Why are Comets so Interesting for Chemical Evolution

The study of comets has, with good reason, intensified over the past few years. Comets are minor bodies in our solar system which are almost as old as the solar system itself. They provide unique information about chemical evolution processes that occurred at early stages of the solar system. Besides that, they are dynamically connected with other small objects all over the solar system (Weissman and Levison, 1996). In addition, within the scenario of the origin of life, Chamberlin and Chamberlin (1908) were the first to suggest that extraterrestrial material could have played an important role in terms of having contributed to the organic matter on the Earth, influencing abiotic synthesis and biological evolution. Later, J. Oró (1961) reworked this hypothesis, implying a specific role for comets as raw material providers for chemical evolution on Earth, and, since then, this line of research has been widely investigated, thus reinforcing this idea (Hartman et al., 1985; Negrón-Mendoza et al., 1994; Oró and Lazcano, 1997; Oró and Cosmovici, 1997; Irvine, 1998, Colín-García et al., 2008 in press). This means that, not only local processes, such as UV irradiation and electrical discharges on the primitive atmosphere, were important in triggering necessary reactions on the early Earth, but extraterrestrial contributions were also of great significance.

The putative contribution of impacts could be paradoxical. Clear evidence of impact events in the past can be observed on the surface of our moon; those events have been extrapolated to understand what occurred on Earth (Chyba and Sagan, 1992). Those impacts could have contributed organic material that was necessary for prebiotic synthesis on the early Earth. Noble gases, for example, were carried mainly by cometary impacts (Owen and Bar-Nun, 2001). On the other hand, comets and asteroids could have annihilated forms of life during various extinction events (Cockell and Bland, 2005).

Extraterrestrial prebiotic molecules (such as aldehydes, CN-containing compounds, etc.) may have played an important role, as raw material, in the origin and evolution of life on Earth (Oró, 1961; Chyba et al., 1990; Bernstein et al., 1997). The contribution of comets in reference to organics depends on two main factors: first, the mass contribution and, second, the survival capacity of carbon containing compounds, while entering the atmosphere, and on impact.

The flux of cometary material to the early Earth has been estimated between 105 to 3x106 kg per year (Chyba et al., 1990). According to Svettsov (2002), bodies of 1 km size could have reached the surface and have contributed largely to the influx of cometary organics to the surface of the Earth. However, major bodies of hundreds of kilometers in diameter had to be completely evaporated in a dense atmosphere.

There are several approaches to the study of comets: observational, theoretical modeling, and experimental simulations. This review intends to bring attention to some experimental models that reflect the processes that may occur on cometary ice and that may have had a direct impact on the synthesis of bioorganic molecules on Earth. The main interest in this paper is to provide a general perspective in terms of data obtained through the high-energy irradiation of cometary icy analogues. A brief summary of some cometary characteristics is presented in the following paragraphs. To obtain a full perspective about what comets are, their properties, and their origins, see, for example, Delsemme (1977).

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