypothesis above):
http://www.sandia.gov/news/resources/relea...7/asteroid.html

So in the Mars spherule case you are talking about possible depositional effects of a distant large crater-forming (ground-burst type) impact, and in the Alaska case possibly about a Tunguska-like small (air-burst type) impact high in the sky, one that might have produced a narrow blast directed downwards.

Just my uninformed suggestion. Thanks to the badastronomy.com website for the original link to that second story.

-- HDP Don