1 INTRODUCTION 2 4 exactly such biochemical pathways evolved; however it is evident that they must have

been constructed from simple prebiotic precursor molecules.

Although there is no general agreement about the source o f prebiotic material on Earth, several theories exist, the most popular being that this material was brought to Earth by impacting comets and asteroids during the early Earth’s bombardment era about 4 billion years ago (Orgel, 1998, Sorrell, 1999). The observation o f some o f the complex organic molecules in the ISM suggest that abiogenic synthesis o f organic molecules, some o f which are recognised as constituents o f important biochemical processes, is indeed possible.

A number o f laboratory experiments have been carried out to investigate this hypothesis. It has been shown that UV irradiation o f interstellar analogues can produce some astrobiologically relevant complex organic molecules. Bernstein, et.al (2002) have created the amino acids glycine, alanine and serine from a 2 0:2:1:1 ice mixture of H2 0:CH3 0H:NH3:HCN (representative o f the composition o f interstellar garain mantles

in dense molecular clouds). Simultaneously, Mufioz-Caro et a l (2002) have reported the identification o f 16 amino acids, 6 of which are protein constituents, created under

similar conditions from a 2:1:1:1:1 ice mixture o f H2 0:CH3 0H:NH3:C0 :C0 2. It has

also been shown, through laboratory work of UV irradiation o f interstellar-type ices (simple ice mixtures of H2O, NH3, CH4 and CO), that it is possible to form complex

organic compounds that are capable of self-assembly into vesicular structures (Dworkin

et.al 2001). This is particularly significant in studying molecular organisation and

primitive cell formation.

A major biochemical puzzle in the origin of life is its homochiral nature (Bailey, 2000). Chiral amino-acid molecules exist in a right-handed or left-handed form, with an arrangement o f functional groups about a central carbon atom such that they are mirror images o f each other. All o f the 19 naturally occurring chiral amino acids on Earth are left-handed, but any laboratory-synthesised sample o f a chiral molecule will produce a racemic mixture. It has been shown that one of the handed isomers can be destroyed in preference to the other by circularly polarised light (Clark, 1998; Sorrell, 1999). As there are no natural sources of circular polarisation on Earth, it has been hypothesised that a racemic organic mixture formed in the ISM irradiated by circularly polarised UV light from a nearby neutron star or a young star surrounded by a dense dust cloud would result in a certain degree of homochirality. In the latter case, unpolarised UV light from

1 In t r o d u c t io n______________________________________________________________________________2 5 a young star can acquire circular polarisation by multiple scattering off dust grains (Clark, 1998).

Although it has been seen through the observation o f over 120 molecular species in interstellar environments and through laboratory simulations of interstellar conditions that abiotic formation o f complex organics is possible, the reaction mechanisms involved in their formation are still not fully understood. It is essential to ascertain the individual formation processes o f simple molecules by first considering simple molecular systems and identifying their reaction rates and pathways.

Understanding the chemistry in dense molecular clouds surrounding protostars will provide us with important clues about the nature of the chemistry within our own solar system during the early stage o f its formation. Furthermore, understanding the chemical evolution o f ices within our own solar system may provide important clues to the understanding of chemistry in other protoplanetary and solar systems and the possibility of life originating elsewhere.

1.5

S

T

O

M

A new apparatus has been designed, built and tested to study the formation of molecules stimulated by photon and ion (and electron) irradiation o f astrophysical ice analogues. Much of the motivation for the construction o f this apparatus has already been detailed in this chapter. A better understanding of the processes and the mechanisms of condensed phase molecular formation is required. Although it is not possible to recreate identical conditions in the laboratory to those found in the astrophysical environments described earlier (See Chapter j), a great deal o f information can nevertheless be inferred from rigorous, systematic laboratory studies.

For over 30 years there have been an increasing number o f laboratories worldwide devoted to the study of molecular synthesis on or within astrophysical ices in a variety of analogous astrophysical environments that can be simulated in the laboratory. Such studies are primarily being carried out using UHV systems in which ices are vapour deposited on reflective or transmitting substrates and are subjected to particle, photon, or thermal processing. The ice samples are then periodically monitored, in-situ, using infrared, UV or mass spectroscopy.

