What does this equation mean?

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.