[meteorite-list] Impact Question
From: Barrett <BarrettWF_at_meteoritecentral.com>
Date: Mon, 25 Apr 2011 02:44:41 -0600 Message-ID: <026e01cc0325$06d99da0$148cd8e0$_at_net> Dear Mr. Sterling K. Webb, Simple questions usually dictate a simple answer, which is what I gave, and is essentially correct. Unlike you, I wasn't nit-picking. If you want to nit-pick, I can do that also. There is no such thing as a perfect conversion when it comes to energy/matter conversion, not even with the so-called BIG BANG. To nit-pick, your statement: " Plasma ball, a certain temperature, a certain energy -- that's the whole story, because that's all there is left." Is incorrect as it assumes a perfect conversion. This simply isn't true in the context of the question he asked. Simple evidence that your statement isn't so is the worldwide iridium deposition at the K/T Boundary is directly attributed to the event cited in the original question- "Chixalub impact" Our (mankinds) best attempt at nuclear conversion is only a few percent of the available fusionable material. Which this is a good thing or the somewhat wrongful name for the HYDROGEN BOMB would have eliminated all life on earth when first tested, as was feared by many. (which would have made the original question, my answer and your uncalled-for drival a moot point as we wouldn't be here for it) Your over-reactive reply to my answer is why many people on lists don't get involved in answering questions. For the most part your reply was technically sound and eloquent, but the attitude " Let's get our physics straight." I feel was uncalled for, offensive and downright nitpicking. For his original question, bringing "(Entropy? Don't ask!" is totally uncalled for, demeaning and poor usage of () marks as you left the closing ) off. NITPICKING huh? ENTROPY as part of your answer IS incorrect as he asked a SIMPLE question, not a technical one. I could keep going, but.... 'Nuff said.... -Barrett -----Original Message----- From: Sterling K. Webb [mailto:sterling_k_webb at sbcglobal.net] Sent: Monday, April 25, 2011 12:28 AM To: Barrett; 'Stuart McDaniel'; 'meteorite list' Subject: Re: [meteorite-list] Impact Question Stuart, Barrett, List, Let's get our physics straight. The mechanisms being talked about here: "burning from entry" and "inertia... travel[ing] into the earth" are missing the point. These are not "causes," but rather "effects." All isolated, disconnected bodies have a certain amount of energy in them. A small asteroid traveling through space has energy that can be described in many way depending on what other body you reference it to. All bodies in free motion in a field, like a gravitational field, have a potential energy determined by their position and their motion. Imagine you are standing in your backyard with a nice chunk of rock in your hand. Motionless in your hand, held up by a force from your muscles equal to the Earth's gravitational pull, it has no kinetic energy at all, because it isn't moving. But it does have potential energy. It you were to release it, that rock would move under the invisible influence of the mysterious gravity field that we all take for granted because we've lived in it all our lives. It would start to fall... It would speed up as it fell and acquire an increasing amount of kinetic energy, more the faster it went. Oddly, its total energy wouldn't change. No, its potential energy would decrease as its kinetic energy increased because it was getting closer to the center of the gravitational field, a position of less potential energy. And the total energy of the rock, the sum of its potential and kinetic energy would be constant, unchanging. Dropping rocks is not that exciting, so let's throw that rock straight up with all your strength, as high as you can toss it. It's going very fast when it leaves your hand, but as it rises and gets further away from the center of the Earth's force field of gravity, it goes slower and slower. Its potential energy is increasing from its new position at the expense of its declining kinetic energy. Finally, all the energy you gave the rock from your muscles will be transformed into potential emery of position in the Earth's gravitational field and the rock's kinetic energy will be zero -- it will be standing still, just for a split second, at the top of your huge toss, before... It starts down, down, gaining speed all the way, as its increased potential energy is turned back into kinetic energy. Better not let it hit you... But through the entire