Final answer:
Three out of four Jovian planets have internal heat sources primarily due to gravitational compression and retained primordial heat. Jupiter has the most significant internal energy, while Saturn's internal heating comes partly from the separation of helium from hydrogen.
Step-by-step explanation:
The internal heat sources of the Jovian planets are primarily due to gravitational compression and primordial heat remaining from their formation. These heat sources can raise the temperatures in the interiors and atmospheres of these planets higher than what would be expected from solar radiation alone. For example, Jupiter has the largest internal energy source, equivalent to 4 million billion 100-watt lightbulbs, mostly coming from primordial heat. Additionally, Saturn has significant internal heating due to the separation of helium from hydrogen in its interior, a process that releases gravitational energy.