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Radiolysis of water: a look at its origin and occurrence in the nature
ARTICLE IN PRESS
Radiation Physics and Chemistry 72 (2005) 181–186 www.elsevier.com/locate/radphyschem
Radiolysis of water: a look at its origin and occurrence in the nature Ivan G. Draganic´ Institute of Nuclear Sciences Vinc˘a, PO Box 522, 11001 Beograd, Serbia and Montenegro Received 5 September 2003; accepted 3 February 2004
Abstract The ubiquitous presence of water and ionizing radiation in the nature point out to the occurrence of radiolysis of water on Earth and beyond. Laboratory experiments and computer simulations of radiation-induced processes are reviewed: action of radioactive 40-K rays in early ocean water 3800 Ma ago and of natural nuclear reactor radiations in underground waters of the primitive Earth 3200 Ma ago. Also some findings relevant to the action of cosmic and radioactive rays in water ice of a cometary nucleus are presented. r 2004 Elsevier Ltd. All rights reserved. Keywords: Water radiolysis; Early ocean; Oklo phenomenon and underground waters; Cometary ice
1. Introduction Radiation-induced decomposition of water molecules, the radiolysis of water, is carefully studied from various reasons. Water is an important constituent of living matter. It has a significant role in nuclear technology. Physical chemists are attracted by a variety of shortliving reactive species in the radiolysis of water and aqueous systems. The radiolysis of water is a phenomenon known for almost one century and well understood (Ferradini and Jay-Gerin, 1999). However, the studies of the origin and occurrence of water radiolysis in the nature are somewhat neglected despite of the well-known fact of the ubiquitous presence of water and radiation in the nature. Such is the case with the studies of the origin of life and aqueous environment where the primitive living matter originated (Brack and Raulin, 1991). The situation is changing with an increasing interest on the role of ionizing radiation in the origin of life E-mail address: [email protected] (I.G. Draganic´).
(Akaboshi et al., 2000). Also, cometary chemistry in space programs, such as NASA’s Deep Impact Mission and similar ESA projects, is stimulating the studies of radiation chemistry of the outer Solar System ices (Domingue and Allamandola, 2001). The present work reviews some basic aspects of the origin and occurrence of water radiolysis on Earth and beyond; specific details of the radiation chemistry of water (Draganic´ and Draganic´, 1971) are outside the scope of this work.
2. Radioactive potassium (40-K) and early ocean water 2.1. Deposit of energy–the dose rate 40-K is a radioactive isotope, which decays (half-life: 1250 Ma) by emitting 1.46-MeV gamma (11%) and beta rays (89%), with an average energy 0.455 MeV.Its relative isotopic abundances on Earth, present and early Earth (4500 Ma ago), are 0.0117% and 0.145%, respectively.
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The amount of energy delivered over a year, the dose rate (eV kg1 a1), depends on the amount of radioactive potassium, i.e., on the composition of early ocean water and its content in the chemical element potassium. A reliable estimate was based on the Isua water-laid sediments of West Greenland, considered as representative of the state of Earth and very likely of ocean waters some 3800 Ma ago (Ernst, 1983; Dymek and Klein, 1988). They suggest that the salt content and the major chemical constituents were not essentially different from the contemporary ones. Consequently, the concentration of the chemical element potassium can be taken as 0.38 gkg1 of ocean water and that of 40-K as 0.38 mg kg–1. These data enable the calculation of the energy deposited annually by 40-K decay: 1:84 1015 eV kg1 a1 ð0:29 mGy a1 Þ: It should be noted that because of the long half-life of 40K, the above dose rate calculated for 3800 Ma ago would vary only by 75% for a shift of 7100 Ma. 2.2. Radiolysis of early ocean water (Draganic´ et al., 1991). The model system was ocean water with sodium, magnesium, calcium, strontium, and potassium as principal cationic, and chloride, bromide, sulphate, and bicarbonate as ionic constituents (Stumm and Morgan, 1981). The radiation-induced process was followed over a geologic time scale of one hundred million years (100 Ma). The beginning of the process is deliberately taken as 3800 Ma ago, the age of the Isua water-laid sediments of West Greenland. Computer modeling takes into account the radical– radical and radical–solute reactions in a process consisting of 87 reactions. Gaseous products (H2 and O2) accumulate with increasing dose but, unlike other radiolytic products, diffuse away and their liquid–gasphase