Evidence for significant differences in the relative amounts of silicates vs. gas compounds of volatile elements between the terrestrial planets and the moons of the gas giants comes from calculating the bodies' .

Densities

Explanation:

A planet is a body which orbits the Sun (or father's it's sun) directly and is massive enough to show a hydrostatic equilibrium ( a spheroid) and be resilient in dominating its orbit

What comes to mind as we mention the word planet is specifically, the eight major bodies of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Note that  Pluto once was considered a planet from the 17th century until 2006 and has now been reclassified as a dwarf planet.

Density is the measure with ratio of one quantity to another quantity.

It can quantitatively be a measure of the mass of matter contained by a unit volume.

The knowledge of density of gases comes in handy to calculate the relative amount of substances.

4d^55s^1 ( third choice)