early core fractionation processes, are also depleted. Niobium/tantalum (Nb/Ta) isotopic ratios, when compared with a chondritic reference frame, show mild depletion in bulk silicate Earth and the moon. Pallasite meteorites Pallasites are thought to form at the core-mantle boundary of an early planetesimal, although a recent hypothesis suggests that they are impact-generated mixtures of core and mantle materials. Dynamics Dynamo Dynamo theory is a proposed mechanism to explain how celestial bodies like the Earth generate magnetic fields. The presence or lack of a magnetic field can help constrain the dynamics of a planetary core. Refer to Earth's magnetic field for further details. A dynamo requires a source of thermal and/or compositional buoyancy as a driving force. Thermal buoyancy from a cooling core alone cannot drive the necessary convection as indicated by modelling, thus compositional buoyancy (from changes of phase) is required. On Earth the buoyancy is derived from crystallization of the inner core (which can occur as a result of temperature). Examples of compositional buoyancy include precipitation of iron alloys onto the inner core and liquid immiscibility both, which could influence convection both positively and negatively depending on ambient temperatures and pressures associated with the host-body. Other celestial bodies that exhibit magnetic fields are Mercury, Jupiter, Ganymede, and Saturn. Core heat source A planetary core acts as a heat source for the outer layers of a planet. In the Earth, the heat flux over the core mantle boundary is 12 terawatts. This value is calculated from a variety of factors: secular cooling, differentiation of light elements, Coriolis forces, radioactive decay, and latent heat of crystallization. All planetary bodies have a primordial heat value, or the amount of energy from accretion. Cooling from this initial temperature is called secular cooling, and in the Earth the secular cooling of the core transfers heat into an insulating silicate mantle. As the inner core grows, the latent heat of crystallization adds to the heat flux into the mantle. Stability and instability Small planetary cores may experience catastrophic energy release associated with phase changes within their cores. Ramsey (1950) found that the total energy released by such a phase change would be on the order of 1029 joules; equivalent to the total energy release due to earthquakes through geologic time. Such an event could explain the asteroid belt. Such phase changes would only occur at specific mass to volume ratios, and an example of such