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Adv. Space Res. Vol. 27, No. 2, pp. 323-328, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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CONCEPTION OF THE 'CHIRALITY-EXPERIMENT' ON ESA'S MISSION ROSETTA TO COMET 46P/WIRTANEN

W. H.-P. Thiemann 1, H. Rosenbauer 2, and U. J. Meierlhenrich 1

lDept. Phys. Chemistry, University of Bremen, Leobenerstrafle, D-28359 Bremen, Germany 2Max-Planck-Institut fiir Aeronomie, Max-Planck-Strafle 2, D-37189 Kat.lenburg-Lindau, Germany

ABSTRACT

In the last years extraterrestrial scenarios for the origin of homochirality in biological structures received considerable attention in the topical literature: Rubenstein and Bonnet postulated a rapidly rotating neutron star emitting circularly polarised synchrotron radiation responsible for the first asymmetric synthesis; the group of Bailey published the observation of circular polarisation caused by Mie scattering from aligned dust grains in the Orion OMC-1 star-formation region that might provide an enantioselective effect on prochiral or racemic organic molecules. Rikken and Raupach observed a magnetochiral effect and considered extraterrestrial magnetic fields of sufficient strengths to introduce biomoleculars parity violation. With the aira to investigate these hypotheses among other theories describing the origin of biological asymmetry, our laboratory participates in the conception and development of ROSETTA's COSAC Experiment, that is designed to identify organic molecules in the cometary matter in situ. Within COSAC's 'Chirality Module' enanfiomers will be separated gas chromatographically with the help of capillary columns coated with chirally active liquid films. This technique will allow the separation of specific chiral organic compounds out of the analysed cometary matter into their enantiomeric constituents. Both thermo conductivity and mass spectrometric detectors will be used to determine each enantiomer's amount and therefore the corresponding enantiomeric excesses. As a consequence of COSAC's 'Chirality-Experiment' far-reaching results are expected to investigate the various hypotheses about the first asymmetric synthesis. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

I N T R O D U C T I O N "Homochirality is the signature of life", so goes a famous saying describing any thinkable form of biosphere,

be it of terrestrial origin, be it of an hitherto unknown extraterrestrial origin. So let us give a brief definition of the term homochirality in the context of this 33rd COSPAR Scientific Assembly, which a number of scientists of the various disciplines attend, united in their passionate desire to discover some signs of life outside this planed Earth. Homochirality is a property of matter, which is made up of exclusively only one "hand' (that is what is literally meant by "cheir" in the ancient Greek language) out of the potentially two "hands" available within confined boundaries of abundance. The boundaries thus defined by this homochirality are the borderline/s between living here and non-living matter there. In other words, life consists of only one handed entities (looking more closely into the problem it seems this is not true in an absolute sense though! There are certain exceptions to the rule!), while synthetic chemicals prepared in laboratories exhibit both, namely right as well as left handed entities in equal abundancy in general. This fact is so well established that it is generally accepted as an axiom, in a similar fashion as the second law of thermodynamics postulating an increase of entropy in spontaneous chemical reactions. Because of its universality this law is generalised to apply equally well to all potential biospheres beyond Earth waiting for their discovery in the nearer or more distant future.

In spite of this well known fact it is astonishing that so far there is no one convincing theory yet explaining the cause for the driving force behind leading to this result during the long process of evolution of such a biosphere as we observed it today on Earth,- astonishing the more while searching through the literature, where a large number of papers of more theoretical or experimental nature are devoted to this very essential problem.

323

324 w.H.-P. Thiemann et al.

This is not the place to spread out the different attempts in great detail to uncover the secrecy behind the phenomenon, why we see the living nature in this one-handed form only today, because several reviews are gfven e.g. in Thiemann (1998) and Meierhenrich et al. (1999). We shall rather take the opportunity to propose some experimental pathways to make use of a so-to-speak "chiral probe" as a safe means to search for living matter outside Earth on space missions toward neighbour planets, comets, and other objects inside and outside our own solar system.

