Effects of atmospheric heating on infalling meteorites and micrometeorites: Relevance to conditions on the early earth

Since micrometeorites may have played an important role in the development of conditions on the early Earth, bringing vital supplies of the biogenically important elements, it is apposite to appraise the effects that atmospheric heating exerts on such particles as they head towards the planet's surface. An expedient way to approach this problem is to undertake an investigation of the effects of pulse-heating on the carbon inventories of micrometeorites arriving at the present time. Unfortunately analytical techniques for carbon have not yet advanced to the point where levels of sensitivity are compatible with a study of this type. However, it is still possible to contemplate the effects of atmospheric heating by utilizing samples of relatively well documented meteorites as analogs of micrometeorites. To this end small samples of three meteorites (Allende, Weston and Goalpara) have been subjected to pulse-heating in order to investigate the effects of atmospheric infall on carbon chemistry. In this preliminary study the samples were heated in air thereby simulating present day atmospheric conditions. The results show, not surprisingly, that heating removes carbon. However, the extent of this effect is dependent upon combustion kinetics, which in turn is presumably related to the nature of individual components and their location within the samples. For instance, the results show that refractory carbon-bearing components are not only able to survive pulse-heating to 1500°C, but may actually become enriched in samples as a result of a heating episode (i.e., as a consequence of preferential removal of other more labile materials). Although carbon is removed by heating, this does not appear to affect the carbon isotopic composition of the material that remains. This means that analyses of micrometeorites falling to Earth at the present time, although recording artificially low carbon contents, will be to some extent valid in terms of carbon isotopic measurements (especially where stepped combustion is used to acquire the relevant data above a certain threshold temperature, which is itself related to the extent of atmospheric heating). Furthermore, a detailed calibration of the extent of carbon removal may assist the development of atmospheric heating models, thereby constraining entry velocities, and ultimately sources. To simulate effectively the nature of infall heating in former times, more sophisticated experiments would need to be carried out involving the use of synthetic “atmospheres” prepared from various gas mixtures.