Originally posted last June (2001), the statistics
of transfer of impact ejecta from planet to planet suggested that there
should be 1 or 2 Mercurian meteorites that have been collected but not
(then) identified. The ratio of Martian to Mercurian meteorites should
be about 15:1. Since basalt requires a differentiated world, Vesta is about
as small as a basalt producer is likely to be, so the candidates for the
origin of these rocks have to be bigger.
If the Mercurian origin of the two current candidates
is confirmed, it would suggest that the statistical simulations of the
study cited below were reasonably in line with the real world evidence.
Interestingly, you can see that the authors include several paragraphs
attempting to explain why there aren't any Mercurian meteorites (Science
Professional Rule 37B: always cover your...), instead of predicting their
discovery. As Gallileo could have told you, boys, sometimes, you've got
to stick your neck out just a little bit.
Another conclusion of the Gladman study is that
there should be some Venusian meteorites, a greater number than Mercurian
meteorites. So, where are they hiding? (Following Rule 37B,
maybe the atmosphere of Venus is too thick for anything to escape, etc.
etc. Following the Gallileo rule, keep looking!)
Here's the original post:
Subject: Mercurian Meteorites, chances of
Date: Sat, 30 Jun 2001 23:23:28 -0500
Hi, List,
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)
mantle.
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
---------------------------------
FROM:
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).
TEXT:
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.
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