[meteorite-list] Re: Earth Originating Meteorites
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:06 2004
Hi, Bob, Vern, List,
I love it when a thread wanders into one of my fondest obsessional
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
Someone (possibly Bernd Pauli) posted the text of this article to
the List a year or so ago. 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.
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
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
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 300-500 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!
Bob V 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."
Yeah, 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 space travel was impossible!) 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
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,
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?
Here's a brief table of Gladman's simulations with how much ejecta ends
Ejecta From Mercury
Ejecta From Venus
Ejecta From Earth/Moon
Ejecta From Mars
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...
Received on Fri 26 Jan 2001 12:57:32 AM PST