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Two reasons: (1) the earth has really huge thermal inertia (2) radiogenic heat production. (2) often gets a lot of the credit (who doesn't like radioactivity),

Two reasons: (1) the earth has really huge thermal inertia (2) radiogenic heat production. (2) often gets a lot of the credit (who doesn't like radioactivity), but (1) is probably doing more of the heavy lifting At present day, Earth is losing heat at a rate of about 40 terawatts. This 40 terawatts drives mantle convection, plate tectonics, and every volcano out there. As best we know, about 35% of that 40TW is radiogenic (decay of Th-232 with a half-life of 14 bilion years, U-238 with a half life of 4.5 billion years and K-40 with a half life of 1.25 billion years, among others). A fair amount of the decay energy is actually lost to neutrinos, but the rest is converted to heat and eventually lost by convection or conduction. The other 65% or so is primordial -- essentially, gravitational potential energy from the accretion of the Earth, converted to heat and slowly lost over the past ~4.51 billion years since the moon-forming impact. Earth's bulk thermal inertia (heat capacity times mass) is about 5.6E27 J/K. In other words, cooling the earth by one degree C releases 5.6E27 joules. That's a lot of joules! Even so, there's a bit of a problem because geochemical evidence suggests that the mantle (which makes up most of the earth by volume) has probably not cooled by more than about 150C over the past 3.5 billion years, which means either one of our estimates is wrong, or the core is cooling pretty fast -- this would actually mean that the inner core might be pretty young, like < 1 billion years.

flukus 8y ago
> this would actually mean that the inner core might be pretty young, like < 1 billion years What are the error bars on that? It seems to be getting quite close to the emergence of complex life, enough that the radiation shielding may be a more important pre-requisite than previously believed.
- cbkeller 8y ago
  Big enough that that's a totally reasonable question! I don't think I can even put a number on it very confidently, but maybe something like 1.0 -0.7/+2.0 Ga? One counterpoint that comes to mind is that the Ediacaran fauna is usually though to be marine, and water is a pretty good shield of radiation. There is one geologist who argues the ediacarans were terrestrial (Greg Retallack), but he interprets any sedimentary rock that's red as a paleosol, which is actually not completely silly, but is a bit more liberal than most others would be comfortable with. In any case, I don't know, but it's not impossible! edit: I should also say that the generation of Earth's magnetic field in the liquid outer core is a bunch of incredibly messy magnetohydrodynamics that is hard to simulate accurately, but we can probably have a pretty reasonable magnetic field with a purely liquid core (no solid inner core at all) -- it's the convection of the liquid outer core that provides the energy to sustain the magnetic field, so it's actually once the core becomes fully solid that we'll be in trouble
marktangotango 8y ago
If radiogenic heating is so important, why are Mars and Mercury not also geologically active? Mercury is only slightly less dense than Earth after all? Thanks for the response, btw, been waiting for a geologist ama for a while!
- cbkeller 8y ago
  Size - Mars and Mercury are both quite a bit smaller than Earth. So they lose heat faster for essentially the same reasons that your cells have to be small: smaller spheres have a higher surface area to volume ratio! While in neither case is diffusion the only process at work, it's a critical limiting factor in both cases. Diffusion of heat, diffusion of nutrients: it's all the same math!
pklo 8y ago
We get 174400 TW from the Sun (1367W/m2 over the Earth cross-section, which pretty much balances out with the outgoing blackbody radiation at something like 5 deg C IIRC. This would be the surface temperature if there was no internal heating. This slows down cooling of the mantle because there's no cold reservoir for it to cool into.

flukus 8y ago

> this would actually mean that the inner core might be pretty young, like < 1 billion years What are the error bars on that? It seems to be getting quite close to the emergence of complex life, enough that the radiation shielding may be a more important pre-requisite than previously believed.

cbkeller 8y ago
Big enough that that's a totally reasonable question! I don't think I can even put a number on it very confidently, but maybe something like 1.0 -0.7/+2.0 Ga? One counterpoint that comes to mind is that the Ediacaran fauna is usually though to be marine, and water is a pretty good shield of radiation. There is one geologist who argues the ediacarans were terrestrial (Greg Retallack), but he interprets any sedimentary rock that's red as a paleosol, which is actually not completely silly, but is a bit more liberal than most others would be comfortable with. In any case, I don't know, but it's not impossible! edit: I should also say that the generation of Earth's magnetic field in the liquid outer core is a bunch of incredibly messy magnetohydrodynamics that is hard to simulate accurately, but we can probably have a pretty reasonable magnetic field with a purely liquid core (no solid inner core at all) -- it's the convection of the liquid outer core that provides the energy to sustain the magnetic field, so it's actually once the core becomes fully solid that we'll be in trouble

marktangotango 8y ago

If radiogenic heating is so important, why are Mars and Mercury not also geologically active? Mercury is only slightly less dense than Earth after all? Thanks for the response, btw, been waiting for a geologist ama for a while!

cbkeller 8y ago
Size - Mars and Mercury are both quite a bit smaller than Earth. So they lose heat faster for essentially the same reasons that your cells have to be small: smaller spheres have a higher surface area to volume ratio! While in neither case is diffusion the only process at work, it's a critical limiting factor in both cases. Diffusion of heat, diffusion of nutrients: it's all the same math!

pklo 8y ago

We get 174400 TW from the Sun (1367W/m2 over the Earth cross-section, which pretty much balances out with the outgoing blackbody radiation at something like 5 deg C IIRC. This would be the surface temperature if there was no internal heating. This slows down cooling of the mantle because there's no cold reservoir for it to cool into.