Answerer

Hi guys, remember the heat death as predicted by the 2nd law of thermodynamics.It goes the same for earth.

Purple Monkey Dishwasher

Yeah, I was just wondering how long it will take until it's cooled enough to be totally solid. It's amazing (to someone with my lack of knowledge) that 5 billion years isn't enough. To what extent is radioactivity keeping it warm? If there wasn't any radioactivity would it have cooled by now?

JohnR

I remember reading that Niels Bohr used this to show the earth had to be at least 250,000 years old. Later when radioactive decay was taken into consideration they upped it to 4 million. The second law was used to refute a young earth.

Answerer

Thats why christinity never become truly successfully as long as science existed on earth.

[ February 26, 2002: Message edited by: Answerer ]

[ February 26, 2002: Message edited by: Answerer ]</p>

Francois Tremblay

So you're saying that Christianity was NOT successful during the Dark Ages ? What ?

Answerer

Hi Franc, Sorry that I did not check properly for my post, I have made corrections already. IT seems christinity also needs force in order to be truly successful.

Corey Hammer

Pressure does play a role in the core, but not in heating it.

The inner core is solid because of pressure that the outer core lacks even though the inner core is hotter.

Tim Thompson

Purple: Yeah, I was just wondering how long it will take until it's cooled enough to be totally solid. It's amazing (to someone with my lack of knowledge) that 5 billion years isn't enough. To what extent is radioactivity keeping it warm? If there wasn't any radioactivity would it have cooled by now?

About 25% of the earth's interior heat comes from radioactive decay. The remaining 75% is a combination of heat left over from formation of the earth, and heat generated by the condensation of the liquid outer core onto the solid inner core (ref: Gravitational energy of core evolution: implications for thermal history and geodynamo power, F.D. Stacey & C.H.B Stacey, Physics of the Earth and Planetary Interiors 110(1-2): 83-93, January, 1999)

The sun deposits energy on the earth's surface, in the form of sunshine, at a rate of about 1.370 Watts per square meter. On the bottom side of that surface, the earth's interior deposits about 0.062 Watts per square meter, which is about 4.5% of the heat deposited by the sun. What this means is that under the current circumstances, heat flow from the earth's interior into space is extremely weak.

On average, the earth is currently cooling at a bulk rate of about 57 Kelvins per billion years. heat transport through the mantle is mostly by convection, which moves heat energy to the surface with a characteristic time scale of about 2x10^8 years. But convection is driven by heat energy at the core-mantle boundary, which is itself generated by the condensation of the liquid outer core onto the solid inner core. Once the inner core has solidified (which will take at least a few more billion years), convection in the viscous mantle will shut down. Once that happens, heat transfer will be by conduction instead of convection, and the characteristic time scale for heat transport in the mantle by conduction is much longer, about 3x10^11 years (where the current age of the earth is about 4.5x10^9 years).

This all adds up to the earth cooling more slowly as time passes, so that average 57 Kelvins per billion year will get even smaller. The rate of heat transfer across the surface of the earth from the interior depends on the temperature at the surface, and that is already dominated by the sun and the atmosphere, not by any interior processes. So even if the earth's interior froze solid, it would have little effect on the heat balance at the surface (except for long term changes in greenhouse gases in the atmosphere that come from natural volcanism).

I would say that as long as the sun shines, the earth's interior probably won't get much cooler than it is now, on average. Most likely, the core will cool and the mantle warm up, making the interior more isothermal. But transfer of heat across the surface will remain very weak. In another 3.5 billion years or so, the sun will be twice as bright as it is now, and when it goes into its red giant stage, about 5.5 billion years from now, it could become as much as 1000 times brighter. When that happens, assuming that the earth is not entirely absorbed by the expanding sun, heat will be flowing from the surface down, and not from the interior up. After that, it will prbably be something like 10 billion years from now, after the sun has become a cooling white dwarf, that heat will begin to flow from the interior outwards (if there is an earth left).

The earth's interior temperature structure is probably fairly well understood, though it's thermal history remains a matter of open research. Bill gave some webpages of interest. I would add to his list, the <a href="http://mahi.ucsd.edu/shearer/SEDI/sedi.html" target="_blank">Study of the Earth's Deep Interior</a> webpage, from the <a href="http://www.agu.org/" target="_blank">American Geophysical Union</a>, and the <a href="http://www.astro.oma.be/SBC/main.html" target="_blank">Special Bureau for the Core</a>. My time scale values come from The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle, Ronald T. Merrill, Michael W. McElhinny & Phillip L. McFadden, Academic Press, 1996 (chapter 7, Processes and Properties of the Earth's Deep Interior: Basic Principles). I also used F.D. Stacey's old book Physics of the Earth, John Wiley & Sons, 1977 (2nd edition). Another good book is Understanding the Earth, edited by G.C. Brown, C.J. Hawkesworth & R.C.L. Wilson, Cambridge University Press, 1992. Each chapter is written by a different author. It's quite an extensive book, designed for the non science major type readers; no mathematics & good graphics.

Cheers.

Friar Bellows

Yes, a combination of radioactivity and residual heat. Pressure? Corwin needs to learn some physics. Nuclear reactor? A reasonable guess, but off by a few orders of magnitude.

As an exercise in geology class one day (long ago) we did some calculations on the radioactive component showing how a particular isotope of Aluminum (forget exactly which one) historically contributed most of the radioactivity (but has sinced mostly disappeared due to its extremely short half-life). Currently it's Potassium, Uranium and Thorium.

[ March 07, 2002: Message edited by: Friar Bellows ]</p>

Corwin

No, if you think all the energy from all that pressure just vanishes, YOU need to learn some physics.