Why Is Luann Becker Not On The Mars Organic Molecular Analysis

Hydrocarbons

L length, ID inner diameter, LT layer thickness

L length, ID inner diameter, LT layer thickness acids were investigated with a derivatization step prior to gas chromatographic analysis.

9.1.3.1 Resolution of Chiral Amino Acids

The identification and resolution of chiral amino acids are probably the most fascinating targets for ROSETTA's chirality-module. However, we have to consider that D,L-amino acids are too polar to get direct access to their analysis by gas chromatography. D,L-Amino acids are affected with insufficient gas chromatographic properties. The volatility of these zwitterionic compounds is too low.

Compared to amino acids, esters are non-polar, much more volatile, and do gas chromatograph well. Esterification is therefore a good choice for a derivatization of amino acids, particularly for the intended gas chromatography on the comet's nucleus. As requested by ESA, solvent chemistry had to be avoided, because the transformation of polar organic compounds into derivatives suited for chromato-graphic resolution has to be performed right on the surface of a cometary nucleus under near zero-gravity. Conventional methods of esterification include reaction with diazomethane or higher diazoalkanes or esterification with methanol-acid mixtures. Diazomethane is unstable, toxic, and explosive, and the transesterification step requires solution chemistry. These properties excluded conventional methods from the ROSETTA-COSAC project.

There was thus a need for a reliable working "dry" method for the preparation of amino acid esters on COSAC's gas chromatograph. Dimethylformamide dimethyl-acetal DMF-DMA was selected being a suitable gas phase derivatization reagent that is also assisting the pyrolysis procedure. Sufficient DMF-DMA was inserted into special alloy capsules that melt at 100° C. These capsules were inserted into half of the COSAC ovens in order to release DMF-DMA when the required gas phase temperature for the derivatization reaction is reached. Amino acids will be transformed into their dimethylamino methyl esters as it is given in Fig. 9.7 for the example of D,L-valine. The elegance of this technique is, that the whole derivatiza-tion procedure will be automatically achieved within the oven/injector system of the COSAC gas chromatograph and sweep the volatile derivatives directly into the capillary column. This fast derivatization technique can be assumed to provide advantages also for Earth-based analytical laboratories, where it may find an increasing number of useful applications.

9.1.3.2 Resolution of Chiral Carboxylic Acids and Macromolecules

Similar to D,L-amino acids, D,L-hydroxycarboxylic acids exhibit a low vapour pressure, because of their high dipole moment and polar character. In a GC injector, they are slow-evaporating and sweep rarely into the analytical column. Apart from this, underivatized D,L-hydroxycarboxylic acids tend to 'tail' because of associative effects with the stationary phase. Compared with the parent D,L-hydroxycarboxylic acids, their alkyl esters are less polar, much more volatile, and well resolved by gas

Dmf Dma Reactions
Fig. 9.7 Chemical equation depicting the pyrolysis assisting reagent dimethylformamide dimethy-lacetal DMF-DMA permethylating non-volatile amino acids like d,l-valine. Permethylated amino acids are volatile and thus "visible" for the COSAC GC-MS instrument

chromatography. Esterification was therefore the first choice for derivatization also in the case of D,L-hydroxycarboxylic acids.

Just like for amino acids, DMF-DMA will be used as a methylating reagent in the heated insert zone directly before their gas chromatographic enantiosepa-ration. Results show that the D,L-lactic, D,L-malic, D,L-mandelic, and D,L-tartaric acids or mixtures thereof were transformed into their methyl esters very fast. These D,L-methylesters could easily be resolved into their enantiomers by gas chromato-graphy employing chiral selectors embedded in stationary phases (Meierhenrich etal. 2001c).

Besides the derivatization of highly polar analytes like D,L-amino acids and D,L-hydroxycarboxylic acids, we have to consider that the organic compounds expected to be abundant on comets might not occur as monomer units. Matthews (1992) expected that macromolecules could be major components of cometary matter, Krueger and Kissel (1989), Kobayashi et al. (1998), and Takano et al. (2007) similarly suggested that complex organic oligomers would make up the main cometary ingredients. We assume that these macromolecules and oligomers would break up into their subunits during pyrolysis with DMF-DMA, because McKinney et al. (1995) pointed out that similar working reagents do not only methylate polar pyrolysis products, but could assist in bond cleavage reactions, too. In lack of samples of cometary matter, the described derivatization experiments were successfully tested with external standards.

