[meteorite-list] FOLLOWUP TO Smallest Possible Earth Impact Crater?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 19 Jul 2007 15:21:48 -0500
Message-ID: <009d01c7ca42$7053a250$ac2ee146_at_ATARIENGINE>

Hi, Charles, List,

    I just went and read your excellent article on Merewether:
http://ottawa.rasc.ca/articles/odale_chuck/earth_craters/merewether/index.html
so now I know Merewether is 200 meters across. However,
the key data is that the crater is NOT in a rock surface, but
a glacial morraine of boulders gobbed up with sand and clay.

    That fact alone would explain the absence of a rim upturn.
Rims are formed by the explosion "pushing" upward against
horizontal strata that are significantly rigid and "resist" being
pushed with a strength beyond the mere weight of the material.
A conglomerate of boulders and mud is not rigid, hence no
tilted rim is produced.


Sterling K. Webb
----------------------------------------------------------------------
----- Original Message -----
From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
To: "Charles O'Dale" <codale0806 at rogers.com>;
<meteorite-list at meteoritecentral.com>
Sent: Thursday, July 19, 2007 2:30 PM
Subject: Re: [meteorite-list] Smallest Possible Earth Impact Crater?


Hi, Charles, List

    The three most important characteristics an incoming
body needs if it wants to get to the surface in one piece
are a) slow entry speed, b) a shallow entry angle, and c)
an aerodynamic (flattened) shape. Calculations performed
by John S. Lewis suggest that, with a shallow entry just
above escape velocity, an an iron of 30 to 100 tons can
"land" without making a crater.

    HOBA is a perfect example at 60 tons. It sits on a flat
surface surrounded by red rusty soil that may contain 25
or more tons of degraded iron shale residue.

    The same calculations for a stone meteorite give an
upper weigh limit of around 40 tons, however there are no
stones known that come anywhere near this mass. Stones
are too fragile; they fragment too easily. (JILIN has the
record at 1.77 ton.) However, if anyone finds a ten-foot
diameter stone meteorite, it's fine with me.

    Of course, with a higher incoming speed and a more
usual angle, that 100 ton iron would make a lovely crater.
Using the excellently handy LPL Impact Calculator:
http://www.lpl.arizona.edu/impacteffects/
we see that (theoretically at least) a 10 cm (4 inch) iron
ball entering the atmosphere at 15,000 m/sec at a 45 degree
angle reaches the surface at about 800 mph with a force
of about 200 pounds of TNT and would make a four-foot
crater in sedimentary rock; the crater would be about 15
inches deep with five inches of broken rock in the bottom.
Oh, and the iron ball would survive intact!

    That is what a good computer model says. Whether this
actually happens in real life is another matter. The computer
model only calculates target surfaces of water or sedimentary
or crystaline rock. Small craters in which the impactor survives
are technically not impact craters -- they are impact pits. An
impact pit has the rock or dirt removed by mechanical force,
not by an explosive event. It takes an explosive event to make
a technically "true" crater. There are a small number of
examples of meteorites recovered from impact pits in dirt,
though.

    What is the size of Merewether? From the look of the picture
it must 100's of feet across. I took my previous example from
the impact calculator and re-ran it with larger and larger iron
balls and got bigger and bigger craters, over 300 feet in diameter,
until I reached the size where the impactor fragments, after which
I get crater fields from the fragments. Of course, that's with one
limited set of parameters; other parameter, other results. The small
"craters" (less than 20-30 feet) were all shallow impact pits with
a surviving object. Above a certain size, they were all explosive
craters, with that characteristically deeper profile.

    Merewether is certainly more than big enough to be an explosive
crater. This does not say that it is, but if there's an objection that it is
"too small" to be an explosive crater, that's a mistake.


Sterling K. Webb
---------------------------------------------------------------------------------
----- Original Message -----
From: Charles O'Dale
To: meteorite-list at meteoritecentral.com
Sent: Tuesday, July 17, 2007 8:58 AM
Subject: [meteorite-list] Smallest Possible Earth Impact Crater?


I am seeking advice on a "small" problem. I am trying to determine what the
smallest possible impact crater on earth would be. In other words, we have
to determine the smallest size of a bolide that would impact earth at cosmic
velosities (>12 km/sec) to create such a structure. Or, the largest size of
a bolide that would be slowed to terminal velosity by our atmosphere (and
not creating a "crater").

The answer may help in adding information to the "enigma" of the Merewether
structure, could it be an impact related crater?

http://epod.usra.edu/archive/epodviewer.php3?oid=315776

Chuck
http://www.ottawa.rasc.ca/articles/odale_chuck/earth_craters/index.html
Received on Thu 19 Jul 2007 04:21:48 PM PDT