METHODS TO PROBE FOR CHIRALITY IN SPACE MISSIONS If the reader were willing to accept the above given assumption that an overwhelming homochirality were a

true sign of living matter anywhere in the universe, then it follows immediately that this unique property of living ("biological") matter should be exploited as a very universal marker at any space mission which is aimed to discover biological matter, - or only "precursor" for such a biology or extinguished biology! - outside Earth! Why that has not been the case so far in the past missions to Mars and elsewhere is not completely clear to us. One of us had proposed this idea already as early as in 1974 within the frame of an international COSPAR Meeting in Sao Paulo! (Thiemann, 1975) Maybe it was the fear of the "unreliability" of the instruments available and discussed so far within the space scientists' community for probing for homochirality, which made the proposers finally retreat from those projects until now, not to speak of the difficulties in interpreting the data coming in in the case, when instruments for probing for chirality had been possibly flown earlier to extraterrestrial objects. But the general opinion in the community is gradually changing nowadays: More and more scientists have accepted the idea today to carry such instrumentation along future space missions, that allow the search for a homochiral chemistry elsewhere in principally 2 ways.

Chiroptical techniques and the SETH project One of the basic options to be performed during any space missions to be carried out consists of an optical

device capable to determine any optical activity, optical rotatory dispersion, resp. circular dichroism of a condensed sample grabbed from a planet's, moon's, or comet's surface to search for the above described homochiral property of the chemicals, of which the condensed matter is made of. Specific "space-suitable" polarimeters have been proposed already by MacDermott et al. (1996 and 1997), are of a most simple, light- weight, robust, and yet sensitive and reliable design resisting disturbing mechanical, pressure, and temperature shocks usually experienced during longer space journeys and automated landing conditions in unmanned missions. The inclusion of such apparatus is foreseen in some Russian Martian Excursions now. We propose to include polarimetric measurements of the SETH or any similar type on all future missions to all solid targets, such as Venus, the giant planets' moons Titan, Europa, and Ganymed, and the accessible comets to be targeted at in the future. Had such a polarimeter instrument been included on the famous Viking Experiment to explore life on Mars already back in 1976, some of ~ahe ambiguous life detecting experiment's results might have given more straight forward answers as to the intriguing question "... does life (or extant life) exist on Mars or not?", if the knowledge about chiral compounds on Martian surface were provided at this time. Under the extremely dry, cold, low pressure conditions of Mars the chiral properties of fossilised (micro-)organisms - if ever existed - would have easily survived over long geological time periods, as laboratory experiments by Bada (1995) on the racemization (i.e. deterioration of homochiral property) of chiral amino acids under Martian conditions suggest. Probing any solid bodies' surfaces in the Universe for optical activity, should result in thscinating results concerning the intriguing search for the existence of life beyond Earth.

Enantioselective chromatography and ROSETTA's COSAC experiment The quest for homochirality outside Earth should be taken up of course by completely different methods

independent on the above discussed purely physical method, because this method suffers somewhat from a number of severe limitations. There is obviously a strong need for an unequivocal chemical identification of chiral compounds within material recovered from extraterrestrial bodies. The most powerful tool for qualitative and quantitative identification of chiral chemicals of an unknown nature is the chromatographic method. Altogether the screening of aliquots of soil, etc. by chromatographic methods requires larger investments in equipment and performance to be carried on space missions, but benefits from a number of great advantages over the physical spectroscopic method alone: Chromatography - both gas-liquid as well as liquid-liquid - is able to identify very small amounts of chemicals, in the order of a few nanograms and below; it allows the reliable identification of unknown compounds, - especially if it is combined with a mass spectroscopic detection unit. We have so far

