The kerogen model also helps us surmise the ultimate fate of carbonaceous dust. While the graphite found in the soil is the final outcome of kerogen/coal evolution, not all of these end up in graphite. Other conditions are necessary for this to be the case (see Bustin et al. 1995): first, a very high anisotropic pressure, as is known to apply very deep in Earth's crust; second, the precursor material must not be O-rich, for this would block early formation of large clusters of pericondensed aromatic rings, thus hindering "molecular orientation" and precluding the formation of graphite crystals (see Papoular et al. 1996). If, as in space, both conditions are not met, the end point of evolution is still H/C=O/C=0 but in "polygranular (or so-called semi-)graphite". Should this be the case for IS dust, as predicted by the present model, it would at least be compatible with the attested inability of crystalline graphite to reproduce the 217-nm feature independent of grain size. This immediately suggests a simple experiment: take a sample of type II kerogen, anneal it up to 500 K for a few days, then make a VUV/vis spectrum and compare with the IS extinction curve.
Another test of the kerogen conjecture could be made at the other end of the spectrum, in the far IR. If kerogen is a valid model of dust, it should also exhibit the three conspicuous bands observed by Hrivnak et al. (2000), towards transient objects, PPNe and PNe, at nominal wavelengths 20, 26 and 30 
m. These features are weak and the measurement will not be easy, but an encouraging omen is that numerical models indeed exhibit these features and ascribe them to OH, NH and -O- groups attached to mainly non-aromatic skeletons, like in kerogens (Papoular 2000).
Acknowledgements
Thanks are due to Dr. Vandenbroucke, Professor Rouxhet and Dr. Robin for interviews, letters, spectra and papers on kerogens.
Copyright ESO 2001