PSRD: When Worlds Really Did Collide

Most iron meteorites are thought to come from over 50 asteroids 5-100 kilometers in size that were melted, differentiated into metallic cores and silicate mantles, and broken open by impacts long after they had slowly cooled. However, we find that the cooling rates for one group of iron meteorites (dubbed the IVA group) and other irons require a much more complex history and much larger parent bodies. The IVA metal was probably derived originally from a body at least 600 kilometers in diameter, and possibly much larger, that was dispersed by an impact leaving a molten metal body 300 kilometers in diameter. The molten metal body solidified and cooled slowly with scarcely any enveloping silicate mantle. This history supports several recent theoretical studies that infer that differentiated asteroids and meteorites are debris from protoplanetary collisions, that protoplanets were abundant in the asteroid belt, and that parent bodies of iron-rich meteorites were broken up early.