in the early Solar System are important aspects in the formation and growth of planets and planetary cores. Earth–Moon system The giant impact hypothesis states that an impact between a theoretical Mars-sized planet Theia and the early Earth formed the modern Earth and Moon. During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core. Mars Core merging between the proto-Mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years (depending on viscosity of both cores). Chemistry Determining primary composition – Earth Using the chondritic reference model and combining known compositions of the crust and mantle, the unknown component, the composition of the inner and outer core, can be determined: 85% Fe, 5% Ni, 0.9% Cr, 0.25% Co, and all other refractory metals at very low concentration. This leaves Earth's core with a 5–10% weight deficit for the outer core, and a 4–5% weight deficit for the inner core; which is attributed to lighter elements that should be cosmically abundant and are iron-soluble; H, O, C, S, P, and Si. Earth's core contains half the Earth's vanadium and chromium, and may contain considerable niobium and tantalum. Earth's core is depleted in germanium and gallium. Weight deficit components – Earth Sulfur is strongly siderophilic and only moderately volatile and depleted in the silicate earth; thus may account for 1.9 weight % of Earth's core. By similar arguments, phosphorus may be present up to 0.2 weight %. Hydrogen and carbon, however, are highly volatile and thus would have been lost during early accretion and therefore can only account for 0.1 to 0.2 weight % respectively. Silicon and oxygen thus make up the remaining mass deficit of Earth's core; though the abundances of each are still a matter of controversy revolving largely around the pressure and oxidation state of Earth's core during its formation. No geochemical evidence exists to include any radioactive elements in Earth's core. Despite this, experimental evidence has found potassium to be strongly siderophilic at the temperatures associated with core formation, thus there is potential for potassium in planetary cores of planets, and therefore potassium-40 as well. Isotopic composition – Earth Hafnium/tungsten (Hf/W) isotopic ratios, when compared with a chondritic reference frame, show a marked enrichment in the silicate earth indicating depletion in Earth's core. Iron meteorites, believed to be resultant from very