[meteorite-list] Carnacas smoke-trail photos

[Chris Peterson]

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" <sterling_k_webb at sbcglobal.net>
To: <meteorite-list at meteoritecentral.com>
Cc: "Chris Peterson" <clp at alumni.caltech.edu>
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