Irradiation of Cometary Ices in Laboratory Experiments

Ample lists of authors have devoted their efforts since the early 60s, when cometary simulation experiments began, to understand the processes and the evolution that comets experience. A Russian group did pioneering work; their samples (of sand and water) were deposited onto a cold finger, and then exposed to a strong light (see Kochan et al., 1998 for references). In 1961, Oro proposed that comets could have been carriers of organic material; then, with experimental data, he reinforced the hypothesis of the importance of comets in reference to the origin of life. Since that time, studies and experimental designs have become greatly diversified, based on new discoveries and information obtained from observation, theoretical modeling, and experimental work.

Increasing knowledge of the composition, dynamics, and component interactions has been reflected in the quality and quantity of existing data on comets. Experimental work has focused on the major constituents of comets: ice and, in some cases, the refractory material. Kossacky et al. (1997), for example, studied the thermal evolution of cometary analogs following the structural behavior of samples made of water ice, carbon dioxide, and dunite. Among their interesting results are the stratification of the samples into well-defined layers of varied chemical composition and cohesion.

Ices may be investigated in pure or mixed solutions by three experimental approaches: following thermal processing, changes in structure and mobility, and by irradiation (with high energy electrons, ions, protons, and UV light) and its effects (i.e. desorption, sputtering). All of these factors are fundamental and have close relationships to one another. However, for better comprehension of the entire phenomenon, it is necessary to isolate each one from the others. Irradiation is fundamental to understand the evolution of comets. Among the effects produced by irradiation on frozen targets are fundamental ones: sputtering, structural and morphological alterations, and the synthesis of new molecular species (Strazzulla and Palumbo, 2001).

Many of these experiments relate not only to comets, but also to other objects in the solar system, where ice is an important component. There are different approaches to the study of the irradiation of ices. Many research groups are devoted to the study of irradiation of thin ice films (see, for example, the works of Greenberg, Strazzulla, Moore and Hudson). Other research groups analyze samples obtained by spraying suspensions of water, silicates and other components in liquid nitrogen (Kochan et al. 1998). Bulk irradiations were carried out by Colin-Garcia et al., 2008. They studied the radiation chemistry of HCN, one of the most important and highly versatile constituent of cometary ices.The behavior of over-irradiated water dominant HCN ice mixtures underline the importance of radiation-induced reactions as an energy source in extraterrestrial scenarios, like comets or other icy objects. Even, when low temperature is a limiting variable for diffusion-controlled reactions, it is evident that reactions occur and produce many organics relevevan. in prebiotic chemistry

0 0

Post a comment