9.1.3.3 Resolution of Chiral Hydrocarbons

It is important to take into consideration the chemical and thermal stability of chiral molecular candidates to be resolved with the ROSETTA mission. Amino acids, carboxylic acids, and carbohydrates disappear comparatively rapidly in geological timescales. They do not survive as stable compounds for very long geological periods of time, in any appreciable amounts on Earth. The relative chemical and thermal stability of carbohydrates, amino acids, carotenoids, porphyrins, and hydrocarbons increases in that order. For hydrocarbons, we have experimental data (AHC = 66.5 kJ/mol and the pre-exponential factor of the Arrhenius equation A = 1014) for the breaking of a carbon-carbon bond. The lifetime required to decrease the amount to 1/e fraction of its original value is 10145 years at 400 K and 1027 years at 300 K. For the destruction by breaking off a pair of hydrogen atoms, using AHh = 63.0 kJ/mol and A = 1013 we can determine the lifetime as 10155 years at 400 K and 1025 years at 300 K (Calvin 1969).

Branched saturated hydrocarbon structures can be chiral. The above calculations are also valid for the stability of chiral hydrocarbons against racemization reactions, which are expected to occur mainly via hydrogen subtractions2. Hydrocarbons can therefore be considered to be the most stable group of cometary compounds and may be expected to retain a significant part of their original molecular structure. The detection of a number of aliphatic hydrocarbons in samples of meteorites such as CI1 and CM2 chondrites has been demonstrated (Cronin and Chang 1993) and their abundance in the interstellar medium is established. It was thus plausible to assume that hydrocarbons occur as organic ingredients in cometary matter.

The preferred method for separating racemic pairs of non-derivatized alcohols, diols, and amines has been a classical gas chromatography on cyclodextrin phases. However, enantiomers of saturated chiral hydrocarbons like 3-methylhexane and its higher homologues could not be resolved in this classical way, because they lack the functional groups necessary to undergo "intensive" diastereomeric guest interactions with the cyclodextrin host molecules.

The first quantitative resolution of a series of branched hydrocarbon enantiomers was achieved recently in our group at the Department of Physical Chemistry at the University of Bremen. Racemates of the aliphatic hydrocarbons R, S-3-methylheptane, R, S-3-methyloctane, R, S-4-methyloctane, R, S-3-methylnonane, and R, S-4-methylnonane were resolved into their enantiomers by enantioselective gas chromatography with new chiral stationary phases (Meierhenrich et al. 2003a). The enantiomers of R, S-3-methylhexane had been separated previously (Konig et al. 1990). Enantiomer resolution has been increased by the use of (a) a special cryostat controlled GC instrumentation3 delivering liquid nitrogen directly into the GC-oven for low-temperature gas chromatography and (b) the Chirasil-Dex CB phase, in

2 Further investigations are required to understand pathways of racemization reactions of chiral hydrocarbons in detail. At present, we have reason to assume that racemizations of chiral hydrocarbons are negligible for the timescale of Gyr on Earth and particularly on comets.

3 For the low gas chromatographic temperatures we often use an Agilent cryostat, which delivers liquid nitrogen directly into the GC oven via a monitored electromagnetic valve. Under such conditions, the stationary phase in the GC capillary column stays liquid down to minus 60°C and allows chromatographic resolution particularly for highly volatile compounds. Consider that the polymer of the stationary phase in gas chromatography with wall-coated open tubular (WCOT) columns is liquid (and not solid!) interacting with analytes via partition processes and not by adsorption of the gas phase analytes on the inner surface of capillary columns.

which the cyclodextrin molecules are chemically bonded to the polysiloxane matrix. This stationary phase was used - as for ROSETTA's COSAC instrumentation - in a length of 10 m only, allowing to focus on highly volatile hydrocarbons by applying very low temperatures. With a combination of these techniques, sufficient separation factors a and resolutions RS were obtained for the enantiomer separation of chiral hydrocarbon molecules like trialkylalkanes resulting in a base-line separation of the enantiomers.

A gas chromatogram of a selected mixture of six chiral aliphatic hydrocarbon pairs is depicted in Fig. 9.8. Racemic mixtures were injected. Peaks of equal areas were obtained for each pair of enantiomers. At present, the absolute configuration of the separated enantiomers is unknown. The enantioselective synthesis of branched hydrocarbon structures started recently in our laboratory. Furthermore, we currently try to resolve enantiomers of the deuterated R, S-[2H1, 2H2,2H3]-neopentane hydrocarbon presented in Chap. 2 by enantioselective cryogenic gas chromatography.

