concept

Moment of inertia factor

A single number telling you how far a planet's mass is concentrated toward its centre — measurable from a distance, just by watching the body spin and wobble. Perfectly uniform material gives 0.4; the more mass is gathered at the middle, the smaller the number. Earth comes in at 0.3307, meaning a dense metallic core. Mars gives 0.365, the Moon 0.393, Io 0.378. This is evidence entirely independent of seismology, and it is the agreement between the two methods that makes the picture of a planet's interior trustworthy.