[meteorite-list] Slow cooling rate of irons in space

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 5 Sep 2009 00:14:40 -0500
Message-ID: <E724405E555A486FB729DF6469EBB6D9_at_ATARIENGINE2>

Hi,

Carl raises a lot of interesting points with his
questions, some of them still unanswered. The
cooling question, for example. The problem is
not how do you keep it hot -- it's how do you
cool it? Take a minor differentiated body like
the Earth. Our iron core is plenty hot after
4.5 billion years of "cooling." But, as with all
differentiated bodies, size matters. The smaller
the body is, the faster it cools.

A lot of the heat that melts cores (and planets)
comes with their formation. It's been calculated
that the kinetic energy of the impacts that accreted
the Earth was more than enough to melt the planet,
or at least the mantle! So, there's heat to start with.

Then, there's the possibility of internal radioactive
heating. When the solar system formed, there
were a much higher proportion of short-lived
radioactive isotopes that release energy as they
decay. There was a measurable amount of Al-26,
which rapidly decays into "normal" aluminum 27,
producing a burst of heating. (There are a lot
of arguments about where the Al-26 came from,
but none about it being there.) There was also
another intense short-lifer -- Hafnium-182 that
may have heated bodies early on.

At the start of the solar system, the proportion
of Uranium 235 in "natural" uranium was much
higher than today. Now, U235 is about 0.7% of
a natural uranium sample. 4.5 billion years ago,
it was 24%. That's enriched to the point that
would be called bomb-grade or fast reactor fuel
grade today!

There was enough U235 in terrestrial uranium
deposits for them to perk along as a natural nuclear
reactor in the early days of the Earth. In Gabon, in
Africa, there was a natural nuclear reactor that only
quit running about 2 billion years ago. The uranium
in the Earth contributes to its heat production, though
again there are arguments as to how it does it. Where
there are antineutrinos, there is actinide decay, and
the Kamioka, Japan, detector counts about 16.2 million
antineutrinos per square centimeter per second
streaming out from Earth's core, the result of 30-36
terawatts of nuclear reactions. Again, where and how
are big questions.

Differentiation is when a body gets big enough to get
hot at its center from the force of gravity-induced
pressure. Iron in the rock-iron mixture melts and drips
down to center forming a molten core surrounded by a
melted (or almost melted) rock mantle. The rock on the
outside cools and insulates the molten core., slowing
its heat loss. We used to think only very big bodies could
differentiate, but it seems that objects "only" 120 miles
in diameter may have been capable of differentiation in
the early solar system. Even small details have a big
effect on slowing heat loss. It turns out that rocky dust
and particles (regolith) is a very good insulator. Just a
few feet of regolith helps keep a body warm.

Whenever you pick up an iron meteorite, you are holding a
piece of the formerly melted core of a differentiated body in
your hand. Because I am biased toward physical processes,
I would call any body that was big enough to differentiate
a "planet." But there's another argument we're all too familiar
with, so I'll just keep typing "differentiated body." But "planet"
is so much shorter...

The point about iron meteorites is that they are, for the most
part, completely iron (and nickel and that stuff) -- they are
100% core material. We all know most meteoroids are chips
off asteroids, so there must be a large number of iron asteroids,
and -- there are. To have a 10-mile or 20-mile long chunk of
differentiated body core orbiting, there must have been a
once-differentiated body that was smashed into bits, or at
the least chunks. And of course, there "worlds" that are
pure iron. The asteroid 16 Psyche has the radar spectrum
of pure refined metal and it's 260 kilometers in diameter!
Is that a "stripped" core or did is form deep in the inner
system, too near the Sun to be anything but iron-nickel?
See, more questions...

Whole "worlds" collided and destroyed. Well, how many such
worlds were there? See, interesting questions always lead to
more interesting questions. We divide up irons into three kinds,
which correspond to a little nickel, medium nickel, and lots of
nickel (hexahedrites, octahedrites, ataxites), but there are many
other elements dissolved in the iron just as nickel is. Gallium,
germanium, iridium are all tough and hard to melt, like nickel,
but they dissolve into the iron melt without loss because of their
high melting points.

