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Author | Topic: A science question | |||||||||||||||||||||||||||
Phat Member Posts: 18338 From: Denver,Colorado USA Joined: Member Rating: 1.0 |
I have a science question. The Earth, as old as it is, is still quite hot in the middle. From what I understand, the crust is roughly 2-10 miles thick and the mantle is hundreds of miles thick before we get to the core. Volcanic activity shows us a hint of how hot be the innards. My question is this: Why does it take a planet so long to cool off? Anything baked in a ceramic kiln may glow red for a minute, yet may be cool in a day. If the earth is as old as they say, why is is still so darned hot on the inside?
Does anyone have an approximation of when the core will be cool to the touch? (Put in Geology?) This message has been edited by Phatboy, 01-26-2005 05:06 AM
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AdminJar Inactive Member |
Thread moved here from the Proposed New Topics forum.
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jar Member (Idle past 421 days) Posts: 34026 From: Texas!! Joined: |
IIRC there are three processes at work. First, radioactivity provides some heat but probably less than many claimed in the past. Second, the earth's gravity is still trying to compress us into a smaller ball. This heats up the interior as it's squeezed. The third mechanism is probably the smallest contributor and that is solar radiation heating the surface.
But hopefully someone with far more knowledge than I have will give you the real answers. Aslan is not a Tame Lion
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nator Member (Idle past 2196 days) Posts: 12961 From: Ann Arbor Joined: |
I really don't know much about this, but wouldn't surface area have something to do with the rate of cooling?
Also, how hot the item was to begin with, density, etc?
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
You missed an obvious one (although it's contribution may be small). Phase changes - as rock goes from molten to solid it will give up heat. IIRC there are some exothermic chemical reactions going on down there, too. And of course the outer layers of the planet act as insulation to some degree.
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Coragyps Member (Idle past 761 days) Posts: 5553 From: Snyder, Texas, USA Joined: |
As I understand it, about half the heat comes from radioactive decay - potassium-40 and uranium, mostly. The other half comes from gravitational potential energy being turned to heat as heavier stuff settles toward the middle, and from the heat of crystallization released as the liquid of the outer core crystallizes onto the inner core.
I don't know when it'll be down to lukewarm, and I'm not waiting around to find out...
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jar Member (Idle past 421 days) Posts: 34026 From: Texas!! Joined: |
I thought about adding that but I imagine that will actually turn out to be something of a wash. While it will release heat to the cooler layers surrounding the core, it will not and cannot transfer heat to the hotter core.
I was trying to stick to those things that could heat the interior. The surface warming actually probably should have been included in the same category as phase changes, increasing the temperature and so insulating the core. Afterall, as two objects approach equlibrium it gets increasingly harder to transfer heat. Aslan is not a Tame Lion
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JonF Member (Idle past 195 days) Posts: 6174 Joined: |
The other mechanism that's listed in most discussions is fractionation; heavier elements are still sinking ot the core and that si another source of heat.
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Percy Member Posts: 22495 From: New Hampshire Joined: Member Rating: 4.9 |
The rate at which a body can radiate heat is a function of its surface area, and the rate at which a body can cool is a function of the ratio of surface area to volume. A sphere is the most efficient shape for conserving heat, which means spheres cool more slowly than any other object.
But I think your issue is actually one of scale. You mention the time it takes a ceramic to cool from red hot, but a ceramic has a very high surface area to volume because it is so small, even if you're making marbles. For example, a one inch radius clay sphere has a surface area of 4πr2 or 12.6 in2, and a volume of 4πr3/3 or 4.2 in3, for a surface area to volume ratio of 3. The earth has a surface area of 8.1x1017 in2 and a volume of 6.8x1025 in3, for a surface area to volume ratio of 1.2x10-8, which is orders of magnitude smaller than for your clay sphere. Naturally it will take much longer to cool. But Lord Kelvin calculated that it would take the earth only about 20 million years to cool from a molton state to its current state (he took the increasing temperature with depth into account by obtaining measurements from deep mines), and while I can offer no precise figure for how long Kelvin might have thought it would have cooled completely, certainly it wouldn't have been on the order of billions of years. The reason the earth hasn't yet fully cooled after 4.5 billion years is due to the contributions others have mentioned, radioactivity primary among them. Radioactivity was unknown to Kelvin, though it's contribution was uncovered not long before his death. Here's an example from the real world. When you take a tour of the Hoover Dam you learn quite a bit about its construction. Given the size and thickness of the dam, had they just layed the concrete and allowed it to cool naturally it would have taken a couple centuries. Not wanting to wait that long, they included water pipes to circulate cold water in every concrete segment. In other words, big objects take a long time to cool. --Percy This message has been edited by Percy, 01-26-2005 11:30 AM
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ohnhai Member (Idle past 5189 days) Posts: 649 From: Melbourne, Australia Joined: |
Dont the internal motions of the plannet also create a whole lot of friction,and thus heat? as well as setting up the dynamo effect that created the earth's magnetic field.