The mechanism of formation of aliphatic hydrocarbons in the interstellar medium and in meteorites is still subject of a controversial debate. A Fischer-Tropsch type (FTT) process was suggested as well as interstellar photoreactions via alkyl radicals, and the decomposition of hydrogen cyanide polymer structures (Minard et al. 1998).

So what about the chirality of hydrocarbon enantiomers? Today, nothing is known on the chirality of interstellar or interplanetary hydrocarbon enantiomers. In this context, however, it is useful to refer the reader to the occurrence of chiral hydrocarbons on Earth, where their origin in geochemical samples gave rise to a remarkable scientific debate: Pristane and phytane are known as the archetypes of

Fig. 9.8 Gas chromatogram of resolved hydrocarbon enantiomers separated on a capillary column coated with permethylated P-cyclodextrin (Chirasil-Dex CB, 25 m, 0.25 mm inner diameter, layer thickness 0.25 |m, Varian-Chrompack). Split injection 1:50 at injector temperature of 230°C, 1.3 mL/min constant flow of He carrier gas, oven temperature programmed 3 min at 35°C, 1°C/min, to 15 min at 70°C. The transfer line temperature was constant and set to 200°C. After solvent delay of 5 min detection of total ion current in mass range 50-350 amu with GC-MSD system Agilent 6890/5973

Fig. 9.8 Gas chromatogram of resolved hydrocarbon enantiomers separated on a capillary column coated with permethylated P-cyclodextrin (Chirasil-Dex CB, 25 m, 0.25 mm inner diameter, layer thickness 0.25 |m, Varian-Chrompack). Split injection 1:50 at injector temperature of 230°C, 1.3 mL/min constant flow of He carrier gas, oven temperature programmed 3 min at 35°C, 1°C/min, to 15 min at 70°C. The transfer line temperature was constant and set to 200°C. After solvent delay of 5 min detection of total ion current in mass range 50-350 amu with GC-MSD system Agilent 6890/5973

chiral hydrocarbons. They show branched aliphatic structures and belong to a group called isoprenoids. In these molecules, most carbon atoms at branching positions are stereogenic centers provoking that pristane, phytane, as well as other isoprenoids are chiral. Isoprenoids are usually quite abundant in geological matter. Pristane for example occurs in ancient sediments, rocks, shales, crude oils, coals, and bitumens (Nip 1987).

Despite the abundance of isoprenoid compounds in geochemical samples, the pathway to the formation of the enantiopure pristane and phytane molecules is not completely understood. Biological and non-biological formation processes have been discussed. Pristane-formation precursors like kerogens (Larter et al. 1979; van de Meent et al. 1980; van Graas et al. 1981), tocopherols (Goossens et al. 1984, 1988; Sinninghe Damste and de Leeuw 1995), and methylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans (MTTCs) (Li et al. 1995) were suggested (Tang and Stauffer 1995). One particular discussion refers to the question whether enan-tiopure isoprenoids in geochemical samples are primarily degradation products from plants, formed e.g. by chlorophyll decomposition. Indeed, chlorophyll's side chain is a branched hydrocarbon structure including two stereogenic centers in ^-configuration. Alternatively, an animal origin was proposed due to the here relevant tocopherol (vitamin E) side chain that is equally composed of a branched alkane with two homochiral branching points in ^-configuration (Meierhenrich et al. 2001e).

Branched alkanes have been intensively used as biomarkers for a variety of geological compounds. Special ratios of the isoprenoid alkanes pristane, phytane, and pristene have often been used as palaeoenvironmental indicators of thermal maturity (Goossens et al. 1988) and as depositional environment indicators (ten Haven et al. 1987; Hughes et al. 1995; Large and Gize 1996). The chiral composition of branched hydrocarbons in crude oil or sedimentary samples may furthermore lead to information on deposition time, oil maturity, etc., since chiral hydrocarbons are assumed to show tremendously long racemization times. Stereochemical studies of isoprenoid compounds were therefore assumed to provide fundamental information on their pathways of formation. The above-presented separation technique might open a door to the resolution of enantiomers for a wide range of chiral alkanes like the branched and chiral isoprenoid hydrocarbons pristane and phytane in petrochemistry and related fields.

Summarizing, the envisaged resolution of enantiomers of chemically and thermally robust chiral hydrocarbons including isoprenoid-type structures in cometary samples is an integral part of the ROSETTA mission and might contribute to a better understanding of the origin of biomolecular asymmetry on Earth.

