Sources of Organic Material

Amino acids are the building blocks of the proteins and enzymes that are essential for life as we know it today, as well as of peptides that could have been fundamental ingredients of the first protocells (e.g., Pohorille, 2002). Many of the protein-building amino acids have been synthesized abiotically in laboratory experiments of the early Earth's atmosphere from simple precursors (Miller, 1953; Schlesinger and Miller, 1983; Stribling and Miller, 1987). The earliest experiments started from the assumption of a reducing primitive atmosphere dominated by CH4 (often including also significant amounts of NH3, H2O, and sometimes H2), and amino acid production occurred via the standard electrical discharge Miller-Urey synthesis (Miller, 1953; 1998).

However, it is now thought that the early Earth's atmosphere was a weakly reducing mixture of CO2, N2, and H2O, probably with a number ratio of H2 to CO2 less, and possibly much less, than unity (Walker, 1986; Kasting, 1993; Kasting and Brown, 1998). In this case, the production of amino acids appears to be very limited (Schlesinger and Miller, 1983), making it difficult to justify an exclusive endogenic source of amino acids on the Earth. The early atmosphere on Mars had probably a composition similar to that on Earth, and thus a similar Miller-Urey type production of complex organic molecules. Other solar system bodies of interest for the origin of life, such as Europa, may have never had an atmosphere.

Hydrothermal systems in the deep ocean have also been proposed as possible environments for the synthesis of prebiotic organic molecules on the Earth (e.g., Holm and Andersson, 1995; 1998; Shock and Schulte, 1998). Laboratory experiments that test this hypothesis suggest formation of amino acids in vent environments, in some cases with even higher yields than that reported for electric discharges (e.g., Hennet et al., 1992; Yanagawa and Kobayashi, 1992). It has been suggested that the high temperatures would preclude the survival of the newly formed organic molecules, but laboratory work to simulate a typical deep-sea hydrothermal system circulation suggests that significant amounts of complex organic molecules can build up (Imai et al., 1999). As Europa may harbor an extensive ocean underneath its icy crust, it is plausible that deep-sea hydrothermal systems similar to the Earth may develop, providing an endogenic source of organics (e.g., McCollom, 1999). Other endogenous sources of organics are also possible on Europa (Chyba and Phillips, 2001). Hydrothermal systems have also been hypothesized for Mars, where organic synthesis may have somewhat greater potential than in Earth case (Shock and Schulte, 1998).

Amino acids are abundant in some extraterrestrial material. Certain carbonaceous chondrites, the most volatile-rich meteorites, contain up to 5% by weight of organic matter, covering over 70 different amino acids, including eight protein-building amino acids (Cronin, 1976; Cronin and Pizzarello, 1983; Shock and Schulte, 1990; Cronin et al., 1988; Cronin; 1998). Comets show substantial organic material (Krueger and Kissel, 1987; Delsemme, 1988; Chyba et al., 1990), although there is no direct spectral identification of amino acids. Evidence for the amino acid glycine in the interstellar medium (ISM) remains ambiguous (Snyder, 1997), but if amino acids do exist in the ISM, comets may inherit them directly during accretion. Since it is improbable that we may uniquely identify many complex organic compounds from cometary spectra (because of the heavy overlapping of their spectral signatures with simpler molecules; Bernstein, personal communication), only an in situ analysis or sample return mission may provide this type of information. Recently, amino acids have been identified in simulated cometary material (Bernstein et al., 2002; Muñoz Caro et al., 2002), a promising indication of the possible presence of these complex organic molecules inside comets. When analysis of Comet Wild-2 samples, returned to Earth in January 2006, becomes available, we

Fig. 5.1. Organic molecules: From biogenic elements to living cells.

may find out for sure. Even if amino acids are not present, however, many of the aspects of this analysis would carry over to other organic molecules of prebiotic interest (see Fig. 5.1) that are certainly present in comets (for Arrhenius parameters for other organics, including some based on shock-tube experiments, see Chyba et al., 1990).

