Evidence for Biodegradation Products in the Interstellar Medium
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International Journal of Astrobiology 9 (1): 29–34 (2010) 29 doi:10.1017/S1473550409990334 f Cambridge University Press 2009 Evidence for biodegradation products in the interstellar medium Kani Rauf and Chandra Wickramasinghe Cardiff Centre for Astrobiology, Cardiff University, 2 North Road, Cardiff CF10 3DY, UK e-mail: [email protected] Abstract: The interstellar absorption band centred on 2175 A˚ that is conventionally attributed to monodisperse graphite spheres of radii 0.02 mm is more plausibly explained as arising from biologically derived aromatic molecules. On the basis of panspermia models, interstellar dust includes a substantial fraction of biomaterial in various stages of degradation. We have modeled such an ensemble of degraded biomaterial with laboratory spectroscopy of algae, grass pigments, bituminous coal and anthracite. The average ulrtraviolet absorption profile for these materials is centred at 2175 A˚ with a full width at half maximum of 250 A˚ , in precise agreement with the interstellar extinction observations. Mid-infrared spectra also display general concordance with the unidentified interstellar absorption features found in a wide variety of astronomoical sources. Received 27 September 2009, accepted 18 October 2009, first published online 20 November 2009 Key words: degradation products of biomateria, interstellar dust, panspermia, polycyclic aromatic hydrocarbon, interstellar molecules. Introduction The attribution of the 2175 A˚ extinction bump by Hoyle and Wickramasinghe to small graphite spheres is now widely Some four decades ago, ultraviolet (UV) stellar spectroscopy accepted, with such a component included in most models was born with the deployment of telescopes and instruments of interstellar grains (Hoyle & Wickramasinghe 1962, 1969; flown above the atmosphere. When stellar spectra were first Mathis 1990; Draine 2003). There are, however, serious pro- examined at wavelengths shortward of 3000 A˚ , an interstellar blems associated with the graphite model (Wickramasinghe absorption band centred at 2175 A˚ with a half-width at full et al. 1992). The requirement of graphite, which has a planar maximum of 250 A˚ was discovered in the spectra of most y lattice structure, to be in the form of small spheres in inter- reddened stars (Stecher 1965; Wickramasinghe 1967). The stellar space appears scarcely plausible, as indeed does the average interstellar extinction curve showing this feature is requirement that the size dispersion around a central size shown in Fig. 1, where the observed interstellar extinction 0.02 mm be vanishingly small. Alternative molecular expla- curve is decomposed theoretically into three components. nations of the 2175 A˚ absorption, first proposed by Hoyle and Component 1 represents scattering by dielectric grains with Wickramasinghe in 1977, involve aromatic organic molecules sizes and properties resembling bacteria, Component 2 is the (Hoyle & Wickramasinghe 1977). In this paper we show that extinction due to an absorber producing the mid-UV band, such molecules are most reasonably interpreted as being and component 3 represents a population of nanometric- degradation products of biology. sized dielectric particles (Hoyle & Wickramasinghe 1999; Wickramasinghe et al. 2009). Microbial dust and polycyclic aromatic Whilst the mean extinction curve for dust in the solar hydrocarbons vicinity is well represented by the uppermost curve in Fig. 1, stars in other regions of the Galaxy and in external galaxies Some recent models of interstellar dust have included an show a modest degree of variability. Such variability is con- abiotic polycyclic aromatic hydrocarbon (PAH) component sistent with differing proportions of the components 1, 2 and that, under appropriate circumstances, could produce a l= 3. Fig. 2, from Boulanger et al. (1994), shows how individual 2175 A˚ absorption band (Witt et al. 2006). However, for stars show varying relative amounts of the 2175 A˚ carrier. PAHs and other complex organic molecules to form abioti- Boulanger et al. (1994) also discuss the correlation between cally poses serious problems, in particular with the require- the strength of the UV absorption peak and the mid-infrared ment that compact PAHs are in high ionization states. An (IR) emission bands that could be interpreted as arising alternative model that fits the general context of panspermia from a common molecular carrier (Hoyle & Wickramasinghe is that organic dust and molecules in interstellar space 1989). include a substantial component derived from biology itself. 