lost early in the Solar System's history. Mercury has a solid silicate crust and mantle overlying a solid iron sulfide outer core layer, followed by a deeper liquid core layer, and then a possible solid inner core making a third layer. Venus The composition of Venus' core varies significantly depending on the model used to calculate it, thus constraints are required. Moon The existence of a lunar core is still debated; however, if it does have a core it would have formed synchronously with the Earth's own core at 45 million years post-start of the Solar System based on hafnium-tungsten evidence and the giant impact hypothesis. Such a core may have hosted a geomagnetic dynamo early on in its history. Earth The Earth has an observed magnetic field generated within its metallic core. The Earth has a 5–10% mass deficit for the entire core and a density deficit from 4–5% for the inner core. The Fe/Ni value of the core is well constrained by chondritic meteorites. Sulfur, carbon, and phosphorus only account for ~2.5% of the light element component/mass deficit. No geochemical evidence exists for including any radioactive elements in the core. However, experimental evidence has found that potassium is strongly siderophile when dealing with temperatures associated with core-accretion, and thus potassium-40 could have provided an important source of heat contributing to the early Earth's dynamo, though to a lesser extent than on sulfur rich Mars. The core contains half the Earth's vanadium and chromium, and may contain considerable niobium and tantalum. The core is depleted in germanium and gallium. Core mantle differentiation occurred within the first 30 million years of Earth's history. Inner core crystallization timing is still largely unresolved. Mars Mars possibly hosted a core-generated magnetic field in the past. The dynamo ceased within 0.5 billion years of the planet's formation. Hf/W isotopes derived from the martian meteorite Zagami, indicate rapid accretion and core differentiation of Mars; i.e. under 10 million years. Potassium-40 could have been a major source of heat powering the early Martian dynamo. Core merging between proto-Mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years (depending on the viscosity of both cores and mantles). Impact-heating of the Martian core would have resulted in stratification of the core and kill the Martian dynamo for a duration between 150 and 200 million years. Modelling done by Williams,