
Confirmation That Earth's Core Is Solid - gscott
https://www.sciencealert.com/seismic-j-phase-shear-wave-detection-reveals-elastic-solid-inner-core
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cbkeller
I'm a geologist. This press release is pure BS. We've known beyond reasonable
doubt that the inner core is solid for more than a decade - for instance:
[http://science.sciencemag.org/content/early/2005/04/14/scien...](http://science.sciencemag.org/content/early/2005/04/14/science.1109134)

We even know the elastic modulus of the solid inner core:
[http://advances.sciencemag.org/content/4/6/eaar2538](http://advances.sciencemag.org/content/4/6/eaar2538)

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marktangotango
So why is the earth so old and still incredibly geologically active? Is it
simply because of the moon and tidal effects? If the crust is thin relative to
other planets why hasn’t the earth cooled more by now?

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cbkeller
Two reasons: (1) the earth has really huge thermal inertia (2) radiogenic heat
production. (2) often gets a lot of the credit (who doesn't like
radioactivity), but (1) is probably doing more of the heavy lifting

At present day, Earth is losing heat at a rate of about 40 terawatts. This 40
terawatts drives mantle convection, plate tectonics, and every volcano out
there.

As best we know, about 35% of that 40TW is radiogenic (decay of Th-232 with a
half-life of 14 bilion years, U-238 with a half life of 4.5 billion years and
K-40 with a half life of 1.25 billion years, among others). A fair amount of
the decay energy is actually lost to neutrinos, but the rest is converted to
heat and eventually lost by convection or conduction.

The other 65% or so is primordial -- essentially, gravitational potential
energy from the accretion of the Earth, converted to heat and slowly lost over
the past ~4.51 billion years since the moon-forming impact.

Earth's bulk thermal inertia (heat capacity times mass) is about 5.6E27 J/K.
In other words, cooling the earth by one degree C releases 5.6E27 joules.
That's a lot of joules!

Even so, there's a bit of a problem because geochemical evidence suggests that
the mantle (which makes up most of the earth by volume) has probably not
cooled by more than about 150C over the past 3.5 billion years, which means
either one of our estimates is wrong, or the core is cooling pretty fast --
this would actually mean that the inner core might be pretty young, like < 1
billion years.

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flukus
> this would actually mean that the inner core might be pretty young, like < 1
> billion years

What are the error bars on that? It seems to be getting quite close to the
emergence of complex life, enough that the radiation shielding may be a more
important pre-requisite than previously believed.

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cbkeller
Big enough that that's a totally reasonable question! I don't think I can even
put a number on it very confidently, but maybe something like 1.0 -0.7/+2.0
Ga?

One counterpoint that comes to mind is that the Ediacaran fauna is usually
though to be marine, and water is a pretty good shield of radiation. There is
one geologist who argues the ediacarans were terrestrial (Greg Retallack), but
he interprets any sedimentary rock that's red as a paleosol, which is actually
not completely silly, but is a bit more liberal than most others would be
comfortable with.

In any case, I don't know, but it's not impossible!

edit: I should also say that the generation of Earth's magnetic field in the
liquid outer core is a bunch of incredibly messy magnetohydrodynamics that is
hard to simulate accurately, but we can probably have a pretty reasonable
magnetic field with a purely liquid core (no solid inner core at all) -- it's
the convection of the liquid outer core that provides the energy to sustain
the magnetic field, so it's actually once the core becomes fully solid that
we'll be in trouble

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olivermarks
'our latest understanding is', not 'confirmation'

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Retra
When your latest understanding is based on new evidence consistent with and
supportive of your previous understanding, then it is a confirmation. It
really isn't necessary to blindly hedge on what we know unless you're willing
to call into question the concept of knowledge itself.

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greglindahl
This 'confirmation' sure doesn't sound like a five-sigma detection of a new
particle at CERN.

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vectorEQ
there is actually no observable proof that anything about the inner inner
earth we 'know' is correct. for lack of a better theory and any kind of
observations, like with any science, we accept the most likely theory untill
it's proven wrong >.> which it might or might not be in the future. to say you
know exactly how the core of the earth looks like is just plain silly. it's
like saying the universe is infinite or finite. there is no proof or fact.
just theory upon theory upon theory. not saying it's bad to theorize, but
confirmation? that would be a sample from earths core...... not some wild
speculation base on observations of processes very far from what is being
theorized about

