
New theory explains how Earth’s inner core remains solid despite extreme heat - renafowler
https://www.kth.se/en/forskning/artiklar/new-theory-explains-how-earth-s-inner-core-remains-solid-despite-extreme-heat-1.705398
======
nonbel
Something I've been wondering. Does the earth's core have negative heat
capacity?[1]

I've read that it is usually modeled as a self-gravitating system[2], and that
such systems can end up in states with negative heat capacity [3].

[1] > "If the system loses energy, for example by radiating energy away into
space, the average kinetic energy actually increases."
[https://en.wikipedia.org/wiki/Heat_capacity#Negative_heat_ca...](https://en.wikipedia.org/wiki/Heat_capacity#Negative_heat_capacity_.28stars.29)

[2] > "The Earth’s outer core is a rotating ellipsoidal shell of compressible,
stratified and self-gravitating fluid."
[https://www.uleth.ca/dspace/handle/10133/3672](https://www.uleth.ca/dspace/handle/10133/3672)

[3] > "Negative heat capacities can only occur in isolated or nearly isolated
systems. They are impossible in truly extensive systems in canonical ensembles
or for that matter in grand canonical ensembles. However, far from being a
strange phenomenon only found in the thermodynamics of black holes they occur
widely on a macroscopic scale in astrophysics and with less precision on a
microscopic scale throughout physics and chemistry. They are the origin of the
large fluctuations that occur at phase transitions and we speculate that they
cause those transitions."
[http://adsabs.harvard.edu/full/1977MNRAS.181..405L](http://adsabs.harvard.edu/full/1977MNRAS.181..405L)

Edit: Also, the link to the paper leads to a 404 page... Stabilization of
body-centred cubic iron under inner core conditions, Nature Geosciences,
nature.com/articles/doi:10.1038/ngeo2892

~~~
pdonis
If you look at the Wikipedia article in more detail, you will see that the
negative heat capacity of a self-gravitating, isolated system arises from the
virial theorem. Heuristically, as a self-gravitating system radiates energy
away into space, it becomes more tightly bound; that means it contracts. But
as it contracts, the "orbital speeds" of its constituents increase (because
decreasing orbital radius means increasing orbital speed), meaning its average
kinetic energy increases, meaning it heats up.

Technically, the virial theorem argument only applies to systems whose
constituents are in free-fall orbits. But I believe you can do a similar
analysis for an isolated self-gravitating system that is in hydrostatic
equilibrium--basically, as it radiates energy away into space, the hydrostatic
equilibrium changes to one in which the radius is smaller and pressure and
temperature are higher. Unfortunately I don't have a handy link to such an
analysis right now, though.

As far as whether any of the above would apply to the Earth's core, I don't
think it would since the Earth's core is not isolated; but it might apply to
the Earth as a whole, to the extent that the Earth can be viewed as being in
hydrostatic equilibrium (i.e., pressure balancing gravity everywhere). AFAIK
it is common in astronomy to treat stars this way.

~~~
nonbel
Interesting, thanks. I still am not sure what the technical meaning of
"isolated" is here. But wouldn't such an object continue heating up
"infinitely" (maybe stopping at some phase-transition temperature) without
constant input of energy?

~~~
pdonis
_> I still am not sure what the technical meaning of "isolated" is here._

Technically, it means an object of finite spatial extent surrounded by vacuum,
and nothing else in the entire universe. Obviously that's an idealization. :-)
But it makes the mathematical model tractable.

 _> wouldn't such an object continue heating up "infinitely"_

Not necessarily; it might reach a stable state where it can't radiate any more
energy. Cold white dwarfs, cold neutron stars, and cold planets are possible
examples of such states. ("Cold" here basically means "at absolute zero",
i.e., in the ground state for its configuration of particles.)

~~~
evanb
"Cold" in astrophysical settings tends to mean T << µ (the chemical potential)
which usually justifies a low temperature expansion in T/µ, although thermal
effects can be qualitatively important.

~~~
pdonis
_> "Cold" in astrophysical settings tends to mean T << µ (the chemical
potential)_

Yes, I know, but this in itself does not preclude energy being emitted by
radiation. To preclude that, the object needs to be in a genuine ground state
--no internal transitions possible that can reduce its total energy. That's a
considerably stronger condition than T << µ.

------
technologyvault
This headline seems a bit overzealous, as if this new theory is the final
answer.

This new theory, noted as contradicting one from 2014 and 30 years of previous
conventional scientific wisdom, came after researchers "looked into larger
computational samples of iron than studied previously". What will be the
headline and tone in several more years, when someone comes along and studies
an even higher number of samples and coming to an entirely different
conclusion?

Doesn't it seem more appropriate and useful for the scientific community and
publishers of these studies to be careful not to state their findings as
conclusive and act as if their most recent study does much more than to
estimate what might be happening?