9.1.3.4 Thoughts on Possible Results of ROSETTA's 'Chirality-Module'

The recent identification of amino acid and diamino acid structures in interstellar ice analogues (see Chap. 7) strengthened the scientific and public interest in the ROSETTA cometary mission (cf. ESA homepage). In order to perform a more reliable estimation on the presence of amino acid structures in comets, their delivery to the early Earth, and their role in life's molecular origins including life's selection for one-handed molecules, supplementary information is required. We need to understand more precisely parameters such as UV exposure, particle fluxes, and number of collisions under interstellar and circumstellar conditions. The in situ analysis of cometary material by space probe ROSETTA is necessary for this understanding. In 2014, ROSETTA's COSAC experiment might find molecular evidence of the presence of amino acid structures in an authentic sample of comet 67P/Churyumov-Gerasimenko.

ROSETTA's 'Chirality-Module' was moreover designed to provide information on the distribution of enantiomers in cometary matter. An observable enantiomeric excess of the 'correct' left-handed amino acids will support the assumption that chiral organic molecules have indeed been delivered onto the Early Earth during the Heavy Bombardment phase via comets and/or other interstellar bodies that are thought to carry along a considerable amount of organic matter. These molecules of an interstellar/cometary origin could have had a seed function in determining the handedness of chiral biotic matter in the Earth's initial biosphere (Thiemann 1998; Thiemann and Meierhenrich 2000; Meierhenrich et al. 2003b)4.

In this case, the enantiomeric excess measured by ROSETTA's 'Chirality-Module' is to be compared with the molecules' circular dichroism reference spectra in order to investigate whether interstellar photochemical processes might have induced the molecular asymmetry. One might be able to deduce from these data wavelength and polarization of the required extraterrestrial circularly polarized radiation. A number of non-racemic organic molecules such as amino acids, diamino acids, carboxylic acids but also alcohols, diols, amines, and hydrocarbons would provide detailed information about the chiral field that would have been necessary to introduce the molecular parity violation.

If the observed biomolecular parity violation on Earth were mainly caused by random mechanisms, one would naturally expect to find enantiomers of different handedness on different planetary or interstellar bodies. If homochirality in biomolecules were however originated by multiple (Mie) scattering or dichroic scattering (Lucas et al. 2005) in star formation regions, these scattering processes would presumably produce antipodic circularly polarized UV-photons as a function of time and location. All planetary bodies within a given solar system, including comets, would have been affected by the same light's handedness and their biomolecules would be produced stereospecifically in the same configuration. In different planetary systems or on different interstellar bodies, one would expect to detect variable enantiomeric excesses. And if biology's optical activity were determined by the universal chiral influence of the weak force, one would expect to find identical handedness of enantiomers throughout the entire universe. The results of ROSETTA's 'Chirality-Module' might give hints to the validity of these various hypotheses.

4 Cometary matter showing right-handed amino acids would also be very exciting, maybe as much as left-handed cometary amino acids; it would be only more unexpected from the origin of life on Earth point of view.

Although - at this stage of investigating cometary surfaces - we do not anticipate the existence of large enantiomeric excesses of organic molecules, not to speak of homochiral samples on comets with their harsh and life excluding climatic conditions as there are low temperatures, near-zero atmosphere, probably no liquid water, and others. 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. We assume that once the comet is formed, processing occurs at the very surface since vacuum ultraviolet light penetrates less than one micrometer and cosmic rays of the order of a few meters. If a small enantiomeric excess is induced on the grain mantles in this decisive comet-forming step, such e.e. may be preserved in the comet nucleus. The long awaited results of ROSETTA's 'chirality-module' on the resolution of different kinds of enantiomers would help understanding at least one important aspect of the development of life on Earth and point out the importance of synthesis of chiral compounds in interstellar space, regardless of the process by which the required enantiomeric excess may have come about.

Besides this, the identification of chiral diamino acid structures, aminoethyl glycine, sugar molecules or their characteristic molecular fragments by ROSETTA's COSAC instrument on comet 67P/Churyumov-Gerasimenko might promote research activities on the eventual contribution of these compounds to the evolution of the first genetic material on Earth, which might be described as a pre-RNA oligonucleotide.

