[meteorite-list] Re: Earth originating meteorites

From: Frank Prochaska <fprochas_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:02 2004
Message-ID: <NDBBICFKNKHAAEEJLDALGELPCGAA.fprochas_at_premier1.net>

Hello,

I am also interested in this topic and have been mulling over some ideas
around the possibility of terrestrial meteorites for some time. I am very
interested in those who have done impact and ejection models for generating
terrestrial meteoriods.

There seems to be another wrinkle to the identification issue though.
Recent experiments with earth rocks attached to heat shields seemed to
confirm some people speculations that sedimentary rocks do not develop
fusion crusts. Some of the minerals in these rocks apparently vaporize and
the heat affected zone apparently sort of "pops" or boils in a way, taking
with it the higher temperature minerals that could otherwise melt and form a
fusion crust. This would certainly help explain why all of the 17 or so
martian meteorites are igneous. If a martian sediment had enough tensile
strength to survive ejection in a sufficient size to survive earth entry, it
would likely never be suspected as a meteorite anyway. The same would be
true for terrestrial meteorites. I am not an expert on Martian geology, I
would guess that a greater percentage of the Earth's surface is covered with
thicker layers of sedimentary material than that of Mars. All other things
equal, if terrestrial meteorites exist, a greater proportion of them would
be nearly impossible to recognize than martian meteorites.

Would cosmis ray tracks or chlorine isotope analysis be helpful in
identifying a sedimentary meteorite (from any target)?

Frank Prochaska





-----Original Message-----
From: meteorite-list-admin_at_meteoritecentral.com
[mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of Robert
Verish
Sent: Monday, January 22, 2001 8:25 PM
To: kelly_at_bhil.com
Cc: meteorite-list_at_meteoritecentral.com
Subject: [meteorite-list] Re: Earth originating meteorites


Kelly,

You have made a good point. A fusion crusted rock
with "no radioactivity" may actually be evidence for
an Earth meteorite. (This is where this discussion
dove-tails with the Takysie Lake thread.)

Here's the problem. The energy necessary to launch an
Earth rock (during an impact) into space - far enough
so that it can stay in space over 10,000 years before
Earth's gravity sweeps it back up - is more than
enough to either melt it or vaporize it completely.
Those Earth rocks which survive ejection without
melting will be the first to be "swept up" by Earth's
gravity, and consequently, have little or no cosmic
ray exposure.

Another problem, there are very few impact events
younger than 10,000-20,000 years that were large
enough to eject Earth rock into space, so we're
probably not going to find any freshly crusted earth
meteorites. If we do find one, it will probably be
re-terrestrialized (hey, I think I just invented a new
word;-).

This may be why we haven't found/recognized one, yet.

Would appreciate reading your comments or opinions,
Bob V.

--__--__--

Message: 3
Date: Sun, 21 Jan 2001 22:30:44 -0600
From: Kelly Webb <kelly_at_bhil.com>
To: v-nord_at_excite.com

Subject: Re: [meteorite-list] Concerning Eath
originating meteorites


v-nord_at_excite.com wrote:

> I was down with the Antarctic Search for Meteorites
team. The team always picks up any unusual rock, even
if they aren't sure it is a meteorite. A terrestrial
meteorite would be collected as long as there was some

unusual property associated with it, such as fusion
crust or visible shock damage.
> Indeed, two years age they returned two samples that
turned out to be just unusual mudstones. Counting in
a low-level gamma-ray counting facility showed that
these samples contained no radioactivity caused by
exposure to cosmic-rays in space, and thus the rocks
were never in space.
>

Kelly Webb <kelly_at_bhil.com> wrote:
    The extent of evidence of cosmic ray exposure
would depend on the duration of the extraterrestrial
exposure time. Times of less than perhaps ten thousand
years would be essentially undetectable.

Kelly Webb






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Received on Mon 22 Jan 2001 07:56:42 AM PST