Hola Sterling, All, > Don't want to do a full-court press here or have > a knock down drag out. The point of "atmosphere > not a factor in that size range" is merely that an > impactor that makes a one kilometer crater is not > going to be stopped from reaching the surface by > the Earth's atmosphere; that's all.
At a shallow enough angle, I might beg to differ...especially if we're talking about friable cometary material, but I really couldn't say for sure given my range of knowledge. > As for the angle of an impact, that is a datum > obscured by the event. There is no way to derive > the angle of impact from the resulting crater in any > except the smallest and weakest events. Any angle > of incidence over 15 degrees produces a round, > level crater. At 15 degrees or less, elliptical craters > are produced. I know as much...my point is that I might expect more damage due to the greater the angle of impact (to the horizontal - it was pointed out to me that we must be clear about whether we're talking about degrees in relation to horizontal vs. vertical) due to the combined effects of less atmospheric drag as well as greater force being turned directly into heat as opposed to moving ejecta farther, as would occur with an elliptical crater. I may well be wrong, but it seems to me that both of these might have something of an effect on the climatological effects of an impact. > Meteor Crater (aka Barringer) is low angle: > http://adsabs.harvard.edu/abs/1995Metic..30Q.567R > Gene Shoemaker's classic 1960 analysis that > established it was an impact crater, still a hot > dispute "in the day." A history of the dispute, > and in particular the finding that angle of incidence > has no effect on crater formation can be found in: > http://www.enotes.com/earth-science/barringer-meteor-crater > "Barringer and his 12-year-old son set out to > experiment with the formation of such craters by > firing bullets into clumps of rock and mud. Regardless > of the firing angle, the Barringers demonstrated > (and published their results in both popular and > scientific magazines) that the resulting craters > were substantially round. More definitive proof > was subsequently provided in 1924 by calculations > of astronomers who determined that forces of > impact at astronomical speeds likely resulted in > the explosive destruction of the impacting body. > Importantly, regardless of the angle of impact, > the result of such explosions would leave rounded > craters." I've been collecting meteorites for about ten years now; I know... > Such experimental crater-forming data show > that impacts above a certain energy level (a rather > low level, too) preserve only the fact of the total > energy required to produce them, as any really big > explosion does. Such factors as angle of incidence, > direction of travel, velocity of impactor, and/or type > of impactor may inferred from circumstantial traces > in a recent, well-preserved crater, if such traces are > present. Usually this only happens in "small" craters, > like the Arizona one. The bigger the crater, the less > the chance of such indicators surviving. I do read books, time permitting....this is a nice breakdown, but I know all of it already. > Jason wrote: > > > the likelihood of an impact occurring at an angle > > of ~30 degrees or less is simply unlikely. Possible, > > but unlikely - as in odds are that it would not occur > > this way. I'm not an idiot. > > Impactors large enough to make a respectable crater > do not have an entry affected by aerodynamic forces > nor altered by gravity after they encounter atmosphere > (and very little before). They do not deviate from their > cosmic vector in any substantial way. ANY angle of > incidence with the surface of the Earth is totally random > and ANY angle of incidence is as equally likely as ANY > other. Basic knowledge. Look it up. The "slow" 16 km/s > 500-meter rock heads for the future 3.98 mile diameter > crater site as straight as a bullet from a gun, pretty much. 1) I know that meteorites coming in at completely random angles. My point was simply that you appeared to have chosen a particular angle of entry for your calculations that in my mind seemed as though it might not represent the maximum possible amount of devastation given the size and composition of the impactor. My point was that the odds of a given impactor entering the atmosphere at *that* angle were small and that the likelihood of a body entering the atmosphere at a different angle that might be more penetrating, at least with regards to an ice impact, might be greater. > If you average all the directions together, you get 45 > degrees, just because you have to pick some direction! > That's what everybody does. I just got tired of typing in > "45," so I typed in "30" -- it makes no difference. 