9.1.3.5 Liquid Water in Comets?

In the above chapters, we have assumed that comets and small icy bodies of the Solar System are - given the low ambient temperature of interplanetary and interstellar regions - composed of pristine, original, and unaltered material. This was one of the reasons to consider the chemical and enantioselective analysis of comets as important for obtaining clues on the origin of life and particular on the origin of biomolecular asymmetry; one of the reasons to envisage cometary in situ measurements with the help of the ROSETTA mission.

However, if we have a closer and more precise look into the interior of comet-forming small icy bodies of the Solar System, we should notice that these objects could alter due to impacts, insolation, and particle radiation. But more importantly, radioactive decay of unstable isotopes incorporated in the inorganic dust might have contributed to a non-negligible thermal evolution of comets via radioactive heating. Particularly the 26Al radioactive decay with an 26Al half-life of 0.72 million years could have transformed the amorphous cometary ice into crystalline ice. Even the melting of water ice was calculated to be possible - if not widespread -for cometary-like bodies of radii between 2 and 32 km. Numerical modelling of cometary thermal evolution led to a broad variety of thermally processed icy bodies and to the knowledge that the early occurrence of liquid water and extended crystalline ice interiors may be a common phenomenon (Merk and Prialnik 2006).

The calculated duration of the liquid water stage in comets and small icy bodies is illustrated in Fig. 9.9. It is clearly shown that liquid water could have been present for a duration of up to 4.5 million years (Ma), which is a function of the heliocentric distance, the radius of the object, and its ice to dust ratio. The bigger the object and the higher the ice content, the higher the temperatures.5 According to Rainer Merk from Tel Aviv University, we can assume for the indicated liquid time spans in Fig. 9.9 that 40-60% of the radius (given at the abscissa) had been liquid. This liquid may be considered as a bygone "three-dimensional internal lake" shaded by a thick pristine crust to the exterior. The pristine nature of comets and small icy bodies seems thus to be restricted to a particular set of initial conditions and should not be taken for granted.

It is evident that a more profound knowledge on the presence of liquid water including its temporal extent, temperature, volume, pressure, concentration of polar organic molecules therein, but also information on the physical, eventual nebular-like, state of liquid water under near zero gravity in a highly porous body is crucial for 'astrobiological' questions and affected with far-reaching consequences. In particular, we assume that organic molecules of high molecular weight are present in interstellar ices and comets releasing amino acids after hydrolysis. We may now figure out if this hydrolysis may have occurred not only after the delivery of cometary organic molecules on the surface of Earth, or other planets, moons and asteroids bearing liquid water, but also in small icy bodies and comets itself in well defined interstellar regions. Nevertheless, experimental data show that no hydrated minerals were found so far in the Wild 2 grains collected by Stardust.

Comet 67P/Churyumov-Gerasimenko, target of the ROSETTA Mission, is a periodical Jupiter family comet classified as short period. This comet has probably originated from the scattered disk, located between Kuiper belt and Oort cloud. The above-mentioned numerical simulation of the early internal thermodynamic by Merk and Prialnik (2006) are in particular valid for objects from this region.

The nucleus of comet 67P/Churyumov-Gerasimenko was estimated to have a radius between 2 000 m and 4 740 m. Assuming a dust-to-ice mass ratio of about 0.8 for 67P/Churyumov-Gerasimenko (Lamy et al. 2007), this comet is to be settled on the left side of the upper Fig. 9.9. At the present state of our knowledge, it is therefore improbable that the surface of the nucleus of this comet, where the ROSETTA space probe is going to touch-down its robot lander Philae, has ever been transformed to liquid water in its history. But with this prediction we have to be careful, since comets might be fragments of bigger 'parent bodies'. If this is the case, comet 67P/Churyumov-Gerasimenko might have seen liquid water in a temperature range of 290 up to 350 K.6 For this estimation, error bars are difficult to number, since

5 The upper image in Fig. 9.9 was calculated for low dust content and consequently a low amount of radioactive 26 Al heating sources. Nevertheless, it shows longer time spans for liquid water (4.5 Ma compared to 2.7 Ma) compared to the lower image that was calculated for a higher dust content. This is not surprising, since dust-rich particles show a higher density, accrete more slowly, and cool down faster.

6 Personal communication with Rainer Merk, Tel Aviv University, in May 2007.

^PcOO 12000 22000 32000

Fig. 9.9 Time spans in million years (Ma) during which comets were calculated to contain liquid water. The heliocentric distance in AU is printed as a function of the object's radius in m. (Top) 1:1 composition of ice and dust; (Bottom) ice-poor bodies. The illustration was first published by Merk and Prialnik (2006). Reproduced with permission of the authors numerical calculations do not give random errors; systematic errors on the other hand are improbable because of the use of sophisticated standard models.