1 In t r o d u c t io n______________________________________________________________________________2 6 The preliminary studies, described in this thesis, carried out using the new experimental apparatus described in Chapter 3 involve both ion and photon irradiation o f pure and mixed (and in some cases layered) H2O and CO2 ices. Irradiation o f H2O and CO2 ice

mixtures has previously been studied at the NASA Goddard Flight Centre (Moore and Khanna 1991; Moore e ta l 1991; DelloRusso et.al 1993, Gerakines et.al 2000) and at the Catania Astrophysical Laboratory, Italy (Pirronello et.al 1982; Brucato et.al 1997). In the experiments carried out by the NASA group, H2 0:C0 2 samples were irradiated using high energy (0.7 - 0.8 MeV) protons at 20 K. A number o f products have been identified using a combination of FTIR spectroscopy and mass spectroscopy including H2CO3 and CO. The focus of the NASA work has primarily been the identification of H2CO3 which is observed both in the irradiated ices at 10 K and as a residue after

heating the samples up to 250 K. Strazzulla, Palumbo and co-workers have also carried out ion irradiation experiments o f H2 0:C0 2 mixtures, confirming the formation of

H2CO3 (carbonic acid) following irradiation at lower ion energies (Brucato et.al 1997).

In a recent study by Gerakines et.al (2000), H2 0:C0 2 ice mixtures were irradiated, for

the first time with UV light and H2CO3 formation was again observed. The yield of H2CO3 was found to be limited by the penetration depth o f the UV photons. Gerakines

et.al (2000) suggested possible H2CO3 production pathways (See Chapter 5) and

attempted to calculate the absorption coefficients o f H2CO3 by dissociating the annealed H2CO3 residue and measuring the yields o f CO and CO2. In Chapter 6 o f this thesis the

first VUV irradiation of H2 0:C0 2 mixtures using synchrotron radiation are described.

The observation o f H2CO3 in laboratory irradiation experiments is of important

astrophysical significance as it has not yet been identified in astrophysical spectra. However a number of IR absorption bands observed in the ESQ spectra of the Mars may be attributed to H2CO3. An interesting result o f irradiation experiments is the fact

that H2CO3 has not been observed as a product in H2 0:C0 ices. Instead H2CO is observed. Likewise, H2CO is not observed in H2 0:C0 2 mixtures (Brucato et.al 1997)!

CO, also a product o f H2 0:C0 2 photolysis and radiolysis, has not yet been described in

this context as attention was primarily given to the synthesis o f H2CO3. However

extensive studies have been carried out to study the interaction of CO with polar and non polar ices. These studies involved CO being either co-deposited with other species or deposited on top of an existing layer of ice and then allowed to diffuse into the underlying ice. The CO molecule is an abundant constituent in astrophysical ices (Table 1-3) and is an important diagnostic of the interstellar ice composition as its infi'ared

1 iNTRODUCnON______________________________________________________________________________2 7 absorption profile is highly sensitive to the environment in which it is found. Experimental studies o f CO mixtures with a variety o f molecules that may be possible astrophysical ice constituents (Sandford et.al. 1988, Schmitt et.al. 1988, Palumbo and Strazzulla 1993, Strazzulla et.al. 2001), have shovm the solid CO infrared absorption profile to generally contain two components, a narrow band centred about - 2 1 4 0 cm"' and a broad band centred about 2136 cm '’. The former is found to be the dominant component in interstellar ices along many lines o f sight (Tielens et.al. 1991) and is attributed to CO found in an ice matrix dominated by non-polar molecules. The lower frequency broad band has been attributed to CO found in an ice matrix dominated by polar molecules. The CO molecule has a unique profile when mixed with H2O. A shoulder which appears at the higher frequency end (-2152 cm '’) o f the main band has been attributed to a possible interaction o f the CO molecule with the 0-H dangling bonds o f H2O in the amorphous ice matrix (Devlin 1992). As yet this band has not been

identified in interstellar spectra. However this may be due to the fact that its intensity is very weak and it is overpowered by the ‘XCN’ feature near 2165 cm ’. Understanding the CO interaction with other molecules in ice matrices is particularly important in the understanding o f the evolution and the physico-chemical properties o f grain mantles and the composition of molecular clouds. An outer layer o f CO-rich ices is believed to be formed in the latter part of the molecular cloud evolution cycle when most atomic hydrogen has been converted to molecular hydrogen.

A detailed study of CO diffusion in H2O has been carried out by Palumbo (1997). These

studies have shown that the CO 2152 cm ’ feature was sensitive to the phase o f water ice and hence the structure of the water ice matrix. Palumbo (1997) also confirmed that CO becomes mobile in an H2O ice matrix above 27 K. In a series o f experiments carried

out by Devlin (1992) the intensity of the 2152 cm ’ band was found to be sensitive to the number o f available 0-H dangling bonds in the ice and is believed to arise due to CO ‘complexed’ with the OH groups. In their experiments a small ether (ethylene oxide) was adsorbed onto the ice, which has the effect o f weakly binding with the dangling 0-H bonds, reducing the number of available binding sites for the CO molecules. Subsequent adsorption o f CO resulted in the disappearance o f the 2152 cm ’ band. In Chapter 4 we report the formation o f CO during carbon ion implantation in H2O ice. The highly structured CO band profile is described in these experiments. In

1 INTRODUCTION______________________________________________________________________________2 8