exchange, its total energy hasn't changed, but it has been transformed from one kind of energy to another and back again. Enough about little rocks! What about BIG rocks, like Chicxulub? Well, the story is the same. If it's going about its business and a planet happens to get in its way, it will come to a very rapid stop, planets being the equivalent of a solid brick wall 8000 miles thick. Whatever kinetic energy that big rock had, it would be transformed into other forms of energy, which would act on whatever matter was present and cause another transformation of energy, etc., until the vast majority of those energies would turn into the Lowest Common Denominator of Energy -- Heat. Heat is just the random motion of everything and because of the tendency of entropy to always increase everything tends toward the same random motion, which we call "temperature." (Entropy? Don't ask! I'm only allowed One Mystery per Met-List Post...) What's odd about kinetic energy is this: it increases with the square of the speed, like this: Energy = Mass x 1/2 x Velocity^2 Double the velocity; four times the energy. Triple it, eight times the energy. Ten times the speed, 100 times the energy... Let's say I take a tiny pellet like a bullet and throw it at you as hard as I can, say, at 35 meters per second, you would jump up and yell, "Ouch! What the heck did you do that for?" If you took the same pellet and mounted it in a cartridge, put it in a gun and fired it back at me at 700 meters per second, I wouldn't say much, being dead. Twenty times the speed, 400 times the energy. Now, let's get to that asteroid. Contact velocity with the Earth, after its initial speed has been augmented by the pull of the Earth's gravity drawing destruction down on itself? Let's say 35,000 meters per second (a perfectly reasonable figure for a mildly eccentric asteroid). Gram for gram, that big asteroid has 1,000,000 times more kinetic energy than the pellet I threw and 2500 times more kinetic energy than the same pellet you fired back as a bullet. It has more energy per gram than the amount necessary to melt it, in fact, many times more than is needed to melt the average rock or a chunk or iron, more energy than is needed to vaporize rock or the chrome-nickel-steel of a meteorite. Energy can't be destroyed, only transformed. It takes a little over a joule to melt a gram of rock; that's the kinetic energy of that gram traveling at the sedate velocity of a mere 2100 m/s. A good-sized, high-speed impactor would turn to plasma with close to 100% efficiency. And since mass and velocity are the ONLY things that matter, it makes no difference at all whether a truly big impactor is made of rock or of iron or of pure water ice or high-density styrofoam composite or tightly bundled goose feathers --- ENERGY is the ONLY thing that will determine the outcome, if the energy is above a certain rather modest threshold. We use the same units of measurement to describe big impacts as we do nuclear weapons, because both consist of the same thing: a super-hot ball of plasma on the surface of the planet. All other defining characteristics of the event have vanished with a single second or two. Plasma ball, a certain temperature, a certain energy -- that's the whole story, because that's all there is left. The mechanisms that caused it, nuclear reactions or kinetic energy, don't matter anymore. Only the result... which is the same. Impacts are, of course more energetic than nuclear firecrackers. Chicxulub specs out at 100,000,000 megatons. The largest thermonuclear weapons, the "super-city-busters, like the US B53 9 megaton device (no longer in service) are of the range of 5-10 megatons. "Ordinary" city-busters, like we would have used on Moscow or they on Washington, were 1.0 to 1.5 megatons, like the B83 1.2 megaton device, the largest US thermonuclear device in service today. You would have to build a pyramid of 80,000,000 B83 city-busters and light them up all in the same instant to create a Chicxulub. I doubt we ever had more than a few thousand B83's and the entire nuclear arsenal of the world at its peak wouldn't have totaled 50,000 megatons, or less than 0.05% of a Chicxulub. Kinetic energy is a more dreadful and potent power than the clever trick of nuclear reactions. Which is why I find it so strange that some geologists dismiss the life-extincting potential of major impacts. Me? I think we've been dam lucky. Sterling K. Webb Received on Mon 25 Apr 2011 04:44:41 AM PDT |
StumbleUpon del.icio.us Yahoo MyWeb |