[meteorite-list] Mercurian Meteorites Low In Metal?

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:28 2004
Message-ID: <3CC3AA46.ED81EC2_at_bhil.com>

Hi, Mark, List,

    Yes, everybody assumes that Mercury has a massive iron core
(~75% of the planet). However, since we have never done the basic
spacecraft investigation necessary to measure Mercury's moment of
inertia, it is just that: an assumption with zero evidence to
back it up. (The measurement of the moment of inertia tells you
the distribution of mass inside a body, whether it's uniformly
distributed or concentrated at the center or somewhere inbetween
and how much.)
    No, a metal-rich core does not form because the sun's heat
melts the planet. Putting a planet together by having millions of
planetesimals slamming down through the growing gravity field of
the new planet one after the other releases more than enough
energy to melt an entire planet. When a planet melts, the iron
separates from the rock and gravity pulls the heavier metals down
to form the core. Elements that are still chemically combined
with the metals go along for the ride. Something still
unidentified makes up about 10% of the Earth's core, so you can
see that even studying a core that's right under our feet for a
century does not give you all the right answers.
    Back to that big iron core for Mercury (if it has one). The
oldest theory of the formation of the solar system is the nebular
hypothesis, the notion that the planets condensed from the solar
nebula. This gave rise later to the more sophisticated notion of
planetary formation by equilibrium condensation, that is, the
different planets formed at different locations in the nebula
where different elemental abundances were in equilibrium with
local temperatures. Since Mercury formed close to the Sun, it was
thought that metals and refractories would be the first compounds
to condense as the nebula cooled, hence the notion of a big iron
core for Mercury.
    In the last fifty years, however, we have realized that
equilibrium condensation only works to form small particles, like
the fascinating little droplets we find in meteorites. After
that, accretion and differentiation take over as a planet forming
process. Millions and millions of accreting planetesimals have a
complex dynamic history that makes unraveling the construction of
planets a lot more fun than simply letting them "condense."
    The current theory of why Mercury would have a big iron core
(again, if it does) is that a Mercury about twice the size of the
present one suffered a huge late collision with a planetesimal
about 20% its own size which came close to disrupting Mercury,
stripping off most of its silicate mantle and giving it its
eccentric and inclined orbit. (A body forming as close to the Sun
as Mercury should have a circular and non-inclined orbit, but
Mercury has neither. A collision that could give Mercury an odd
orbit for 4 billion years can be defined pretty much as one hell
of a whack!)
    But basically the reason why we think that Mercury has a big
iron core is because Mercury is very dense (uncompressed density
of 5.3 gm/cm^3 for Mercury versus 4.0 gm/cm^3 for Earth and
Venus). But one fact in isolation has a dangerous attraction for
the mind of a theorist. We should remember that the fact that the
Moon has a bulk density virtually identical with the bulk density
of the Earth's mantle caused theorists to spend more than a
century spinning out theories that derived the Moon from the
Earth's mantle. The Moon of course is radically different from
the Earth's mantle and in fact, it appears that the Moon is just
a little planet we bumped into long ago. Bonk. So, we should
beware of building theories on a single fact.
    Mercury is odd in lots of ways. Photos of its surface look a
lot like the Moon at first glance, but there are huge
differences. The most obvious thing about the Moon's appearance
is the contrast in color (reflectivity) between the bright and
dark areas, the cratered highlands and the basins with their lava
plains. The two types of areas are formed from different
materials with different histories. Mercury has plains, basins,
highlands, craters, just like the Moon, but they're all the same
color. The surface of Mercury has little or no albedo contrast.
Mercury is brighter in absolute terms, too. In general a
Mercurian feature is twice as bright as the same type of feature
on the Moon.
    We assume (there we go again!) that smooth plains on Mercury
are formed by flood vulcanism, just like all the other rocky
planets, and that the lava that flooded out and smoothed those
plains was basaltic, just like all the other rocky planets.
Actually, this is just another unjustified assumption, derived
from theory in the face of contradictory evidence. All basalts
are dark. Mercury's "lava" plains are not dark. If Mercury's
plains are volcanic, then Mercury is certainly unique in all the
solar system (and a mystery).
    If the theory of a late collision blowing off most of
Mercury's mantle is correct, the surviving mantle would be a
combination of Mercury's original mantle (composition unknown)
with the mantle of the impactor (composition unknown). Lavas
derived from the resulting Mercurian mantle could be really odd.
Whatever the "lava" is, it's the same color as the rest of the
planet, so by implication, the rest of the surface is pretty much
the same material (composition unknown). That's a lot of
unknown's.
    If Mercury has a big iron core, then that core is liquid,
since Mercury has a weak but definite magnetic field and a solid
iron core would have frozen out by now. The chief (and dam near
only) candidate for keeping an iron core liquid is sulphur (FeS).
But if Mercury condensed next to the Sun, where did substantial
quantities (2% to 3%) of a volatile like sulphur come from?
That's a real puzzler.
    Additionally, Mercury has a very thin atmosphere composed of
sodium, another element too volatile for such a hot neighborhood.
To make matters worse, Mercury has ice at its poles. Yes, that's
ice, frozen water, on the planet tucked under the Sun's armpit.
It could have all been brought there by comets, of course, or it
could be that Mercury is surprisingly volatile-rich. More
mystery.
    So your question of "what a Mercurian meteorite should look
like and/or otherwise contain" is a real good one. It's so good
that it doesn't have a good answer. We assume (there's that word
again) that Mercury is highly differentiated then the Mercurian
surface would be very iron-poor (and nickel-poor and generally
depleted in a long list of elements that like to hang out with
iron). If that assumption is good, the candidate meteorite would
be iron-poor, like an aubrite.
    The current candidate is a eucrite (all eucrites are
nickel-poor and iron-poor) with an oxygen isotope ratio different
from all its country cousin eucrites from the backwoods of Vesta.
So, it's got to come from somewhere else and that somewhere else
has to be big enough to differentiate. As big or bigger than
Vesta. The list of suspects is not long. Admitting (again) that I
haven't read the original article, I like to know what all the
isotope ratios are for this rock (particularly rare gasses).
    The problem is that Mercury is more than enough of a mystery
on its own that what we know about Mercury doesn't help much with
the question, "does this rock come from Mercury?"



Sterling K. Webb
-------------------------------------------------
Mark Fox wrote:

> April 21, 2002
>
> Greetings Meteorite Enthusiasts!
>
> A question has emerged concerning what a Mercurian
> meteorite should look like and/or otherwise contain.
>
> While reading the recent discussions, it was stated
> that such space rocks would contain a low metal
> content. However, it is currently accepted that
> Mercury contains a rather large core of molten iron.
> Being the second smallest planet in the solar system
> and without a useful atmosphere, wouldn't such a
> planet normally be less equilibrated than Venus or
> Earth if it wasn't for its close proximity to the sun?
> To clarify this, wouldn't one expect there to be more
> free iron-nickel near or in the planet's mantle if it
> wasn't for the sun? I am guessing that to be the
> reason why such a prediction was made concerning
> Mercurian meteorites: the sun's heat causing the
> heavier elements to migrate to the core. Is this so?
> (Venus, by the way, if I read correctly and which is
> further away from the sun, is supposed to be hotter
> due to its thick clouds.)
>
> Before I go, I wish to thank Al for his input on my
> last post about the European fireballs!
>
> Long strewn fields!
>
> Mark Fox
> Newaygo, MI USA
>
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Received on Mon 22 Apr 2002 02:14:30 AM PDT