What Is a Meteorite? Free Samples From the Early Solar System

The terminology is precise and frequently muddled. A meteoroid is the object while it is in space, ranging from dust grains to bodies a few metres across. A meteor is the luminous phenomenon produced when it enters the atmosphere, which is a visible streak caused by the object compressing and heating the air ahead of it and by the resulting ablation of its surface, and it is the light rather than the rock. A meteorite is what reaches the ground. The great majority of incoming material burns up entirely, and a typical shooting star is produced by a particle the size of a grain of sand travelling at tens of kilometres per second. Larger bodies decelerate sharply in the lower atmosphere, and by the time a surviving fragment lands it is generally falling at ordinary terminal velocity and is not hot enough to start fires, contrary to a persistent belief, since the intense heating affects only a thin surface layer that fuses into a dark glassy crust and then largely ablates away.

Meteorites divide into families that correspond to different parent bodies and different histories:

• •Chondrites, the most common, containing small spherical chondrules formed as molten droplets in the early solar nebula, which have never been melted since and therefore preserve the original composition
• •Carbonaceous chondrites, a subset rich in carbon compounds including amino acids and water-bearing minerals, which are central to questions about the delivery of organic material to the early Earth
• •Achondrites, which come from bodies large enough to have melted and differentiated, so their chondrules were destroyed
• •Iron meteorites, from the cores of shattered differentiated bodies, whose interiors show a distinctive crystal pattern that forms only over millions of years of extremely slow cooling and cannot be faked
• •Stony-irons, mixtures from the boundary region between core and mantle, including pallasites containing olivine crystals set in metal
• •Lunar and Martian meteorites, identified by matching their composition and trapped gases to known planetary material, which is how samples of Mars reached laboratories before any mission returned one

The age of the solar system, generally given as about four and a half billion years, was determined from radiometric dating of meteorites rather than of Earth rocks, because the Earth's surface has been reworked continuously and no original material survives. Chondrites also supply the reference composition against which planetary compositions are compared, since they approximate the bulk material the planets formed from. Isotopic analysis has revealed that the early solar system contained short-lived radioactive isotopes, implying a nearby supernova shortly before or during its formation. Carbonaceous chondrites contain amino acids with a composition unlike terrestrial contamination, demonstrating that complex organic chemistry occurs in space. Martian meteorites carry trapped atmospheric gases matching measurements made on Mars, which is how their origin was proven, and one of them became the subject of a widely publicised claim of fossil evidence in 1996 that has not been sustained. Iron meteorites were worked by humans long before smelting, and beads made from meteoric iron have been found in Egyptian burials predating the iron age.

Meteorites fall uniformly across the Earth and are found in very particular places, because recovery depends on being able to recognise a dark rock against its surroundings and on the rock surviving weathering. Antarctica is the richest hunting ground by a wide margin, since falls accumulate in ice and are concentrated by ice movement into blue ice fields where they sit dark on a white surface, and systematic expeditions have recovered tens of thousands. Hot deserts are the second source, offering pale ground, low rainfall and slow weathering. Camera networks now track bright fireballs from multiple stations, calculate the fall area and the original orbit, and allow recovery within days, which produces samples with a known origin in the solar system rather than an unknown one. Identification relies on a dark fusion crust, unusual density, magnetic response in most types, and internal features, while the most common false alarm by a wide margin is industrial slag, and the practical test used by museums is that a genuine specimen requires laboratory confirmation rather than appearance alone.

The meteor is the light and the meteorite is the rock, and most incoming material is sand-grain sized and burns up entirely. Chondrites were never melted, so they preserve the original nebula composition and supplied the radiometric date for the solar system. Carbonaceous types carry amino acids, and Martian ones were identified by trapped gases matching measurements on Mars. Antarctic ice concentrates falls where they are easy to see.