Micrometeoritic Sulfur and Ferrihydrite in Exobiology

A more detailed summary of previous works supporting the role of unmelted micrometeorites (about 25% of the incoming flux of micrometeorites) in prebi-otic chemistry is given elsewhere (Maurette, 1998). Krueger and Kissel (1987) quoted thermodynamic computations suggesting that cometary dust grains of the Halley type, when added to a preexisting "soup" of organics, could trigger the formation of nucleic acids. Anders (1989) relied on the characteristics of the tiny micrometeorites collected in the stratosphere (10-20 |m), which amount to about 1% of the micrometeorite mass flux, to argue that micrometeorites played a major role in the delivery of organics to the Earth. As first quoted by Ponchelet (1989), we proposed that hydrous-carbonaceous micrometeorites might have been functioning as microscopic chemical "reactors" on the early Earth during their interactions with gases and waters (Maurette, 1990; Maurette et al., 1991). Subsequently, Chyba and Sagan (1992) ceased supporting the role of the direct impact of comets in the delivery of such organics and started to also quote that of micrometeorites.

All these previous contributions did focus on the role of micrometeorites that survive unmelted upon atmospheric entry. However, our work on the formation of the early Earth's atmosphere (Sect. 4 and 5) suggested that even micrometeorites, which are destroyed upon atmospheric entry, could have contributed in various ways to prebiotic chemistry (Maurette et al., 2001; Maurette et al., 2003a; Maurette et al., 2004b). We discuss below some of these recent contributions.

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