Chirality Experiment on Rosetta Mission 325

collaborated already with the Max-Planck-Institute for Aeronomie at Katlenburg-Lindau, Germany, in optimising suitable gas chromatographic specifications .for the search of chiral volatile or pyrolysable organic compounds on comet Wirtanen's surface in situ, called COSAC project (Cometary Sampling and Composition Experiment) within the ROSETTA/RoLand mission prepared by an international ESA team described by Rosenbauer et al. (1999 and 2000). The goal of this experiment is to test an automatic device which would allow to sample solid aliquots from a solid surface, dissolve and/or pyrolyse it in small temperature steps, let it pass over a suitable GC column coated with a chiral phase and have it detected and quantified according to its chemical constituents and stereospecificity by both a heat conductivity detector and a mass spectrometer. The analytical procedure for this enantioselective chromatography of extraterrestrial molecules will be presented at this 33rd COSPAR Scientific Assembly in the presentation of Meierhenrich et al. (2001). Underivatised racemic pairs of non-complex alkanes, alcohols, diols, and amines were successfully resolved into their optical antipodes by this analytical approach in our laboratory as depicted in Figure 1 for a racemic mixture of 2-hexanol and 1,2-pentanediol. Little volatile carboxylic and amino acids were transformed online into their methyl esters by a thermally assisted chemolysis step within the injector port of the gas chromatograph. Based on these experiments enantiomers of a wide range of specific organic

compounds were separated and identified. The feasibility of this analytical method for the identification of extraterrestrial molecules will allow us to determine enantiomeric ratios, and consequently produce information about potential extinct extraterrestrial life and the origin of life linked closely with its homochirality on Earth.

s R

2-Hexanol

1,2-Pentanediol

L__

Start :1 2 ;3 4 5 '" ~} ........... -T 8 t/rain

Fig.1. Gas chromatographic enantiomer separation of 2-hexanol and 1,2-pentanediol.

326 W.H.-P. Thiemann et al.

DISCUSSION: WHICH INFORMATION IS INCLUDED IN THE DETERMINATION OF AN ENANTIOMERIC EXCESS DETECTED IN EXTRATERRESTRIAL MATTER?

Although - at this stage investigating cometary surfaces - we do not anticipate the existence of large enantiomer excesses of organic molecules, not to speak of homochiral samples on comets with their harsh and life excluding climatic conditions (low temperatures, near-zero atmosphere, probably no liquid water ...), yet we believe in a strong chance to find partial enantiomeric excesses of small molecules due to the treatment of the comet's surface with extraterrestrial chiral fields from the Universe during its long journey with no atmosphere around protecting from this radiation. In this context it is reasonable to assume that optically active organic compounds were transferred to Earth on its fly-by path, and would serve as very suitable precursors for the build- up of the homochiral biosphere of today. The determination of six non-racemic ct-methyl-t~-amino alkanoic acids in the Murehison and Murray carbonaceous chondrite by Pizzarello and Cronin (2000) support the models based on 'transmitter-properties' of comets and meteorites.

The Rubenstein-Bonner neutron star hypothesis Circularly polarized radiation which is required for an introduction of an enantiomeric excess into racemic

mixtures of organic molecules or into prochiral educts within photoreactions was suggested to be emitted from a rotating neutron star according to speculations of Rubenstein and Bonner (1983). This model has been further developed by Bonnet and Rubenstein (1987), Bonner (1991, 1992 and 1995), and Bouner et al. (1999a) and was resulting in a topical discussion: Mason (1997) argues against this model, considering the 'Kuhn-Condon zero-sum rule' (Kuhn 1930, Condon, 1937), that the integral in the circular dichroism (CD) over the whole wavelength range is zero. In case that the neutron star would emanate the required radiation over the entire spectrum, hence no enantiomeric enhancement could be produced by photochemical interactions with organic molecules. Very recently Bonner et al. (1999b) replied that only the CD band within the characteristic absorption spectra of prebiotie molecules is relevant for the incriminated enantioselective photochemistry by synchrotron radiation. However, it seems to be difficult to observe and detect circularly polarized synchrotron radiation in supernova remnants of pulsars (Bailey et al., 1998 and Bailey 2000).

