[Lewis Thomson (OP)]

Donald has another question for the Naked Scientists forum.

"The sand on a beach can get very hot, that is absorb a lot of heat energy easily.  When water flows over the sand, the sand cools quickly and, presumably, the water quickly absorbs that heat. So if heat can be quickly absorbed by solid earth and transferred to water, why hasn't the Earth's molten core transferred all it's heat thru the earth into the oceans by now? And what percentage of the heat energy of the Earth's core been diminished since it's formation, and how long before it cools to a solid?"

What do you think the scienctific reasons are for this?

[chiralSPO]

The earth's core will eventually cool down, but it will take several billion years.

The core is much, much hotter than the hot sand described in the OP. It is also much, much, bigger, and the heat has much, much farther to go to escape.

Also, there is the additional complication that the core generates much of its own heat. There are large quantities of radioactive elements, like uranium, in the earth. They are so dense that they have sunk into the core of the earth, where they decay, releasing significant amounts of energy (there's likely what is basically a large nuclear reactor down there, but even if there isn't a high enough concentration of fissile isotopes for a chain reaction, they still decay and release heat). Many of these elements have half-lives that are hundreds of millions of years or longer, so the core will continue producing its own heat for billions of years.

https://www.nature.com/articles/news.2008.822
https://newscenter.lbl.gov/2011/07/17/kamland-geoneutrinos/

[evan_au]

It is estimated that the heat flowing from the Earth's interior into space is about 0.1 Watts per square meter.
-This can be estimated from the temperature rise of rocks as you go down deep mines
- A similar experiment is being attempted on Mars as we speak - but they had trouble drilling a deep enough hole.

In contrast, the Sun's daily input peaks at around 700 Watts per square meter - on a clear day at the equator (and nothing at night). The temperature variation due to the day/night cycle penetrates no more than half a meter into soil.

There are some suggestions that another source of heat for Earth's core is heat released by the crystallizaion of iron/nickel "hail", which rains down from the liquid outer core onto the inner core. So the radius of the inner core grows slowly, but the temperature of the outer core remains fairly stable. Since the outer core is believed to be the main source of Earth's magnetic field, running out of liquid outer core has implications for Earth's habitability (possibly this contributed to the loss of atmosphere on Mars)...

See: https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Global_internal_heat_flow
https://en.wikipedia.org/wiki/Earth%27s_inner_core#Growth