30, > 60, all the same. This is an event with only ONE parameter: > Kinetic Energy. Of course... > You've obviously read too much about the little rocks > that leave littler rocks behind for us to collect, with their > long curving paths and their aerodynamic braking, their > "falls" and all that meteorite frew-fah. Crater-makers are > not like that at all. For all practical purposes, think "straight > line" segment of a very large radius curve. I understand the bloody frozen rope trajectory of these things but what I'm saying is that regardless of whether or not they travel on a straight line, atmospheric breaking will have an impact (small, maybe only a few km/s difference in velocity, if that) on the objects velocity and thus the amount of KE released. > Jason wrote: > > > An airburst of a ridiculously large (lets say a 1 km > > object, for the hell of it) size would devastate, what, > > maybe a thousand square miles? Few thousand? > > Not enough to do any real damage... > > Tunguska was an object of perhaps 100 meters; its > zone of greatest destruction, the tree-flattening-and- > charring zone, was 2200 square kilometers, or 850 > square miles. The lethal-to-humans zone (LD 50) was > about ten times bigger than that. The flash was as bright > as the Sun 80 miles away. It was observed 600 miles > away. The barometric shock wave traveled around the > planet 2.5 times before dying out. (Stop beating the > horse...) But I just don't see how or why a 1km body is going to vaporize in such an explosion in the atmosphere. Don't get me wrong - it sounds like it would be an awesome event were it to happen, but I just don't see it physically occurring. > And you think that an object one thousand times > bigger (10^3) would be "not enough to do any real > damage"? My only suggestion: think again. If such an explosion were to take place, I don't doubt it. I doubt the explosion in the first place... > As for the general cause of all this argument, some > possible impact events in "recent" times proposed > by Firestone and an increasing number of collaborators, > that can only be determined by decades of field work > yet to be done and, hey! we can't do it. All we are doing > is sitting around like a bunch of ancient Greeks arguing > from "basic principles." This inductive approach (the > Greeks' greatest logical flaw) is essentially of very limited > utility. We can't do it? Eh, I hear the Greeks made some progress with all their sitting around and doing nothing...they may not have had quite the right idea about the atom, but they did manage quite a bit in such a manner. > What we can do is follow the hunt and wait for the > results, science spectators that we are, and enjoy the > arguments in the meantime. Then, even if the evidence > favors impact, we'll have unending decades of argument > about whether it could have "exterminated" anything. I > expect the present Dino and Permian disputes to extend > well beyond all our lifetimes. Hmmm, so this is all probably pointless anyways... > What I do know is that, twenty miles north of my house, > there was once a wall of ice that towered into the sky above > low clouds and bird flight, and it ain't there any more. And > I haven't seen a mammoth in ages, even tho Thomas Jefferson > asked Lewis and Clark to keep an eye out for'em. Right...for whatever reason. > Jason wrote: > > > Floods would leave geologic evidence, no? > > I live on the 200-foot-high edge of a canyon 20 to 30 miles > wide that was carved out in a geological wink of the eye (few > hundred years) by the melting of a mile-thick glacier, and I > think, as I watch an ice cube drip, that was awfully quick. > The mechanism proposed is a long slow melt as conditions > warmed, the formation of vast glacial lakes, the rupture > of ice dams and gravel-and-rubble dams, followed by very > and rapid drainage. The evidence is the Mississippi River > Canyon with its wave-formed canyon walls that undulate in > 500-foot-long periods. Hey, there's still a little water left in > the bottom of the Canyon -- the Mississippi River. It's just > a trickle now... Such an impact would probably shatter surrounding ice for hundreds of miles, were it to occur, and, I would bet, cause massive flooding as several km^3 of ice turned into boiling water and steam (much of which would condense on contact with colder air, forming a relatively warm rain on surrounding glaciers). I don't see how large flash-floods would be avoidable in such a scenario...say what you will about the Mississippi river, but I view your analogy as rather unrelated. > The Mississippi River is a spanking brand-new river; > there was never any river here before the end of the last > glaciation, nor any of the rivers between the Rockies and > the Appalachians; North America always drained sluggishly > north to Hudson Bay before the glaciations. As for impacts, > even huge ones, melting the glaciers? Forget it. You know > how much energy it takes to melt a cubic mile of ice? The > equivalent of 330 MegaTons of TNT, which is the thermal > portion of a GigaTon impact, to remove one cubic mile > from the thousands and thousands of cubic miles of ice > that made up the ice cap. > What does a GigaTon impact take? A 300 meter rock, > more or less, leaving a 3-mile crater on the ice cap, less > than a quarter-mile deep. A pinprick to a glacier, a scratch, > a mosquito bite, a jumbled melt zone that quickly re-freezes > and doesn't leave a trace. But it might have considerable > atmospheric effect and deposit traces for thousands of > miles in every direction. You spoke of a 10km impactor a while back...it really depends on the dynamics of each individual impact, but I would suppose that it might be possible for one not to leave a trace were it to occur far enough North. That said, I still favour saying "I don't know," seeing as you're simply putting forth the solitary scenario that would provide us with no solid evidence as opposed to looking for any traces of any other sort of event