Hi Sterling-
I don't place much faith in seismic data as an accurate estimator of
impact size. It is poor at distinguishing between the energy dissipated
at impact and the much larger energy dissipated in the atmosphere. And
there are many uncertainties with respect to the efficiency of energy
conversion during small impacts. I've got three well accepted models,
and they vary over more than an order of magnitude in predicting the
size of impactor in a case like this (but are much closer with respect
to crater aspect ratio).
Anyway, at this point these calculations are really just useful for
estimating a reasonable range of possibilities for the impactor.
Hopefully there is some real data out there somewhere. If all we ever
have to work with is a little shocked material and a few witness
reports, we'll probably never know what happened with any precision.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Sterling K. Webb" <[EMAIL PROTECTED]>
To: <[email protected]>
Cc: "Chris Peterson" <[EMAIL PROTECTED]>
Sent: Wednesday, October 03, 2007 12:45 AM
Subject: Re: [meteorite-list] Carnacas smoke-trail photos
Chris --
The seismic measurement is of a 20-21 GJ event.
The Russian formulas for scaling crater energy,
developed from their work with the various sizes of
the Sikhote-Alin craters, would make it about 18 GJ.
The ground at Carancas is not merely wet soil, it is
wet rocky soil, a different kettle of resistance. You
can see the strata in the walls of the crater.
You specify a 5 GJ event, but your 10 ton and
1500 m/s example would have 11.25 GJ, not the
5 GJ you specifiy. Even a 5 GJ event would be
500 joules per gram of meteorite when it only
takes 100 joules per gram to powder even harder
terrestrial rock. The actual energy of the 10 ton,
1500 m/s example would be 1125 joules per gram
of meteorite, very close to the energy required to
completely melt ten tons of rock.
Of course, that's assuming all the energy is released
within the impactor and so, is only true for the leading
portion of the impactor. As the crater evolves, it takes
its share of the energy away.
The heat of vaporization for most earthly rocks is
around 18,000 joules per gram of rock. That's the
figure used to calculate vaporization for underground
bomb blasts. Silica is quite tough; it takes 22,000
joules per gram. Meteoritic material with a lot of
dissolved iron would also be hard to vaporize, but
after much Googling I can't find a value, so I will
be scientific and assume it's similar to the terrestrial
average. (Anybody know the actual figure?)
To be vaporized by a 21-22 GJ impact, a one ton impactor
would need ~6500 m/s impact velocity. In fact, for any
rock impactor to be vaporized, it needs to convert 18,000
joules of KE to heat for each gram, so roughly 6000 m/s
is the speed needed to vaporize any rock on impact,
regardless of its size. That's a high velocity to get all the
way to the surface.
Sterling K. Webb
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