Iron meteorites seem common only because people notice lumps of metal
Around 5 percent of witnessed meteorite falls are iron, yet in Antarctica, where nearly everything that lands can be recovered, iron makes up just 0.4 percent of finds. Metal is simply easier to spot. Most space rocks are stony chondrites about 4.55 billion years old, leftovers from the birth of the planets.
A meteorite is a piece of space rock that survives its plunge to a planet or moon. On the way down it heats up and glows as a meteor, the brightest known as bolides. Most disintegrate; only five to ten falls a year are seen and then recovered for science. Recovered falls are called falls, and everything else found later counts as a find. Traced back, most meteorites come from a handful of asteroid break-ups in the main belt.
About 86 percent are chondrites, named for chondrules, tiny beads of silicate that melted while drifting freely in space. Some carry organic matter and grains older than the Sun, and they are often described as the building blocks of the planets. Achondrites, about 8 percent, resemble volcanic rock from the crusts of differentiated bodies; two small, younger groups come from the Moon and almost certainly from Mars, the only planetary samples humans have ever held. Iron meteorites come from the shattered cores of once-molten planetesimals, and stony-irons such as pallasites from the boundary above those cores.
The journey leaves marks. Melting surfaces can develop thumbprint-like dimples called regmaglypts, and a stone that stops tumbling may be shaped like a nose cone before a thin, usually black fusion crust hardens. Witnesses report fireballs rivalling the Sun, sonic booms heard more than a hundred kilometres away, and puzzling whistles. Fragmenting objects scatter across elliptical strewn fields, with the heaviest pieces landing farthest along the path.
Iron bodies cause most of Earth's hypervelocity craters, including Barringer and Wolfe Creek, because they stay intact through the air; stony objects up to millions of tons usually break apart, as probably happened at Tunguska. Meteorites also bear on life's origins: in 2019 researchers reported ribose sugar in them, and in 2022 a Japanese team found all five nucleobases used in DNA and RNA.
Source: Meteorite
