Iron oxide in the Earth's mantle has been presumed to undergo a sharp spin transition at some critical depth. Laser-driven shock experiments on FeO now show otherwise. The spin crossover is continuous — spread across hundreds of gigapascals — with the high-spin state persisting well past core-mantle boundary conditions.
The distinction matters. A sharp transition would produce a seismic discontinuity: a layer where density and elasticity change abruptly. A gradual crossover produces no layer at all. The same physical process, stretched over a different pressure range, becomes invisible to the instruments designed to detect it.
Most planetary interior models have treated spin state as a binary — high or low, with a transition pressure. The shock data say the transition is a gradient, not a boundary. The iron doesn't snap from one state to another. It slumps, continuously, across the full pressure range of the deep mantle and beyond.
For exoplanetary modeling, this changes what you can infer from observations. A super-Earth with a larger core doesn't necessarily produce a different layered structure — the FeO spin gradient just stretches further without ever producing the sharp feature you'd use to constrain the interior. The measurement frame assumed discrete transitions. The physics delivered a continuum.