Introduction

Meteorites have so far provided our best analytical window into the cosmochemical evolution of the elements that make up Earth's complex chemical systems. They are for the most part fragments of asteroids, i.e. of the small-sized and odd-shaped planetesimals that orbit the Sun in great numbers between Mars and Jupiter. According to the regular spacing of inner planets from the Sun (the Titius–Bode law), this orbit should be occupied by a planet, but it appears that the smaller lumps of early Solar-System material on their way to form a planet in this region fell under the strong gravity of the gas-giant Jupiter, which slung many away and left the rest unable to fully coalesce. Asteroids, therefore, are the remnants of a planet that never was and, just like comets and other smaller bodies in the Solar System that avoided the geological reprocessing of planet formation, may offer a pristine record of early Solar-System material. Yet, they have the important distinction of being concentrated in a crowded orbit and, subjected still to the gravitational pull of planets nearby and many collisions, regularly send their fragments to the Earth as meteorites, whose direct laboratory analyses secure unequivocal data of their extraterrestrial material.

In the case of carbonaceous meteorites, the delivery has taken on an astrobiological significance because this subgroup of meteorites contains abundant and diverse organic material, including compounds having identical counterparts in the biosphere, such as amino acids (e.g. Pizzarello et al., 2006).