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interplanetary dino debris



(hey, maybe that "Interplanetary Gazette" title was on to something!)

There have been numerical studies of where the debris from giant
impacts would go, once it gets ejected from its planets. To quote
from the abstract by Melosh and Tonks (1993, Meteoritics 28, 398):
"The results show very little dependence on velocity of ejection. Mercury 
ejectais nearly all reaccreted by Mercury or eroded in space--very little ever 
evolvesto 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 
spenta median time of 5 m.y. in space before falling again on the Earth) and 
from 
Venus."

So there's about 5 tims as much Mars junk falling here than our detritus
falling there. Fold in the 3.5x smaller surface area of Mars and it's
not too unequal. Not that the Martian surface is a very friendly environment
for organic matter near the surface of such a rock, but I've heard 
poeple worry about whether we could misinterpret remnants of
meteoric Earth forms as fossils of indigenous organisms. I'll
be happy to see the matter discussed when some such samples are in
hand (robotic claw, whatever). 

So why don't we see dinos on Mars? Maybe small, skinny, very low-metabolism
types? Hmm, maybe we know what kicks up those dust devils that paint
some of the plains in MGS images...

Bill Keel
Astronomy, University of Alabama