that might exist... > No meaningful damage to the physical planet is not > the same as no meaningful damage to the creatures that > live on it. Your hypothetical one kilometer impactor is > a 50 GigaTon impact with an 8 to 10 mile crater about > a half-mile deep; it leaves a big lake for a while, but its > atmospheric effects spread around the entire globe and > might have serious effects on climate. But neither of these > will leave any trace "on the ground" other than the distant > deposits of the type talked about. A physically trivial > event but perhaps not biologically trivial. Hmmm, possible, and an intriguing thought, but one that I think should be thoroughly researched/explored, and other evidence looked into before a conclusion is reached... As I said before, the airburst model seems highly unlikely to me, but the ice-impact one...I view the odds of it having occurred as small, but the dynamics of it at least appear to be logical. Regards, Jason > Sterling K. Webb > ----------------------------------------------------------------------- > > ----- Original Message ----- > From: "Jason Utas" <[EMAIL PROTECTED]> > To: "Meteorite-list" <meteorite-list@meteoritecentral.com> > Sent: Thursday, December 20, 2007 3:11 AM > Subject: Re: [meteorite-list] Mammoth Stew - first you cut up the Mammoth > > > Hola Sterling, E.P., All, > > > Concerning recent impacts (<12,000 years old), what > > I've noticed over the years is that some people go > > into denial, and those denial mechanisms are sometimes > > really pretty bizarre. It's tough to accept on a gut > > level that as things now sit you, your family, your > > friends, everyone you ever knew or loved can be blown > > off the face of the Earth in an instant without a > > minutes warning. > > I think you missed my point, because I accept this fully. > > > But that's exactly how it is, > > Yeah... > > > and > > that's exactly what happened to some of our fellow > > human beings in the recent past. > > Well, probably, though we have no real proof of their having been > blasted to death *anywhere.* > > > So, Jason, you wrote: > > > > >Right, but seeing as the effects from the event of > > >which we speak differ greatly from those of your > > >comparison, it seems an unworthy one to make. Yes, > > an >unknown phenomena might create such a set of > > effects > > >as are geologically evident, but just saying "it's > > >possible" is something that I acknowledge as well; we > > >all know that Tunguska events occur and that, > > >evidently, astronomical events that create the > > >geological evidence that we've found occur. But that > > >still in no way ties the two together. > > > > Fact is, Jason, the Tunsguska impactor was viewed > > coming in, and spherules from it have been recovered. > > Sorry to disappoint you, but it wasn't a flying > > saucer. > > No shit. It was most likely a piece of a burnt-out comet. My point > was that saying that there was some sort of cosmic event that left no > geologic trace of an impact and saying that Tunguska left no trace > (whatsoever) proves nothing, given that 1) the geologic evidence for > each event is completely different, and that 2) we have no proof that > what occurred was due in any way to an event similar to that which > occurred in Tunguska. > > > Sterling, you wrote: > > > > >Atmosphere not a factor in that size range. > > > > Yes it is. Another factor in lunar crater distribution > > is later coverage by dust and removal by later > > impacts. > > I'm not sure what you're trying to say here...that craters on Earth > are hidden by ejecta blankets from other craters that have hidden > them, or that ejecta doesn't move as easily (or moves more easily?) > through an atmosphere...what? > > > Jason, you wrote: > > > > >I don't know where you draw the 1km crater line, as, > > >in my opinion, such a body might well break up if it > > >entered the atmosphere at a shallow angle, but who's > > >to say.... > > > > Well, airbursts can be more devastating then ground > > hits, in terms of overall effect. We know compression > > propagation in impactors, and 1 kilometer crater seems > > to be a good guess as to airburst versus ground > > impact. > > No. > Sorry, but that's only true to a limit. A 10km impactor isn't going > to explode in the atmosphere for any reason that I could conceivably > think of, and it's going to make one hell of a lot of destruction upon > impact... > A 1km object would most likely break up (as opposed to explode in a > violent airburst) if it managed to enter the atmosphere at a slim > angle without glancing off, as the energy required to vaporize such a > large amount of matter is simply too large as to be reasonable. I > don't think there's a great chance of it breaking up in general, but > if we're talking about cometary material, I do hear that it's > friable... > > > Jason, you wrote: > > > > > A thirty-degree impact is highly unlikely, > > > > unsupported and most likely wrong. > > It's less likely than an impact occurring at a greater degree measure, > unless I'm mistaken. > My point is that looking at all possible impacts, the likelihood of an > impact occurring at an angle of ~30 degrees or less is simply > unlikely. Possible, but unlikely - as in odds are that it would not > occur this way. I'm not an idiot. > > > > and I'm thinking that an iron impactor would do a > > bit > more damage than a comet. > > > > wrong. See