RADIOACTIVE HEATING

Some atomic nuclei are not stable. If they are allowed to sit, they will spontaneously break apart into smaller nuclei releasing particles and energy. For example, the isotope ₁₃Al²⁶ is not stable. (Oh, ; the 13 says that the Aluminum nucleus contains 13 protons and the 26 says that the Aluminum nucleus contains 26 protons and neutrons, that is, the nucleus contains 26 - 13 = 13 neutrons.) The Aluminum decays to magnesium ₁₂Mg²⁶ as shown below
₁₃Al²⁶ ---> ₁₂Mg²⁶ + positron + energy

This process has a half-life of 720,000 years. The half-life measures the amount of time it takes for 1/2 of the Al nuclei in a box to change into Mg nuclei. This property of radioactive decay lends itself well to the age dating of rocks and other materials.

Some important short half-life radioactive nuclei are

₁₃Al²⁶ ---> ₁₂Mg²⁶ with t = 0.72 M years


₅₃I¹²⁹ ---> ₅₂Xe¹²⁹ with t = 16 M years

Some important long half-life radioactive nuclei are

K⁴⁰ ---> Ar⁴⁰,Ca⁴⁰ with t = 1.25 B years


Th²³² ---> Pb²⁰⁸ with t = 14 B years


U²³⁵ ---> Pb²⁰⁷ with t = 0.7 B years


U²³⁸ ---> Pb²⁰⁶ with t = 4.5 B years


The shorter half-life nuclei add to the initial heating of the planets while the longer half-life nuclei add to the current heating of the interiors of the planets (the interiors of some of the Terrestrial planets are still quite hot.