The average density of the massive Jovian planets (Jupiter, Saturn, Uranus, and Neptune) is significantly lower than that of the terrestrial planets (Mercury, Venus, Earth, and Mars). This is because the Jovian planets are primarily composed of lighter elements like hydrogen and helium, while the terrestrial planets are mostly made up of heavier elements like iron and silicate minerals.

Here's a general overview of the average densities:

* Jovian Planets: Around 1.3 to 1.6 grams per cubic centimeter (g/cm³)

* Terrestrial Planets: Around 5 to 5.5 g/cm³

Let's compare the densities of individual Jovian planets:

* Jupiter: 1.33 g/cm³

* Saturn: 0.69 g/cm³ (The least dense planet in the solar system)

* Uranus: 1.27 g/cm³

* Neptune: 1.64 g/cm³

As you can see, even within the Jovian planets, there's some variation in density. This is mainly due to differences in their atmospheric composition and the presence of heavier elements in their cores.