At present, we should accept that the substantial question on the ancient presence or absence of liquid water in comets cannot be answered for individual comets with sufficient precision and will remain subject to ongoing research. Direct evidence for liquid water in comets has never been seen and today we should not expect it for comet 67P/Churyumov-Gerasimenko due to the limited half-life of the radioactive heating sources.

9.2 Chiroptical Techniques and the SETH Project

Information on the eventual homochirality of organic enantiomers in interplanetary, interstellar, and extraterrestrial samples in general can be used as an elegant way to finding life traces somewhere in the universe. Focussing on this ambitious endeavour, a complementary option to the above described enantiomer-separating chromatography was proposed: Space missions were suggested to implement an optical device able 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 a homochiral property of the chemical compounds, of which the condensed matter is made of. Specific "space-suitable" polarimeters are of a most simple, light-weight, robust, and yet sensitive and reliable design. They were shown to resist mechanical, pressure, and temperature shocks usually experienced during longer space journeys and automated landing conditions in unmanned missions. Alexandra MacDermott from the University of Houston-Clear Lake and colleagues have proposed such kind of instrument (MacDermott et al. 1996; MacDermott 1997) investigating the stereochemistry of extraterrestrial chiral organic molecules under the title Search for Extraterrestrial Homochirality (SETH).

The inclusion of this kind of chiroptical instrument is foreseen in a Russian Martian Excursion, now. Moreover, one may include polarimetric measurements of the SETH or any similar type on all future missions to all solid targets, such as Mars, Mercury, and Venus, the giant planets' moons Titan, Europa, and Ganymed, and the accessible comets to be targeted in the future.7 Had such a polarimeter instrument been included on the famous Viking Experiment to explore life on Mars already back in 1976, some of the 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.

7 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.

9.3 The Search for Chiral Molecules on Mars

So let us turn towards Mars and ask the question whether or not it might be achievable to identify and quantify chiral organic molecules on its surface. Martian organic molecules might be relicts of stellar evolution brought to Mars via meteorites, they might be remnants of chemical evolution on Mars itself, but also remains of extinct (or present-day) life on the Red Planet. In order to distinguish between these three potential pathways of formation, the chirality of Martian organic enantiomers would be of high importance to study.

As indicated above, the current discussion on organic molecules on Mars is still deeply influenced by experiments of the Viking Landers 1 and 2 that arrived at the surface of Mars in 1976. Viking's search for organic matter in two soil samples at each landing site showed surprisingly, that no organic molecules were found at the parts per billion (ppb) level (Biemann et al. 1976). Very recently, it was proposed that Viking's experimentation might have been blind to detect (a) life (Navarro-Gonzales et al. 2003) and in particular (b) to detect low levels of organics due to the instrument's thermal volatilization (Navarro-Gonzales et al. 2006) suggesting that the Martian surface could have several orders of magnitude more organics than the stated Viking detection limit (see also Wu 2007). Moreover, oxidation-reactions of Martian organics by inorganic oxidants such as superoxides, peroxides, or per-oxynitrates forming non-volatile compounds might have caused their non-detection (Benner et al. 2000). This accusation forced Viking scientist Klaus Biemann from the Massachusetts Institute of Technology in Cambridge USA, to republish an Antarctic soil standard used for the calibration of Viking's GC-MS (Biemann 2007). The gas chromatogram shows aliphatic, aromatic, as well as nitrogen-, and oxygen-bearing compounds and seems thus to proof design concept, operation, extensive tests, and validity of the recorded Viking GC-MS data.

So for us, it remains an open question whether or not organic molecules are present in the Martian surface or its subsurface. Upcoming space missions such as ExoMars will tackle this issue, focussing additionally on their chirality.

9.3.1 Space Invaders: Traces of Life in Martian Meteorites

Spectacular was the recent claim of detecting traces of past Martian biota in the meteorite called ALH84001 (McKay et al. 1996) supported also by isotope ratios on the meteorite EETA79001 announced by Colin Pillinger and Ian Wright from the Open University in Milton Keynes (MacDermott 1997). "If this discovery is confirmed," US-President Bill Clinton said, "it will surely be one of the most stunning insights into our universe that science has ever uncovered." Serious reinterpretations of the presented ALH84001 data today conclude that the observed results do not necessarily indicate the presence of living organism on Mars. Neither today nor to any previous times. Surprisingly, any claims on extinct or present life on Mars have so far never been supported with information on chirality and eventual enantiomeric excesses of chiral structures.

