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Thread: Heating of planets and asteroids

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    Question Heating of planets and asteroids



    Many asteroids and planets have undergone enough heating so that they were at some point completely or partially melted. What physical mechanisms account for this heating? I'm assuming a solar system such as our own.

    Thanks!

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    Default Re: Heating of planets and asteroids

    Gravity is the short answer.

    Asteroids that are irregularly shaped, rubble piles for instance, have not undergone any heating whatsoever. But if the mass is sufficient that gravity overcomes solid state forces and squeezes the matter into a spheroidal shape, then the object is in hydrodynamic equilibrium. The pressure differences in the material counteract the force of gravitational compression.

    One can think of this as the energy for heating comes from the gravitational potential energy released by assembling the object. Upon assembly the thermal energy is minus half the negative gravitational potential energy. (The Virial Theorem guarantees this.)

    After assembly the planet cools and the energy is maintained by radioactivity of the elements composing it. The Earth has long since radiated away all the energy it gained from assembling and the current molten core is due to radioactive elements decaying.

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    Default Re: Heating of planets and asteroids

    For smaller celestial bodies radioactive heating is also an option. In the early solar system there were enough radioactive isotopes with a half-life of a few million years (such as aluminium-26 and Iron-60) to cause enough heat and thus internal melting of some moons and asteroids.

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    Default Re: Heating of planets and asteroids

    Quote Originally Posted by Gmetric View Post
    For smaller celestial bodies radioactive heating is also an option. In the early solar system there were enough radioactive isotopes with a half-life of a few million years (such as aluminium-26 and Iron-60) to cause enough heat and thus internal melting of some moons and asteroids.
    But the rubble piles are pretty well exempt....
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    Default Re: Heating of planets and asteroids

    Quote Originally Posted by not_Fritz_Argelander View Post
    But the rubble piles are pretty well exempt....
    No melting, to be clear. Gravity will dominate radioactive decays in any body where heating is sufficient to melt.
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    Default Re: Heating of planets and asteroids

    The flow of heat from the earths interior to the surface is primarily due to the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth of which these contribute roughly equal amounts to the earths 47 TW of heating budget. There are multiple factors for the thermal heating of planets and asteroids such as that caused by gravity (the Kelvin–Helmholtz mechanism), nuclear fusion, tidal heating, core solidification (heat of fusion released as molten core material solidifies), and radioactive decay. Yes gravity dominates in the main but radiogenic heating still plays its part.
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    Default Re: Heating of planets and asteroids

    Quote Originally Posted by Gmetric View Post
    The flow of heat from the earths interior to the surface is primarily due to the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth of which these contribute roughly equal amounts to the earths 47 TW of heating budget. There are multiple factors for the thermal heating of planets and asteroids such as that caused by gravity (the Kelvin–Helmholtz mechanism), nuclear fusion, tidal heating, core solidification (heat of fusion released as molten core material solidifies), and radioactive decay. Yes gravity dominates in the main but radiogenic heating still plays its part.
    Radioactive decay can account for all of the Earth's heat budget. The point is perhaps controversial.

    https://en.wikipedia.org/wiki/Earth%...al_heat_budget

    The section on radiogenic heating in the above link is suspect since it ignores radiogenic heating in the core where the abundance of actinides is non negligible. If you count the core as a radiogenic source, the entire heat flux can be accounted for. I did the calculation as a homework problem a few decades ago.
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    Default Re: Heating of planets and asteroids

    GMetric's source account of primordial residual heat in the Earth coincides with the majority mainstream dogma that has prevailed for 5 decades or more. Is this a myth?

    NFA's homework problem is worthy of a peer reviewed paper and rewriting of some textbooks, imo, seriously.

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    Default Re: Heating of planets and asteroids

    What about the kinetic energy of bombardment by asteroids, etc.? Does this provide substantial heat energy?

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    Default Re: Heating of planets and asteroids

    Quote Originally Posted by Boatman View Post
    What about the kinetic energy of bombardment by asteroids, etc.? Does this provide substantial heat energy?
    Not these days. In the early solar system it's lumped into the "gravity" piece.
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