What does this equation mean?

Gravity. A planet’s external gravity field is not sensed with a gravimeter sitting on the planet; it is read off the motion of an orbiting or flying-by spacecraft, whose trajectory bends in response to mass distribution beneath it. In practice this means tracking the spacecraft’s radio carrier signal from Earth and measuring its Doppler shift with extreme precision as the spacecraft accelerates and decelerates along its orbit. A denser, non-uniformly distributed interior produces small, characteristic deviations from a simple point-mass orbit, expressed as gravity harmonics ( $J_2$ , $C_{22}$ , and higher terms) layered on top of the dominant 1/$r^2$ term. From those harmonics and the body’s known radius and rotation, one can compute the moment of inertia factor , $\frac{C}{MR^2}$. where C is the polar moment of inertia, M the mass, and R the mean radius. A uniform sphere has C/M$R^2$ = 0.4 ; a value below that indicates mass concentrated toward the centre — a core. This is exactly the method Cassini’s radio tracking used to constrain Titan’s interior, returning a moment of inertia factor near 0.34 and pointing to incomplete separation of rock from ice rather than a fully differentiated body [ 6 ] , and it is the same logic Galileo’s gravity measurements applied to Europa, where the data supported a metallic core, rocky mantle, and outer ice–water layer [ 7 ] . The method is powerful and also intrinsically limited: gravity alone cannot uniquely fix a full density profile, because different internal arrangements can produce the same low-degree harmonics. Gravity constrains bulk structure; it does not, by itself, resolve boundaries.