~~~
beevai142
I expect the article itself is more carefully worded than the press release.

I don't think people in the scientific community in general really care much
about the press releases. The publicity is more something that the funding
organizations and universities have started to require in recent years --- you
may even get some quantified "career points" of some sort from them depending
on the grant. The press releases are at best an afterthought for the
researchers. As they are also often edited by the PR staff of the university,
there may be some standardized level of hype injected.

~~~
jessriedel
There are many researchers who are eager to have their work publicized, both
for reasons of pride and grant-writing. They often are far from diligent in
working with the journalist to make sure the article is written with
sufficient modesty. Yes, universities and funders have began directly
soliciting hype journalism, but the researchers themselves still benefit, and
follow incentives.

------
jcoffland
> The ultimate goal of Earth Sciences is to understand the past, present and
> future...

That is the ultimate goal of all science.

~~~
egeozcan
You are right, science is not bound to Earth and also even "the past, present
and future" seem too narrow to me. Science is about understanding anything
which we are curious about, and that may include alternative timelines which
do not lay neither in the "the past, present nor the future" and also the time
itself.

~~~
cgriswald
Without a way to access those alternate timelines, no science can be done
there, because no hypothesis will be falsifiable.

------
lngnmn
How do they know that it is solid?)

~~~
nickjarboe
To the first order it is the fact that a liquid only propagates pressure waves
while a solid can propagate shear and pressure waves (which propagate a
different speeds).

~~~
rcthompson
That's true, but can shear waves be transmitted to the earth's solid core
through the liquid iron surrounding it?

~~~
Florin_Andrei
Pressure waves go through the liquid part, hit the solid part, some of their
energy is converted into shear waves which go through the solid core, convert
back to pressure waves on the other side, and eventually hit the surface
again. By measuring propagation time, direction, etc, the full picture is only
compatible with the pressure / shear / pressure double conversion.

TLDR: It's complicated.

------
sundvor
Funniest ad placement seen in a while:

"Spinning within Earth’s molten core is a crystal ball"

... with the Canadian Foreverspin.com ad just above this. I want one, and
please, keep on spinning.

~~~
hermitdev
If you ever want to f' with TSA in a completely legal way: carry lead crystal
in your carry-on luggage. I've done it a number of times carrying Christmas
gifts to/from family. My favorite was when my parents gifted me a set of lead
crystal tumblers from Waterford. The cut on the crystal sorta makes them look
like a pineapple grenade. Thing is, they're opaque on TSA scanners, so they
make you pull them out (and they're really cautious at this point), then they
pull the items out, and they're completely transparent. Their minds: blown. If
you're lucky, you get one you can explain it to, if you're unlucky, you get a
doofus that has to call for backup.

~~~
sundvor
Haha. :) Thing is, I'd probably have to show my HN credentials and whatever
an^T^T digital probing they do to let you in, so they'd know it was all
premeditated and then I'd be in real trouble. (I'm in Straya).

------
DrScump
Blogspam of:

[https://www.kth.se/en/forskning/artiklar/new-theory-
explains...](https://www.kth.se/en/forskning/artiklar/new-theory-explains-how-
earth-s-inner-core-remains-solid-despite-extreme-heat-1.705398)

~~~
dang
Ok, we changed to that from
[http://sciencebulletin.org/archives/10367.html](http://sciencebulletin.org/archives/10367.html).

I'm pretty sure most of these are press releases intended to be republished,
which isn't exactly the same as blogspam (where one site rips off another).
Still, original URLs are preferable.

~~~
DrScump
I never comment against actual press releases, because, hey, that's what
they're meant for.

"Blogspam" is appropriate for literally ripped content without specific
attribution; to then proceed to _edit out_ a byline or other author credit is
simple _plagiarism._

Press Releases always have either an embargo date/time or "For Immediate
Release" and a press contact for the originator, in my experience.

~~~
dang
I'm not sure that's right. We see countless articles, usually originating on
university websites, that are PR pieces about research studies. These get
reprinted across a wide range of science-content farm sites, which tend to be
pretty low quality, but I'm not sure they're copying without permission. It
seems more that the system is designed to work this way.

