Why do iron meteorites come from so many parent bodies?
Iron meteorites come from the cores of many small planetesimals that melted early; their strength lets them survive while the rocky outer layers were destroyed.
Iron meteorites are a small fraction of falls but represent many more parent bodies than stony meteorites, perhaps 50 or more. There are several reasons.
Early melting. Isotope dating, especially with the hafnium–tungsten method, shows that many iron meteorite parent bodies formed and melted within about a million years of the first solids. These early planetesimals contained abundant aluminium-26, which heated them enough to melt and separate a metal core. So the early Solar System produced many cores.
Toughness. Iron-nickel metal is far stronger than rock. When parent bodies were broken up by collisions, the rocky mantles and crusts shattered into fragments that were further destroyed over time, while metal cores survived. In space, iron meteoroids last much longer. Iron meteorites often have cosmic-ray exposure ages of hundreds of millions of years, compared with tens of millions for stony meteorites.
Missing mantles. Strangely, we have few meteorites from the olivine-rich mantles that should have surrounded these cores. This is sometimes called the 'missing mantle problem'. It may be because mantle rock was ground down to dust, or because the parent bodies formed closer to the Sun and their cores were scattered into the asteroid belt later.
Inner and outer Solar System. Isotopes divide iron meteorites into two families, inner and outer Solar System, showing that cores formed in both regions.
Together, these factors mean iron meteorites are a rich record of many vanished planetesimals.