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Re: Hell Creek (long)
>> , for massive impactors the approach angle doesn't have too
>> much of an effect on crater shape or primary direction of blast
>> wave (jrc)
> 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.
Then why is it that most all very large impact craters throughout the
solar system are essentially (nearly) circular?
> 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
Not entirely. It also has to do with the diameter of the impacter
versus the effective depth of the atmosphere.
> How much atmospheric drag does it suffer and for how long?..... How
> much deceleration takes place?...
Well, for an object coming in at a zenith angle of 30 degrees at a
velocity of roughly 30 miles/second, the path below 60 miles up will be
about 69 miles long, so the exposure will be about 69/30=2.3 seconds,
more or less. However, the atmospheric density only becomes really
substantial below roughly about 40,000 feet (approximately one diameter
for a K/T type impactor), so that travel time becomes about 8.75/30=0.3
seconds, loosely. Assuming normal densities for a stony asteroid (or
those for a comet), computing the approximate atmospheric deceleration
should be fairly straightforward. I'll leave that as an excercise for
you, if you're interested.
> . 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...
Usually somewhat faster, yes. Although it will also be less dense than
an asteroid, which will offset the additional kinetic energy somewhat.
> .. And then there is the factor of where the comet was coming from to
> begin with..... The Oort Cloud and the Kuiper Belt.
Don't forget the possibility of interlopers (extrasolar objects). But
for a body from within the solar system, it is unlikely that the impact
will be at much less than 11 miles/second, or much more than 50
miles/sec. Say 25-30 miles/sec as a loose average.
> The slower an object is traveling, the more likely it will survive its
> passage through the atmosphere in order to impact the ground.
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
> 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
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 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..
Don't you mean 'carbonaceous' (sp?)? And carbonaceous condrites are not
comets, though comets may contain some cc's.
> . 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
You make my point perfectly. Thanks.
> 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...
Quite right. Some of them are elliptical. But the big ones are pretty
> .. This was a clue that was used to figure out what was going on with
> the odd elliptical appearance of Chicxulub..
What is the e of Chicxulub? It looks pretty durned circular to me.
> .. 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.
You are a little more certain of this than I. Though I do agree that it
likely came in at a pretty good angle. I notice that we're describing
the melt sheet and ground impact, but Chicxulub impacted in water with
enough energy to convert maybe 30,000 cubic miles of water to plasma
(not exactly the same as just boiling it), so there likely was more
going on than just popping a melt sheet downrange. My own personal
interest is in the intermediate-term effects of the recombined water on
the hydrologic cycle over the following 30-50 years.
> It's not the size of the body that dictates the crater's shape
> (circular or elliptical)..... It's the angle of attack..
For small, slow objects, that's true. For large, fast ones, apparently
not quite so true.
> .. and that's if it even surviv! ! ! ! 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.
Are you saying that a 6-10 mile diameter stony object won't survive to
reach the ground? I find that hard to believe. But even if it didn't,
the kinetic energy would still have been mostly transferred to the
planet (less the energy of that splash material ejected at greater than
> See this website from Brown University...
Thanks for the ref. I appreciate it.
> .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.
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.
> 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.
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.
> Anyway, to sum this up... Angle of attack does have bearing on what
> the impact crater looks like, regardless of impactor mass and size.
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?
> 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....
My point exactly. Thanks for making it.