[meteorite-list] Mercury, Venus, and Other Planetary Meteorites

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:34 2004
Message-ID: <3F975615.6400AE32_at_bhil.com>

Hi, List,

    I love it when a thread wanders into one of my fondest obsessional
areas! And it's done it again! This is a re-post from the LAST go-round
on Venusian, Mercurial, and other planetary meteorites!

The best reference on the transfer of impact materials from one planet
to another is:
Science, March 8, 1996 v271 n5254 p1387(6).
Title: The exchange of impact ejecta between terrestrial planets.
Author: Brett J. Gladman, Joseph A. Burns, Martin Duncan, Pascal Lee
and Harold F. Levison

    First, there is the question of "Earth" meteorites: chunks blasted
off the Earth and then returned to it as meteorites. These arguments
apply to Lunaites as well, of course.
    Gladman is using large scale step-by-step integration simulation
for this, which requires a huge amount of very heavy computer-time
lifting. His findings are not based on abstract dynamic arguments
(which have a lousy track record for prediction). He has done a lot
more work on the question, which was summarized in the
September, 1999, issue of Sky & Telescope.
    Fully 50% of the simulated Earth (or Moon) ejecta returns to the
Earth (or Moon) on a time scale of 10,000 years up to 10,000,000 years.
The 10,000 year time scale applies to objects that wander around in the
vicinity of the Earth-Moon system; the longer time scales for objects
that achieve independent heliocentric orbits but get swept up again.
    Frankly, if you admit that the Earth does or has ever been hit with
impactors big enough to produce ejecta, then I can't really see how you
could rule out "Earthites!" The real question is a) how to identify
them, and b) how to prove it.

Where Are the Venusian meteorites?
    Just as interesting (to me at least) is that Gladman's runs show
that there should three times as many chunks of Venus reaching the earth
as chunks of Mars. So, if there are 15 Mars meteorites, where in the
hell are the 45 Venusian meteorites? Jay Melosh calculated that over the
history of the solar system there should have been about 500,000,000
chunks of Mars arriving on Earth. I cross-wire these two conclusions and
ask, how could we have misplaced 1,500,000,000 chunks of Venus? And, oh,
yeah, what about the 50,000,000 chunks of Mercury? If we have 15 chunks
of Mars, we should have 1.5 chunks of Mercury to go with the 45 chunks
of Venus. [Note: the number of Martian meteorites is greater now than
when I originally wrote this. Adjust the figures accordingly.]
    Since the current conventional wisdom on Venus regards the planet as
essentially terrestial but with a variant history, there would be little
to distinquish a chunk of Venus from a chunk of Earth. The Russians'
published table of bulk crustal composition for Venus are virtually
identical to what would be found on much of the terrestial crust.
Personally, I think contemporary planetary science has a bad case of
looking at the Worlds with Earth-colored glasses, but, hey, that's just
my opinion. So, if you find a rock that appears terrestial but has a
fusion crust, check its argon isotope ratios (40/38/36) along with
everything else. PLEASE.

Rocks would be Destroyed by Being Blasted off a Planet, Wouldn't They?
    The truth is that theory is not truth; facts are truth. Here are
these humble little pieces of Mars. We can hold them in our hands. They
made it. They got here. They didn't get melted. They didn't get
vaporized. (Well, okay, they got whacked good.)
    An Earth or Venus rock accelerated to escape velocity only
needs 10 to 15 seconds to traverse the atmospheric blowout path to
atmosphere-free heights. Another problem we have is trying to visualize
some poor little rock getting booted to excape velocity with one swift
kick.
    An Earth rock accelerated to escape velcity in one second only
needs about a 1,120 gee kick. There are a lot of terrestial rocks
that could withstand 1000 gee's without even fracturing. Given a
2-3 second acceleration, they would need to hold up to only 300 to
500 gee's. The critical factor is not that first derivative
(how velocity changes with time) but the second derivative (how
acceleration changes with time); as long as the force is not applied
"instantaneously" but builds up, even over a few tenths of a second, the
rock will survive. Anyone got a mass driver and a bucket of rocks? It
would be fun to find out!
    But there's a simpler argument. Ever seen a rock wall 40 or 50 feet
high? Sure you have. Well, every cubic inch of rock at the base of that
wall has a 500 inch high column of rock sitting on it, pressing down
with
the force of 500 times that bottom cubic inch's weight. That's 500
gee's,
my friend, showing that a rock isn't bothered by 500 gee's. For a 100
foot high stone wall, it's 1200 gee's equivalent. Now, think about the
Washington Monument... Ouch!