airburst versus ground impact, above. > > Wrong, because such a comet would (in all likelihood) not airburst. See > above. > Also, since we're talking about airbursts, why not look into them vs > craters. > > An airburst of a ridiculously large (lets say a 1km object, for the > hell of it) size would devastate, what, maybe a thousand square miles? > Few thousand? Not enough to do any real damage on the scale that you > mention (death of 90% of all humans on the continent, etc). Climate > change due to dust released by such an airburst might cause trouble I > suppose, but it was an ice-age anyways; they knew how to deal with > cold. > > An impact, on the other hand, should it hit either water or land, > would be infinitely more destructive. Instead of 1km^3 of material in > the atmosphere (since we're talking about a comet anyways, much of > that would be water, not particulates), you would get that 1km^3 as > well as whatever ejecta that was thrown out or vaporized (causing > global wildfires, etc - the sort of fires that would occur *only > locally* should we be talking about an airburst, because the fires > would only occur in the area underneath the airburst). The climate > change potential is exponentially higher, as is the amount of general > damage produced, not to mention how widespread it winds up being. > > > >Do you, by any chance, know what the composition of > > >the dust layer (if it would suggest such a thing) > > >points towards the composition of the body having > > >been? > > > > You're confusing two different impacts here, the iron > > one at 31,000 BCE and the cometary one at 10,900 BCE. > > Why? > > I've already taken care of the iron thing; you failed to address any > of my thoughts regarding the physical impossibilit*ies* of these > traces of iron embedded in the tusks having come from an airburst of > any sizable nature, and, since your only hope for a cometary event at > 10,900 BC revolves around some sort of super-airburst that seems > (according to the laws of physics), impossible, unless, of course, > we're talking about an ice-impact (which I view as physically > possible, though unlikely, given that I would believe that other > geologic evidence would result from such a necessarily large impact > (though I suppose the Great Lakes might have been formed by the > subsequent flood following the impact), etc, etc. > You'd get a big volume of water from such an impact, and I would guess > that surrounding glaciers would be fractured by the impact for > hundreds of miles. > Floods would leave geologic evidence, no? > > You have a dust layer, I'll grant you that, but drawing conclusions > from this without any other evidence seems like nothing more than > folly to me; find a piece of evidence and draw off of it, but simply > saying "this happened" because there's a layer of cosmic dust (the > likes of which we have never really seen before and thus cannot justly > identify) is pointless. By all means, explore, excavate, dig, do > whatever you'd like. > But try working out the physics of it. A cometary airburst of a body, > say, 1km in diameter, simply doesn't make any sense. Physically > speaking (I'm currently taking college-level physics), it just doesn't > make sense. Maybe you know of some laws regarding atmospheric > resistance that I don't, but unless some such laws exist, I'm > disinclined to believe just about everything you say. > > > >I just don't see much metallic residue coming from a > > >comet, though I suppose there would be some. > > > > It seems to me that the cores of the cometissimals in > > a comet have a nice metal content. That's where the > > iridium is, after all. > > No. > We don't know much about cometary composition, but there's no reason > (at all) to suspect that they formed around iron cores, as they are > undifferentiated bodies. Even chondrites, which formed much closer in > to the newly born sun (and thus have higher metal contents overall), > such as the H or L chondrite parent bodies, didn't have concentrated > iron cores (they'd be achondrites if this were the case). If they had > such cores, comets would look like Eucrites, and they wouldn't contain > any water. In general, it takes a body ~300km or more in diameter to > form an iron core. > Comets don't have metal cores, as far as we know. They're probably > CI1 material, through and through, though maybe with a bit more ice > thrown in. > > > Sterling, you wrote: > > > > > 5. You say, "most of the craters were formed before > > > the [recent?] timeframe." Well, that's exactly what > > > the argument's about, isn't it? This is the > > > comfortable, "that's all in the past" argument. > > > > You've got it: denial. Did I ever tell you the one > > about Homo wushan? > > They were mostly in the past. Impact rates have been declining. I'm > not saying that there's no chance that we could be wiped out tomorrow. > What I'm saying is that the odds are better for us living out the > next day than they were, say, two billion years ago. > That's a fact. > > > Jason, you wrote: > > > > >It's not all in the past, nor have I ever said such a > > >thing. That said, there were more impacts two > > billion > years ago than there are today, and you know > > that as > well as I do. > > > > Jason, if you're not in denial, then why try to make > > statements about the recent impacts, and then divert > > from the two impacts under