Overall, four mechanisms are responsible for heating up the insides of planetary bodies. The first two mechanisms—accretional heating and core formation—are called primordial sources of heat because they are only associated with the initial formation of a planetary body. This differs from the latter two mechanisms—radioactive decay and tidal heating—which are processes that can operate long after a celestial object has formed. General descriptions of these four sources of heat are listed below:

1. Accretional heating (primordial source): Heat came from collisions. When objects hit Earth, some of the energy from their motion went into deforming Earth, and some of it was transformed into heat. Clap your hands vigorously to experience this on a much smaller (and safer!) scale.
2. Core formation (primordial source): During differentiation, gravity pulls iron and nickel inward, towards the center of a planet, to form a core. This process of squeezing nickel and iron into a smaller volume (i.e., compression) generates heat.
3. Radioactive decay: The process of unstable atoms breaking down randomly and spontaneously into more stable atoms produces heat.
4. Tidal heating: Under the right orbital conditions (read below), gravitational attraction between two bodies can cause the smaller object to be repeatedly stretched and unstretched. This continuous deformation creates heat.

The internal heat budget of the terrestrial planets primarily comes from primordial sources and radioactive decay—for Earth, each contributes roughly a 50%. On the other hand, tidal heating is an extremely important heat source for many moons in our solar system.

5.1.1 Radioactive Decay

As mentioned above, a major source of Earth’s heat is radioactivity, the energy released when the unstable atoms decay (their nucleus breaks apart, or they lose particles from their nucleus). The main source of the radiation heating is the decay of the radioactive isotopes uranium-235 (²³⁵U), uranium-238 (²³⁸U), potassium-40 (⁴⁰K), and thorium-232 (²³²Th) in Earth’s mantle.

Radioactive decay produced more heat early in Earth’s history than it does today, because the more decay that happened, the fewer radioactive atoms were left to decay in the future. Heat contributed by radioactivity today is roughly a quarter what it was when Earth formed (Figure 5.1).

Watch the below video to learn about the process of radioactive decay.

5.1.2 Tidal Heating

Tidal heating provides Io (one of Jupiter’s moons) with most of its internal heat budget. Additionally, this process probably helped form subsurface oceans on icy moons like Europa. Let’s use Io as a case study for understanding this process.

Despite its small size, Io is the most volcanically active body in our solar system (making it very geologically active!). How can Io continue to produce so much volcanism? The answer, lies in the effect of gravity, through tidal heating. Io is about the same distance from Jupiter as our Moon is from Earth, but Jupiter is more than 300 times more massive than Earth. Tidal forces created by Jupiter’s strong gravity pull Io into an elongated shape, with a several-kilometer-high bulge extending toward Jupiter, but it is not tidal forces alone that cause tidal heating within a body; the tidal bulge must change for heating to actually occur. Io’s orbit is not exactly circular due to gravitational perturbations (tugs) from Europa and Ganymede. In its slightly eccentric orbit, Io twists back and forth with respect to Jupiter, and the degree to which Io bulges changes as the moon moves nearer and farther from the planet. This twisting and flexing (or deformation) of Io causes frictional heating, just like repeated flexing of a wire coat hanger heats the wire. (You could also imagine stretching a rubber band. Holding a rubber band in a stretched position does not generate heat, but if you repeatedly stretch and release the rubber band, it will heat up.) After billions of years, this constant flexing and heating have taken their toll on Io, driving away water and carbon dioxide and other gases, so that now sulfur and sulfur compounds are the most volatile materials remaining. Its interior is entirely melted, and the crust itself is constantly recycled by volcanic activity.

If we move inward toward Jupiter from Callisto to Io, we encounter more and more evidence of geological activity and internal heating, culminating in the violent volcanism on Io. Three of these surfaces are compared in Figure 5.2. Just as the character of the planets in our solar system depends in large measure on their distance from the Sun (and on the amount of heat they receive), so it appears that distance from a giant planet like Jupiter can play a large role in the composition and evolution of its moons (at least partly due to differences in internal heating of each moon by Jupiter’s unrelenting tidal forces).

5.1.3 Internal Heating of the Giant Planets

Read the textbook section Thermal Radiation from Gas Giant Planets by Chris Impey available on the Teach Astronomy website.

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