Meteorite Weathering

Most meteorites date from the oldest times in the solar system and are by far the oldest material available on our planet. However, despite their age, they are pretty vulnerable to terrestrial environment: water, salt, oxygen attack the meteorites as soon they reach the ground.

The terrestrial alteration of meteorites is called weathering. In order to quantify the degree of alteration that a meteorite experienced, several qualitative weathering indices have been applied to Antarctic and desertic samples.

The most known weathering scale, used for stone meteorites, ranges from W0 (pristine state) to W6 (heavy alteration).

Source : http://en.wikipedia.org/wiki/Meteorite