Under the extremely dry, cold, low pressure conditions of Mars the chiral properties of fossilized (micro-) organisms - if ever existed - would have easily survived over long geological time periods, as laboratory experiments by Bada and McDonald (1995) on the racemization of chiral amino acids under Martian conditions suggest. Probing solid bodies' surfaces in the Universe should include measurement on optical activity or chirality aiming to obtain results concerning the intriguing searches for the existence of life beyond Earth.

9.3.2 Chirality and Mission ExoMars

Officials from the European and North-American Space Agencies ESA and NASA understood this immense research opportunity to be of outstanding interest. In December 2005, ESA-representatives of 17 member states signed to support ESA with 8.36 billion Euros in the upcoming years. Based on this funding, ESA itself decided to design a mission to Mars ready to start in 2013. The mission is called ExoMars and includes a Mars Rover (Fig. 9.10).

The scientific objective of the ExoMars mission is to obtain information on extinct and/or present life on Mars and thus to detect carbon-containing organic molecules on the surface and subsurface of the Red Planet (Brack 2000). Amino acids, carboxylic acids, oxidized compounds, and other defined and prioritised molecular building blocks of life (see Parnell et al. 2007) should give hints to the organic and prebiotic chemistry on Mars (and Earth). The aim of the ExoMars Mission is to identify a "chemical fingerprint" of extinct and/or present life on Mars.

A large number of scientific proposals for suitable ExoMars instruments from all over the world were developed and examined by an international Science-Team at ESA headquarters in Noordwijk, the Netherlands. In 2007 the decision was taken. ExoMars will be a joint ESA-NASA mission and it will include two (sic!) instruments dedicated to the resolution of enantiomers on the surface and subsurface of Mars: MOMA and UREY.

9.3.2.1 The Enantioselective Mars Organic Molecule Analyser (MOMA)

One of these instruments is called the Mars Organic Molecule Analyser (MOMA). MOMA combines a sophisticated gas chromatograph-mass spectrometer (GC-MS) with a Laser desorption mass spectrometer (LD-MS) system (Goesmann et al. 2007a).

The idea of this integrated approach is to unite the different methods and their strengths from very high specificity at extremely low detection thresholds at the one extreme to a very wide range of detectable families of different molecules at higher detection limits at the other. The central part of MOMA is an ion trap mass

Fig. 9.10 Artist's impression of the ExoMars Rover drilling into the Martian surface. The vehicle will be able to travel a few kilometres with its 16.5 kg exobiology payload. Surface and subsurface samples of Mars will be subjected to chemical analysis in a miniaturized laboratory onboard the Rover. Here, chiral organic molecules can be separated into their enantiomers, identified, and quantified with the help of (a) enantioselective gas chromatography by MOMA and (b) enantioselective capillary electrophoresis by UREY to search for signs of past or present life. Credits of ESA -AOES Medialab spectrometer (ITMS) for the detection and identification of molecular ions. The source to be investigated will be Martian soil samples from the surface and maximum 2 m subsurface obtained from a specifically developed drilling system. In order to transfer molecules from the soil sample to the MS two methods are employed. Firstly, Laser desorption mass spectrometry (LD-MS) will be used, where the Martian sample will be subjected to intense Laser flashes producing molecules and ions directly, even from refractory material, but in a mixture.

Secondly, in GC-MS, Martian samples will be heated (pyrolysed) or subjected to a combustion procedure. The evolving volatiles are transferred to a GC where the compounds are separated and then fed into the MS to be measured individually. The combination of methods to feed the MS in MOMA (a) via GC and (b) via LD-MS covers a wide rage of molecules from the very light (example: methanol) to medium sized (example: naphthalene) by GC-MS up to more complicated (example: peptides) by LD-MS. Rather low detection limits in the 20ppb range for GC-MS and in the pico-mole range for LD-MS can be provided. The MOMA instrument will be designed and constructed by an international research team, the coordinators of which are Fred Goesmann at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and Luann Becker at the University of California, Santa Barbara, USA (Goesmann et al. 2007a).

