What makes some planetary cores hot and others cold?

[so-crates]

Is it know why the Earth's core, and indeed the cores of other planets, are "hot" while some are cold? Is it possible that fusion or fission processes are taking place there ?

 

[Mk]

No, there's not a chance of any thermonuclear fission or fusion reactions there.

How do scientists know if the core is not? If a celestial body is big enough the core will be hot inevitably, because of the compression of internal materials by gravity. Bodies such as satellites and Pluto have cold cores because of insufficiency of gravity, I'm guessing.

 

[Mk]

Is it that, the sun contains thermonuclear fusion specificly because it is of high enough mass?

 

[Andre]

Compression of mass is static and the terrestrial planet would cool off eventially abeit in hundred millions years.

Temperature of the core is around 5500K, way too hot to be residual from the initial compression. Hence there is a heat generator inside. What is it? take your pick, the following are candidates:

radiogenic decay from heavy atoms U - Th series
40K decay.

Or perhaps Core - mantle friction (section 3.2)?

 

[Mk]

Could you please explain why it would cool off? How do geologists know how hot the core is?

 

[Andre]

Sure, Why it would cool off is also answered here although it seems that the links to the original is broken.

http://www.geo.lsa.umich.edu/IHFC/heatflow.html you can see the interior heat flow at the surface of the Earth. It appears to average about 100 milli watt per second per square meter. And there are a lot of square meters.

Anyway, this seems to be an excellent place where the heat balance of the Earth is presented.

More later

 

[Phobos]

Is it that, the sun contains thermonuclear fusion specificly because it is of high enough mass?

Yes, the enormous mass of a star is required to generate the necesasry pressure/temperature to get atoms to undergo fusion (occurs in the central core of the star). Actually, astronomical objects are classified as stars only if they can sustain fusion in their core.