This message has been edited by ohnhai, 01-26-2005 11:00 AM
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JonF Member (Idle past 195 days) Posts: 6174 Joined: |
The Earth is so hot on the inside because heat is still being generated there, and because the mechanisms for transporting heat away from the Earth take time. I have no idea when, if ever, the core will be cool to the touch; we may be engulfed by the Sun turning into a red giant (in five billion or so years) before the core cools.
There's a long and interesting history of the answer to your questions. Isaac Newton speculated on how long it would take a globe of red-hot iron, the size of the Earth, to cool. He pretty much guessed, and came up with 50,000 years. Georges-Louis Leclerc, Comte de Buffon, made an attempt in the late 1700's. He had a foundry build him iron spheres of different diameters (1/2 inch to 5 inches). He heated them to white heat and measured the cooling time. He found a roughly linear relationship between diameter and cooling rate. Assuming the Earth was like an iron sphere and extrapolating to the Earth's size, he came up with 96,670 years (the precision is obviously not justified). Later he repeated the experiment with rock spheres and including the ffect of the Sun's heat, and modified his result to 74,832 years. This wildly wrong result was actually very important in the development of modern science and geology. From "The Age of the Earth", G. Brent Dalrymple, Stanford University Press, 1991, page 31:
quote:The most famous early result about the Earth's cooling was from William Thomson (later Lord Kelvin) in 1862. He used measured properties of rocks, measurements of temperature from wells and mines, and the relatively new science of thermodynamics to calculate a cooling time of 98 million years. He realized that his data was inadequate, and estimated that the cooling time was no less than 20 million years and no more than 400 million years. This was a tremendous problem for geologists and "evolutionists". Lord Kelvin was the Einstein of his day, and his results and pronouncements were held in high repute. But the results of geologists studying the Earth and "evolutionists" studying the fossil record strongly suggested the Earth was much older. There were lively debates. Later papers by others refined Kelvin's calculations, and probably the "best" estimate based on the assumptions and knowledge of the day was 24 million years, by Clarence King in 1893. So, we can see that if the Earth is as old as we say it is today, then there must be some source of heat other than the influx of solar energy and the initial heat content when the Earth formed. And that is ... radioactivity. Well, mostly. Dalrymple puts it well on pages 46-47:
quote: We know somewhat more now that we did in 1991, but Dalrymples remarks are still pretty much on the money.
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JonF Member (Idle past 195 days) Posts: 6174 Joined: |
Radioactivity was unknown to Kelvin, though it's contribution was uncovered not long before his death. Rutherford spoke before the Royal Insitution in 1904, and recalled:
quote: Copied from The ‘demise’ of Kelvin.
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Silent H Member (Idle past 5846 days) Posts: 7405 From: satellite of love Joined: |
Conduction brings the heat to the surface, where it is lost into space. Correct me if I am wrong, but heat cannot be lost into space, except through radioactivity, electromagnetic phenomena (light), or physically projecting material into space (to be lost from that body). Vacuum is an insulator and space is a vacuum. It seems to me much of the "cooling" is about transferring heat from the inner compacted areas (as well as radioactive areas) to outer regions including the hydrosphere and atmosphere. Since we are not creating much light, it would generally be losing atmosphere that would count as the only real "loss" of heat, otherwise it is just a distribution of heat to all parts of the planet. holmes "...what a fool believes he sees, no wise man has the power to reason away.."(D. Bros) "...don't believe I'm taken in by stories I have heard, I just read the Daily News and swear by every word.."(Steely Dan)
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Coragyps Member (Idle past 761 days) Posts: 5553 From: Snyder, Texas, USA Joined: |
We lose a good bit of infrared radiation to space - "light" after a fashion, just not visible. Losses of atmosphere are surely tiny in comparison.
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JonF Member (Idle past 195 days) Posts: 6174 Joined: |
Vacuum is an insulator and space is a vacuum. Vacuum essentially prevents the transfer of heat by conduction, which requires molecules that physically touch each other (at least occasionally). Nothing prevents transfer of heat by radiation (not the radiation produced by radioactivity, the radiation produced by all things that are above absolute zero). Note that we receive quite a bit of heat, via radiation, from the Sun ... through the vacuum of space. That warm feeling on your skin at the beach in the summer? Radiation.
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