Meteorites

Where do they come from?

Only in the late 18th century did scientist begin to recognize that meteorites were extraterrestrial matter. Some come from the Moon and others from Mars. When a huge meteorite carves a big crater, especially if hitting the surface with a low angle, little rock fragments can escape the gravity field of the parent body. They orbit the Sun during millenia, and if some are finally intercepted by the Earths' orbit, they fall as meteorites. Today we know that most meteorites are remnants of the formation of the solar system 4.6 billion years ago. A lot are fragments produced when asteroids collide.

The smallests burn and disintegrate when enter the Earth's atmosphere, but some are big enough to resist air friction and aerodinamic pressure, crashing and sometimes producing a crater. Meteor showers happen when our planet crosses the trail of a comet, often densely populated with little fragments of rock, most the size of sand grains or smaller.

martian meteorite
ALH84001

Pallasite
Pallasite
(Robert Haag)

Tektites
Tektites
(Sisse Brimberg)

 Meteorites are classified as follows:
  • Stony meteorites (92.8%):
    • Chondrites, the most common meteorites (85.7%). Its age, 4.55 billion years, is approximately the age of the solar system. Ordinary chondrites are thought to have formed in the inner asteroid belt, while Carbonaceous chondrites, wich have the highest proportions of volatile elements are thought to have originated in even greater solar distances.
    • Achondrites (7.1%), formed by melting and recristallization within their parent bodies. Martian meteorite ALH84001 belongs to a kind of achondrites named SNC.
  • Iron meteorites (5.7%):
    • Rarer, but easy to identify, they consist primarily of iron-nickel alloys, and probably come from the core of broken asteroids.
  • Stony iron meteorites (1.5%):
    • Pallasites, composed of olivine enclosed in metal. Probably originated from broken asteroids as well, from the deepest layers of their mantle.
    • Mesosiderites.
  • Tektites:
    • Are thought to be melted ejecta of major impacts, launched by explossive collision on ballistic trajectories across hundreds or thousand of miles and scattered across the earth in vast fields. Some look like black glassy buttons.


Visit Meteoroids and Meteorites Web Page.