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8.17 Origin of Meteorites

Every once in while a strange, half-melted, often mostly metal rock gets found on the surface of the planet. These non-terrestrial impactors have many different origins, but thanks to science we are able to tell their stories. Called meteorites, these rocks may come from asteroids, Mars, or even the Moon. After decades of study, scientists have begun to understand how meteorites formed and what they can tell us about the ancient history of the Solar System. The vast majority of meteorites come from asteroids, and they are the most ancient rocks studied by geologists. These asteroids formed at the beginning of the Solar System, and at various times in the last four and a half billion years they were broken apart or had fragments chipped off during collisions. Some of these resulting fragments were thrown into orbits that eventually intersected Earth's. Luckily, the majority of these objects are small, although occasionally many kilometer across objects have threatened our planet.

Not all the pieces of space debris that intersect the Earth's atmosphere make it to our planet's surface. The majority of pebble and rock sized meteors burn up in the Earth's atmosphere as so-called "shooting stars." Only larger (and often the densest) asteroids and asteroid fragments can survive passage the journey through the atmosphere without the frictional heating tearing them complete apart or vaporizing them. While it is a very bad thing when full-sized asteroids hit the planet — this is what killed the dinosaurs, after all — it is scientifically beneficial to have a steady supply of smaller rocks make landfall.

We are just starting to be able to sample asteroids directly, and today the majority of what we know about these building blocks of the Solar System we've learned from the bits and pieces that have made it on their own to the surface of the planet Earth. Scientists have been able to connect different groups of meteorites with classes of asteroids by comparing their reflectance spectra (by how they reflect light of different colors). Certain types of stony meteorites correspond to stony asteroids, iron meteorites match the spectra of metal-rich asteroids, and carbonaceous meteorites are consistent with the dark C-type asteroids. The stony meteorites are samples of asteroids’ outer rocky layers, or of undifferentiated asteroids, and the metallic ones are samples of dense cores inside differentiated bodies, analogous to the Earth's iron core. The carbonaceous ones are samples of the black, carbonaceous asteroids (and perhaps comets) of the outer Solar System. (see the article on types of asteroids to learn more.)

More than 80% of meteorites are from stony bodies that never differentiated, which indicates that their parent asteroids (or portions of them) never got hot enough to melt completely. About 1 in 10 meteorites are lava-like rocks that once melted and then solidified. This gives us proof that some asteroids reached high enough temperatures to melt, while others did not. The heating mechanisms are unknown; maybe it was concentrations of radioactive minerals, magnetic or electrical effects, or shock waves — all these sources have been suggested. Whatever the heat source, the iron meteorites (4%) and stony-iron mixtures (1%) show that some asteroids differentiated completely (like the oil and water separating in Italian dressing left to stand in the refrigerator), forming iron cores with a stony-iron interface between the core and a silicate mantle at the surface.

Scientists are particularly interested in the primitive, unmelted types of asteroids called chondrites. Most stony meteorites, and all carbonaceous meteorites, are chondrites. These rocks are primeval fragments of the Solar System's original mineral grains. Dating by the technique of radioactive decay shows that the most primitive meteorites formed during a "brief" interval of only 20 million years, some 4.6 billion years ago. In other words, if the history of the Solar System were reduced to one year, the majority of asteroid-sized building blocks of planets had formed and melted by January 2. This interval marked the birth of planetary material. Other types of radioactive dating show that major of collisions among these bodies happened throughout the interval from 4.6 to about 4.4 billion years ago. While sporadic collisions continued to smash other asteroids together with one another and with planets throughout Solar System, this are much safer today then they were in the earliest days of our Solar System. While asteroids continue to attack, we have no reason to fear planet destroying object will wreck any near-future tomorrows.

A meteor falling earthwards. Commonly called a "shooting star," meteorites have very little to do with stars. Click here for original source URL.