Hello List,

A few comments (perhaps naive speculations) on statements by Francis
(Graham),  Sterling (Webb) and Robert (Wolard) about meteorites on Mars.

1) I assume (unless I miss something important) that a meteorite falling on
Mars would not carry any kind of fusion crust because of the particular tiny
atmosphere (friction and melting minimized or nil, especially if oxygen is
hardly present). As a consequence, if Spirit would start examining Mars
"rocks", the examination should not be based on any of their external
aspects because a meteorite (from a place other than Mars) would not show
any external difference if compared to any other Mars rock. This is
similarly true from a rock ejected from Mars as meteorite and falling again
on Mars (3% according to data reported by Sterling) 
This would be even more valid if the rocks were brought eventually on Earth
through any means. 
In other words, detecting a ameteorite among oll those rocks would imply the
use of sophisticated measurements (either in situ in Gusev, in this case or,
later, on Earth) such as, e.g. the age of each rock, or composition or...
I agree with Francis that this would be an interesting task and if, by
chance, one of those rocks proves to be a meteorite, its fusion crust (that
is, to some extent, another kind of planet-specific weathering) would be
very interesting to compare with any of terrestrial fusion crusts found on
meteorites. The fusion crust would a priori be characteristic of the
atmosphere composition and density of a given planet (although many other
factors such as meteorite speed etc... would also contrubute). 

2) Regarding the computer simulation of rocks knocked off planets (data
reported by Sterling), the tables show the percentages of chunks ejected
from Earth, Moon and Mars, on Mars. Indeed, Mars is supposed to receive only
0.1% ejecta from Earth/Moon and 3% from itself. 
But these data do not report the % of meteorites coming from ("nearby")
asteroid belt. Here I agree with Robert Woolard that there should be MANY,
at least (far) more than on Earth. 
For a mission examining (or bringing back to Earth) Mars rocks, chances that
the latter include some "old good ordinary chondrite" from asteroid parent
bodies can then be at least larger than if this is achieved on Earth, though
probably very very small. Unless the mission is able to pick up a specific
true meteorite through some "visual" selection such as the above mentioned
fusion crust (if any formed on Mars) in a way the meteorite hunters do on
Earth's deserts ? Tricky probability indeed....

Am I missing something ?



At 11:24 07/01/04 -0800, Francis Graham wrote:

>  I guess this is a question/comment mostly for Ron
>and others concerned with Spirit, although it seems
>like something for all to consider and input.
>   Looking at the large field of rocks in Gusev on the
>Spirit images undoubtedly a very small fraction may be
>meteorites on Mars.
>   Certainly the major reason to go to Mars is to
>study Mars rocks. But is there any plan to examine a
>meteorite on Mars (from a place other than Mars) if
>one is encountered by sheer chance? 
>   Such an examination might not be a waste of
>precious rover time. We know how meteorites,
>especially iron meteorites, of known composition,
>weather on Earth in various climates. Meteorites of
>identical composition should weather differently on
>Mars (with little oxygen in the atmosphere, different
>pressure, water vapor content, etc.) I'm thinking,
>that a comparison of the two may well help us
>understand Mars in the past as well, if well-weathered
>meteorites are found on Mars. Especially the recent
>past, while the Earth was going through the last
>glacial epoch, alteration of meteorites on Mars might
>tell us what was happening on Mars in that time-scale.
>   Anyway, it seems like a reasonable idea to ponder,
>in the  improbable, but not impossible, situation that
>the Spirit rover or any future geophysical rover
>encounters a substantial meteorite on Mars.
>   I know such meteoric fragments are common on the
>Moon. What do the Mars experts have in mind?
>Francis Graham
On Thu, 08 Jan 2004 00:22:01 -0600, Sterling K. Webb wrote:


         In the largest scale computer simulation (by Bret Gladman) of
     rocks knocked off planets, the following tables show what
     percentages of chunks make it to what planets:

     Ejecta  From Earth/Moon
     Mercury         0%
     Venus          15%
     Earth/Moon     50%
     Mars          0.1%
     Escapes      34.9%

     Ejecta     From Mars
     Mercury         0%
     Venus           4%
     Earth/Moon      5%
     Mars            3%
     Escapes        88%

         You can see that the Earth/Moon system is very good at
     sweeping up its own debris (50%) while Mars only re-captures 3% of
     its ejecta. The Earth (and Venus, too) get more chunks of Mars
     than Mars does!
         That means that, oddly enough, of the two places in the
     universe that look exactly like the Mars Rover images, namely,
     Mars and Arizona, you'd be better off looking for Mars meteorites
     in Arizona than on Mars itself.
         On the other hand, Mars gets only a tiny fraction of the Earth
     rocks (one in a thousand). The Earth/Moon gets 50 times more Mars
     rocks than Mars gets Earth (and Moon) rocks. So the likelihood of
     finding a "terrestrial" meteorite on Mars is small indeed!
         If the Earth has 25 Mars rocks (discovered), a similarly
     intensive search of Earth meteorites on Mars would have been
     unlikely to have found even one. Of course, meteorites may persist
     on the Martian surface for much longer than they could survive on
     a terrestrial surface and in that case the incidence of Earth
     meteorites would be multiplied by a time factor.
         If what we're looking at in the Rover images is a surface
     unchanged for 3 billion years (which some would say it is), there
     would be 60,000 times more Earth meteorites on it than if it
     degraded meteorites as fast as the Earth does! But that's still
     only one Earth (or Moon) meteorite for every 500 square miles of
     Mars (scaled to the 25 Martian rocks found on Earth).
         All of this assumes equal "landability" for meteorites on Mars
     and the Earth, which as has been pointed out is not likely to be
     the case.
         Packing for Mars? It's probably a waste of time. Just put that
     Universal Achondrite Detector back in the closet, hang up your
     space suit, drain the fuel from your lander, and point the SUV for
     Arizona instead.

     Sterling K. Webb
Date: Wed, 7 Jan 2004 15:06:54 -0800 (PST), Robert Woolard wrote:

Hello List,

   There was an article, it seems like within the last
2-5 years but could be more, in either an issue of
Astronomy or Sky and Telescope magazine, that
predicted there would be a VERY high concentration of
meteorites per square kilometer on Mars. I've tried
searching both magazines archives on line, but I was
unable to locate the article. (Perhaps someone else
with more "savvy-searching-skills" will be able to
find it.) But I DO know it was in one of these mags.,
and I DO remember that it was a short article stating
that meteorites would be very plentiful on the surface
of Mars. It gave a figure for the estimated number per
square kilometer that I cannot recall precisely, but I
DO remember being impressed. 

   Also, in my searching. I did find this different
article that might be of interest:



   Robert Woolard
Prof. Zelimir Gabelica
Groupe SÚcuritÚ et Ecologie Chimiques (GSEC) - ENSCMu
3, rue A. Werner
Tel: +33 (0)3 89 33 68 94
FAX: +33 (0)3 89 33 68 15

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