If - and this is promising - organic molecules will be detected in Martian surface and subsurface samples by the MOMA-instrument, we would like to learn about the organics' chirality by measuring enantiomeric excesses. Will there be any deviation from the racemic enantiomeric distribution expected for abiotic synthesis of organic molecules? The pathways of formation of eventual chiral organic molecules on Mars, biotic versus abiotic and extra-Martian versus Martian, might be deduced from the enantiomeric ratios in different families of organic molecules.

The challenge is that no organic molecules were found on Mars by the Viking mission in 1976 but within a solid meteorite of possible Martian origin on Earth in 1996. This means that organic molecules are likely to be very rare on Mars. Furthermore, only a fraction of the conceivable organic molecules do have stereogenic centers. Hence, enantiomers will be even more rare. There is consequently a strong need for sample enrichment in order to improve detection limits for trace abundances of organic molecules. Furthermore, as the possible molecular structures are unknown, the instrument must not only be sensitive, but able to characterize the detected molecule and identify them unambiguously.

Summarizing the above, the MOMA instrument requires to be capable of performing enantioselective analyses with different chiral molecules on the surface of Mars. However, for interpretation of the results we will have to be patient until ExoMars measurements after landing which is also foreseen for 2014.

9.3.2.2 The Enantioselective UREY Instrument

The other officially selected apparatus for the ESA-NASA Mission ExoMars which focuses on the phenomenon of chirality is called UREY, referring to Nobel Prize laureate Harold Urey from the University of Chicago in whose lab Stanley Miller simulated the early Earth's atmosphere showing the synthesis of various amino acids. The UREY apparatus for the ExoMars Mission consists of a Sub-Critical Water Extractor (SCWE) to solubilize organic molecules from Martian rock/soil materials and deliver the enriched extract to the Mars Organic Detector MOD (Kminek et al. 2000). Here an analysis of amino group containing compounds such as amino acids, amines, nucleobases, amino sugars, and polycyclic aromatic hydrocarbons

(PAHs) will be performed. The detection will be done by laser-induced fluorescence, a technique capable of providing a parts-per-trillion sensitivity.

If chiral amino acids will be found, they will be further analyzed using the Micro-Capillary Electrophoresis (|CE) Unit to determine their chirality. The precise enantioselective-working mode of this instrument is not yet published. Enantiose-lective capillary electrophoresis is a well-known and well-proven technique for the resolution of enantiomers, however, this technique is also very vulnerable since it requires fresh buffer solutions combined with the controlled handling of various liquids. These measurements will be made at a thousand times greater sensitivity than the Viking GC-MS experiment, and will significantly advance our understanding of the organic chemistry of Martian soils.

The MOD will be fed with 1-3 mL aliquots of aqueous extract from the SCWE and start working by removing the water by freeze-drying. MOD will then slowly sublimate all volatile organic compounds including chiral amino acids onto a fluorescamine-coated target held at -10°C with a cold-finger. The chemical constituents will then be separated by controlled sublimation at increasingly higher temperatures. It is well-known that, for example, the sublimation of amino acids occurs in the temperature range of 125-350°C, whereas polyaromatic cyclic hydrocarbons sublime at approximately 450°C. Laser-induced (395 nm) fluorescence will be used to excite organic molecules such as polyaromatic cyclic hydrocarbons and amino acids that are bound to the fluorescamine on the cold-finger, in order to detect their presence. These compounds are fluorescent under ultraviolet illumination. Each analysis will be followed by a cleaning step at 900°C, in order to remove material from the sublimator and prevent contamination and memory-effects in the upcoming analysis.

What about the chirality of eventually identified amino acids or other chiral organic molecules? The Micro-Capillary Electrophoresis Unit (|CE) of the UREY-module will also be fed with liquid extracts from the SCWE. This device consists of a laminated multilayer ('lab-on-a-chip') wafer stack that performs fluidic manipulations and electrophoretic analysis. Buffers and other reagents are used to detect the chirality of any amino acids present in the liquid sample. Using this technique, amino acids and nucleotide bases of biotic and abiogenic origin can be detected and differentiated. One unknown parameter remains the long-term stability of amino acids in the Martian regolith.

Jeffrey Bada from the Scripps Institute of Oceanography in San Diego scientifically guides the UREY team. From this team, we also anxiously expect data on the chirality of extraterrestrial organic molecules that are probably not "contaminated" by the homochirality of living organisms on Earth. We'll just have to wait for the landing of the ExoMars probe until 2014.

Chapter 10

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  • salla
    Why is luann becker not on the mars organic molecular analysis?
    1 year ago

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