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 ?
Regards,
Zelimir
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
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On Thu, 08 Jan 2004 00:22:01 -0600, Sterling K. Webb wrote:
Hi,
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%
Mars3%
Escapes88%
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
