Prehistoric meteor shower? Options

[djellison]

Yeah - the maths of this makes the likelihood of a particle of that size, traveling fast enough to do that damage very very small.

Doug

[dvandorn]

Sounds like we ought to move this discussion to the "Tiny Craters" thread over in the MER forum...

-the other Doug

[nprev]

...makes the likelihood of a particle of that size, traveling fast enough to do that damage very very small.

Doug

I'm inclined to agree, but damned if I can think of another mechanism more effective then an airburst. This may have been an extremely rare event (thank God; it sounds truly ghastly).

oDoug...yeah....makes you wonder a bit, doesn't it? This sort of thing may well be far more common on Mars, since the atmospheric drag would be so much smaller on frags. Better make sure future explorers bring Kevlar flak jackets...I'll donate mine in six months when I retire from the Reserves!

[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.

[PDP8E]

Fascinating mystery!

Airburst? rocky meteors maybe...but iron ??

The power to launch these little BBs so near to beasts and it doesn't kill them?

We need to find the site.. somehow...and let the geologists take over...

this is a juicy story, I hope it pans out for some lucky young ground pounder!

[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

[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.

[dburt]

...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.

Thanks much for the Christmas present (compliment), but I'd hardly classify myself as an impact expert. I had no particular interest in or experience with impacts until the very first MER images started coming back. Then, when the images didn't look very much like what they were supposed to (i.e., evaporitic lake deposits with concretions), I had to ask myself, what else could they possibly be? And impact deposits seemed like the only reasonable alternative to me and to my colleague Paul Knauth, and still do. Since then I've been trying to educate myself, but it hasn't been easy. I doubt if anyone, least of all me, could fully envisage a cosmic impact, although personally I might refer to the "wonder" of it unless I was in the target area. But that's just the scientist in me speaking.

-- HDP Don

[dburt]

Yeah, I read this one the other day. Very peculiar (and, really, more-or-less unbelievable)...

I'll also quote Helvick: "I can't see how this could be possible." These two early reactions here at UMSF (very different from mine, I freely admit) have been formulated in far more detail, with documentation and numerous literature citations, in the January, 2008 issue of GSA Today, the monthly newsletter of the Geological Society of America, p. 37-38, here:
http://www.gsajournals.org/perlserv/?reque...;issn=1052-5173
The authors are Nicholas Pinter and Scott Ishman of SIU and the full 2-page article (amazingly outspoken) is here:
http://www.gsajournals.org/perlserv/?reque...2FGSAT01801GW.1

Basically, using phrases such as "observations and claims so wild," "Frankenstein monster," "runs roughshod over ... evidence," "ignore extensive literature," and "played out primarily in the popular press," they ascribe the microparticles and microspherules to the normal constant infall of sand-sized micrometeorites, plus normal wildfires, and the extinctions of giant mammals to conventional causes (e.g., climate change and the influence of early humans). Just FYI, and DBETYR (don't believe everything you read).

-- HDP Don

[nprev]

Boy howdy... ...yeah, I'd call that "outspoken" all right, Don.

Can't disagree, though; always like to see pushback & a rally cry for critical thinking, esp. when presenting very unusual (in this case, nearly anomalous) findings. Seems like there's a very human tendency to assign recently recognized phenomena--in this case, terrestrial impact events--as probable causes for poorly understood observations.

Still...this particular case is bizarre. I'm keeping my mind open.

[TheChemist]

If I understood well, this refers mainly to a 12,9 ka impact that seems to have been proposed by the same author in different abstracts of the same (?) conference. I think these abstracts are not available yet on the net.

The abstract which is the subject of this thread refers to a supposed 30-35 ka impact. Would not a "normal constant infall of sand-sized micrometeorites " result in a more common occurence of mammoth skulls looking like those described in this thread ?

So although the criticism is good and absolutely necessary against the general "easy solution through impact" fever, for the specific case discussed in this thread I remain in the dark.
And I am sure there are a lot more scientists (maybe 1 out of 2) responsible for the loss of scientific credibility in the recent decades through "extensive advertising" of their own work. A bit too outspoken.

[dburt]

If I understood well, this refers mainly to a 12,9 ka impact that seems to have been proposed by the same author in different abstracts of the same (?) conference. I think these abstracts are not available yet on the net.

The abstract which is the subject of this thread refers to a supposed 30-35 ka impact. Would not a "normal constant infall of sand-sized micrometeorites " result in a more common occurence of mammoth skulls looking like those described in this thread ?..

Actually, as I recall (I don't have it in front of me), the GSA Today article discusses both events - the older one is supposed to have produced mega-tsunamis (debunked by the article as eolian dunes); the younger one the extinctions. Recent posts here seem to have been referring to the younger extinction event.

-- HDP Don

[nprev]

Just adding this as a data input: seems that fragments from a conventional grenade (0.5kg of explosive) can travel as much as a mile (1.6km) from the detonation point. (Would cite the reference, but it's not publicly accessible, though not classified in any way.) Don't know if anyone's ever measured the frags' temperatures, though.