A large fraction of interstellar molecules is incorporated in the solid state in refractory grains and ice mantles, however most of the known interstellar molecules are revealed in the gas phase only (Woon 2008). Gas phase molecules are free to rotate; this produces narrow and characteristic bands which fall in the radio and sub-millimeter portions of the spectrum. From these narrow bands a finer and more sensitive identification of molecules is possible with respect to the infrared ones. This explains the much larger number of interstellar molecular identifications in the gas phase.
The first interstellar molecules were discovered in the gas phase in the 1940s with the observation of CH, CH+, and CN at optical wavelengths (McKellar, 1940; Adams 1941; Douglas & Herzberg 1941). At a later stage, many molecular detections such as NH3, H2CO were made at radio frequencies (Cheung et al. 1968; Snyder et al. 1969), but it was in the 1980s, with the development of new millimeter receiver technologies, that the study of interstellar molecules blossomed. As of today, more than 150 different interstellar molecules have been identified in the ISM, most of them in giant molecular clouds such as Sagittarius B2. The sources include circumstellar envelopes around evolved stars, hot cores and corinos, outflows from young stars, etc. A list of the interstellar molecules detected in space in all the spectral domains is presented in Table 1.2. The molecules detected include light hydrides, ions, chain molecules, cyclic molecules, complex molecules, etc., many of them also common on Earth. One may note from this table that, as already discussed in this chapter, the observed gas phase chemistry is essentially made of the most abundant atoms and will favor organic chemistry. In this list the number of molecules decreases with the number of atoms, which is indeed not surprising. By interstellar standards, molecules containing six or more molecules are viewed as complex. With this definition, about one third of the molecules detected in space are complex and the more complex ones are those in the smaller relative concentrations. The largest organic molecule (13 atoms) that has been unambiguously identified is HC11N (Bell et al. 1997). Among these complex molecules, methanol (CH3OH) is the only one detected unambiguously also in the solid state, while all the rest have been detected exclusively in the gas phase. This remark reflects the fact that absorption infrared features from molecules in the solid state are broad and cannot be assigned with as much certainty as narrow gas-phase features. Among the complex molecules detected, some of them are thought to be involved in processes useful for prebiotic chemistry on Earth. Molecules of this category have some structural elements in common with those found in living organisms. Of the interstellar prebiotic molecules, many were first detected toward Sgr B2(N-LMH). For example, the first interstellar sugar glycolaldehyde (Hollis et al. 2004), ethylene glycol (Hollis et al. 2002), acetamide (Hollis et al. 2006), and amino acetonitrile (Belloche et al. 2008), a direct precursor of the simplest amino acid, glycine. Furthermore there is a strong evidence for the presence of PAH (polycyclic aromatic hydrocarbons), large aromatic molecules containing up to 100 carbon atoms, although specific assignments to any peculiar molecule have not yet been possible (Léger & Puget 1984; Allamandola et al. 1989). Conversely, the detection of glycine, claimed by Kuan et al. (2003) remains still controversial.
In summary, many complex molecules are observed in the ISM (mainly in the gas phase) and some of them contain up to 13 atoms. Moreover, it is likely that many additional molecules exist both in the gas and in the ice phase. Unfortunately, in the ice phase the large complex molecules cannot be observed since line broadening and intermolecular reactions caused by the solid environment leads to overlapping features that are dominated by the most abundant ice components. In the next section we will see how these complex molecules can be formed.
Energetic processing and thermal evolution of ices
Non-energetic reactions, such simple atom additions on grain surface, are necessary to initially form water-based ice mantles; however, additional energetic mechanisms appear to be necessary to further form molecules and, in particular, complex molecules. These mechanisms are often concomitant and include energetic processing by cosmic ion irradiation and UV photolysis, and thermal processing (Fig. 1.8).
Figure 1.8 – Scheme of the different processes occurring on interstellar grains (from Guss 2013).
UV photons are widely present in space environments, and those with energy greater than ~4 eV can drive ice photochemistry. Indeed, ice photochemistry is now accepted to occur deeper into the dense clouds than thought earlier and at much lower energies than in the gas phase (Gudipati & Allamandola 2003). On the other hand, energetic UV photons produced by newly formed stars do not penetrate deeply into the surrounding molecular cloud and those from external stars affect only the outer edges of these clouds. However, internal young stars can be important sources of UV radiation especially during the T-Tauri phase. A high flux as 1012 photons cm-2 s-1 can penetrate the circumstellar disk and efficiently photolyze the ice. Moreover, there is another internal source of UV photons that can affect the entire cloud, which is the decay process of the H2 molecule after cosmic ion bombardment (Shen et al. 2004).
