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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).
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
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...
Astronomy, University of Alabama