|Oh the joys of being so sick that you can't breathe and your head feels like it's floating..... Instead of sleeping as I should be...... I stay up and write things for the DML :-)
In a message dated 5/30/2002 12:48:27 AM Eastern Daylight Time, firstname.lastname@example.org writes:
Then why is it that most all very large impact craters throughout the solar system are essentially (nearly) circular?
Graydon pretty much explained that one for you..... Thanks Graydon
> For the most part, it all has to do with the time the object is
Yes...... as I said..... Pretty much the time the object is passing through the atmosphere...
I'll leave that as an excercise for you, if you're interested.
Nice try..... But no thanks.... :-) I have nothing to prove here...... Those were nice computations though....... I vaguely remember doing things just like it in Mechanics class when we were toying with Orbital Dynamics and when I was doing papers on this subject..... The nightmares are coming back......
Objects a few miles in diameter will likely survive to reach the ground since they are a substantial fraction of the effective depth of the atmosphere.
Large objects generally don't have time to vaporise substantially prior to impact. Shearing affects mostly intermediate sized objects on a near-grazing trajectory and large objects on a grazing trajectory. Large objects don't slow substantially due to aerodynamic drag (not enough displaced mass in the traversed atmosphere to be truly significant to a really large impactor).
Your comments such as these 2 really didn't have to be made.... I guess I needed to completely clarify things when I was talking in general about impactors?????...... I mean really?.... Do you seriously think I don't realize that a huge body would survive to hit the ground???.... regardless of trajectory angle or composition????? Let alone one that can extend into the upper atmosphere????..... I don't think you did... Which makes me wonder why you needed to write what you wrote.........
Don't you mean 'carbonaceous' (sp?)? And carbonaceous condrites are not comets, though comets may contain some cc's.
Yes..... I meant carbonaceous chondrite..... I'm so use to dealing in paleo terms that I often get wires crossed (Carboniferous as in a geological period)... So, sorry about that. And I also know that comets are not carbonaceous chondrites, who's black color and absorption lines are indicative of a carbon-rich, organic-bearing, hydrated mineralology, and who are thought to be the most "primitive" meteorites. That "hydrated mineralology" and "most primitive meteorites" is where the comet thing comes into play. Carbonaceous chondrites, if I remember right, have experienced alterations via water. Nuff said there. Chondrites are the only known meteoritic material to be traced back to the very birth of the solar system, contain mostly matter that formed within the flattened cloud of gas and dust that surrounded the infant sun, also have interstellar grains that predate the solar system, also contain other presolar material in the form of organic molecules more complex then th! os! ! e that formed in interstellar clouds when charged atoms collided with gas atoms and interstellar dust grains, and that some contain refractory inclusions telling of the high temps first experienced when minerals condensed from cooling gases of solar composition and are thus the oldest solid matter that formed in the nebula, and finally..... that many carbonaceous chondrites remained cool, only to have aqueous fluids produce a complex assortment of water-bearing clays, veins of sulfate, and carbonate, and other secondary minerals.
Comets, as you know, are usually just labeled as icy conglomerates made up of different types of ice including water, carbon monoxide, carbon dioxide, polymerized formaldehyde, ammonia, and other substances. The dust on the other hand....... and here is the important part.... is predominately made up of C, H, O, and N, which I'm sure you know is labeled as CHON particles. Other dust has a silicate mineralology similar to rocks in the crust of the terrestrial planets and meteorites. BUT...... the most common type of dust consists of particles that are very, very similar to the primitive carbonaceous chondrites... The only difference is that they are enriched with the CHON. All of these dust grains are thought to be remnants of the original icy-organic particles from the nebula that formed the solar system..... This data comes from observations of Halley's Comet and Hale-Bopp.
Since some comets do fragment, as did West in 1976 and Shoemaker-Levy 9 in 1992, comet nuclei could be fragile collections of smaller bodies, loosely bound together..... rubble pile aggregates..... But since we don't have an understanding of how such fragmenting occurs, and since most comets do not break apart, at least in obvious ways, they could also be coherent solids. All in all, cometary nuclei may come in a variety of structures.
The original comet factory is thought to have been in the realm of the solar nebula occupied now by the gas giants and the Kuiper Belt. There the icy mantles of primordial dust gains were not sublimated away, as occurred near the Sun (A carbonaceous chondrite formation area?????). Over time these ice-coated grains collided and accumulated, producing objects that were to become comets and the major planets. Gravitational perturbations dispersed them into the Oort Cloud or completely out of the solar system. those int he region of the Kuiper Belt were left pretty much alone.
So, you can easily see the connection between comets and carbonaceous chondrites.... Both formed during the very beginning of the solar system, elements of one are found in the other, and one has the habit of showing chemical alterations via liquid water. I've read that it's possible that the Tunguska object was just a large chunk of carbonaceous chondrite that used to be part of a comet.... One of our periodic comets happens to have a debris stream that the earth was moving through right at the time of the Tunguska Event.... I still can't remember the name of the comet...... Go to this website to read about all the fun and games involved in this idea, along with references to look up:
So, I didn't make this up........
