[meteorite-list] Extreme melting event defines Earth's early history

Fri Jun 17 02:43:53 EDT 2005

Hi,

    With all the usual caveats about this being a press release and with the full admission that I agree
with and welcome the results of this study, I have a complaint about a particularly ridiculous statement,
namely:
    "One possible explanation of the difference in 142Nd/144Nd between Earth and chondrites is that the
Earth's average composition is not chondritic, but on the basis of several chemical arguments this
explanation is unlikely."
    The notion that you can "make an Earth" out of 6000 billion billion tons of chondrites is as
thoroughly discredited a notion as I can imagine.  This a very old notion (XIXth century) and was once
very popular but it's like saying your findings "could mean that the Earth is not flat, but this
explanation is unlikely."
    I feel strange even attacking it.  It's like you were kicking a dead dog, one that stopped twitching
hours ago.  Somewhere, I have a source (which, after an hour of searching I cannot find) that outlines
what the Earth would be like if it actually were made of chondrites, some of the most scathing paragraphs
I've ever read.  It wouldn't even vaguely resemble our planet.  I think the author may have been John S.
Lewis, but I can't find it.  It'll turn up under my nose as soon as I post this, of course.
    A chondrite is by definition a planetesimal (or portion thereof) from 2 to 4 AU's out, formed there
and lived there, until some happy (for us) accident sent it our way.  Everything about the isotopic
composition of every body for which we have any knowledge indicates little or no lateral mixing of
planetesimals.  I only say "little" to be polite.  Every bit of evidence suggests a close precise zoning
of compositional differences, with the only mixing occurring because of the ability of the accreting
planet to pull in material over an wider zone as it "bulks up."
    The chirpy little meteorite (and excuse me, earth science) websites that give a "recipe" for the Earth
by mixing a certain percentage of chondrites, a dash of eucrites, a certain percentage of carbonaceous, a
certain percentage of this and that, are like medieval fairy tales because no such mixing ever took place
and is not even possible.
    Every planet has a unique compositional origin, modified by a unique history.
    Yes, you can say the Earth is "chondritic" if, and only if, by "chondrite" you mean every condensed
chunk of our accretion disk that was not iron or ice, from the intra-Mercurian boundary to the edge of the
Kuiper Belt, is a "chondrite."  That's not what I mean by a chondrite.  Is that what you mean by a
chondrite?
    If you mean anything you can pick up a chunk of today, forget it.  The "chondrites" that made the
Earth are no longer available for sale or trade, even by the estimable STEVE.  Those chondrites?  We're
standing on them.  The term "chondrite" and "planetesimal" do not mean the same thing and are not
inter-changeable.
    This study -- hey! I like it -- is not a breakthrough, Mr. Press Agent.  It's a nail in a coffin so
studded with nails it's hard to find room for a new one.  You're proud; you're happy; we understand that.
Don't go overboard.
    As for the melting, the press release says, "this evidence fits the growing number of observations
from the Moon and Mars."  Yeah, like the last forty years worth of evidence, which is most of what we
know.
    Long before an accreting Earth reaches the size of the finished planet, the amount of kinetic energy
delivered to its surface by every gram that falls in is greater than the energy required to melt that
gram, greater than the energy required to vaporize that gram, great enough to turn that gram of vapor into
a gram of 20,000 degree plasma!  And the bigger the planet gets, the faster they fall, in a fury that is
hard to comprehend.
    Even delivered over tens of millions of years, this accretional kinetic energy is more than sufficient
to melt the entire planet completely through to the core several times over and keep it melted.  The
coolest spot on our lovely planet at this point is the top of the atmosphere, of course, only the
atmosphere is rock vapor at 2500 degrees or more. (It's only as cool as 2200-2500 degrees at the top.)
Since rock vapor is completely opaque to almost every wavelength and is refectively ionized to boot, it is
a perfect insulator, so little of that heat can transfer to the top and escape.
    Yes, there are all these other heating mechanisms, like radioactive decay of short-lived isotopes, and
the rest of a long list, but they're like pouring gasoline on an already raging fire -- they help out, but
they're not necessary.
    As for the complimentary material required "if the Earth is to have an average composition matching
chondrites," it's a fantasy.  Show me a piece of it.  It's like phlogiston, an invented imaginary
substance, here used to explain why pigs have wings.  The only problem is that they don't.  Let me see
those pigs perched in the tree-tops!  Then, I'll believe.
    Sorry if this seems cranky.  Ok, sorry that this IS cranky.  Sore spot.
    Wonderful study!


Sterling K. Webb
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Darren Garrison wrote:

> http://www.eurekalert.org/pub_releases/2005-06/ci-eme061505.php
>
> Extreme melting event defines Earth's early history
> Could Earth have had an even more violent infancy than previously imagined? New isotope data suggest
> that the Earth not only had a very violent beginning but also point to new information about our
> planet's chemical evolution.
> New and precise measurements of a neodymium isotope ratio (142Nd/144Nd) led Maud Boyet and Rick
> Carlson of Carnegie Institution's Department of Terrestrial Magnetism to the discovery that all
> terrestrial rocks have an excess of 142Nd compared to the expected building blocks of the planet.
> The results will appear in the June 16, 2005 edition of Science.
>
> Prior research suggested that the Earth formed by the accumulation of planetesimals -- small cold
> bodies present in early solar system history. The chemical composition of these early bodies is
> reflected today in a type of stony meteorite called chondrites. Scientists had expected that the
> Earth would have a composition similar to these meteorites. However, this new research challenges
> these earlier conclusions by showing that terrestrial rocks have excess 142Nd caused by the
> radioactive decay of the now extinct isotope 146Sm.
>
> One possible explanation of the difference in 142Nd/144Nd between Earth and chondrites is that the
> Earth's average composition is not chondritic, but on the basis of several chemical arguments this
> explanation is unlikely. More probable is that the portion of the Earth involved in creating crustal
> rocks was chemically differentiated very early in the planet's history â€“ Boyet and Carlson's results
> suggest within the first 30 million years, or less than 1%, of Earth's history. As such, this
> evidence fits the growing number of observations from the Moon and Mars that the early history of
> planets was a very violent one, where collisions with planetesimals, the release of radioactive
> heat, and the energy involved in separating a metallic core all provide enough energy to melt the
> planet. Cooling and crystallization of the molten planet over timescales of millions to a few tens
> of millions of years then result in its chemical differentiation, segregating material according to
> density. This differentiation left most of the Earth's mantle similar in composition to the
> present-day upper mantle from which volcanic rocks are derived.
>
> There must then be material that is complementary in composition to the bulk of the mantle. This
> complementary region, if the Earth is to have an average composition matching chondrites, must be
> enriched in potassium, uranium, and thorium -- radioactive elements that have provided most of the
> heat generation in the Earth's interior throughout its history. Furthermore, this complementary
> mantle reservoir must be very deep, because none of the magmas that have erupted at the Earth's
> surface have ever sampled it. Boyet and Carlson suggest that the reservoir coincides with the
> so-called D" layer imaged seismically at the very base of the mantle, just above the core. A
> radioactive-element-rich layer deep in the Earth is like a heating plate at the bottom of a pot: it
> will keep the bottom of the pot hot for a long time. Such a layer will also keep the top of the core
> hot and hence delay its cooling and crystallization. The scientists postulate that the early
> differentiation of the Earth and the deep layer produced by that process may be the reason that the
> Earth still has its magnetic field. The deep layer may also be responsible for generating hot plumes
> of upwelling mantle material that give rise to volcanic island chains such as Hawaii.
>
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