Introduction

Since July 1984, “large” micrometeorites with sizes ranging from ≈50 to 500 μm were successfully collected in large number on the Greenland and Antarctica ice sheets (Maurette et al. 1987, 1991, 1994). It was shown that they represent by far the dominant source of extraterrestrial materials that “survive” upon their hypervelocity capture by the Earth.

The cleanest collections of micrometeorites have been recovered from Antarctica ices. These Antarctica micrometeorites (AMMs) are mostly carbonaceous objects, related mainly to a relatively rare class of carbonaceous meteorites (CM-type chondrites). They contain high concentrations of carbonaceous material (average C-content ≈7 wt%), including complex organics such as amino acids (Brinton et al. in press) and polycyclic aromatic hydrocarbons (Clemett et al. in press).

The characteristics of the AMMs' constituent grains were acquired in the early Solar System, before their accretion in the micrometeorite parent bodies (comets and/or asteroids). But some of them have been since altered as a result of interactions effective either in these parent bodies or in the interplanetary medium and the terrestrial environment.

Such interactions have to be understood, first, to identify the primitive characteristics of micrometeorites (which can be used either to relate them to other types of Solar System bodies, or to find new constraints on the early history of the Solar System), and secondly, because they might have produced new features improving the functioning of micrometeorites as microscopic “chondritic chemical reactors” to synthesize prebiotic molecules on the early Earth.

The major objective of this chapter is to present this “early micrometeorites” scenario, as well as the major difficulties and speculations still facing it.