[meteorite-list] Million's of Dollars of Tunguska Meteorites may be located, just like Chelyabinsk's Meteorites: OUTSIDE of, NOT INSIDE, the Blast Zone.

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sun, 5 May 2013 20:25:12 -0700
Message-ID: <000001ce4a09$51702c30$f4508490$_at_cox.net>

Hi Steve,

> Robert Beauford and I were talking about Chelyabinsk shortly after the
> fall and he asked how many Chelyabinsk meteorites were being found
> in the city of Chelyabinsk, where all the windows had been blasted out?
> I told him that, as I understood it, the strewnfield was farther "down
> stream" because inertia carried the rocks further beyond the blast,
> like what almost always happens with fireballs.

The real reason nothing will be found in the city of Chelyabinsk itself
is that the meteoroid did not pass over the city. The closest point to
Chelyabinsk under the fireball path was 35 km to the south-southeast.
Even Korkino was north of the fireball's path. The main burst was
about 26 km above the city of Pervomayskiy, but prevailing winds
were to the southeast, so the closest meteorites would have fallen
south of that city.

> We later talked about how the Tunguska event of June 30, 1908,
> was bigger than Chelyabinsk it seemed, but probably not as big as
> the 101 crater forming event of Sikhote Alin, Russia of Feb. 12,
> 1947. Certainly, Tunguska was not as big as what caused the
> near-mile-wide Barringer Crater in Arizona. All of a sudden, it
> hit Robert..."Maybe there really should be Tunguska meteorites,
> but not where everyone has been looking for the last 105 years!"
> ... So why would there be Tunguska meteorites amongst the
> fallen trees at the Tunguska blast zone?

I agree that if meteorites made it to the ground, you would not
expect them to be concentrated at the epicenter of the terminal
burst (presumably the center of the fallen tree zone). But they
would almost certainly be within it. The Tunguska terminal burst
was at rather low altitude (likely 10 km or lower), while the
radius of the zone of devastation is something like 25 km. So
unless Tunguska's entry angle was very shallow and/or upper
atmospheric winds were extremely high, it would be difficult
for meteorites to travel 25 km downrange of that terminal
burst.

I do not believe the entry angle for Tunguska was particularly
steep or shallow; I think I remember the consensus is that it was
average, e.g. 30 degrees from horizontal. [Side note: 30 degrees
*is* the exact average entry angle, not 45 degrees.] So even if
the terminal burst was as high as 10 km, and there was no
atmospheric drag, meteorites could only travel about 17 km
downrange from the terminal burst.

Where meteorites would end up relative to the epicenter of
devastation depends on a combination of the original flight
direction, and the prevailing winds at the time and location of
the fall. There isn't consensus on that flight direction, though
based on the evidence I've seen I would estimate that it was
to the west-northwest. Unfortunately, the prevailing winds
are unknown, but you could probably bound them by examining
several years of historical data for mid-June to mid-July for
that general region of Siberia at around 0h UT. In fact, I think
that would be an excellent research project! I might even
tackle it myself...

> If we are right, WHERE should someone be looking to actually
> find the potentially millions of dollars of meteorites that have
> been waiting to be found all this time?

Based on my arguments above, *inside* the tree devastation
zone (which isn't very helpful, given that it covers some
2000 square kilometers!) Flight direction favors the western
half of that zone [hey, down to 1000 sq. km ;-) ] I would want
to run some Monte Carlo cases with different assumptions
for wind, terminal burst altitude and flight direction to better
constrain the fall zone. Ultimately, the choice of wind speed
and direction is going to drive the answer to your question.

--Rob
Received on Sun 05 May 2013 11:25:12 PM PDT