Meteorite

Work in progress.

A solid natural object reaching a planet’s surface from interplanetary space.

Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and textures. Meteorites range in size from microscopic to many meters across. Of the several 10s of tons of cosmic material entering Earth’s atmosphere each day, only about one ton reaches the surface.
Heating begins at an altitude of 100-120 km when a meteoroid encounters the Earth’s atmosphere. An object’s chance of survival depends on its initial mass, speed and angle of entry, and friability (tendency to break up). Micrometeoroids radiate heat so effectively that they are dramatically slowed without being vaporized and fall as a continuous, gentle, invisible rain. Meteoroids with masses between 10-6g and 1 kg tend to burn up completely as meteors. Friable meteoroids break up and are destroyed at altitudes of 80 to 90 km. Those which are tougher survive longer and produce fireballs as their surface undergo melting and ablation at temperatures of several thousand degrees. If meteoroids avoid destruction high up, they enter the lower, denser part of the atmosphere where they are rapidly decelerated. Finally, at subsonic speeds the fireball is extinguished and what remains falls to the ground as a meteorite. The last melted material on the surface of the object solidifies to form a thin, usually black, rind known as a fusion crust.

A natural solid object larger than 10 μm in size that was ejected by the impacts between meteoroids and/or larger natural bodies, was then transported by natural means from the body from which it was ejected to a region outside the dominant gravitational influence of that body, and later, when their orbits intersected, collided with a natural or artificial body larger than itself (even if it is the same body from which it was launched). Meteorites found on Earth, or any planet with a sufficiently dense gaseous atmosphere, will have experienced ablation due to frictional forces experienced during the object’s high velocity entry through the planet’s atmosphere. An object loses its status as a meteorite if it is incorporated into a larger rock that itself becomes a meteorite at a later time (xenoliths found in meteorites are one example).

Weathering processes do not affect an object’s status as a meteorite as long as something recognizable remains of its original minerals or structure. These highly altered materials that have a meteoritic origin but are dominantly (>95%) composed of secondary minerals formed on the body on which the object was found are called Relict Meteorites. (Meteoritical Society, 2006)