The New Space Race Chiral Molecules on Comets and on Mars

9.1 In Search of Chiral Molecules in Comets

Comets are fascinating celestial objects. But they are much more than that. It is probable that comets, to a large extent, consist of pristine interstellar material (Greenberg 1982; Irvine et al. 2000).1 This material could have been delivered to the early Earth by comet dust, as well as asteroids, and interplanetary dust particles, during the epoch of heavy bombardment (Oro 1961; Chyba and Sagan 1992). Comets were thus discussed to hold the key of the origin of life on Earth and they may have played a crucial role in the origin of biomolecular asymmetry. It is therefore of importance to understand the chemical composition of comets and particularly the stereochemistry of cometary organic ingredients more in detail. The determination of enantiomeric ratios in the matter of such extraterrestrial bodies in situ is long overdue (Thiemann 1975; Brack and Spach 1986). The cometary sampling and composition experiment (COSAC) is part of the payload of the Lander Philae in ESA's cometary mission ROSETTA (Biele and Ulamec 2008). It will provide the first opportunity of this kind when it begins its measurements on the nucleus of comet 67P/Churyumov-Gerasimenko after its touchdown in November 2014 (Glassmeier et al. 2007). Previous to the discussion of the 'Chirality-Module' of this mission, we will have to understand: What is a comet? and how does it form itself?

1 As we will see later in this chapter, at least some comets are not so pristine as the Greenberg-model proposed. This is what recent results from cometary missions tell us. The presence of crystalline silicates, for example, inferred from astronomical observations (Bregman et al. 1987; Hanner 1999), and minerals formed at high temperatures present in the dust of comet Wild 2 (Brownlee et al. 2006) suggest that at least some comets are not that pristine as previously thought. Such minerals probably formed in the hot inner regions of the solar nebula. Comets likely contain a mixture of interstellar and solar nebular materials.

U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 161

and Biogeophysics, © Springer-Verlag Berlin Heidelberg 2008

9.1.1 Comet Formation: Theory and Experimental Results

Comets show a visible coma (a dust hull) and a nucleus, invisible from Earth and Earth based telescopes. In 1986, the cometary mission GIOTTO took the first detailed image of a cometary nucleus (Fig. 9.1). Our knowledge of the composition of cometary nuclei is predominantly based on the evaporation of volatile species and thermal emissions from siliceous and carbonaceous dust when bright comets approach the sun. Sometimes comets such as 73P/Schwassmann-Wachmann 3 split into fragments and offer a look onto fresh material from the comet's interior (Dello Russo et al. 2007). From photospectrometric measurements of this kind, we know today that comets are made of silicates (~25%), organic refractory material (~25%), and ^50% of water ice with small admixtures of other ice species. Comets are assumed to include 33 to 50 % of carbonaceous material (Lisse et al. 2006). Moreover, comets can be observed from Earth when they are far away from the sun and therefore inactive. Spectroscopic methods already allowed the identification of many elements and specific compounds in comets' comae, including hydrogen, carbon, nitrogen, oxygen, sulphur, water, carbon monoxide, carbon dioxide, and formaldehyde (A'Hearn and Festou 1990), methanol (Greenberg et al. 1994), and furthermore hydrogen sulphide, sulphur dioxide, methane, ethane, formic acid, ammonia, and acetonitrile. Polycyclic aromatic hydrocarbons, water vapour and ice, and sulphides were found by spectral observations after NASA's Discovery mission Deep Impact had sent an impactor into the nucleus of comet 9P/Tempel 1 (Lisse et al. 2006)

The nucleus of a comet is thought to be built up of aggregates of interstellar dust particles (Greenberg 1993). Interstellar dust particles are silicate grains surrounded by a mantle of H2O, CO2, CO, and other ices, which may serve as a matrix for many kinds of atoms and molecules (Fig. 9.2). Due to the results of laboratory simulation experiments, we have reason to assume that specific chiral organic molecules like amino acids can be found in cometary matter too (Munoz Caro et al. 2002; Bernstein et al. 2002).

Fig. 9.1 Comet Halley pictured by the Halley Multicolour Camera onboard ESA's GIOTTO spacecraft. Coma and nucleus are visible. The ROSETTA mission's target is the nucleus of comet Churyumov-Gerasimenko. The ROSETTA mission was launched on 2 March 2004 by the Ariane 5 launcher. "Chury" will be reached by May 2014, the robotical lander Philae will then detach from the orbiter and start the landing manoeuvre. Credit: Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany. Image courtesy: Dr. H. U. Keller

Fig. 9.1 Comet Halley pictured by the Halley Multicolour Camera onboard ESA's GIOTTO spacecraft. Coma and nucleus are visible. The ROSETTA mission's target is the nucleus of comet Churyumov-Gerasimenko. The ROSETTA mission was launched on 2 March 2004 by the Ariane 5 launcher. "Chury" will be reached by May 2014, the robotical lander Philae will then detach from the orbiter and start the landing manoeuvre. Credit: Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany. Image courtesy: Dr. H. U. Keller

Fig. 9.2 A piece of a fluffy comet manufactured by the Greenberg research team at the Leiden Observatory for Astrophysics, the Netherlands: Model of an aggregate of 100 average interstellar dust particles. Each particle consists of a silicate core, an organic refractory inner mantle, and an outer mantle of predominantly water ice in which are embedded the numerous very small (<0.01 |m) particles responsible for the ultraviolet 216 nm absorption and the far ultraviolet extinction. Each particle as represented corresponds, in reality, to a size distribution of thicknesses starting from zero. The packing factor of the particles is about 0.2 (80% empty space) and leads to a mean comet mass density of 0.28 g cm-3. Once the ices removed, the dust mass density is & 0.1 g cm-3 and the porosity is & 0.96 (Greenberg 1996). The overall dimension is scaled to about 4 |m (Greenberg 1986)

Fig. 9.2 A piece of a fluffy comet manufactured by the Greenberg research team at the Leiden Observatory for Astrophysics, the Netherlands: Model of an aggregate of 100 average interstellar dust particles. Each particle consists of a silicate core, an organic refractory inner mantle, and an outer mantle of predominantly water ice in which are embedded the numerous very small (<0.01 |m) particles responsible for the ultraviolet 216 nm absorption and the far ultraviolet extinction. Each particle as represented corresponds, in reality, to a size distribution of thicknesses starting from zero. The packing factor of the particles is about 0.2 (80% empty space) and leads to a mean comet mass density of 0.28 g cm-3. Once the ices removed, the dust mass density is & 0.1 g cm-3 and the porosity is & 0.96 (Greenberg 1996). The overall dimension is scaled to about 4 |m (Greenberg 1986)

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