concentration equilibrium was taken into account. It has been shown that the absence of minor constituents, or minor presence of new compounds, would not significantly affect the reaction mechanism because of the dominance of radiation–chemical reactivity of the chloride ion in the system considered. Also, the reaction mechanism is not sensitive to concentration changes if the concentration ratios of the major constituents are not affected, for example in the case of some local dilution or concentration (by freezing or evaporation). Analysis of computer simulation results shows that the annual generation of oxidizing radicals was modest, about 1011 mol dm–3 a–1. Depending on the presence of oxidizable substrates, these could have been more or less efficiently consumed leading to the syntheses of new compounds. A simple estimate
shows that about 1014 g of organic compound, with a molecular mass M p ¼ 75 a:m:u:; could have been synthesized in primitive oceans over 100 Ma. The total amount is significant even with a low-radiation chemical yield assumed in the above calculation: 0.15 molecules per 100 eV absorbed. The main issue in primitive ocean water is its decomposition with the formation of molecules of oxygen, hydrogen peroxide and hydrogen. When considering these data the total mass of ocean water was assumed to be M ocean ¼ 1:4 1024 g: Table 1 shows the total amounts of radiolytically generated radiolytic products obtained by the computer modeling. For comparison, the total oxygen content in the contemporary atmosphere is 1.2 1021 g. A significant amount of hydrogen peroxide is generated over 100 Ma. It is disappearing by reactions with primary free radicals (H, OH, e aq) and with oxidizible substrates present in the ocean water (ferrous ions, etc.); these reactions have been taken into account in the computer modeling. The generation of molecular hydrogen may also be worth noting: its amount is similar to that released by volcanic activity over the same period of time. Primitive hydrosphere contained very likely other constituents, inorganic and organic compounds, in addition to those taken into account in the reviewed computer simulations. Their origins were in various processes of different intensity and duration such as electric discharges, volcanic eruptions or cometary impacts (Miller and Orgel, 1974). It should be noted, however, that these chemical compounds were deposited locally and have been involved in chemistry before an efficient mixing occurred. Computer modeling may certainly provide a better insight also in such cases but this remains to be done.
Table 1 Radiolysis of primitive ocean water 3800 Ma agoa Radiolytic product
Amount generated over 100 Ma
Molecular oxygen
3.0 1019 g O2 /Mocean/ 100 Ma 1.1 1018 g H2O2 /Mocean/ 100 Ma 3.8 1018 g H2 /Mocean/ 100 Ma
Hydrogen peroxide Molecular hydrogen a
Computer modeling concerns a process of 87 radical–radical and radical–solute reactions (Draganic´ et al., 1991). A total mass of ocean water is assumed to be Mocean=1.4 1024 g and its chemical composition is taken according to Ernst (1983) and Dymek and Klein (1988).
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3. Oklo phenomenon and radiolysis of underground waters of early Earth The phenomenon of Oklo concerns the old geological arrangements of water and uranium in which the fission chain process of uranium could be triggered. These particular nuclear fission zones are also known as natural nuclear reactors (IAEA, 1975, 1978). Underground waters had an important role in making of uranium deposits a natural nuclear reactor like in the 2000 Ma-old deposit in Oklo (Gabon, Africa). They contributed in propagating and controlling the fission chain process. At the same time the water was radiolyzed and the dissolved substances chemically altered. Underground water drain was of help to circulate efficiently the radiolytic products and take part in the chemistry on the primitive Earth. 3.1. Deposit of nuclear reactor radiation energy (Draganic´ et al., 1983) Natural nuclear reactors were sources of mixed radiations, charged particles (fission fragments, beta rays), neutrons, and gamma rays. Operating at a kilowatt-level power, their dose rates were similar to those available in small irradiation units in routine use in laboratories. Inpile dosimetry shows that within a reactor core with a diameter of 2 m, a natural nuclear reactor operating 3200 Ma ago at a power of 3 kW provided the dose rate of –1 8.1 1020 eV kg–1 h1 (1.3 kGy h ). 3.2. Radiolysis of sandstone underground water (Bjergbakke et al., 1989) The system used in the computer modeling was based on the Oklo phenomenon data: chemical composition of water, its high temperature and pressure, and a 3 kw-level power.
