Planetary Interiors Under Pressure The interiors of Earth and other rocky planets generally consist of a few common minerals. Depending largely on the size of the planet, the distribution and relative abundance of these minerals varies; for example, MgO is abundant in the mantles of Earth and large Earth-like planets, but is present in Jupiter's core. The properties of MgO also vary with planetary size as a function of temperature and pressure. McWilliams et al. (p. 1330 , published online 22 November) performed laser-shock experiments at pressures over three times higher than Earth's inner core. MgO underwent two phase transformations, first to a solid with a modified crystal structure, and then to a conductive liquid. In terrestrial planets greater than eight Earth masses, MgO in the mantle could generate a magnetic field–generating dynamo such as those that typically found in planetary cores.