[meteorite-list] Early Mercury Impact Showered Earth

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu Apr 6 00:53:18 2006
Message-ID: <001a01c6591c$7a381550$50342b41_at_ATARIENGINE>

Hi,


> Ron Baalke posted:
> http://www.spacedaily.com/reports/Early_Mercury_Impact_Showered_Earth.html
> Early Mercury Impact Showered Earth
>
> > Given the amount of material that would have been ejected in such a
> catastrophe, Horner said, Earth could contain as much as 16 quadrillion
> tons of proto-Mercury particles.
>
    That's 1.6 x10^17 tons or 1.6 x 10^20 kg. The rough
estimate I posted Monday (before this article was out of
embargo), made by extrapolating from Bret Gladman's 1996
simulation of meteorite transfer, was roughly 3 times greater
(5 x 10^20 kg).
    That's about one gram per 10,000 grams of Earth. This
study would put it at one gram per 31,250 grams of Earth.
I'd say being within a third of an order of magnitude amounts
to pretty much the same thing when you're talking either
computer simulations or back-of-the-envelope scribbles!
It's actually a pretty remarkable correspondence when
two different methods produce such a similar result.

> The group found that the fate of the debris depended
> on where Mercury was hit, in terms of its orbital position
> and the angle of the collision.

    The two studies are quite different in their approaches.
To do this new study, they had to decide on ONE impact at
ONE angle and calculate from that. Hard to know which
impact angle is "right" after 4 billion years. Gladman's
original study was based on simulating 200 random
impacts and so creates essentially an "average" impact.

    It's a fascinating notion, whether the Earth is 1/10,000th
Mercury or 1/30,000th Mercury. One assumes that the
Mercurial debris were well mixed into the early Earth,
or at least its upper crust, and since have been churned
by tectonics for billions of years. It would seem very
unlikely that anybody could identify any component
of the Earth as Mercurian in origin, particularly in such
a small concentration.

    Making matters worse is that we don't have any way
of knowing what the "original" Mercurian crust (which would
have been blasted away to the Earth and Venus) would
have been composed of, and so have no clues as to
what to look for in the Earth's composition. Anyone
have any rare isotopic ideas?

    Timing is another factor. Did the Earth get splatted
with Mercurian stuff before or after our own Big Impact
which formed the Moon? If it was before, then the
Mercurian deposits would have been redeposited
on the Moon! (There are an awful lot of refractories
on the Moon, really.)

    Even going to Mercury wouldn't be much help, as the
"missing" material of its former crust isn't there any more!
Even the theory of the Moon's formation by a big impact
has testable implications (that the Moon should be very,
very dry, which it is, and highly refractory, ditto), but
it's hard to think of any evidence for the Mercury Crust
Removal Scenario that we didn't already have an
explanation for before the theory came along.

    If Gladman's simulation is correct, Venus would
have received 13 times more of Mercury than we did.
Since Venus's crust is young (~500 million years),
any Mercurian traces are gone now, even if you could go
down into the Venusian Hell to look for them!

    We always expected Mercury to be dry and refractory
because it formed near the Sun, and the big metal core
used to be explained the same way. The Giant Impact Theory
may be improvable in the strictest sense, at least without
going to Mercury, which of course we will do eventually.

    Nevertheless, The Big Whack Theory is important in
another sense. Belief in it implies a much more collisional
history for the early Solar System than previously thought.
That in turn implies a much more "mixed" origin for the
planets than the usual view. All the differing theories of
solar system origin (a la the 1990's anyway) agree on the
fundamental concept of the planets having been formed
from very narrow zones of fairly uniform composition.

    There is a body of "clues" accumulating that suggest "it
ain't necessarily so." Here are a few:

    a) Stardust shows us LOTS of cometary silicates. But
silicates are inner solar system products. If comets were
formed beyond Neptune, where'd all those silicates
come from?

    b) Closer looks at individual asteroids show us a very
wide range of compositional differences. How could they
have formed in narrow zones of uniform composition?

    c) Many of the newly discovered Trans-Neptunian
or Plutonian "planets" have densities that show they
must be mostly or entirely rock. How could they have
formed out where there was nothing but ices? Did they
"move around"? Or are some of them captured from
other stars? (Nobody likes that unlikely idea...) To
make matters even worse, the discovery of a nice neat
multiple satellite system around one makes a violent
capture or "planet-move" almost impossible to
conceive of. So there must have been enough
inner solar system materials out beyond Pluto to
form whole planets. That's enough to give a roomful
of cosmogonists lifelong migraines...

    d) A recent dynamic study claims the multitude
of iron and iron/rock asteroids now found in the
Asteroid "Zone" all come originally from the inner
Solar System. Another nail in the coffin?

    a) Most of the extra-solar planets detected in the
past decades have their "gas giant" planets in very,
very close to their system's stars. So, is our solar system
just a whacko oddball? Or are they, all 104 or so
of them, the oddballs?

    Now we have collisional mixing proposed for
Mercury and the Earth, albeit small scale (one part
in 10,000), but for Venus it may be one part in
750. At an earlier stage of solar system evolution,
were there immense episodes of material transfer
that churned the solar system (yet to form) into an
incalculateable mixture?

    If this were so, or even if the early system was merely
heavily mixed by eccentric planetesimals, it would
blow the neatness of our theoretical considerations
of "equilibrium condensation" and the other ordered
notions we have concerning our origins into a cocked hat.

    I love the smell of a paradigm shift in the morning...


Sterling K. Webb
Received on Wed 05 Apr 2006 09:50:26 PM PDT