Mie scattering from aligned dust grains Alternatively to a rapidly rotating neutron star circular polarisation of ultraviolet (UV) photons caused by Mie

scattering on aligned interstellar dust particles in star-formation regions was suggested as a plausible original source for biomoleeular's handedness. At infrared (1R) wavelengths in Orion OMC-I circularly polarized radiation was indeed recently observed as described in Bailey et al. (1998). Bailey (2000) reports that the OCM-1 region is known to have grains aligned by a magnetic field. Observations of circularly polarized light are only possible in the IR, because of the high dust obscuration in Orion's massive star formation regions. However calculations of the scattering of light from aligned grains show that substantial circular polarisation could still be produced at UV wavelengths. So it is plausible to assume that there are locations in these massive star forming complexes where light from a UV emitting star could scatter off the aligned grains. The circular polarisation in OCM-1 shows regions of both signs but the polarisation structure is on a larger scale than the expected size of a protostellar disk. With respect to ROSETTA's COSAC experiment this means, that Bailey's model would generate the same handedness of chiral molecules within our Solar System.

Interstellar magnetic fields A highly fascinating idea is to investigate the potential of magnetic interaction in chemical reactions with the

aim in mind to produce enantiomer excesses in the products to be expected: From whatever point one is looking at it, the research in this area covers a vast field and deserves attention. Hypotheses and rather wild speculations on such a stereospecific interaction have been always around in the literature, but only since the advent of a few fundamental papers of Barron of the University of Glasgow (1994a and b), a solid foundation and hence a firm justification for such endeavours have been laid down.

A few papers have been published based on hypotheses of this kind, making use of an interaction of magnetic fields in chiral reactions: Among them some did not yield the wanted results (cf~ Teutsch and Thiemann, 1986 and 1990), some which were greeted with enthusiasm first due to their rather dramatic positive results, were soon after their appearance in the literature withdrawn from the panel because Zadel and Breitmaier (1994) had to admit that they had manipulated the results intentiously.

Chirality Experiment on Rosetta Mission 327

Very recently Rikken and Raupach (2000a and b) reported the sound experimental observation of a magneto- optical effect, that again provides the link between magnetism and chirality. Within their experiments the handedness of the enantiomeric excess in organic molecules was determined by the relative orientation of the light beam within the magnetic field. This process consists of a polarisation independent refractive index difference between left- and right-handed systems, which is proportional to Bxk. In a photochemical reaction with unpolarised light in a magnetic field, this magneto-chiral anisotropy was shown to give rise to a small enantiomeric excess linear in B×k in the order of 1.0 × 10 -5. With regard to these authors (1998) and Barron (2000a and b) in principle two mechanisms might introduce an enantiomeric excess into racemic organic compounds or into prochiral educts: 1) The magnetic field may 'convert' the unpolarised light firstly into circularly polarized light as a result of the Faraday effect and the obtained circularly polarized light interacting with the molecule produces circular dichroism, which is well-known to result in an excess of one of the enantiomers. The whole procedure is called a "cascaded mechanism", which is however calculated to be too small to end in the observed enantiomeric excesses, because a magnetic field of 1 T may introduce an enantiomeric excess of only e.e. = 5 x 10 7. 2) Due to this a so-called 'pure' mechanism for the observed magnetochiral anisotropy was taken into consideration that might produce much larger enantiomeric excesses, because it involves less electric dipole interactions. This means that the observed magneto-chiral dichroism is a phenomenon distinct from Faraday rotation.

CONCLUSIONS In investigating the described as well as other hypotheses we expect fascinating and far-reaching results from

the 'Chirality-Experiment' of ESA's cornerstone mission ROSETTA. A determined enantiomeric excess of a specific cometary organic molecule would support the idea that the biomolecular parity violation on Earth is originated from extraterrestrial sources: The enantiomeric excess were to be compared with the molecules' circular dichroism reference spectrum in order to determine wavelength and polarisation of the required extraterrestrial circularly polarized radiation. A number of non-racemic organic molecules would provide detailed information about the chiral field that had been necessary to introduce the molecular parity violation; therefore the results would indicate the validity of the various hypotheses mentioned above.

A C K N O W L E D G E M E N T S We were grateful for scientific support of the whole COSAC team and the participation of Alexandra J.

MacDermott in the conception phase of the COSAC experiment. In addition to this we thank for intensive discussions with Laurence D. Barron and Geert Rikken concerning the theoretical complexity of magneto-chiral effects, and Jeremy Bailey with regard to extraterrestrial sources of circularly polarized light.

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