discussion to the long term > > impact rate? > > Because you're misunderstanding just about everything I say? > > > Sterling, you wrote, most excellently: > > > > >Let's review the cratering history of the solar > > >system. After initial accretion, a tapering off. > > >Then, at 3.8 to 3.9 billion years ago, an intense > > >episode, the "Late Bombardment," followed by an > > >exponential decline for more than 3 billion years. > > >Then, at 0.6 billion years ago, cratering rates begin > > >to rise dramatically, until 0.4 billion years ago, > > >when they have increased fourfold in 0.2 billion > > >years. They again decline. until 125-100 million > > years >ago, when they increase, roughly doubling. > > > > A great summary, which leaves us with significant > > questions concerning meteorites and their parent > > bodies - and answers which are only now gradually > > being accepted. > > So you say that impact rates are less today than they have been > (generally) in the past. > Hmmmm....sounds like...... > Denial! > > ..Right. > > > > The role of comets, stellar encounters, Oort Cloud > > > shenanigans, asteroidal family dustups is all > > unclear > and yet to be pinned down. Good old > > ignorance. > > > > Yes indeed. Lack of science budget helps in this too, > > as well as the behaviour of certain individuals > > (ahem!) who control those budgets. > > Well, that and the fact that we simply haven't been observing for long > enough. > We have lots of information, but no Kepler to put it all > together...much of the research we're doing today is being wasted on > trying to find planets like Earth that exist unattainable hundreds or > thousands of light years away, instead of trying to figure out to save > Earth itself; we seem to be sidetracked...there's enough money to > manage it, but we really don't want to because, well, it's boring. > > > Jason, you wrote: > > > > >What I'm trying to say is that given that there are x > > >number of craters on the moon, we would need to know > > >the age of each to determine a good number for the > > >rate of impacts over the past, say, 50,000 years, > > >which would be relevant to this discussion. > > > > Undoubtedly this information will be provided by the > > next series of manned landings on the Moon. > > Well, hopefully, but the point is that I find it odd to believe impact > estimates in general, when I have little understanding of how > (predictably, or in general) they have been conceived. > > > Jason, you wrote: > > > > >Well, then climate change could well have been the > > >culprit - but what cause the climate change is, I > > >think, still open to discussion. > > > > Actually, no it is not. That discussion has finished. > > When you have a layer of extra-terrestrial material > > and impactites such as has been demonstrated to exist, > > fact replaces speculation. > > Well, assuming that the age of the layer and the climate change > correspond, and from my understanding, with regards to the supposed > 10,900BC event, they do not. > > > >And that those iron bits, if they are from a > > >meteorite, didn't come from the culprit that caused > > >the climate change, > > > > No one said they did, except you. > > You strongly implied a connection between the two, above in this same > message, not to mention previous emails. > > > Two different impacts, two different impactors. One > > that killed some mammoth, and another one that > > extincted them. Two impacts. > > "killed some mammoth" > You're looking at an event which was, at best, something like > Sikhote-Alin, and might have sniped a herd or two of mammoth. > Anything bigger and you would get a crater as well as significant > heat damage to the bones. Physics. The fact that the metal was hot > when it hit the bones, and that it occurs in such tiny fragments - on > the top of the bones, implies that you're looking at a very low > airburst of a very friable body at a very high speed. > And yet it can't have been too high of an impact because the particles > are quite spread out, considering that they were moving still at a > great speed and would thus have deviated little from their original > path due to gravity. Oh - and it was a rather homogeneous, friable, > iron body, as there have been no large masses found embedded in > mammoth corpses, etc...seeing as we're talking about cosmic velocities > here, it seems logical to conclude that size distribution would matter > little in such an airbust that 1) must have been very low and 2) must > have been, not an airburst, but rather a breakup event such as > Sikhote-Alin, because none of the tusks show signs of being burned. > > Regards, > Jason > > > good hunting all, > > E.P. Grondine > > Man and Impact in the Americas > > > > > > > > > > > > ____________________________________________________________________________________ > > Be a better friend, newshound, and > > know-it-all with Yahoo! Mobile. Try it now. > > http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ > > > > ______________________________________________ > > http://www.meteoritecentral.com > > Meteorite-list mailing list > > Meteorite-list@meteoritecentral.com > > http://six.pairlist.net/mailman/listinfo/meteorite-list > > > ______________________________________________ > http://www.meteoritecentral.com > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list > > ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
