[meteorite-list] Re: Any Meteorites of Earth Origin?
From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:10 2004
Here are comments from a US scientist closely involved with the collection
of Antarctic meteorites (he/she wished to remain anonymous):
"Dismissing out of hand" is a strong overstatement, particularly for
Antarctic searches. In many of the places we go (far out on the icesheet),
any rock we find is a meteorite, and we pick it up. Doesn't matter if it
looks like sandstone or slate or an ordinary chondrite, we pick it up.
Similarly, if there is terrestrial rock around it's usually very limited
in terms of lithology; so unless the "earth meteorite" or mars meteorite
or what have you is an identical match for the local rock and has zero
fusion crust, we're very likely to recognize it. As of today, out of
nearly 25,000 specimens, zero earth meteorites have been found.
"Furthermore, there ARE "earth meteorites" but they've been highly
altered. Most tektites fit this bill pretty well; they've been totally
melted and show exposure to solar cosmic rays but still they are clearly of
earth origin. In spite of the abundance of tektites, there are reasons to
think that unmelted Earth meteorites might be pretty rare in collections
we're picking up today. Ejection events are rare in recent history (let's
arbitrarily pick 10 million years, a very maximum guess at how long an
object ejected from Earth might stay in an earth crossing orbit) - there
are no craters younger that are big enough. Ejecta from much older craters
has almost certainly evolved out of earth crossing orbits or already been
swept up by the moon and earth, and younger events are not obviously
recorded. Most of the earth's surface is covered by materials that won't
easily survive ejection intact (water, ice, unconsolidated sediments), and
those that could survive have to fall on the surfaces we collect from AND
survive additional weathering until they're found. With all these factors
against them I'd argue unmelted earth meteorites are going to be a very
very small proportion of the stuff swept up by the earth.
"So don't blame the meteorite hunters for "out of hand" dismissals.
The fact that we've identified so many martians (18 so far, all of which
are similar to many volcanic earth rocks) suggests that we'd find unmelted
Earth meteorites too if they were at all common."
[end of reply]
At 08:41 PM 11/12/2001, you wrote:
>Of course someone already pointed out that at first people said that some
>computational model "proved" that meteorites couldn't be ejected from Mars
>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.
>[mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of Robert
>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.
>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
>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. - ??
Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Tue 13 Nov 2001 09:43:58 AM PST