> . and only about 50m across. It smacked the atmosphere at an oblique
Ummmmm.........Well yes..... You were right in things that you said..... just as I stated you were in my post..... No need to state what I already stated again, unless you are seriously trying to bury me beneath your feet....
Quite right. Some of them are elliptical. But the big ones are pretty much circular.
Graydon answered the why of that one too......
I notice that we're describing the melt sheet and ground impact, but Chicxulub impacted in water......
Yup...... and I touched that a bit in another post. Sandia did a simulation of an oblique 30 degree angled impact from an asteroid some 1.2 km in diameter off the coast of New York. It was interesting in that there was mostly just super heated plasma and vapor involved in the resulting blast directed right at the city from an elliptical cavity in the ocean..... very little sea bottom was excavated. There was very, very little in the way of material being directed up into the atmosphere for global trajectories or a dust cloud. Sandia also did a simulation with a 1.2 km comet striking the ocean off the coast of New York. Once again there was very little sea bottom involved in the resulting blast..... It was mostly all super heated plasma and vapor.... But this time the impact was at a 45 degree angle which caused ALL of the blast force to be directed straight up and right out of the atmosphere from the round cavity of water that was sculpted out of the ocean...... T! he! ! blast force was hundreds of magnitudes larger then that of the oblique asteroid impact..... a ridiculous 400 gigatonnes or something like that.... Really amazing stuff...... And these results fit oh so nicely with my post to Rob about the dust cloud.
Nice to know that you are interested in the hydrology after the fact.... I've never read anything about it before.
> It's not the size of the body that dictates the crater's shape
Well, refer back to what Graydon and I have said....... then you can take it up with the astrophysical community I guess..... :-)
Excuse me, but you have just said that the craters will be circular for all but very low impact angles, and if I'm following you correctly, that appears to be contrary to what you have been saying previously.
Gettin a bit testy at this point huh?..... I don't know where the confusion is at..... Maybe I left out a word or something in my original post somewhere.... I'm not looking over it again to see where the problem is since I thought I was clear on the main idea here (Then again, to you at least, I didn't seem to have been clear on a lot of things). I'll just fix it here....... Oblique angles, especially those low to the horizon... 30 degrees and less.... experimentally result in craters that are elliptical...... and in extreme cases, horseshoe shaped...... regardless of projectile size and mass. Studies done to derive the shape of Chicxulub and of other noncircular craters found on other planetary bodies, confirm this fact.
I notice that you say that Chicxulub is only slightly elliptical, though the impact trajectory was very flat (substantial zenith angle). To me, only slightly elliptical translates as pretty near circular.
Well, this is the only thing you got me on.........
You know..... Chicxulub is actually horseshoe-shaped according to the researchers in that link I posted....... I double checked some things and it does look slightly elliptical at first, but when you stare at it, it is actually a warped horseshoe.... Anyway, yes..... most of your larger impact craters known from other bodies in the solar system are nearly circular.... Oblique impacts are rare..... Not nonexistent...... Just really rare..... But as that website states, and other sources I've read before similarly say, "As evidence, the researchers show that the horseshoe-shaped Yucatan crater matches the structure of craters on the moon and Venus that were created when objects struck those heavenly bodies at oblique angles.".......... "Schultz used a high-powered gun to recreate the dynamics of an object striking Earth's surface at a 20- to 30-degree angle. The experiment produced horseshoe-shaped craters, while high-speed film captured gas and materials jettisoned downra! ng! ! e."........ "The researchers suggest that the relatively low angle of the Yucatan impact propelled a ballistic fireball downrange into North America. The fireball carried a two-mile-deep layer of vaporized rock and other material sheared off the Yucatan. The killing zone of matter cascaded through the atmosphere at near orbital speed, across North America and eventually around the globe."
So, I'm not making this up......
How many horseshoe impacts are known in the solar system with widths (or lengths) on the loose order of say 150-200 miles or more?
Well, I haven't personally surveyed all the other planetary bodies in the solar system.... I'm only using the info already compiled for me that I find in books and the like. (None of which I have in my possession at the moment.... So I cannot honestly say much more about this..... Convenient huh?)
> At the moment of impact, the impactor's huge amount of kinetic energy
Well...... if you post the rest of what I said......... "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 in time. Thus, instead of releasing energy at a single point, the energy is spread out along a line.... and ellipse." ......... It turns out that I didn't actually make your point...... So if I were you, I wouldn't thank me. :-)
That info is from an associate professor of physics at Harvey Mudd College:
And that's about it....... I'm too dizzy to continue...... which I'm sure you are happy about :-)
Now I can cough myself sleep........