Which set lists the four radioactive isotopes commonly cited for Earth's radiogenic heat?

Which set lists the four radioactive isotopes commonly cited for Earth's radiogenic heat?

• A. K-40, Th-232, U-235, U-238
• B. K-40, Th-232, U-235
• C. U-235, U-238, Th-232, Pu-239
• D. K-40, U-238, Th-232, Cm-244

Answer: (D)

Radiogenic heat in the Earth mainly comes from the decay of long‑lived radioactive isotopes present in rocks. The four isotopes most often cited as the primary heat sources are potassium-40, uranium-238, uranium-235, and thorium-232. Potassium-40 provides heat because potassium is relatively abundant in minerals, even though each decay releases modest energy. Uranium-238 and uranium-235 release a substantial amount of heat through their long decay chains, and thorium-232 also decays with a long half-life, continuously producing heat over geological timescales. Together, these four account for the bulk of the planet’s radiogenic heat budget. The other options aren’t regarded as major natural contributors. Plutonium-239 and curium-244 are not present in significant natural abundances in Earth’s crust; their primary relevance is in human-made contexts or in very limited natural processes, and their decay characteristics don’t add meaningfully to Earth’s radiogenic heat over long timescales. That’s why the standard four cited for Earth’s radiogenic heat are potassium-40, uranium-238, uranium-235, and thorium-232.

Radiogenic heat in the Earth mainly comes from the decay of long‑lived radioactive isotopes present in rocks. The four isotopes most often cited as the primary heat sources are potassium-40, uranium-238, uranium-235, and thorium-232. Potassium-40 provides heat because potassium is relatively abundant in minerals, even though each decay releases modest energy. Uranium-238 and uranium-235 release a substantial amount of heat through their long decay chains, and thorium-232 also decays with a long half-life, continuously producing heat over geological timescales. Together, these four account for the bulk of the planet’s radiogenic heat budget.

The other options aren’t regarded as major natural contributors. Plutonium-239 and curium-244 are not present in significant natural abundances in Earth’s crust; their primary relevance is in human-made contexts or in very limited natural processes, and their decay characteristics don’t add meaningfully to Earth’s radiogenic heat over long timescales. That’s why the standard four cited for Earth’s radiogenic heat are potassium-40, uranium-238, uranium-235, and thorium-232.