Nuclear Decay Beneath Your Feet Accounts for Half of Earth's Heat Output

By Joseph Castro | July 21, 2011 8:01 am

spacing is importantAtoms sometimes release alpha particles during radioactive decay.

What’s the News: An international team of researchers has completed the most precise measurement of the Earth’s radioactivity to date. By analyzing subatomic particles streaming out of the interior of the planet, the geologists and physicists discovered that the radioactive decay of several elements generates roughly half of the Earth’s total heat output. Their results were published recently in the journal Nature Geoscience.

What’s the Context:

  • Radioactive decay is a natural process where an unstable atom loses energy by emitting particles (thus decaying into smaller atoms). Radioactive decay can sometimes release neutrinos: tiny, electrically neutral, and nearly massless elementary particles that pass through most normal matter with little to no interaction. Because of their ability to phase through matter mostly unaffected, these ghostlike particles are very hard to detect.
  • Like all other particles, neutrinos have anti-siblings, called antineutrinos. A proton can sometimes capture an antineutrino, which then splits into a neutron plus a positron (an anti-electron). That positron will annihilate when it hits an electron, releasing energy that can be detected with very sensitive instruments, such as the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) deep in the Japanese Alps. If absorbed by a proton, the neutron will emit a gamma ray, providing a secondary signal for KamLAND and allowing researchers to make sure they’re not just detecting background noise.

How the Heck:

  • Between 2002 and 2008, particle physicist Itaru Shimizu of Tohoku University in Sendai, Japan, and his colleagues detected antineutrinos with KamLAND. Overall, they counted over 800 antineutrinos. By looking at the amount of energy released in the antineutrino interactions, the researchers determined that a little over 100 of the particles detected came from natural radioactivity within the mantle and crust of the Earth, particularly from the decay of uranium-238 and thorium-232 (the rest resulted from other sources like nuclear reactors and cosmic rays).
  • The team then estimated that for each second that passes, about 4.3 million of the uranium and thorium-generated particles pass through each square centimeter of Earth’s surface, according to Science NOW. Knowing the energies of the neutrinos, the researchers calculated that the radioactive decay of uranium-238 and thorium-232 contributes about 20 trillion watts to the amount of heat the planet radiates into space.
  • Previous studies estimated that the radioactive decay of potassium-40 contributes 4 trillion watts. Put together, heat from radioactive decay accounts for about 54% of the Earth’s total heat output.

The Future Holds: Scientists still don’t know where the rest of the Earth’s heat from—this is a subject for future research. “One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth’s heat energy,” physicist Stuart Freedman, who collaborated on the project, said in a prepared statement. “Whether the rest is primordial heat or comes from some other source is an unanswered question.”

Reference: A. Gando, et al. Partial radiogenic heat model for Earth revealed by geoneutrino measurements. Nature Geoscience, 2011; DOI: 10.1038/ngeo1205

Image: Wikimedia Commons/Inductiveload

CATEGORIZED UNDER: Environment, Physics & Math
  • Georg

    What abot the heat of crystallisation of the innermost solid iron core from the
    surrounding melt?

    Are there some realistic figures of that heat at the temperatures/pressure “downunder”?

  • John Lerch

    How can the rest be primordial heat? Before radioactivity was discovered, that was the only possibility and that gave an age of the earth of 20 million years. IOW–IT AIN”T MUCH.
    How about the manufacture of exotic isotopes under the intense pressure which then decay with a net output of energy? The most likely candidate would be enhanced electron capture.
    Re heat of crystallization: That would come under the heading of primordial heat; it could explain some of the present heat. You would then have to explain why on the average the heat lost seems to be constant in geological time.

  • Paul

    John: you’re right, it can’t be primordial. That heat left town long ago.

    So maybe there’s some new physics here. From dark matter, perhaps?

  • Roland

    The earth aborbs a tremdous amount of heat from the sun. How much of that heat is stored; how much re-generated over time by plants and animals, elements, compounds, chemical reactions, etc.? Can this be calculated?

  • daniel I.

    Clearly some of the other heat comes from the dwarven forges under the surface. They are busy mass producing axes so that one day they will be able to take over the surface.
    The heat that doesn’t come from radioactive decay and the dwarves is most likely to be from dragon fire. Everyone who knows it knows that the dragons moved underground long ago to escape human-caused extinction. Duh!

  • Duncan

    and don’t forget daniel, 80% of the heat emanating from the west coast is from bong hits. it all adds up.

  • Patrick

    Uh, tidal forces, uh lingering kinetic energy from impacts…?? Just where did that moon come from?

  • rob

    The moon is my choice for the rest of the heat. the moon doesn’t get enough credit for what it is .our sister planet. if mars had a moon as large as ours it would probably still be geologically active and even have a magnetic field and an atmosphere.

  • Mladen

    So, how much heat is generated by tidal forces?

  • Bob

    Most of the remainder of the heat process is caused by the US Congress. So don’t be alarmed if we burn. I know this is anacdotal, but I couldn’t resist.

  • http://Home andras

    Does’nt anybody remember the article about the Nuclear scientist Marvin Herndon, He said his research shows that we have a 5 mile ball of uranium at the center , and it is in fusion, That’s what fuels the molten lava. to me its the only thing that makes sense, (After 4 billion years, everything else has cooled off) he finds the isotopes that he predicted , when he proposed the theory… Look him up! Andras Schoffer

  • Matt B.

    When you say “A proton can sometimes capture an antineutrino, which then splits into a neutron plus a positron”, the word “which” has to refer to “antineutrino”, not “proton”. So you’re telling us that a chargeless antineutrino splits into a chargeless neutron and a charged positron, both of which have much greater masses than an antineutrino.


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