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]

Thank you for that, Don, very informative (and very pretty movies). So there is a clear distinction between hot melt spherules being impelled downward by the original momentum of an incoming bolide and cooler slower-moving accretion lapilli that form in the cloud after a ground blast - and these latter do not fit the present case.

[dburt]

Thank you for that, Don, very informative (and very pretty movies). So there is a clear distinction between hot melt spherules being impelled downward by the original momentum of an incoming bolide and cooler slower-moving accretion lapilli that form in the cloud after a ground blast - and these latter do not fit the present case.

I think you basically have it, although I can think of no compelling reason why the hot particles being impelled downward need be melts, or spherules, as far as that goes (especially by the time they hit the poor mammoth). After major impacts, uniformly sized spherules can form by accretion of bulk solids from vapors, melts, and/or sticky particles while they are tumbling chaotically in a turbulent cloud. They are spherical because of the chaotic manner in which they grow (whereas tektites, glassy impact melt droplets that "splashed out" of the crater and congealed in the air, tend to be teardrop shaped or even more irregular). Perfectly spherical carbonate ooids (in oolitic limestone) are believed to form chaotically also, as wave action rolls them about during growth.

Of course, spheroidal growths called concretions (compare carbonate pisoids) can also form by direct chemical precipitation from aqueous fluids, but this requires rather special conditions (slow or no fluid flow, homogeneous physical medium, widely-spaced uniform nucleation, chemical driving force uniformly applied, etc.). Usually, concretions (also pisoids) are rounded but non-spherical (flattened and/or elongated), of various mixed sizes up to quite large, and concentrated (clumped together) at some sort of chemical reaction front (as where different brines have mixed with or diffused into each other). Pardon the mini-lecture, but I just can't stop myself when it comes to those misunderstood lumps and clumps ...

-- HDP Don

[ngunn]

I think you basically have it, although I can think of no compelling reason why the hot particles being impelled downward need be melts, or spherules, as far as that goes (especially by the time they hit the poor mammoth).

Indeed not though the bone/tusk damage suggests roundish and rather uniformly sized 'shot', which was why I particularly wanted to consult you as our in-house expert on Earth impact phenomena (and particularly sherules). I think most of us have a lot of imagining to do before we can envisage the full horror of a cosmic impact. You've probably done it already.