Why is Earth denser than Mars?

Earth has a larger iron core relative to its size. Mars, being smaller, lost more of its internal heat and has a proportionally smaller core.

Can density reveal a planet's composition?

Yes. A density above about 5 g/cm3 suggests an iron-rich rocky body. Below about 2 g/cm3 indicates a gas or ice giant.

Saturn, with a mean density of about 0.687 g/cm3, less than water.

Calculate the mean bulk density of a planet from its mass and radius: rho = 3 M / (4 pi R^3).

Planet Mass

Planet Radius

Mean Density

5,513.26 kg/m3

Density (g/cm3)5.5133 g/cm3

Mean Density vs Planet Mass

The average density is found by dividing the total mass by the total volume.

rho = M / V = 3M / (4 pi R cubed)

Density helps distinguish rocky planets (about 5 g/cm3) from gas giants (about 1 g/cm3). Earth's mean density is about 5.51 g/cm3.

Earth: M = 5.972e24 kg, R = 6.371e6 m.

Escape speed from a planet.

Surface gravity of a body.

Tidal acceleration from a nearby mass.