[meteorite-list] Re: Any Meteorites of Earth Origin?

From: Frank Prochaska <fprochas_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:10 2004
Message-ID: <NDBBICFKNKHAAEEJLDALMEONCLAA.fprochas_at_premier1.net>

Of course someone already pointed out that at first people said that some
computational model "proved" that meteorites couldn't be ejected from Mars
either.

I don't think it is that hard to believe that hunters are dismissing out of
hand possible terrestrial-meteorite candidates, even in Antarctica. Most
clues that people look for to identify a rock as a meteorite are factors
that make them different from earth rocks. The more like an earth rock a
sample is, the less likely it will be noticed as meteoritic. Note the find
vs. fall ratios of achondrites compared to other classes of meteorite.
There may well be tell-tales signatures that a earth rock was ejected and
then reaccreted, such as high pressure polymorphs of various silicates,
cosmic ray exposures, etc., but if the candidate does not someone's
subjective "that's interesting enough to test more closely" standard, then
that sample will be subjected to the more time consuming and in some cases
expensive scrutiny that would make a quantitative rather than qualitative
determination. No one is going to pick up a rock and identify cosmic ray
exposure in a hand sample any more than they will identify high pressure
polymorphs of quartz or olivine, or twinned mineral crystals.

One of these "interesting enough" triggers would be something that looks
like fusion crust on what otherwise looks like normal earth rock. There is
a theory, that I believe was backed up by an experiment where rock samples
were glued to a re-entry heat shield, that sedimentary rocks in general do
not develope fusion crust. The surface heats up and flakes or pops off
rather than melting homogeneously at the surface and flowing. A quick look
at a world geologic formation map seems to indicate that if terrestrial
meteorites exist, a majority of them, based simply on percentage of the
target surface, would be of this category. Even so, something that looks
like fusion crust can be duplicated by various weathering effects. How many
experienced meteorite hunters have not given a rock another thought because
it looked like a normal igneous earth rock except for a coating of something
that resembles a fusion crust?

I doubt the way collecting is done in Antartica necessarily overcomes this.
It is my understanding that not every rock found on the surface in the
targeted collection areas is cataloged and analyzed. My understanding is
that the rocks found are given a cursory examination in the field, and if
the sample appears (subjective determination again) to merit further work,
it is collected. There are other sources for some of these rocks; this is a
place of active glaciation with mountain ranges either protruding through or
below the ice.

Given the small sample size of martian meteorites, the fact that terrestial
ejection events are probably more rare due to gravity and atmosphere
thickness differences, and these issues with sufficient suspicion of a
candidate to warrent further detailed study, I think it's likely that they
do occur but we simple haven't been able to either find or identify one yet.

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, November 12, 2001 3:06 PM
To: Meteorite-list Meteoritecentral
Cc: Ron Baalke
Subject: [meteorite-list] Re: Any Meteorites of Earth Origin?


--------------------------------------------------
[meteorite-list] Re: Any Meteorites of Earth Origin?
Ron Baalke baalke_at_zagami.jpl.nasa.gov
Mon, 12 Nov 2001 13:04:26 -0800 (PST)

In fact, the impact event that created the Moon
probably ejected a lot of material that escaped
both the Earth's and Moon's gravity.

Ron Baalke
--------------------------------------------------

That "ejected material", were it to return to Earth
now as a meteorite, would be much more like a Lunar
rock, and nothing at all like a [present] Earth rock.


After all is said and done, the bottom line is this:

Dozens of Martian and Lunar meteorites have been
found.
Not one terrestrial meteorite has been recognized.

I find it difficult to accept the notion that
meteorite hunters worldwide are "missing" them.

If your point is that researchers are turning away
terrestrial meteorites brought to them by meteorite
hunters, as being meteor-wrongs, I would need to know
a lot more before I would "second-guess".

But I find it even harder to believe that, among all
those well preserved Antarctic meteorites that have
been recovered, not one of them has been identified as
being anomalous enough to be a potential terrestrial
meteorite. - ??

Bob V.



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Received on Mon 12 Nov 2001 08:41:10 PM PST