Cosmic ions are dominated by protons, mainly in the low MeV range, with an estimated flux value of 1 proton cm-2 s-1 in dense quiescent regions (Mennella et al. 2003). Cosmic ions penetrating the cloud lead to internal production of UV photons via a mechanism in which secondary electrons excite H2, which then re-radiate photons when they decay. In dense clouds this internal UV flux is estimated to be 1.4-4.8 x 103 photons cm-2 s-1 with a typical energy of ~10 eV (Prasad & Tarafdar 1983; Mennella et al. 2003), much smaller than that one present in the diffuse ISM but strong enough to drive the photochemistry in the ices. This energy is released to the ice through a single photo-dissociation or photo-excitation event. As a consequence of these events, molecular bonds are broken, formed radicals diffuse through the ice and react with other molecules, giving rise to a rearrangement of the chemical structures and to the formation of new molecular species. In this way, a complex chemistry occurs. The energy dose from this internal UV source is thought to be similar in value to the energy contribution from the incoming cosmic ions. Moreover, this is the right dose of irradiation required to form new species in the ice preserving them from further irradiation that could led to their destruction. Recent studies suggest that the production of molecules, such as C2H5OH, HCOOCH3 and CH3OCH3, relies on UV irradiation of interstellar ices (Öberg et al. 2009, Modica & Palumbo 2010).
Within the cloud, several circumstances such as the formation of a new star, stellar outflow winds, and shock waves can lead to warm-up of the ices. The gradual warm-up can promote the mobility of species, which diffuse through the ices and trigger new reactions. Some proposed reactions studied in the laboratory include polymerization to form polyoxymethylene (Schutte et al. 1993). Moreover, with the further increase of the temperature, the more volatile components of the ice mantle can sublimate while the more refractory organic material remains.
Evidences for energetic processing of ices arise from the analysis of the infrared spectra of observed sources. The interstellar 4.62 µm absorption band was discovered in the ground based spectrum of W33A (Lacy et al. 1984). Commonly referred to as the “XCN” feature (Gibb et al. 2000), this band is observed in a large variety of objects (Soifer et al. 1979; Tegler et al. 1993, 1995; Demyk et al. 1998; Pendleton et al. 1999; Gibb et al. 2000; Keane et al. 2001; Whittet et al. 2001; Chiar et al. 2002; Spoon et al. 2003). The 4.62 µm absorption is now unambiguously attributed to OCN- formed by UV photolysis or cosmic ion irradiation of NH3 bearing ices, as supported by laboratory works (Palumbo et al. 2000; Bernstein et al. 2000; Hudson et al. 2001) and is considered as a key indicator of such processing. However, besides energetic processing, thermal evolution is also a possible formation alternative for the OCN-(van Broekhuizen et al. 2004). The most probable counter ion of OCN- is NH4+ (thus forming the ammonium cyanate salt) which is properly observed in evolved processed ices (Schutte & Khanna 2003). Another indication for energetic processing of interstellar ices is that of CO2, a molecule detected for the first time by d’Hendecourt & Jourdain de Muizon (1989) by its 15.2 µm absorption in three distinct infrared sources with the IRAS telescope. Interestingly, this molecule was been predicted from laboratory experiments on UV photochemistry of ice mixtures of water and carbon monoxide, revealing the potentiality of this laboratory approach (d’Hendecourt et al. 1985, 1986; Allamandola 1988). Several attempts have been performed to detect the effects of UV photochemistry in space with some tentative successes, such as in NGC 7538-IRS9 where the 5 to 8 micron region may be interpreted by the presence of complex molecules such as urea, formamide and glycerol (Raunier et al. 2004). Some features, observed in the astronomical spectra, still remain not fully explained such as the 6.8 µm feature, attributed to NH4+ formed by UV photolysis of ice analogs (Shutte & Khanna 2003) but more likely due to multiple contributors (Boogert et al. 2008). However, the most emblematic spectroscopic signature which is considered the sign of interstellar ice processing is the series of absorption near 3.4m in the diffuse interstellar medium. These features have been attributed to the C-H stretching in aliphatic hydrocarbon chains carrying the CH2 and CH3 functional groups (Sandford et al. 1991; Pendleton et al. 1994). The correlation of these absorption features with those from different interstellar analogs (hydrogenated amorphous carbon, quenched carbonaceous condensate, and various organic residues including the ones described in this thesis) is remarkable. However, the best candidate material for the 3.4m feature is likely to be hydrogenated amorphous carbon (Chiar et al. 2000; Pendleton & Allamandola 2002; Mennella et al. 2002).