    In that earlier discussion, Bob Verrish wrote:
    "I need references to counter those that say that there are
calculations which show that the energy required to launch an Earth rock
into space is more than enough to completely melt or vaporize it."

    OKay, it takes about 1.25 x 10^10 ergs per gram to melt rock and the
kinetic energy of escape is about 1.2 x 10^12 ergs per gram, so
obviously it is impossible for ANY object to escape the earth, whether
it be rock or rocket. (In fact, this argument was used in the early 20th
century to "prove" that ALL space travel was impossible!) But John Glenn
didn't melt!
    Yeah, if you fired the Space Shuttle out of Jules Verne's Space
Cannon built by the Baltimore Gun Club in 1867, it would melt.
So what? The "those who say" are operating under the assumption
that the transfer of kinetic energy to the object must be instantaneous
and equally obviously, it not only is it not instantaneous, there is
probably no way to make it really instantaneous, which is a mathematical
fiction. It's strictly a straw man argument: let's propose an
impossible case and then prove it's impossible. GIGO.

    Maybe the escapee rocks lie a way back from the crater. The
landscape as a whole at the impact starts to move, shoved aside until it
reaches hypersonic velocity carrying chunks of buckling terrain with it.
The impactor vaporizes and the landscape REALLY starts to move, then the
full shock wave arrives in a another second and boots a rock already
going mach 10 up to mach 30 or so. Yes, a lot of rock gets crushed,
melted, and even vaporized, but the lucky rock just ahead of the wave
get flicked away like a drop of spray into space to surf the void. Bye,
bye, world.

    Anyway, I don't worry too much about how it happens exactly, because
what we do know is that it does happen. Details would be nice and I
would relish them, but I can wait.
    So, how does one convince the world? Only one way I know of. Find
the rock and prove it. Rub their noses into it until they notice. We've
been through this over and over again; how do you prove to the French
Academy that rocks fall from the sky?


What Goes Where?
    Here's a brief table of Gladman's simulations with how much ejecta
ends up where:

Ejecta From Mercury:
Mercury 80%
Venus 7%
Earth/Moon 0.5%
Mars 0%
(Only 4% falls into the Sun!)

Ejecta From Venus
Mercury 0.5%
Venus 50%
Earth/Moon 9%
Mars 1%

Ejecta From Earth/Moon
Mercury 0%
Venus 15%
Earth/Moon 50%
Mars 0.1%

Ejecta From Mars
Mercury 0%
Venus 4%
Earth/Moon 5%
Mars 3%

    You'll notice the totals don't equal 100%. The remainder suffers a
variety of fates, but a lot of it leaves the solar system at about 30
km/sec, to arrive at alpha Centauri in 50,000 years, Sirius in 90,000
years, epsilon Erdani in 112,000 years, and so forth, carrying certain
small living specks with them in some cases, only to be blasted off
those newly life-infected worlds by more impacts, and so on, which would
transfer earth based life to the entire Milky Way Galaxy in only a
billion years or so. Hello, cousins...


A Quote from the Gladman paper:

---------------------------------

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.


Other Sources of Material Transported to Earth: EUROPA?!
    Read Freeman Dyson's delightful piece in The Atlantic Monthly:
<http://www.TheAtlantic.com/issues/97nov/space.htm>
where he points out that impacts on Europa would just smash
through the ice and splash the sub-surface ocean out into Jupiter
space by the cubic kilometer, and IF there were aquatic life on
Europe, it would get splashed out with the water and there would
be freeze-dried "fish" orbiting Jupiter, some of which would be
deflected out of Jupiter's orbit, and what about those crazy
Fortean reports of strange sea-life falling from the sky?
    Yeah, sounds whacky, but am I going to argue with Freeman
Dyson? Not on your life. You tell him.
    Or maybe I should have said, it sounds fishy? :-}

And Lastly: Meteorites From Outside the Solar System?
    It could happen...


Sterling K. Webb
Received on Thu 23 Oct 2003 12:16:21 AM PDT