

Earth's core far hotter than thought - groundCode
http://www.bbc.co.uk/news/science-environment-22297915

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aashaykumar92
This article is fascinating but as someone who doesn't know much about this
discipline, I'm hoping for someone to clarify my confusion:

In the article, it states that our core has temperatures comprable to that of
the sun. However, the sun is 93 million miles away while our core seems to
only be approximately 4000 miles deep. Shouldn't the Earth be hotter than or
are there just so many layers that the heat decreases at such a fast rate as
it approaches the surface?

~~~
Someone
A lot of the radiation that the sun's core emits gets through its surface
without interacting with it much.

You can infer that from <http://en.wikipedia.org/wiki/Sun>:

    
    
        Temperature:
        - Center (modeled): 1.57×10^7 K[1]
        - Photosphere (effective): 5,778 K[1]
        - Corona: ~5×10^6 K
    

=> The sun's surface is about its coolest part. Its core is about 3000 times
as hot. That's where the solar heat comes from.

~~~
uvdiv
_A lot of the radiation that the sun's core emits gets through its surface
without interacting with it much._

This is false. The sun is very opaque; photons from the core could take
_millions of years_ to reach the surface.

<https://en.wikipedia.org/wiki/Radiation_zone>

<https://en.wikipedia.org/wiki/Solar_core#Energy_transfer>

This is the article you're looking for:

<https://en.wikipedia.org/wiki/Corona#Coronal_heating_problem>

~~~
jacquesm
> photons from the core could take millions of years to reach the surface.

And it would be most unlikely they'd be the same photons that started on the
journey. Likely they'd be absorbed and different photons re-emitted many
times.

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edwinkite
Geologist here. Earth's thermal evolution is unsolved decades after we figured
out the stars. The Sun's temperature adjusts to energy production and loss on
a 10^6 yr timescale (scaling as the time for a photon to diffuse from the
heart of the Sun to the cold surface): << the Sun's age. The Earth's
temperature adjusts to internal energy production and surface losses on a
billion year timescale: of order the Earth's age. Planets have long memories
and history matters. The Sun is reasonably well-mixed. Surface spectroscopy
probes the make-up of the whole star. Earth is less well-stirred. Seismic
imaging of the deep earth maps the edges of vast pods of material,
radioactivity unknown, composition unknown (but definitely distinct from the
near-surface stuff), age unconstrained but plausibly as old as the planet [1].
Structure and composition matter [2]. It's a hard problem.

But it matters. When you look up at the night sky far from cities, for every
star you see there's a habitable-zone Earth-radius planet that's closer [3].
We didn't know that six months ago. We think (for good, but circumstantial
reasons) that complex life requires volcano-tectonic resurfacing -
necessarily, a hot interior. Given that habitable-zone Earth-radius planets
are not in short supply, the difference between fast and slow cooling for
planets like Earth is the difference between a Galaxy where most every star
system is habitable and one where almost all the planets are cinders.

The core-mantle boundary heat flux Q_CMB estimated in this paper constrains
the mantle energy balance

d(E_mantle)/dt ~ Q_CMB - Q_surf + H_radioactive

Surface heat flux Q_surf is ~46 terawatts. Mantle radioactivity H_radioactive
is not well constrained but about 10 terawatts [2]. The implication is that
despite the high core heat flux, Earth's mantle is cooling fast - maybe 100
microkelvins per century. Volcanism will therefore shut down in much less time
than the remaining main sequence lifetime of the Sun. Absent human
intervention, the reddening of the Sun won't kill the biosphere, the Earth
will.

As the mantle cools, the temperature contrast between the mantle and the core
will no longer sustain core convection. Then Earth's magnetic field will power
down. Without geo-dynamo shielding against galactic and solar radiation, bad
things may happen: the rapid shutdown of Mars' dynamo is one hypothesis for
the deterioration of Mars climate ~4 Gyr ago [4]. On the other hand, Earth's
magnetic field strength decreased by a factor of 20 during the Laschamp Event
~41000 years ago [5], with no known effects on biology (or human culture).

Diamond-anvil experiments are tough; few grad students make it past quals
without breaking a diamond or two. The diamond-anvil technique is hitting
diminishing returns, so modest advances are (rightly) celebrated. The same is
true for deep-earth seismology and mantle geochemistry. A good new method is
mapping the antineutrino flux from Earth. Antineutrinos are produced by
radioactive decay and move in a straight line from source to surface. Mapping
the Earth with geoneutrino observatories in the deep sea would help determine
the power source for plate tectonics [6].

\----

[1] Garnero & McNamara:
[http://mcnamara.asu.edu/Publications/pdfs/Garnero_and_McNama...](http://mcnamara.asu.edu/Publications/pdfs/Garnero_and_McNamara_Science_2008.pdf)

[2] Korenaga, "Urey Ratio and The Structure and Evolution of Earth's Mantle",
[http://people.earth.yale.edu/sites/default/files/korenaga08d...](http://people.earth.yale.edu/sites/default/files/korenaga08d.pdf)
Korenaga is the best mid-career theorist actively working on this problem.

[3] New result, from several teams working independently to analyze the Kepler
dataset: Caltech <http://arxiv.org/abs/1303.3013> (read this one first);
Harvard team #1 <http://arxiv.org/abs/1302.1647>; Harvard team #2
<http://arxiv.org/abs/1301.0842>; Berkeley <http://arxiv.org/abs/1304.0460>.
I'm assuming 0.1 stars per cubic parsec.

[4] Lillis et al: <http://seismo.berkeley.edu/~manga/lillisetal2008b.pdf>.
Later work broadly supports his conclusion that the dynamo died fast and early
in Mars history. SETI Institute talk:
<https://www.youtube.com/watch?v=REiKzxWbzrQ> It is not known whether loss of
the magnetic field had a big or small effect on the Great Drying of Mars.
Measuring modern atmosphere/water loss rates from modern Mars is the goal of
the MAVEN mission, which launches this Nov -
<http://lasp.colorado.edu/home/maven/>

[5] Known from ice-core spikes in beryllium-10 (isotope produced by cosmic
radiation hitting Earth's atmosphere) as well as magnetic paleo-intensity
measurements in sediments.

[6] <http://www.phys.hawaii.edu/~sdye/hanohano.html>. A knuckle is that SSBN
reactors also emit neutrinos and neutrinos cannot be shielded, so deep-sea
geoneutrino detectors could be strategically destabilizing. In practice either
angular resolution or massive size would be needed to make deep-sea neutrino
detectors useful to militaries.

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bcbrown
So the previous estimate was 5000 C, and now it's 6000C +/- 500C, and that's
"far hotter"? That doesn't seem like a huge adjustment to me.

