[meteorite-list] Mercurian Meteorites, chances of
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:28 2004
On the question of how much ejecta from Mercury falls into the Sun,
here is the results of the largest-scale, most complete computer
simulation of knocking rocks off planets.
As you can see below, only 4% of rocks knocked off Mercury fall into
the Sun. The statistics suggest that there should have been 1-2
Mercurian meteorites collected by now, but that number is so small, it
could just as well be zero.
On the other hand, would we know (or suspect) a Mercurian meteorite
if we had one? At least one theory of Mercury's formation says that it
was whacked with a more-than-Mars-sized impactor (like the one that
became the Earth's Moon) which blew off much of its original (silicate)
Mercury's surface shows a great lack of contrast in reflectivity
(unlike the Moon which it superficially resembles). It has been
suggested that the intercrater and smooth plains of Mercury are all
basin ejecta rather than volcanic in origin. If the smoother areas are
basaltic vulcanism, it is unique and unlike that of the other planets.
The exact nature of the Mercurian mantle (and core) is largely a
matter of assumption and supposition. It is easy to fit Mercury into
current theories of early solar system history because of its high
density. But we ought to remember that the density "match" between the
Moon and the Earth's mantle misled and bamboozled theorists into an
incorrect theory of the Moon's composition for more than a century.
What we need is... MORE MISSIONS.
Sterling K. Webb
Brett J. Gladman, Joseph A. Burns, Martin Duncan, Pascal Lee, and
Harold F. Levison; The exchange of impact ejecta between terrestrial
planets. Science, March 8, 1996 v271 n5254 p1387(6).
Table 2. The fates of meteoroids after a [v.sub.infinity] = 1 km/s
launch from Mars and Mercury. The simulation for Mars included 900
particles and ran for 100 Myr; the simulation for Mercury included 200
particles over 30 Myr. No collisional effects were included. The
position of Mercury was not tracked in the martian simulation, so
collisions with it were not possible.
Particles (% of total) from parent body
Meteoroid fate Mars Mercury
Impact Mercury N.A. 76
Impact Venus 7.5 6.5
Impact Earth 7.5 0.5
Impact Mars 9.0 0
Sun-grazing 38 4
Reach Jupiter 15 2
Survivors 23 11
Just one of the 200 particles was found to hit the Earth, after 23
Myr. This 0.5% delivery efficiency is 50 times higher than previously
suggested but is based on poor statistics. It is about an order of
magnitude smaller than the efficiency for Mars. If we accept this
efficiency and if the mercurian impactor flux is comparable to that of
Mars, the existence of 12 martian meteorites should lead us to expect a
few mercurian meteorites.
However, a purely gravitational model may not be sufficient to
accurately simulate the transfer of material from Mercury to Earth.
Radiation forces in the inner solar system cause significant orbital
evolution over tens of millions of years, times like that required for
our single meteoroid to reach Earth. Orbital collapse as a result of
Poynting-Robertson (P-R) drag at Mercury's heliocentric distance takes
only 5 Myr for a meteoroid 1 cm in radius with a density of 5
g/[cm.sub.3]. On the other hand, the Yarkovsky effect, which dominates
P-R effects for particles of this size with spin periods longer than 1
second, may induce some mercurian meteoroids to spiral outward to Earth.
However, mercurian meteoroids may be catastrophically fragmented by
dust-sized impactors, which, because of gravitational focusing, increase
significantly as the sun is approached. Collisional lifetimes of 100-g
bodies at Mercury's distance are estimated to be less than [10.sup.5]
years. Because of these complications, the likelihood of finding
mercurian meteorites is difficult to quantify.
Received on Sun 01 Jul 2001 12:23:28 AM PDT