30 K. Rauf and C. Wickramasinghe 8 7 Observations 6 5 4 3 1 3 Extinction (mag/kpc) 2 2 1 012345678910 Wavenumber λ–1 (µm–1) Fig. 1. Interstellar extinction observations from a compilation from Sapar & Kuusik (1978) interpreted as a composite of (1) dielectric grains of radii 0.3 mm, (2) molecular absorption due to aromatics and (3) dielectric particles of radii 0.01 mm. 4 HD 98143 2 HD 94414 HD 110336 0 HD 96675 –2 HD 108927 )/A(V) λ A( –4 HD 103536 –6 HD 103875 –8 HD 102065 –10 02468 λ–1(µm–1) Fig. 2. Normalized interstellar extinction curves for stars in our Galaxy showing varying strengths of the 2175 A˚ absorption feature adapted from Boulanger et al. (1994). Crosses and the dashed curve represent mean interstellar extinction. Evidence for biodegradation products in the interstellar medium 31 1.6 1.4 216 1.2 1 0.8 Abs 0.6 427 0.4 0.2 662 0 190 290 390 490 590 690 790 nm Fig. 3. UV-visible spectrum of algae dissolved in cyclohexane. 1.8 1.6 216 1.4 1.2 1 Abs 0.8 0.6 0.4 0.2 0 190 290 390 490 590 690 790 nm Fig. 4. UV-visible spectrum of grasses dissolved in cyclohexane. 2.5 2 213 1.5 Abs 1 0.5 274 0 190 290 390 490 590 690 790 nm Fig. 5. UV-visible spectrum of anthracite dissolved in cyclohexane. 32 K. Rauf and C. Wickramasinghe 2.5 2 219 1.5 Abs 1 0.5 0 190 290 390 490 590 690 790 nm Fig. 6. UV-visible spectrum of semi-anthracite dissolved in cyclohexane. Average spectrum 1.8 217.5 Cyclohexane 1.6 1.4 1.2 1 Abs 0.8 0.6 0.4 0.2 0 190 290 390 490 590 690 790 nm Fig. 7. Average spectrum of a putative interstellar microbiota in various stages of degradation. According to this point of view, microbial cells replicate in degradation we took algal cells and grasses as a starting liquid interiors of primordial comets and are distributed point, and measured the absorption properties of the follow- widely across the galaxy (Wickramasinghe et al. 2009). Inter- ing: stellar material would then include the detritus of biology – (1) algae and grasses; microbial cells in various stages of degradation and break-up. (2) semi-anthracite; The degradation of cells in HII regions and in other high (3) anthracite. radiation environments is of course inevitable. Whilst Samples (2) and (3) represent the last stages in a putative wholesale destruction, if it occurs, would vitiate a panspermia biodegradation sequence. model, the requirement for panspermia to operate unhin- The sources of the material were as follows: dered is that a fraction, less than one part in 1020, survives . Oedogonium sp (a fresh water species of green algae) col- from one amplification site to the next; this condition would lected from Roath Park, Cardiff; seem impossible to violate. grass from the Santa Cruz island of the Galapagos pro- visionally identified as Panicum maximum; . both bituminous coal (semi-anthracite) and anthracite Experiments and materials used were obtained from Geo Supplies Ltd., UK. Our model thus leads to a component of interstellar dust We were primarily concerned with the precise placement grains that start off as intact viable cells and end up as of a 2175 A˚ absorption feature in all of these systems, which coal-like grains. In order to model the effects of sequential were dispersed and dissolved in a non-polar solvent. We also Evidence for biodegradation products in the interstellar medium 33 30 NGC4826 10 NGC5033 25 8 20 15 6 10 4 5 2 0 0 12 NGC5055 15 NGC5194 10 8 10 6 4 5 2 0 0 20 20 NGC5195 NGC5713 15 15 10 10 /Sr) /Sr) 2 2 5 5 0 0 W/m W/m 5 50 –6 –6 NGC5866 NGC6946 4 40 (10 (10 ν ν l l 3 30 ν ν 2 20 1 10 0 0 10 8 NGC7331 60 NGC7552 6 40 4 20 2 0 0 2.5 NGC7793 5 10152030 2.0 1.5 1.0 0.5 0.0 5 10152030 Rest Wavelength (µm) Fig. 8. Recent compilation of Spitzer telescope emission spectra of a variety of galactic sources showing PAH emissions (Smith et al. 2007). examined mid-IR spectra of the same systems dispersed Table 1. Distribution of two astronomical observations in KBr discs, with a view to comparing them with the (UIBs and proto-planetary nebulae (PPNe)) and major IR Unidentified IR Bands (UIBs) in interstellar material. absorption bands in laboratory models of terrestrial origin. Methods of sample mounting and preparation Bituminous Anthracite UIBs PPNe Algae Grasses coal coal For UV and visual spectroscopy, the samples were dis- solved in cyclohexane – a non-polar solvent. Algae, grasses, 3.3 3.3 3.3 – 3.3 3.3 – 3.4 3.4 3.4 3.4 3.4 semi-anthracite and anthracite (approximately 70 mg in each 6.2 6.2 6.0 6.1 6.2 6.2 case) were individually added to cyclohexane (500 ml) and – 6.9 6.9 6.9 6.9 6.9 dissolved. The use of a non-polar solvent eliminates the effect – 7.2 7.2 7.2 7.2 7.2 of wavelength shifts. All samples were agitated for a few 7.7 7.7 – 7.6 – 7.7 minutes and allowed to stand still on the bench for 15 minutes – 8.0 8.0 8.0 – – 8.6 8.6 8.6 – – – to allow the sedimentation to complete.