Comets and the Origin and Evolution of Life

6 J. Oró, A. Lazcano, and P. Ehrenfreund
and Charnley (2004). Molecular ices and the gases released upon sublimation,
silicate dust, and solid state carbonaceous materials are the major components
of dusty cometary comae that can be studied by astronomical observations
in combination with laboratory simulations (Rodgers and Charnley, 2004,
Hanner and Bradley, 2004, Colangeli et al., 2004). More than 25 molecules
have been identified in the coma. Their abundances in percent relative to
water molecules are listed in Table 1.1.
The inventory of cometary species is certainly not yet complete in the range
0.01–1%. Many small reactive molecules important in prebiotic aqueous chemistry
are detected: H2O, CO2, formaldehyde (H2CO), ammonia (NH3), hydrogen
cyanide (HCN), acetonitrile (CH3CN), isocyanic acid (HNCO) and hydrogen
sulfide (H2S). The most complex species identified to date are acetaldehyde
(CH3CHO), methyl formate (HCOOCH3), and formamide (NH2CHO).
Glycine, the simplest amino acid, has not yet been detected in comets; the
present upper detection limit is 0.5%. Given the relative high abundance of
chemical precursors used in the laboratory synthesis of amino acids, the absence
of detectable levels of these biochemical compounds in comets is surprising.
However, as argued elsewhere (Oró and Mills, 1989), the final steps
of abiotic formation of amino acids require a liquid phase, as in the Streckercyanohydrin
synthesis (Miller, 1957). If this is the case, only minor amounts
of amino acids can be predicted to be present in comets. Interstellar glycine
has been recently reported and is likely formed through gas–grain reaction
pathways in dense clouds (Kuan et al., 2003).
Apart from a few volatiles measured in the cometary coma, our knowledge
on bulk carbonaceous material of comets is rather limited. IR observations of
dusty comae ubiquitiously detect a strong near-IR featureless thermal emission
that is well-fitted by amorphous carbon. The detection of a polycyclic
aromatic hydrocarbons (PAH), phenantrene (C14H10), has also been suggested
in Halley on the basis of UV spectroscopic data (Moreels et al., 1994). However,
no PAHs have been detected in the Infrared Space Observatory (ISO)
infrared spectra of C/1995 O1 Hale-Bopp (Crovisier 1997/99), which were
taken at larger heliocentric distance. Comet Hale-Bopp has a strong distributed
source of CO within the heliocentric range ≤ 1.5 AU, i.e., only 50% of the
CO originates from the nucleus (DiSanti et al., 2001). The other 50% of the
CO may from the desorption of organic components of the dust. A possible
candidate for distributed CO, polyoxymethylene (POM), that is a polymer
of formaldehyde was suggested (Boehnhardt et al., 1990, Cottin et al., 2001,
2004).
In the ISM, the distribution of carbon is still an unsolved question. In order
to understand the C fraction in the cometary bulk material, we should look
into the interstellar precursor material. A large fraction (>50%) of the cosmic
carbon is unaccounted for in the interstellar medium. Laboratory simulations
in combination with interstellar observations argue that this missing carbon
is incorporated into solid state macromolecular (Ehrenfreund and Charnley,
2000; Mennella et al., 1998, Colangeli et al., 2004). The same predominance of