Finally, the presence of organic refractory residues has long been postulated as a candidate for the carbon-rich components observed in interstellar dust at 3.1, 3.4, and 6 µm (Greenberg et al. 1995; Greenberg & Hage 1990; Pendleton et al. 1994; Whittet et al. 2001). This residue, that is expected to be rich in complex organics, is thought to be the result of ice energetic processing. As suggested by Gibb & Whittet (2002) this residue, which is not widespread in the diffuse interstellar medium, could in part contribute to the 6 µm band observed in YSOs. This hypothesis, well documented in the laboratory, is supported only by one observation of evolved ices in Mon R2 IRS3, the only observational evidence for organic refractory residues formation (Shutte & Khanna 2003). It is interesting to note how much these observations are biased because evolved ices can be tentatively observed only in evolved objects when the surrounding molecular cloud starts to dissipate. Finally, observations are quite limited in number as well as in performance. At this stage it is clear that a much more sensitive spaceborn telescope such as the JWST is needed.
Complex molecules in hot molecular cores
The formation of a new star has many effects on the chemistry of the surrounding material. One of the most extensively studied regions of star formation is the Orion molecular cloud (OMC-1), which contains a number of bright mid-infrared sources, thought to hold embedded protostars (Wynn-Williams et al. 1984). These sources, called hot molecular cores due to their high temperature (T > 100 K), are compact (< 0.1 pc) and massive (10-103 Msun) condensations of gas and dust surrounding a protostar in the early phases of its evolution (Brown et al. 1988). The Orion hot core is one of the most studied objects of this type (Genzel & Stutzki 1989). The low-mass versions of the hot cores are known as hot corinos (Ceccarelli 2004). Hot cores and corinos are natural laboratories in which the most complex molecules of the interstellar medium can be observed and studied. In these regions grains experience thermal processing that lead to molecular diffusion, structural changes, and subsequently to the complete sublimation of ices. Sublimation allows the molecular species previously formed in the solid state to be released into the gas phase, where they enrich the inventory of detected species and participate in new gas phase reactions. Because of the high temperature of the hot cores, many reactions can occur efficiently, including endothermic processes and exothermic processes with barriers. Hot cores are, in fact, particularly rich in complex organic molecules such as HCOOCH3, HCOOH, CH3OCH3, and CH2CHOH (Snyder et al. 2002; Hollis et al. 2000, 2002; Remijan et al. 2003; Kuan et al. 2003; Cazaux et al. 2003; Kuan et al. 2004; Bottinelli et al. 2004a, 2004b, 2007).The origin of these complex molecules was initially thought to be the rich gas phase chemistry of evaporated or sputtered species (e.g., Millar et al. 1991; Millar & Hatchell 1998; Herbst et al. 1977). However, recent experimental works have shown the inefficiency of gas phase reactions (Horn et al. 2004; Geppert et al. 2006) and that, according to models many of the observed molecules must be primarily produced in ices prior to evaporation (Charnley et al. 1995; Garrod & Herbst 2006; Wakelam et al. 2010). In particular, methyl formate (HCOOCH3), a complex organic molecule widely observed toward numerous different sources, has been suggested to form in the solid state after ion irradiation of ice mantles containing methanol and carbon monoxide (Modica & Palumbo 2010), and released after sublimation into the gas phase, where it can be observed.
In summary, first generation complex molecules form in the ice as a consequence of UV/ion irradiation; they will eventually sublimate once the grain temperature rises above the ice sublimation temperature of ~100 K. Subsequent to ice sublimation, hot core gas phase chemistry between sublimated molecules can drive further complexity in second generation species (e.g., Millar et al. 1991; Charnley et al. 1992, 1995). However, after the formation of a new star and its planetary system, some material from interstellar or protostellar origin, could remain relatively unaltered in the colder edges of the disk. This material could be accreted into small bodies such as comets and asteroids, which are considered leftovers from the formation process of a planetary system. These small bodies are not subjected to extensive processing as the rest of the planetary system, as currently believed for the small bodies of our Solar System.
