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More about Martian meteorites, seeding between planets,...

And the MCF thread gives another virtual invitation for the astronomer
typoes out here to comment. Not on the mocropaleontology or yet the
chemistry, but on some of the dynamics and timescales. Martian 
meteorites are not a new discovery, but getting the idea established
took some work on the plausibility - is it likely that a rock
could be accelerated from an impact to Martian escape velocity
(0.43 of Earth's or 4.8 km/s after leaving the atmosphere) without being 
(a) melted, (b) pulverized, or (c) metamorphosed beyond chemical recognition?
The clinchers were numerical modelling which showed that, at the right
distance from the impact, near-surface material was indeed accelerated
gradually enough to survive almost unaltered (see H.J. Melosh 1995 Meteoritics
30, 545), and bits of solar-system dynamics showing that indeed there existed 
orbits reachable from Martian ejection that cross the Earth's in a reasonable 
time (since the cosmic-ray exposure times for the meteorites are only millions 
of years, much less than the age of last melting for the rocks). This is
discussed by Wetherill 1984 (Meteoritics 19, 1), who finds that the
typical time in between planets is of order 10 million years, and that 
ejection as small fragments is more efficient than as large fragments which
must then be fragmented by collisions with other (inner asteroid-belt?)
pre-existing objects.

It has been a bit of a puzzle that meteorites of lunar and Martian
attribution are about equally numerous. Martian meteorites are more
massive, seem to come from deeper in the interior, and are geologically
unusually young compared to the Martian surface as a whole. Gladman and
Burns have proposed an explanation (Lunar Plan Sci 27, 421, 1996) -
but I can't tell what it is since the abstract doesn't say and we don't
have the journal. It is also interesting that the possibly aquaeous
origin of carbonates and the presence of PAHs in ALH84001 has been
reported in other papers by overlapping sets of authors - apparently a
few people saw this coming.

The issue of crossing between Terrestrial planets was examined by Melosh and
Tonks (1993, Meteoritics 28, 398), who explicitly note the biological
implications. Their abstract states, in part, 
"The results show very little dependence on velocity of ejection. Mercury ejecta
is nearly all reaccreted by Mercury or eroded in space--very little ever evolves
to cross the orbits of the other planets (a few percent impact Venus). The 
median time between ejection and reimpact is about 30 m.y. for all erosion 
models. Venus ejecta is mostly reaccreted by Venus, but a significant fraction 
(about 30%) falls on the Earth with a median transit time of 12 m.y. Of the 
remainder, a few percent strike Mars and a larger fraction (about 20%) are 
ejected from the solar system by Jupiter.  Earth ejecta is also mainly 
reaccreted by the Earth, but about 30% strike Venus within 15 m.y. and
5% strike Mars within 150 m.y. Again, about 20% of Earth ejecta is thrown out 
of the solar system by Jupiter. Mars ejecta is more equitably distributed: 
Nearly equal fractions fall on Earth and Venus, slightly more are accreted to 
Mars, and a few percent strike Mercury. About 20% of Mars ejecta is thrown out 
of the solar system by Jupiter.

The larger terrestrial planets, Venus and Earth, thus readily exchange ejecta. 
Mars ejecta largely falls on Venus and Earth, but Mars only receives a small 
fraction of their ejecta. A substantial fraction of ejecta from all the 
terrestrial planets (except Mercury) is thrown out of the solar system
by Jupiter, a fact that may have some implications for the panspermia mechanism 
of spreading life through the galaxy. From the standpoint of collecting 
meteorites on Earth, in addition to martian and lunar meteorites, we should 
expect someday to find meteorites from Earth itself (Earth rocks that have spent
a median time of 5 m.y. in space before falling again on the Earth) and from 

The atmospheric barriers from Venus and Earth also seem formidable, so it seems
easier for Mars to seed inward than the innermost planets to send rocks to 
Mars (on top of the current larger density of potential impactors near
Mars-crossing orbits).

As in the subject, this is more than people probably want to know, and Mickey 
may not let this through, but I didn't start this thread...

Bill Keel
Astronomy, University of Alabama