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

From: Jerry Flaherty <grf2_at_meteoritecentral.com>
Date: Sat, 5 Sep 2009 17:56:38 -0400
Message-ID: <F3675F1126924D09A89753F1E2019E6E_at_ASUS>

so informative thank you
The classification of irons is less of a mystery but still needs further
"distilling" for the masses [e-r-r-r, me] Ahh I see an addendum. perhaps it
will elucidate.
Jerry F

--------------------------------------------------
From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
Sent: Saturday, September 05, 2009 1:14 AM
To: "Carl 's" <carloselguapo1 at hotmail.com>;
<meteorite-list at meteoritecentral.com>
Subject: Re: [meteorite-list] Slow cooling rate of irons in space

> 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 05:56:38 PM PDT