METEORITES

 
 

Meteorites are the remnants of rocky or metallic extraterrestrial bodies, called meteoroids, which hit the terrestrial ground. Many of them are asteroids, other are pieces of disrupted cometary nuclei. A few meteorites, finally, have lunar or martian origins. One of the martian meteorites, known as ALH84001, is believed to bear traces of life on Mars, but presently there is no definite evidence of this.
  Meteorite ALH84001. (NASA Johnson Space Center)
 

Types of meteorites

 

The variety of these objects reflects their origin from different celestial bodies.
Five main types of meteorites are considered, on the basis of their chemical composition.
 
 

Iron meteorite found at Derrick Peak, Antarctica. It is mostly composed by iron and nickel.   (NASA/JPL) 

 1) Iron meteorites: they are mainly composed by nickel and iron, they are similar to type M asteroids. 

2) Meteorites of iron-rock type: they are composed, as the type S asteroids, by iron an rocky material. 
3) Chondrites: they are the most numerous type and have a composition similar to the terrestrial  mantle and crust.
  Chondritic meteorite found in Antarctica. (NASA/JPL)
 

4) Carbon chondrites: very similar to the Sun in chemical composition, apart from the gaseous elements such as hydrogen and helium; they are similar to type C asteroids. 

5) Achondrites: similar to terrestrial basalts . Meteorites coming from Moon and Mars are achondrites.
  Achondritic meteorite found at Reckling Peak,  Antarctica. It has a basaltic composition and has probably an age of 4 billion years. (NASA/JPL) 

Meteoric impacts

A large number of meteoroids hits the Earth each day, at speeds comprised between 35,000 and 250,000 km/h: we are talking about several hundreds of tons of material, most of which disrupts into the high atmosphere due to friction and reaches the ground as dust. The largest ones, after being slowed down by the atmospheric friction to speeds of a few hundred km/h, hit the terrestrial ground, producing a crater.
At least 120 impact craters have been discovered up to now on the surface of our planet.
 

The Wolfe Creek crater in Australia. (V. L. Sharpton, LPI)

  

The  Barringer crater in Arizona. (D. Roddy, LPI) 

There are at least 1,000 asteroids whose dimension are above one kilometer, and whose orbit crosses the Earth one; the probability that one of them will fall on the Earth is just of one each 300,000 years, but the consequences would be disastrous. For example, it has been proposed the hypothesis that the extinction of dinosaurs from our planet was due to an event of this kind.
 
 
Microscopic image of a martian meteorite.  The colors yellow, pink, green and black are those of olivine, a common mineral in basaltic rocks. (Allan Tremain, LPI) 		  
Meteorite likely coming from the Vesta asteroid.  (R. Kempton, New England Meteoritic Services) 
 
 
 

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