I'm watching Naked Science on the NAtional Geographic Channel, and they're talking about a theory that Mars' core cooled, stopped rotating, and turned it into what it is today, and later they will get into detail about how supposed evidence that the Earth'c core is cooling will subject us to the same fate.

After a certain point, the simple fact that I've never heard this theory is suspect. Any thoughts on this?

Any planet starts out hot and cools off by radiation later, unless the energy it's getting from somewhere else (like the sun) exceeds the energy the planet is dumping into space. Space is cold, and you can't put a hot thing in a cold place and not have it cool off unless you're giving it some other energy source.

The only question is how fast it happens, which can be influenced by a handful of separate factors. One is that bigger objects change temperature more slowly than little ones (which is why planets do it in billions of years instead of the few minutes/hours it would take for a ball you threw out the window of a spaceship). Receiving more sunlight also slows the cooling down even if you're not getting enough sunlight to actually reverse it and gain heat overall. And composition matters because different substances absorb, reflect, and pass different amounts of different wavelengths of solar energy (which is what makes them look different colors), and they also change temperature at different speeds under the same circumstances (which is what makes metal feel hotter than air on a hot day and colder than air on a cold day).

This is a somewhat complicated subject because of the scope.

It is true that the Earth's core is cooling off. This is mainly due to ongoing decay of the radioactive elements that generate heat in the core. Heat generated within the core is not very important in heating the planet's surface, but the heat drives convection cells in the mantle that provide the planetary magnetic field that protects us from the Solar wind. Likewise, outgassing from the mantle replenishes atmospheric gasses lost to chemical reactions or that escape into space.

Mars, being smaller and farther from the Sun, will have had a lower initial quantity of heavy elements like iron and radioactive isotopes. Such a small core would have cooled off long ago causing the loss of a protective magnetic field (if indeed Mars ever had one) and atmospheric replenishment. This has left Mars without much atmosphere to help retain it's surface heat.

Energy from the Sun has provided almost all planetary surface heating for the last two billion years. Mars, being so much farther from the Sun, will have always recieved less energy than the Earth and thus been colder. The fact is that the Sun has slowly been heating up since it's beginning, which would have made the surface of Mars even colder in the past.

More info.

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"Those who cannot remember the past are condemned to repeat it" - George Santayana

It is worth noting that the initial heat of the planets and their cores came from the heat of formation (that is, the conversion of kinetic energy to heat as the parts were assembled in an increasingly deep potential well). Radioactive decay is a factor, but not the only one. Concerning the time it takes to cool, think of the core as being surrounded by a three thousand mile thick insulating layer.

I agree with the above posters, the core of the Earth is cooling, and might get to the same situation as Mars if it happens before the Sun leaves the main sequence.

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Forming opinions as we speak

Does any one have any guestimates? Earth's core rotates in 24.1 hours or 23.9 hours? Temperature the of boundry at the core equator is 10,000 c = 18,032 f due to very high friction heating. Poles of core and center of core is much cooler? A one degre difference in the tilt of the surface poles and the core poles? Neil

I read somewhere that Mars never had tectonic plates as the Earth does but instead had one large static plate which allowed the underlying hot spots to push up the huge volcanoes.

My educated guess is that the core is rotating slightly faster, simply because the tidal drag would be strongest at and near the surface. I have no means of estimating how much faster, but I would be flabbergasted if it were anywhere near your hypothetical amount.

Earth also has a different composition than Mars, and a large Moon, and is deeper in the Sun's gravity well; it may make all a slight difference.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

The Core of Mars
Whether the core of Mars is solid or fluid is unknown. It is normally assumed that the core of Mars is solid or mostly solid, on the grounds that Mars has no global magnetic field. If Mars had a significant fluid core, it should have a global magnetic field. Since Mars is so much smaller than Earth (only 10% of Earth's mass), one would expect it to cool much faster than Earth after its formation. So it makes sense on both counts, the small mass & absence of a global magnetic field, that the core of Mars should have frozen solid by now. Either seismology, or precision observations of the nutation of Mars should reveal the state of the core, but neither of these are observed as yet with sufficient precision to answer the question (i.e., Dehant, et al., 2003; Defraigne, et al., 2003).

The Core of Earth
The solid inner core of Earth superrotates, meaning that it rotates faster than do the coupled mantle & crust. The inner core leads the rest of the planet by about 3 degrees in angle, or about 45 minutes in time, per year. That's about 2.5 seconds per day (Su, et al., 1996).

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

I'm curious. What causes it to rotate faster?

Core rotation has been predicted by Glatzmaier-Roberts geodynamo model among othe sources.


For a bit of historic reference:
http://www.ldeo.columbia.edu/~richards/Jefflec.html

__________________
"Those who cannot remember the past are condemned to repeat it" - George Santayana

Earth started out faster, but the surface is being dragged by Lunar tide, slowing it down. The core is just trying to catch up with our outer slowness. (don't use my guesses as a citation. I'm not a reliable source.)


ADDED: The core's also lubricated by a layer of slippery liquid magma.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Even should your guess not be a reliable citation, that's some good brain work!

My mistake was in trying to figure out what caused an acceleration in Earths core... Instead of what caused an acceleration in the opposite direction at the crust. And yes with a fluid in the middle, the core wouldn't immediatly suffer the efftct the outer surface does.

Sounds very plausible to me.

Thanks, if my brain works so good why did I have to go back and correct my spelling?

Wouldn't have got this except I was just talking about tidal drag on another thread, so it was still fresh in my mind.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

As much as I have to go back and correct my typoes etc, I dare not make any claim that typo's mean an ill working brain

In fact I see "effect" has one, but I'm too lazy to correct it.

I followed some of the links to this presentation, where they apparently reversed themselves two years later.

To put a slightly different twist on how to think about this problem I suggest that if it were claimed that there was no differential motion between the inner core and the subsequent layers, it would be an extrordinary claim that would require extrordinary evidence that "it does not move".

Now, determing how much, that is much harder.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

So the core finishes it's day about 2.5 seconds quicker. The upper atmosphere has about 3 hours longer day? a Mars upper atmosphere day is even longer? Is the exosphere = less than one millibar, even slower? Neil