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-boun...@meteoritecentral.com
[mailto:meteorite-list-boun...@meteoritecentral.com] On Behalf Of
bernd.pa...@paulinet.de
Sent: Friday, September 10, 2010 3:26 AM
To: Meteorite-list@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
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