[meteorite-list] Meteor Crater Shape and Entry Angle

From: Meteorites USA <eric_at_meteoritecentral.com>
Date: Fri, 10 Sep 2010 10:37:44 -0700
Message-ID: <4C8A6CE8.1060702_at_meteoritesusa.com>

Hi John, Thanks for the details. It's interesting to note that the angle
of descent is not known, though there are educated guesses or
calculations. What can probably be agreed is that an impactor with an
entry angle of 45* degrees could produce a round crater. Meaning of
course that Meteor Crater, since it's not perfectly round as evidenced
by the "bulges" in the NW and SE corners, must have been produced by an
impactor with a trajectory much shallower than 45 degrees. Logically.

Here's another question. Which direction was it traveling SE to NW or NW
to SE?

According to the Shoemaker paper here:
http://arrowsmith410-598.asu.edu/Lectures/Lecture16/i0-8137-5402-X-2-0-399Shoemaker.pdf

"...Somewhat greater energy was
required if the projectile struck at an oblique angle, as suggested
by the presence of faults with underthrust displacement on the
north and west walls of Meteor Crater...."

It suggests a NW direction of travel... is this correct? And how do we know?

Does the "underthrust displacement" imply that the impactor was
traveling from the SE toward the NW?

Eric



On 9/10/2010 8:05 AM, Kashuba wrote:
> Eric, Bernd, Sterling, List,
>
> David Kring of LPL put together a great guidebook for the 2007 MetSoc tour
> of the crater (150 pages). He is Gene Shoemakers successor as advisor to
> the Barringer family. He and family members lead the tour. Carolyn
> Shoemaker was there too.
>
> Chapter 9. "Trajectory" begins and ends thusly:
>
> The trajectory of the impacting asteroid is another issue of considerable
> debate and still unresolved.
> Historically, circular plan views of impact craters confounded many
> investigators who assumed a circular
> crater requires a vertical impact. They wondered why more craters are not
> elliptical. Gilbert and
> Barringer both realized that 45 degree impacts are the most probable
> trajectories for meteoritic material.
> Yet Gilbert, like many of his contemporaries, mistakenly thought a 45 degree
> impact produces an oval
> crater (Hoyt, 1987). Barringer, on the other hand, realized that a 45 degree
> impact will produce a round
> crater (Hoyt, 1987). Despite this insight, Barringer, like Gilbert,
> initially assumed that the northern
> Arizona impact had been vertical or nearly vertical and that the asteroid
> was buried beneath the center of
> the crater floor.
>
> When extensive drilling did not locate a main mass beneath the crater floor
> and instead only
> produced traces of the projectile, Barringer began to consider other
> options. He had already noted several
> features that seem to have a directional symmetry.
>
> - snip -
>
> More recently, techniques similar to those of Sutton were applied by
> Holliday et al. (2005) to the
> Odessa impact site. They estimated the Odessa craters were produced
> approximately 63,000 years ago.
> Although the ages of Barringer and Odessa craters are still not precisely
> known, these approximate ages
> suggest Odessa formed earlier, with the caveat that the Barringer crater may
> be older than 49,000 yrs.
> (See discussion in Chapter 11). Thus, the two impact events may not be
> directly related and may not have
> any bearing on the issue of trajectory.
>
> Nonetheless, several other potential indicators of trajectory survive (and
> even the Odessa connection
> might be revived). Unfortunately, these indicators cannot be reconciled at
> the present time and I think it
> fair to conclude that the trajectory of the impacting asteroid that produced
> Barringer Crater remains
> uncertain.
>
> Chapter 9:
> http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/chapte
> r_9.pdf
>
> Whole "guidebook":
> http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/index.
> shtml
>
>
> Regards,
>
> - John
>
> Ontario, California
>
>
> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com
> [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of
> bernd.pauli at paulinet.de
> Sent: Friday, September 10, 2010 3:26 AM
> To: Meteorite-list at meteoritecentral.com
> Subject: [meteorite-list] Meteor Crater Shape and Entry Angle
>
> Eric wrote:
>
> "The crater is not perfectly round as would be expected from an impactor
> coming in at a sharper angle. In fact the crater is more elliptical in
> shape."
>
> SHOEMAKER E.M. and KIEFFER S.W. (1974, 1979) Guidebook to the
> Geology of Meteor Crater, Arizona (Publ. No. 17, Center for Meteorite
> Studies, Arizona State University, Tempe, Arizona):
>
> "Regional jointing has controlled the shape of the crater, which is somewhat
> squarish in outline; the diagonals of the "square" coincide with the trend
> of the
> two main sets of joints. The largest tears occur in the "corners" of the
> crater."
>
> Eric also inquired:
>
> "What would a "relatively low" impact angle be? 10 degrees, 20 degrees?"
>
> I tried to find more precise information on that but was unable to find
> something
> that might be of help here. Maybe someone else can shed more light on this!
>
> Regards,
>
> Bernd
>
> ______________________________________________
> Visit the Archives at
> http://www.meteoritecentral.com/mailing-list-archives.html
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
>
> ______________________________________________
> Visit the Archives at http://www.meteoritecentral.com/mailing-list-archives.html
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
>
Received on Fri 10 Sep 2010 01:37:44 PM PDT