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In a message dated 5/29/2002 4:14:09 PM Eastern Daylight Time, jrccea@bellsouth.net writes:


As you know, for massive impactors the approach angle doesn't have too
much of an effect on crater shape or primary direction of blast wave
(the butterfly blast pattern from the Tunguska impactor appears to have
been due to a quite small event).



Actually, there is much to the angle of attack when it comes to impacting and the resulting blast wave..... no matter what the size of the impacting body. For the most part, it all has to do with the time the object is passing through the atmosphere... regardless if it is large or small.... How much atmospheric drag does it suffer and for how long?..... How much deceleration takes place?.... There is also the matter of where the object is coming from to begin with. A comet obviously would be traveling much faster than an asteroidal body..... And then there is the factor of where the comet was coming from to begin with..... The Oort Cloud and the Kuiper Belt. The slower an object is traveling, the more likely it will survive its passage through the atmosphere in order to impact the ground. Too fast, it will vaporize or be sheared apart and then slowed down big time by aerodynamic drag as the pieces fall away from the main body. The steeper the angle an object is traveling! , ! ! the better chance it has of impacting the ground and not skimming out of the atmosphere or shearing apart before the main body hits. If a projectile is large enough, it can survive passage through the atmosphere more or less intact and strike the ground or ocean at a high velocity.

The real, main factor in the survivability of the impactor is the material strength and density of the body and its velocity at the time of its initial encounter with the atmosphere. For example, when it comes to a stony body, the size for survivability appears to be about 100m. Your Tunguska Fireball in 1908 appears to have been from a debris field of a comet who's name escapes me at the moment.... It was probably a carboniferous chondrite... and only about 50m across. It smacked the atmosphere at an oblique 30-35 degree angle and detonated with a force of 40 megatons at about 6km above the ground..... generating a blast wave in the butterfly pattern you mentioned that felled trees across some 2,000 km of tundra and flashed burned some multiple thousands of reindeer... leaving not a crater to be found...... let alone pieces of the object itself (though there have been reports of cosmic particles found in tree resin).The probable high speed, low angle (long time flying i! n ! ! the atmosphere), and weak nature of the material most likely was the cause for its detonation. Your iron meteoroids sometimes reach the ground with a substantial fraction of their infall velocity. If loosely bound together they can form swarms of small impact craters.... like that of the Sikhote-Alin in 1947 (I have pieces from it)..... If large and/or compact, one massive crater will form. At 1.2 km across and 200m deep, the roundish Meteor/Barringer Crater in Arizona was formed from an iron asteroid only 40-50m across that entered the atmosphere at a little less than 90 degrees to the horizon. It detonated with only a fraction of Tunguska's power..... coming in at only about 3.5 megatons. The assortment of relatively unshocked meteorites found in the crater's vicinity suggests that numerous fragments were detached and decelerated during atmospheric passage of the principle body, which may have broken apart just before impact. This is evident in the shape and structure of t! he! ! crater, along with the resulting circular blast field.

This brings me to the angle of attack issue and the resulting blast field. If you look at certain craters on the moon..... and on other planets such as Mercury, Venus, and Mars, (we see this on Europa and Calisto too) not all of the craters are circular..... Some are elliptical with ejected blast material and such blown downwind from the crater..... This was a clue that was used to figure out what was going on with the odd elliptical appearance of Chicxulub.... Resulting experiments showed that to form a crater like that of Chicxulub, the body traveled through the atmosphere at about an oblique 20-30 degree angle to the horizon. The blast energy didn't go into the ground as with a steeper degree impact..... but instead sheared off straight into the atmosphere.....the melt-sheet blowing downrange.... right into North America. It's not the size of the body that dictates the crater's shape (circular or elliptical)..... It's the angle of attack.... and that's if it even surv! iv! ! es to hit the ground...... which is determined by its velocity, composition, density, and once again, by its angle of attack relative to the earth.

See this website from Brown University.... It has some details on the experiment done to determine what was going on with Chicxulub along with pictures of the impact angles and their results...
http://www.brown.edu/Administration/News_Bureau/1996-97/96-041.html

During the formation of the simple bowl crater, a compression wave spreads outward from the center into the impacted material. A rarefraction wave moving behind the shock front mobilizes the ejected material in a way that forms a conical sheet. Being engulfed in this shock wave, the meteoroid melts and is partly vaporized. Simulations indicate that for all but very low angles of impact, less than 30 degrees, the crater produced is circular, regardless of impactor size and mass.

