[meteorite-list] "Supergiant" Asteroid Impact

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue, 12 May 2009 17:58:34 -0500
Message-ID: <2520D7911A2F449E9E60734A1786C2D3_at_ATARIENGINE2>

Hi, Super-Impactors,

    A 2000-mile crater is No Big Whoop. It requires
only a 375-to-400 kilometer asteroid if it's a dense
rocky body and a 240-kilometer asteroid if an iron
(but an iron that size is unlikely).

    The little bitty Moon has a 2000-mile crater (we
call'em "basins" because anything that big floods
with lava afterwards). Scaling for target size and
gravitational focusing effects, the Earth should have
(or should have had) EIGHTEEN 2000-mile craters.

    To make one, it takes a 400-kilometer asteroid
on a gentle 20 km/s impact. But if it was a 28 km/s
impact, it would only need to be a 250-kilometer
asteroid. And if it was a long-period comet at right
angles and top speed, it wouldn't have to be any
bigger than the dozen or so biggest comets of the
last two centuries.

The depth of the initial crater would be 275 miles!
It soon collapses and fills with molten planet, leaving
a two-mile deep "basin" 2000 miles across. The safe
place to be is exactly halfway around the planet,
of course. Even there, you will be rained on by rock
vapor as it condenses. The pressure peak of the shock
wave will be about 225 pounds per sq. in. or 15 times
normal pressure. The wind of the shock wave will be
about 2200 mph, Mach Three... halfway around the
planet.

    The hydrothermal vent bacteria will be just fine,
though, unless it lands on them.

    If you're interested in Really Big impacts, I suggest
a book called "Comets and the Origin and Evolution
if Life," which contains a paper by Zahnle and Sleep
on larger impacts. They model one big enough to boil
the oceans away, one big enough to melt the entire
mantle, one big enough to give the Earth a long-term
atmosphere of rock vapor.

    Almost every trace of such impacts, from initial
accretion through the Late Bombardment, are gone.
It's amazing what Mother Earth can do with her
tectonic make-up. The odds of such an impact now
(meaning in the last half-billion years) are small...
but not impossible by any means.


Sterling K. Webb
--------------------------------------------------------------------
----- Original Message -----
From: "Darren Garrison" <cynapse at charter.net>
To: <meteorite-list at meteoritecentral.com>
Sent: Monday, May 11, 2009 5:15 PM
Subject: Re: [meteorite-list] "Supergiant" Asteroid Impact


> On Mon, 11 May 2009 13:01:28 -0700, you wrote:
>
>>This raises the question that seems to be getting bigger and bigger.
>>What would such an impact do to Earth?
>
> An impact big enough to make a 2,000 mile crater? Think the oceans
> boiled away
> to their floors, everything bigger than a hydrothermal vent bacterium
> killed
> instantly, and maybe them, too.
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Received on Tue 12 May 2009 06:58:34 PM PDT