What source of internal heat does the Moon rely on for any residual activity, contrasting with Io's primary source?

Answer

Radioactive decay

The Moon relies on slower, finite radioactive decay for residual internal heat, whereas Io's tidal flexing provides an almost inexhaustible supply of energy.

What source of internal heat does the Moon rely on for any residual activity, contrasting with Io's primary source?
SpaceJupiterMoonVolcanismIo