Small Solar System bodies
Comets are icy objects of frozen gases, rocky materials, and large organic particles (Mumma 1997; Irvine et al. 2000). It is suggested that they formed from accretion of planetesimals in the colder parts of the protosolar nebula from a mixture of interstellar and nebular material (Irvine 1999, 2000; Ehrenfreund & Schutte 2000). Comets are the most volatile-rich objects of the Solar System and hence believed to be the least thermally processed material for the study of its formation (Delsemme 1982). After an observational campaign on comet Hale-Bopp, it was noted a remarkable similarity between the composition of cometary ices and hot cores (Bockelée-Morvan et al. 2000). This reinforced the idea that the processes at work in the interstellar medium, particularly in the dense molecular clouds, could play a major role in the formation of cometary ices. On the other hand, molecular cometary abundances can vary greatly among different comets (Bockelée-Morvan et al. 2004). This indicates that comets have different origins and that some cometary materials are the result of a certain degree of processing experienced in the interstellar cloud or in the protosolar disk.
There are two major reservoirs of comets: Kuiper Belt and Oort Cloud. Comets formed beyond Neptune constitute part of the Kuiper belt, a disc-shaped region extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. Comets from this region have a short period (less than 200 years) and low inclination on the ecliptic plane. Comets formed in the Jupiter-Neptune region have been subsequently ejected by gravitational interaction with the giant planets forming the Oort cloud, which is a vast, spherical shell of icy bodies surrounding the Solar System at 50000 AU from the Sun.
Table of contents :
1 Overview on the evolution of ices in space
1.1 The Interstellar Medium
1.1.1 Dust grains
1.1.2 Interstellar ice mantles
1.1.3 Interstellar molecules
1.2 Energetic processing and thermal evolution of ices
1.2.1 Complex molecules in hot molecular cores
1.3 Small Solar System bodies
1.3.2 The exogenous organic delivery to Earth
1.4 Laboratory simulations
2 The MICMOC experiment at IAS
2.1 The experimental set-up MICMOC
2.2 Infrared spectroscopy
2.3 The standard experiment
2.3.1 Preparation of the gaseous mixture
2.3.2 Deposition and irradiation
2.4 Infrared analysis of interstellar/circumstellar analogs
2.4.1 Infrared ice spectra before irradiation
2.4.2 Infrared spectra of an irradiated thin ice film
2.4.3 Infrared analysis of a typical residue
3 Asymmetric UV photochemistry of cosmic ice analogs: induction of enantiomeric excesses in amino acids
3.2.1 Experimental campaigns
3.2.2 Preparation of residue-enlarged samples
3.2.3 Analytical procedure
3.3.1 Distribution of identified amino acids
3.3.2 Enantiomeric excesses in alanine
3.3.3 Enantiomeric excesses in five different amino acids
3.4 Astrophysical implications
4 Analysis of laboratory organic residues by FT ICR mass spectrometry
4.1 FT ICR mass spectrometer
4.1.1 Instrument description
4.1.2 Basic principles
4.2 Mass spectra description
4.2.1 Analysis at low and medium mass-to-ratio range
4.2.2 Analysis in the high mass-to-charge ratio range
4.3 Mass defect versus mass diagrams
5 The amino acid content in the Paris meteorite and in laboratory residues
5.1 Carbonaceous chondrites
5.1.2 Organic matter content
5.1.3 The Paris meteorite
5.2 Analytical procedures
5.2.1 Amino acid extraction and derivatization
5.3 Results and discussion
5.3.1 Amino acid content in the Paris meteorite
5.3.2 Enantiomeric measurements in chiral amino acids
5.3.3 Amino acid content in laboratory organic residues
6 Laboratory simulations using silicate surfaces
6.1 Astronomical silicates
6.2 Experiments and discussion
6.2.1 Laboratory make-up of icy silicate grain analogs
6.2.2 Spectral comparison with the Paris meteorite
6.2.3 Hot water extraction test on laboratory samples
6.2.4 The search for HMT in the Paris meteorite
Conclusions and perspectives