Gallium, germanium, and iridium exist in proportions that cover
a wide range of concentrations. There's an iron (Butler) with 0.2%
germanium and there are irons with 1/100000th of that amount.
Obviously, they didn't come from the same core! Even gallium
has a thousand-fold range. If you plot the abundances of all
four elements, even allowing for poor mixing, you end up with
at least 16 total different source bodies for iron meteorites.
Well, Wasson decided it was 16 groups.

Wow! 16 parent bodies! Well, no. There is a large group of irons
that don't fit into the 16 groups. They not a 17th group; they're
oddballs. None of them share enough in common with the other
70-80 odd irons to be related. They're orphans. Each is its own
group. So, we have 16 parent bodies represented by 10-20
specimens and 70 or so that are the only representative of their
parent body. That's 85 or 90 parent differentiated bodies that
were disrupted.

Personally, I think that the molten metal cores of rapidly
spinning parent bodies would mix quite uniformly within
a few millions of years, much less 100 million. There would
be early vigorous convection, blah, blah. I think the estimate
of <100 parent bodies is hedging. I think it's more like 200
to 250, but who cares what I think? (I get that figure by
assuming that any elemental concentration variance in a
"group" greater than an order of magnitude is "lumping"
and the group ought to be split at that point.)

Another point to be made is that even the smaller estimate
of parent bodies for iron meteorites is noticeably larger
than the number of parent bodies for stone meteorites.
That could mean a lot of things. It is a puzzle -- more large
disrupted differentiated bodies than there are surviving
undifferentiated bodies (chondrites by definition have
not been differentiated). Most early bodies differentiated?
Most early bodies were destroyed?

Obviously, the early solar system was a rough neighborhood.
It takes a really powerful impact to fracture and splinter iron
cores that were probably already cooled to solidity. Isotope
dating helps place some of the events in time. Those IAB irons
which are breccias of iron and chondrites? They have very
old isotope dates; they formed very early in the history of
the solar system, ancient events. Most irons have formation
dates between 4.5 and 4.6 billion years ago. But others,
mysteriously, do not. The Weekero Station IIE has a Rb/Sr
age of only 4.38 billion years and the Kodaikanal IIE of only
3.8 billion years. I would sure like to know their story.

And since Carl's original question was about mesosiderites,
one last puzzle about them. The isotopic dates of mesosiderites
(argon 40) cluster very tightly around 3.9 billion years old.
Some attribute that to an immense collision and re-assembly
between a naked iron core and a basaltic crusted asteroid,
both of them of very large size. Others attribute the dating
to the formation of the mesosiderites in a very large almost
Ceres-sized asteroid with very, very slow cooling that was
suddenly disrupted. One asteroid or two -- it must have
been one heck of whack!

Whenever you pick up an iron meteorite, you are holding a
piece of the dead heart of a world... That's a thought.


Sterling K. Webb
----------------------------------------------------------------------------
----- Original Message -----
From: "Carl 's" <carloselguapo1 at hotmail.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Friday, September 04, 2009 8:18 PM
Subject: [meteorite-list] Slow cooling rate of irons in space




Hi Elton and All,

I've read about the very slow cooling rate of the molten iron in various
books but I don't understand why this is so. Why would it take millions
of years for just a few drops of degrees? It's hard for me to envision
this even accounting for bombardments and radioactive decay.
Radioactivity from the original super nova event, right? Maybe it's
because I think of space as being so darned cold it wouldn't take
anything long to lose heat and freeze up. I realize radioactivity takes
a long time to decay but would it take a lot or so little to keep a
large planetary body hot for so long? Thanks.

Carl



Eman wrote:
>I think this theory has a potential fatal flaw if what we think we know
>about
taenite/kamacite growth is valid. Without an insulating blanket the
molten
pool will not exist in a molten state long enough to permit
crystallization aka
Widmanstatten patterns.

Be it remembered that Widmanstatten pattern/crystal growth is very very
slow on
the order of 10's of degrees cooling per million years. It is difficult
to
develop a scenario that integrates a large crater on an Goldilocks
Asteroid
which works.. ..


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Received on Sat 05 Sep 2009 01:14:40 AM PDT