CA Preprint -- submitted to Astrophysics and Space Science, 1 May 2000.
Cross-Linked Hetero Aromatic Polymers in Interstellar Dustby N.C. Wickramasinghea, D.T. Wickramasingheb and F. Hoylea
a School of Mathematics, Cardiff University, PO Box 926, Senghennydd Road
Abstract: The discovery of cross-linked hetero-aromatic polymers in interstellar dust by instruments aboard the Stardust spacecraft would confirm the validity of the biological grain model that was suggested from spectroscopic studies over 20 years ago. Such structures could represent fragments of cell walls that survive 30km/s impacts onto detector surfaces.
The existence of heterocyclic aromatic compounds in interstellar dust was first proposed as early as 1977 as a possible explanation of the interstellar absorption band at 2200A (Hoyle and Wickramasinghe, 1977a). At about the same time it was shown that cross-linked heteroaromatics in the form of polysaccharide chains might account for the detailed structures of interstellar absorption/emission features over the 9-13 and 18-30 micron infrared wavebands, the detailed structures of which were not satisfactorily explained by the then fashionable silicate model of interstellar dust (Hoyle and Wickramasinghe, 1977b).
A decisive identification of both aromatic and aliphatic compounds in interstellar dust followed from spectroscopic studies over the 2.9-3.9 micron spectral region of the galactic centre source GC-IRS7 (D.T. Wickramasinghe and Allen, 1980; Allen and D.T. Wickramasinghe, 1981). Aliphatic and aromatic CH stretching at 3.35-3.42 and ~ 3.5 micron were clearly identified in this source, and from laboratory measurements of prospective organic polymers it was possible to conclude that a large mass fraction of all the interstellar dust along the 10kpc pathlength to GC-IRS7 was comprised of condensed material of this general type (Allen and D.T. Wickramasinghe, 1981; Hoyle et al, 1982).
A prime candidate for grains that emerged from the new observations was a model that possessed the spectral properties of desiccated biomaterial. The detailed profile of absorption over the entire 2.9-3.9 micron waveband observed for GC-IRS7 was shown to match a laboratory spectrum of dry E.coli to a remarkable degree of precision (Figure 1).
The laboratory measurement of a mass absorption coefficient of ~ 500 cm2 g-1 at 3.4 microns combined with the observed astronomical optical depth of ~ 0.3 mag led to an average space density of such material amounting to over 25% of the canonical mass density of interstellar dust (Hoyle et al, 1982). Thus a large mass fraction of interstellar dust showed evidence of both aromatic and aliphatic CH stretching, and this material, to all intents and purposes, was spectroscopically identical to bacterial matter (Wickramasinghe and Hoyle, 1998), some fraction of which may comprise of degraded biomaterial similar to coal or kerogen. More recent studies of the source GC-IRS7 at higher resolution by Pendleton et al (1994) and Whittet and Tielens (1997) have not substantially altered the conclusions that we reached in 1981-82.
Two of the present authors have argued for over two decades that in order to explain the above facts one is forced to invoke the well-attested properties of biology. Given the right inorganic nutrients and ambient conditions biological replication can proceed with exponential speed. Essentially all the complex organic molecules on the Earth are biogenically derived, and it is clear that no inorganic process can compete with biology in its efficiency of generating such organic matter. In our models biotic material accounting for some 25% of interstellar carbon is generated within the warm liquid interiors of cometary objects that serve as sites of amplification of cosmic biology (Hoyle and Wickramasinghe, 1993). Further support for these ideas followed from 2.9-3.9 micron infrared spectroscopy of Comet P/Halley (D.T. Wickramasinghe and Allen, 1986) as well as from the in situ mass spectroscopy conducted with instruments aboard the spacecraft Giotto and Vega (Kissel and Kruger, 1987).
The Stardust Mission to Comet P/Wildt2 launched in February 1999 was primarily designed to collect dust from the comet in the year 2004 returning it to Earth in 2006. The collection procedures and techniques aboard this spacecraft were not intended to ensure the viability of any microorganisms that might have been present. At the time of planning this mission such a concept as cometary life would have been thought too wildly heretical to merit serious consideration. The climate of opinion has, however, changed to some degree in the intervening years, particularly following the analysis of the Martian meteorite ALH84001 (McKay et al, 1996). In addition to the first ever comet sample return, Stardust was also designed to conduct in situ studies of interstellar dust from out of the ecliptic plane. The results of the first such mass spectroscopic studies of interstellar grains were reported on April 26, 2000 (Kissel, 2000). The provisional conclusion is that the interstellar grains consist mostly of "3-dimensionally cross-linked organic macromolecules, called polymeric-heterocyclic aromates", a result that validates a model we have discussed from as far back as 1977 and invalidates some other models of interstellar dust. The new result from Stardust is described by the investigators as "puzzling", particularly because:
(It has not been mentioned that the new data is consistent with biologically generated aromatics.)
The Stardust experiment leading to these startling conclusions involves the collection of interstellar dust on to detectors at relative speeds of some 30km/s, corresponding to energies of ~ 5 eV per H atom. This is enough to break weak CH linkages in aliphatic side chains, but would preserve the integrity of cross-linked aromatic structures. An original bacterial particle subject to such high-speed impact would be largely destroyed except for components of cell-wall structure with cross-linked polysaccharides and proteins. Such components which could well have molecular masses in excess of several thousand AMU would accord remarkably well with the attributes now claimed for interstellar dust from the Stardust experiment.
Finally we wish to point out that in plausible astronomical scenarios where carbonaceous particles can form, e.g mass flows from carbon stars and supernovae, the condensation of heteroaromatic structures that incorporate O and/or N atoms to the extent of some 10% would appear to be exceedingly unlikely on energetic grounds. Far more likely condensates are graphitic type PAH's (Leger and Puget, 1984) which are evidently ruled out by the data.
This work was supported in part by a grant from Acorn Enterprises LLC, Memphis, TN.