For craters excavated by those impactors being several km across, typically, the sides of the transient cavity slump inward, enlarging the crater's final width and often forming terraces. The inward slumping mass and upward rebound of lower lying rock creates a central peak or plateau. The largest impact basins also sometimes exhibit multiple, concentric rings. Chicxulub has this type of appearance, though it is elongated into a slight elliptical form, showing that the impacting body transversed the atmosphere at roughly less than 30 degrees.

An oblique impactor with an incident angle of only 10 degrees or less will produce a crater that is shaped like a horseshoe. These observed semicircular craters on earth and other planets/moons can be reproduced in the lab, showing that low incident angles are needed to produce them.

Anyway, to sum this up... Angle of attack does have bearing on what the impact crater looks like, regardless of impactor mass and size. At the moment of impact, the impactor's huge amount of kinetic energy is unleashed at a single point in the planet's crust.... This sudden, focused release is like a detonating bomb.... resulting in a typical round crater with ejecta thrown equally in all directions regardless of the direction for which the bomb arrived..... This doesn't make sense if you throw a rock in mud. The rigid impactor makes a crater that forms to its dimensions. Astronomical impacts are completely different since the tremendous amount of kinetic energy that the body carries makes the shape and direction of approach of the impactor irrelevant..... UNLESS the impactor approaches at a very low, grazing angle. If the angle of impact is close to horizontal, the bottom, middle and top parts of the impactor will actually strike the surface at different points i! n ! ! time. Thus, instead of releasing energy at a single point, the energy is spread out along a line.... and ellipse.


> Too bad for the impactor lovers..... The blast wave apparently didn't
> make it to Montana.

Proof?  Quantification?



Well, as I mentioned throughout my original post, the salamanders, turtles, birds, etc, didn't all burn alive in Montana. So, the melt sheet must not have reached them. The blast wave.... as in shock waves and such?.... Sure... The hurricane type winds probably did blast the area. If the melt sheet did indeed reach them, I guess one could say that some of them could have hid somewhere..... But at the temps we are talking about, hiding wouldn't have done you much good.... The air itself would have been on fire... worse then a pyroclastic flow I suppose.... and nothing survives in the paths of those things.... even if it hides underground. The articles I mentioned from the "Encyclopedia of Dinosaurs" also lends proof and quantification to why I said what I said. I don't know if there is any indication in the fossil record of massive fires from this particular area either...... Soot, ash and such ! sh! ! ould be preserved if such a melt sheet did pass, incinerating everything in its wake. This would be true even if we had a Tunguska type thing happen where the fires burned for less than a minute but were then blown out by the blast wave. There is always more to read, so I could have missed something and my assessment is thus wrong.


What about the possible earth-splitting impact from a Mars-sized
impactor that is conjectured for the origin of our moon?



Oh, that most definitely happened... No need for me to get into it here since there's no real place for it on the DML..... Only talk dinosaur science they tell me... :-)


>  As far as I know, we don't have a firm understanding of the weather
> patterns during this time which would have dictated the environmental
> effects caused be the volcanics and the impact(s). This means that the
> dinosaurs were not under stress before the impact.

I must be interpreting this incorrectly.  If I understand you correctly,
you are saying that our lack of a firm understanding of the cocurrent
weather patterns means that the dinosaurs were not stressed before the
impact?



Take out my obvious typo...... What I meant to type was "This means that the dinosaurs might not have been under stress before the impact."

In looking at what was going on in Montana before the KT... and saying that the evidence says that the dinosaurs of at least that particular area were already under stress do to changing environmental factors before the impact.... This lead to their recovery from the added impact imposed stresses to be a hopeless one. In comparing this scenario to the Triassic/Jurassic extinction, for which dinosaurs obviously survived, I'm saying that maybe the dinosaurs at this point in time were not being stressed very much by the Siberian traps and etc.... and when the impact(s) occurred, they were "ready" for it per se... and thus suffered very little or not at all from the impact(s) effects.   All I'm talking about here is that your weather patterns...... how the air is moving around the globe.... high and low level winds..... jet streams.... all that jazz...... would have dictated how, where, and why certain areas were effected by volcanics and impacts... and why ! ot! ! her areas were not affected.

Kris