Density of the Earth

The average density of Earth is calculated from its mass (5.97×10²⁴ kg) and volume (1.083×10¹² km³), yielding 5.51 g/cm³.¹ This value is a fundamental constraint for models of Earth's interior, as it is far higher than the density of surface rocks (2.7–3.0 g/cm³), implying that the interior must contain much denser material.² The first precise measurement was made by Henry Cavendish in 1798 using a torsion balance to measure the gravitational attraction between masses, a method that also yielded the first accurate value for the gravitational constant.

Earth's density increases with depth due to increasing pressure and compositional changes. The crust has an average density of about 2.7 g/cm³ for continental and 3.0 g/cm³ for oceanic crust. The mantle, composed of silicate rocks, ranges from 3.4 to 5.6 g/cm³. The outer core, a liquid iron-nickel alloy, has a density of about 9.9–12.2 g/cm³, while the inner core, solid iron, reaches up to 13.1 g/cm³ at the center.³ This density stratification is inferred from seismic wave velocities and laboratory experiments on minerals under high pressure.⁴

Earth is the densest planet in the Solar System, with an average density of 5.51 g/cm³, compared to Mercury (5.43 g/cm³), Venus (5.24 g/cm³), and Mars (3.93 g/cm³).⁵ The high density is partly due to Earth's large iron core, which constitutes about 32% of its mass. The density of exoplanets is often estimated from transit and radial velocity measurements, providing clues to their composition; for example, the super-Earth Kepler-452b has a density consistent with a rocky composition.⁶

One lesser-known fact is that Earth's average density is not constant; it changes slowly over geological time due to the redistribution of mass, such as the growth of the inner core and mantle convection. The density of the Moon is only 3.34 g/cm³, which is lower than Earth's, reflecting its smaller iron core. The concept of 'free-air' and 'Bouguer' gravity anomalies allows geophysicists to map density variations in the crust, revealing hidden structures like mantle plumes. Additionally, the density of Earth was historically used to test Newton's law of gravitation, and Cavendish's experiment was described as 'weighing the Earth' because it allowed the calculation of Earth's mass.

Average density

Total mass divided by total volume, for Earth about 5.51 g/cm³.

Crust

The outermost solid shell of Earth, with densities around 2.7–3.0 g/cm³.

Mantle

The thick layer between crust and core, composed of silicate rocks, density increasing with depth.

Core

The central part of Earth, composed mainly of iron and nickel, with densities up to 13 g/cm³.

Torsion balance

An instrument used by Cavendish to measure gravitational attraction, leading to the first accurate density of Earth.