system
Given the following post, respond as if you're commenting on Reddit:
human
Can a planet sustain life without a hot core?

I was under the impression that any heat the planet core provides is negligible compared to the heat from the sun. Does the core's heat significantly affect life on the planet?
gpt
The actual heat is totally inconsequential. At the surface, geothermal heat is less than 1/10,000th the strength of sunlight.

However, Earth would be *very* bad off if the interior cooled to the point that the mantle solidified. That would mean no more plate tectonics, and that would be a lousy day for things that like to live.

The problem is the carbon cycle. Carbon dioxide is continually replenished into the air through volcanic outgassing and carbonate rock weathering, but then gets sequestered by chemical reactions in the ocean to eventually get subducted by tectonic plates back into the mantle.

If the mantle cools, that means no more volcanoes outgassing...but that also means no more plate subduction. All that leaves is carbonate weathering - we've essentially just turned our carbon "cycle" into a carbon "arrow" that only flows one way, from rocks to atmosphere. That weathering would continue unabated will nothing to pull that carbon dioxide back out of the atmosphere.

We're actually fairly sure this is exactly what's happened to Venus - tons of carbonate weathering, but no plate tectonics to pull that carbon back out of the atmosphere. The end result is an incredibly thick CO2 atmosphere, with accompanying runaway greenhouse effect that would raise global temperatures 500 degrees.