CARBONACEOUS MICROMETEORITES AND THE ORIGIN OF LIFE

Giant micrometeorites (sizes ranging from approximate to 50 to 500 mu m), such as those that were first recovered from clean pre-industrial Antarctic ices in December 1987, represent by far the dominant source of extraterrestrial carbonaceous material accreted by the Earth's surf ace, about 50 000 times the amount delivered by meteorites (sizes grea ter than or equal to a few cm). They correspond to large interplanetar y dust particles that survived unexpectedly well their hypervelocity i mpact with the Earth's atmosphere, contrary to predictions of theoreti cal models of such impacts. They are related to relatively rare groups of carbonaceous chondrites (approximate to 2% of the meteorite falls) and not to the most abundant meteorites (ordinary chondrites and diff erentiated micrometeorites). About 80% of them appear to be highly une quilibrated fine-grained assemblages of mineral grains, where an abund ant carbonaceous component is closely associated on a scale of less th an or equal to 0.1 mu m to both hydrous and anhydrous minerals, includ ing potential catalysts. These observations suggest that micrometeorit es could have functioned as individual microscopic chemical reactors t o contribute to the synthesis of prebiotic molecules on the early Eart h, about 4 billions years ago. The recent identification of some of th eir complex organics (amino acids and polycyclic aromatic hydrocarbons ), and the observation that they behave as very efficient 'cosmochroma tographs', further support this 'early carbonaceous micrometeorite' sc enario. Future prospects include identifying the host phases (probably ferrihydrite) of their complex organics, evaluating their catalytic a ctivity, and assessing whether synergetic interactions between microme teorites and favorable zones of the early Earth (such as submarine hyd rothermal vents) accelerated and/or modified such synthesis.