Biomolecules like amino acids are thermally fragile; that is, they tend to be destroyed into constituent gases (CO2, H2O, NH3, CO) and a variety of volatile organic compounds (amines, nitriles, hydrocarbons, etc.) when subject to temperatures above about 700-800 K (e.g., Basiuk and Navarro-Gonzales, 1998). In the solid phase, the thermal behavior of typical amino acids is influenced by their structures. Overall, thermal degradation of amino acids follows an Arrhenius-like decay law:

where Ea (cal/mol) and A (l/s) are the activation energy and preexponential constant, respectively, for the organic molecule; R=1.987 cal/mol is the gas constant; P(t) and T(t) are the time-dependent pressure (in Pa) and temperature (in K); V is the activation volume of the molecule (in m3/mol); and M is the mass of the organic material (in kg). New experimental values in the solid phase for Ea and A for the biological protein amino acids (Rodante, 1992) differ substantially from previously available kinetic parameters for thermal degradation in solution (Vallentyne, 1964). Neither case provides the best simulation of impact shock heating. The ideal kinetic parameters for these simulations would derive from shock-tube or impact gun experiments, but such data for amino acids are not currently available. We believe that Ro-dante's kinetic parameters in the solid phase are more appropriate for impact simulations (as amino acids are contained inside solid projectiles) than those from solution (where hydrolysis will occur, especially during slow heating experiments; e.g., Bada, 1991).

The suggestion that a substantial fraction of the Earth's prebiotic inventory of organic matter may have been delivered by incoming comets and asteroids was first proposed in 1908 by Thomas and Rollin Chamberlin, (Cham-berlin and Chamberlin, 1908). This theory acquired greater importance when spacecraft missions to comet Halley revealed it to be nearly 25% organic by mass (consistent with its elemental cosmic abundances; e.g., Krueger and Kissel, 1987; Delsemme, 1988; Chyba et al., 1990). However, a conservative numerical investigation by Chyba et al. (1990) indicated that with the exception of some thermally hardy compounds, such as polycyclic aromatic hydrocarbons (PAHs), the high temperatures involved in large comet and asteroid impacts would probably destroy most organic molecules (this does not preclude the possibility of substantial postimpact organic synthesis in an expanding impact plume: e.g., Oro, 1961; Barak and Bar-Nun, 1975; Oberbeck and Aggarwal, 1992; Chyba and Sagan, 1992; McKay and Borucki, 1997). This conclusion seems to be reinforced by recent laboratory studies (Basiuk and Navarro-Gonzales, 1998; Basiuk et al., 1999), suggesting that simple amino acids (and some nucleic acids) cannot survive rapid heating to temperatures substantially higher than about 1,000 K. It should be pointed out, however, that these experiments also showed survival of amino acids at the 1-10% level when subject to temperatures of about 700-800 K, for periods of time exceeding 10 min. It appears, then, that on the Earth the most promising source of exogenous organics would have been interplanetary dust particles (IDPs), which appear to be roughly 10% organic by mass, and which decelerate gently in the atmosphere and can thus deliver their organics intact (Anders, 1989; Chyba and Sagan, 1992). This idea has recently been challenged by an investigation that simulated the heating experienced by IDPs during atmospheric entry (Glavin and Bada, 2001). After subjecting grains (<100 |m in size) of the carbonaceous chondrite Murchison to heating to about 820 K under reduced pressures, Glavin and Bada (2001) found that only glycine was able to survive heating by sublimating while other amino acids appeared to be completely destroyed.

Organic survival in large impacts has received new life from a combination of theoretical and experimental studies. Early incentive was provided by the identification of what appear to be extraterrestrial amino acids, a-aminoisobutyric acid, and racemic isovaline, at the Ir-rich Cretaceous/Tertiary (K/T) boundary layer at Sussex and Raton Basin sites (Bunch, personal communication), and not only above and below the boundary, as it was found, early on, at the Stevns Klint (Denmark) K/T boundary layer (Zhao and Bada, 1989). Since then, laboratory experiments have shown that significant fractions of amino acids can survive large shocks (e.g., Tingle et al., 1992; Peterson et al., 1997; Blank et al., 2001). Finally, theoretical studies suggest that significant amounts of some amino acids can be delivered to the Earth's surface via a large comet impact (Blank and Miller, 1998; Pierazzo and Chyba, 1999a).

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