Why is the Earth so Hot?

Poulgorm · 09-01-2012 10:37pm #1

Why is the interior of the earth so hot? What caused it ? Why has it not cooled down over the 3.7 billion years (or whatever) that it has existed? What is keeping it on the boil?

Is the interior of the moon hot also ?

Huge gaps in my education.

Maximus Alexander · 09-01-2012 10:48pm

As far as I am aware, a combination of tidal forces and radioactive decay. Mostly radioactive decay.

The interior of the moon is hot too, though not as hot as Earth's. Because it is smaller its surface area is proportionately larger, which has allowed it to radiate away a larger proportion of its heat. If I recall correctly it is still believed to have a somewhat molten mantle.

Capt'n Midnight · 10-01-2012 1:41am

some of the heat is from when all the dust combined to form a planet
stuff at the centre of the earth has fallen several thousand km so plenty of energy there , at one stage the earth was totally molten , and then later on got hit by something the size of mars and melted again

the core of the earth is solid due to the pressure
the core of jupiter is probaby contains a lot of diamond for the same reason

also radioactive decay produces some heat

and also the whole surface to volume thing

Capt'n Midnight · 10-01-2012 9:41pm

http://en.wikipedia.org/wiki/Geothermal_gradient

The Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%)

But note that the heat flow is very small 10.1 60 mili Watts /m2 compared to sunlight 1,000 W/m2 so apart from hotspots it's the poor conductivity of tens of miles of rock that means surface is fairly cool

[edit - wrong heat transfer figure]

Ben D Bus · 13-01-2012 3:20pm

And it's the swirling molten metal that provides our magnetic field? Which in turn protects our atmosphere from being blown away by the solar wind. And deflects harmful radiation. Any idea how long before the earth cools to the point that human life is no longer sustainable?

Maximus Alexander · 13-01-2012 4:16pm

Good question, to which I have never found any sort of conclusive answer.

Apparently the core is not cooling at all at the moment because it is generating heat through radioactive decay at roughly the same rate that it radiates heat into space.

The half lives of the isotopes responsible for the heating are on the order of billions of years, so it seems unlikely that the Earth will have cooled significantly by the time it is swallowed by the sun.

krd · 14-01-2012 12:46am

Over time there have been various theories. And they've all got melded together.

In 19th century geology, the idea of primordial heat was given as an explanation - the heat at the formation of the planet. This was at a time where thinking the earth was older than 8,000 years was very radical.

The earth was formed billions of years ago. It would take the molten earth a long time to cool down - probably not that long.

I don't think Uranium explains the heat either.

A better explanation may be friction. The earth as it orbits the sun, and any other planet, doesn't do it in a perfectly smooth pattern. It wobbles. The wobble puts stresses and strains on the planet. And it's likely the stresses and strains, give the core a constant temperature. It's hard to comprehend - the surface is literally wafer thin - most of the planet is boiling ball of fire. Some active process keeps the core molten - there would need to be a colossal amount of Uranium to do that, if it was uranium. If it were primordial heat, we'd have a much thicker crust by now. So, it has to be something else.

Capt'n Midnight · 14-01-2012 3:40pm

krd wrote: »

A better explanation may be friction. The earth as it orbits the sun, and any other planet, doesn't do it in a perfectly smooth pattern. It wobbles. The wobble puts stresses and strains on the planet. And it's likely the stresses and strains, give the core a constant temperature. It's hard to comprehend - the surface is literally wafer thin - most of the planet is boiling ball of fire. Some active process keeps the core molten - there would need to be a colossal amount of Uranium to do that, if it was uranium. If it were primordial heat, we'd have a much thicker crust by now. So, it has to be something else.

The earth has a much more circular orbit than Mars, Venus is even more circular than the earth so no.

http://www.windows2universe.org/our_solar_system/planets_table.html orbital eccentricity

heat transfer is 100mW/m2 under the sea and 60mW/m2 under the land , so all the heat from all the radio active elements in a pyramid wedge 6,000 Km long extending from the centre of the earth to the surface because it can't go sideways because it's just as hot that way.

There is a collossal amount of radioactive materials, most of the heavy elements sank to the core, some speculation exists that most if not all of the heavy elements in the crust arrived on meteroites after the earth cooled enough so they wouldn't sink

if you wanted you could do the maths about 60mW/m2 as to how far you would need to be from a 100watt bulb to feel the same amount of heat

krd · 14-01-2012 7:10pm

Capt'n Midnight wrote: »

heat transfer is 100mW/m2 under the sea and 60mW/m2 under the land , so all the heat from all the radio active elements in a pyramid wedge 6,000 Km long extending from the centre of the earth to the surface because it can't go sideways because it's just as hot that way.

if you wanted you could do the maths about 60mW/m2 as to how far you would need to be from a 100watt bulb to feel the same amount of heat

Looking at some of the figures, it would seem the heat is very well trapped in the core. Only a tiny amount leaks out at the surface. And more solar energy would seem to be going in than geothermal energy coming out. It may never cool.

