[meteorite-list] Meteoroid entry angle

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sat, 18 Sep 2010 17:02:54 -0700
Message-ID: <GOEDJOCBMMEHLEFDHGMMKEJMEFAA.mojave_meteorites_at_cox.net>

Hi All,

Playing message catch-up after 10 days' vacation/biz trip in Maui.
Saw John's message below, and thought I'd make a small correction
to a common misconception about meteoroid entry angles.

"Gilbert and Barringer both realized that 45 degree impacts are
the most probable trajectories for meteoritic material."

I find many people make this mistake in solving all kinds of
problems that depend on spherical geometry. In *two* dimensions,
if the range of possible angles is from 0 to 90 degrees, and
the angle is uniformly random, then the mean angle is of course
45 degrees: the midpoint of the uniform distribution. In three
dimensions, this is no longer true.

If you look at a globe, it is easy to see why. Compare the
surface area of the earth from the equator up to 45 degrees
north latitude with the surface area from 45 N up to the North
Pole. The latter is a much smaller area. In fact, for a sphere,
the surface area from the equator up to *30* degrees north
latitude is the same as the area from 30N to 90N. Similarly,
the average entry angle for a meteoroid coming from a uniformly
random direction is 30 degrees from horizontal.

That said, the entry angle distribution is not quite uniformly
random because earth's gravity bends all trajectories slightly
toward the center of the earth. This has the effect of making
all entry angles a little bit steeper than they would be if
gravity didn't play a part.

Bottom line is that the average entry angle for a meteoroid is
a little bit more than 30 degrees, but definitely not as much
as 45 degrees. To answer Eric's question below about what's
considered shallow vs. steep, I consider any entry angle less
than 20 deg to be "shallow", anything greater than 40 deg to
be "steep", and anything from 20-40 (which is about 30% of all
meteoroids) to be average.

--Rob

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of Kashuba
Sent: Friday, September 10, 2010 8:06 AM
To: bernd.pauli at paulinet.de; Meteorite-list at meteoritecentral.com
Subject: Re: [meteorite-list] Meteor Crater Shape and Entry Angle


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
Received on Sat 18 Sep 2010 08:02:54 PM PDT