Vdq

° Cosmogenic.

° Cosmogenic.

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 21Ne/22Ne

Figure 4. Three isotope diagram of Ne.

The possibility of abiogenic synthesis of complex biochemical compounds on surfaces of dust particles.

Michael B. Simakov and Eugenia A. Kuzicheva

Institute of Cytology, Russian Academy of Sciences, St.Petersburg 194064, RUSSIA

The small bodies in the Solar system are rich in organic compounds. Different classes of organic molecules, many of which have common biochemical significance, such as amino acids, their precursors and nucleic acids bases have been identified in comets, meteorites and interplanetary dust particles. It is important to test how far the processes of the chemical evolution could develop on the surface of these bodies under the action of different open space energy sources. The abiogenic synthesis of nucleotides and dipeptides in solid films has been investigated under the action of VUV (A=145 nm) and high energy protons. In order to test the processes of chemical evolution in open space, the investigated mixtures are exposed to space conditions on board satellite "BION-11". The abiogenic synthesis of nucleotides is also observed. Our results indicate that such complex compounds as nucleotides and dipeptides could be synthesized in the early stages of Solar system evolution and safely transported to Earth by comets, meteorites and IDP, where they could take a place in prebiological evolution.

1. introduction

The problem of life origins is a very intriguing question in contemporary science. Geological data indicate that complex photosynthetic organisms, possibly related to cyanobacteria, existed 3.5 billion years (Gyr) ago [1]. Moreover, there is evidence for biological mediated carbon isotope fractionation in the 3.85 Gyr old Isua metasediments, which indicates that the photoautotrophy biochemical processes must have existed at that time [2].

On the other hand, it is stated that impacts by meteorites and comets at an early stage of the Earth's history until 3.8-4.2 Gyr ago would have made our planet uninhabitable [3,4], So we can see, that life arose rapidly on Earth once conditions became clement and this process could be very rapid, almost instantaneous on the geological time scale. Only 300 million years appear to be left for terrestrial chemical evolution to lead to primitive, yet fully developed organisms. There is a very short time interval for production of the biochemical compounds in the atmosphere and on the surface of planet, but this kind of matter's evolution had to occur before the biological one. Besides, geological data and theoretical models point to the Earth's early atmosphere to be composed mainly of carbon dioxide, carbon monoxide, nitrogen and water vapor [5,6], It is not easy to form biologically important compounds such as components of peptides and nucleic acids from these gases by any energy sources.

We can suggest that the chemical evolution which had proceeded in the early stages of the Solar system origin, would lead to the appearance of very complex organic compounds. These compounds have been delivered to early Earth at the stage of the "late heavy bombardment"

and later [7]. So, the highly complex organic molecules have been proposed as a starting material for early biological activity on Earth, or on other planets in the Universe [8,9],

The main attention in experimental work is now devoted to energetic chemistry in "simulated space ice conditions", such as ice mantles of interstellar dust particles (IDP) [10,11] and cometary ices [12]. These laboratory and theoretical studies were stimulated by related astronomical observations showing the signature of complex carbonaceous material in circumstellar and interstellar environments [13]. The laboratory experiments yielded some interesting results. Kobayashi with co-workers [14] performed a proton irradiation experiment using a gas mixture of CO, N2 and H20 and found a wide variety of amino acids with high yield in irradiating products after acid-hydrolysis. A mixture of a source of carbon (CO, CH4 or C3H8), H20 and NH3 after irradiation with high energy protons also yielded several amino acids [15]. The another "simulated ISD ices", ice mixtures of methanol, ammonia and water also yielded amino acid precursors after irradiation with high energy protons [16]. One of the RNA bases, uracil, was identified in the product after irradiation of a mixture CO, N2 and H20 by high energy protons [17]. However, the reactions of complification of organic compounds could also take place on the solid surfaces of small bodies in the Solar system, such as asteroids, meteorites, IDP and satellites of other planets (Mars, Jupiter, Saturn). These bodies have tremendous total surface area and are subjected to irradiation by ultraviolet photons, the solar wind, solar flare ions and galactic or extra-galactic cosmic rays.

In space most of the chemical evolution toward complex molecules takes place in the solid phase. The aim of our work is to understand better, using laboratory simulation and spacecraft opportunities, the chemical evolution of biologically important compounds on the surfaces of small bodies in the Solar system. We present laboratory data relevant to ultraviolet irradiation and cosmic ray bombardment of the solid organic films in such environments. In our experiment in laboratory and on board of the space satellite "BION-11" we have planned to establish how far the chemical evolution could develop on the surfaces of small bodies under the action of open space energy sources.

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