RichieC · 16-01-2012 8:51pm

Very good documentary on the topic here:

http://topdocumentaryfilms.com/the-core/

chrismartin4u · 16-05-2012 3:10pm

Because million years ago condition of earth was like a fired ball. So the inner part of earth is still so warm.

shizz · 17-05-2012 11:24am

I suppose the next question is what differs the Earth from Venus and Mars for instance? How come they have cooled down much quicker than the Earth and, for all intents and purposes, no longer have any geothermal action's taking place? Was it the theoretical collision that allowed for this?

Capt'n Midnight · 17-05-2012 2:24pm

The temperature at the core is determined by radioactive decay and primordial heat from decay and gravitational collapse.

The temperature at the surface is determined by heat from the core 0.1 Watts/m2 and the sun which is up to 1,000 watts/m2

so unless you live in a volcanic region you can ignore the heat input from the earth.

temp on earth is determined by Stefan's law of radiation, it's proportional to the darkness of the object and the absolute temperature to the power of 4

djpbarry · 18-05-2012 11:44am

shizz wrote: »

I suppose the next question is what differs the Earth from Venus and Mars for instance? How come they have cooled down much quicker than the Earth...

Venus is cooling at approximately the same rate as Earth. Not much is known about it's internal composition, but it is thought to be similar to Earth's, with some notable differences, such as a lack of plate tectonics and no magnetic field.

What is known is that the planet's surface is a volcanic landscape, with a mean temperature well above 400 degrees!

shizz · 18-05-2012 11:55am

djpbarry wrote: »

Venus is cooling at approximately the same rate as Earth. Not much is known about it's internal composition, but it is thought to be similar to Earth's, with some notable differences, such as a lack of plate tectonics and no magnetic field.

What is known is that the planet's surface is a volcanic landscape, with a mean temperature well above 400 degrees!

Venus' interior is cooling at the same rate? How can it be so similar if it lacks plate tectonics? I thought the lack of plate tectonics would come hand in hand with being cooler overall?

I know about it's volcanic landscape and how it's atmosphere came to be, but wouldn't it's lack of volcanism currently relate to its interior cooling?

krd · 19-05-2012 3:50pm

Maximus Alexander wrote: »

As far as I am aware, a combination of tidal forces and radioactive decay. Mostly radioactive decay.

The interior of the moon is hot too, though not as hot as Earth's. Because it is smaller its surface area is proportionately larger, which has allowed it to radiate away a larger proportion of its heat. If I recall correctly it is still believed to have a somewhat molten mantle.

I'm not sure this is correct. I remember hearing that on the lunar mission they carried out an experiment, setting off an explosion on the moon's surface. The result seemed to tell them that the moon is solid the whole way through.

Capt'n Midnight · 19-05-2012 4:20pm

krd wrote: »

I'm not sure this is correct. I remember hearing that on the lunar mission they carried out an experiment, setting off an explosion on the moon's surface. The result seemed to tell them that the moon is solid the whole way through.

They left seismometers on the moon to detect earth moonquakes.

no need for an explosion with all that surplus space hardware falling back

http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/pse/

However, the third stages of several Saturn 5 rockets and the ascent stages of several lunar modules were deliberately crashed into the Moon after these spacecraft were no longer needed. These man-made crashes produced seismic events of known times and locations and helped to calibrate the network of seismometers.

...
he core is probably composed mostly of iron and sulfur and extends from the center of the Moon out to a radius of no more than 450 kilometers; i.e., the core radius is less than 25% of the Moon's radius, which is quite small. In comparison, the Earth's core radius is 54% of the Earth's radius. However, the size of the lunar core is not well constrained by existing seismic observations. Better constraints come from the laser ranging retroreflector and magnetometer experiments.

shizz · 19-05-2012 4:51pm

They did use explosive experiments on apollo 17 http://www.lpi.usra.edu/lunar/missions/apollo/apollo_17/experiments/lspe/

Maximus Alexander · 26-05-2012 8:33am

krd wrote: »

I'm not sure this is correct. I remember hearing that on the lunar mission they carried out an experiment, setting off an explosion on the moon's surface. The result seemed to tell them that the moon is solid the whole way through.

Fair enough. Just going by some things I read a while back, here is an example. I'm certainly not in a position to say who is right or wrong.

Science Journal wrote:

Our results suggest the presence of a solid inner and fluid outer core, overlain by a partially molten boundary layer.

Why is the Earth so Hot?

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