~~~
ecto
Perhaps a ~17% change is not significant on the scale that we inhabit (20C
today depending where you are), but 1000C is a lot of degrees.

~~~
jlgreco
Indeed. It seems this new estimate puts it into "surface of the sun" territory
(5700-8800K).

~~~
samstave
I know nothing about this, but I wonder if core temp would have any impact on
the habitability of a planet. We know there is the habitable zone around a
star, I wonder if that body in that zone would also need a certain core temp
to support life.

~~~
jlgreco
I don't know much about this, but my understanding is that a hot core is not
itself something that life particularly needs. The effects of that hot core
however (Spinning core to make a magnetosphere? Volcanic activity changing
atmospheric chemistry?) are probably more important.

On the other hand, a hot core gives you deep sea thermal vents. Although I
think the current prevailing theories don't have life originating near those
on Earth, a hot core is certainly essential to life down there and, perhaps on
another world, life could originate there.

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bttf
Was anyone else led by the title of this post to contemplate the difference in
heat between thought and the Earth's core?

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tocomment
It's a shame there's no good way to harness that.

~~~
ovis
Although it's admittedly difficult when the higher temperatures are hundreds
or thousands of kilometers away, geothermal power plants do in fact harness
that.

~~~
dkhenry
If I am not mistaken Iceland uses geothermal to a large extent and is very
successful in doing so.

~~~
mtviewdave
Northern California, as well:

<http://en.wikipedia.org/wiki/The_Geysers>

