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From: jon@oddhack.UUCP
Newsgroups: sci.bio,sci.astro
Subject: Re: Mass extinctions
Message-ID: <2330@cit-vax.Caltech.Edu>
Date: Mon, 13-Apr-87 17:49:58 EST
Article-I.D.: cit-vax.2330
Posted: Mon Apr 13 17:49:58 1987
Date-Received: Wed, 15-Apr-87 03:36:28 EST
References: <6760@alice.uUCp> <1489@kontron.UUCP> <1244@husc2.UUCP> <1510@kontron.UUCP> <2329@cit-vax.Caltech.Edu>
Sender: news@cit-vax.Caltech.Edu
Reply-To: jon@csvax.caltech.edu (Jon Leech)
Organization: California Institute of Technology
Lines: 39
Keywords: sanity check
Xref: utgpu sci.bio:279 sci.astro:837

In article <2329@cit-vax.Caltech.Edu> fritz@polecat.Caltech.EDU (Fritz Nordby) writes:
>>> In article <1489@kontron.UUCP>, cramer@kontron.UUCP (Clayton Cramer) writes:
>>>> 1. No great surprise, but the estimates for one of the Canadian astroblemes
>>>> is 17,500,000 megatons.  (No, not 17.5 megatons -- 17.5 gigatons).
>
>Uh, gee, isn't that kind of large?
>Assuming the creature was about as dense as water
>I work out that these creatures must have had a
>volume of >16500 cubic kilometers!
>That's a cube of more than 25 kilometers on a side.

	No, this is eminently reasonable. A simple relation of impact
energy to crater diameter derived by Shoemaker [1] is:
		   1/3.4
	D(km) = K W

	where 					  1/3.4
		K = 0.074 km / (kT TNT equivalent) 
		W = kinetic energy of impact body, kT TNT equivalent

	so for a 17.5e12 T = 17.5e9 kT impact, a crater diameter of 76 km
is obtained, which passes sanity checks. Note that this model is for
alluvium, a main constituent of the earth at the Nevada nuclear test
range where data for the model was developed; it scales weakly with the
density of the material at the impact site. If the impact in question
was on the Canadian shield, the predicted diameter would be somewhat
smaller. The odd (1/3.4) exponent is more of a fit to observed explosions
than a theoretical model. Geometry suggests that crater diameter goes
as the 2/3 power of yield, but there are other factors involved.

[1] Eugene Shoemaker & Ruth Wolfe, ``Cratering Time Scales for the Galilean
	Satellites'', in Satellites of Jupiter, ed. David Morrison,
	Univ. of Arizona press 1982. 
    The actual model was developed elsewhere; this is just a reference 
	which applies it.

    -- Jon Leech (jon@csvax.caltech.edu || ...seismo!cit-vax!jon)
    Caltech Computer Science Graphics Group
    __@/