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Forty reactions were used in the computer simulation of radiation-induced processes in sandstone water. Bicarbonate ions dominate the radiolysis process. Main molecular radiolytic products are O2, H2O2, and H2 (Table 2). Data obtained concern the equilibrium concentrations in a closed radiation zone (gases cannot escape) after 1 month of operation. The results also show that small amounts of carbon dioxide, formate, and oxalate are formed. To estimate the total amounts produced over one reactor life the average operation time of Oklo reactors was taken as 0.7 Ma, with an average water flow-rate of 3 m3/ month. The data in Table 2 show the amounts produced during the operation time of one natural nuclear reactor only. A reliable estimate of the total production of oxidizing chemical species in underground water flowing through natural nuclear reactors on our planet remains to be made. It depends on the estimates of the potential number of reactors that could have been active between 4100 and 1800 Ma ago. The estimates vary from a conservative 103 to a more realistic value of 108 operating natural nuclear reactors (Draganic´ et al., 1993). Hence, the production of molecular oxygen could be up to 1015 g, and that of hydrogen peroxide up to 1013 g. It is worth noticing that these amounts of oxidizing species were generated, and contributed to chemistry, in underground waters of early Earth. One aspect of natural nuclear reactors as energy sources is also worth mentioning: during 1 year of criticality at a power of 3 kW, 6.9 1021 fission product radionuclides are produced in the reactor core. About 14% have half-lives longer than 1 year and could provide irradiation during reactor ‘‘shut-down’’ periods. Also, some of them would migrate (7% in the Oklo case) and continue to irradiate after their fixation in the environment or their uptake by the Precambrian biota.
Table 2 Radiolysis of sandstone underground water in Oklo natural nuclear reactors 3200 Ma agoa Radiolytic product
Molecular oxygen Hydrogen peroxide Molecular hydrogen
Amount After 1 month of reactor operation 1.5 104 mol/dm3 13.4 107 mol/dm3 3.2 104 mol/dm3
Total amounts produced over one reactor life The average operation time of Oklo reactors was assumed to be 0.7 Ma, with an average water flow-rate of 3m3/month. Molecular oxygen 1.2 108 gO2 Hydrogen peroxide 2.9 105 g H2O2 Molecular hydrogen 1.6 107 g H2 a Computer modeling concerns a process of 40 radical–radical and radical–solute reactions (Bjergbakke et al., 1989). It is based on information on the Oklo phenomenon: chemical composition of water, its high temperature and pressure, and a power of 3 kW.
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4. Radioactive radiation, cosmic rays, and extraterrestrial ices Radiation-generated free radicals and radical ions are efficient promoters of chemical processes in space, occurring even in conditions considered hostile for chemistry known in the laboratory (Akaboshi et al., 2000; Domingue and Allamandola, 2001; Draganic´, 2003). Space projects have provided some general information on the chemical composition of cometary matter helping in reliably defining a model system for simulation experiments. Understanding of extraterrestrial radiation chemistry is presently based mainly on laboratory studies.
nuclides are involved (secondary, tertiary, etc.). An analytical formula is developed for the proton energy deposit and the dose-depth curve calculated down to 20 m (Ryan and Draganic´, 1986). The contribution of other cosmic rays (gamma, electrons, heavy particles) raises only the total dose within the first 1-meter layer (by as much as 100%); it decreases rapidly to become less than 1% already at a depth of 2 m. Radiation damage is also an important aspect of the interaction of cosmic ray-protons with cometary material. On its way through the solid icy matrix the energetic particle collides and knocks out the atoms encountered with formation of vacancies. The displaced atoms end in the interstitial space between atoms. In such events, also hot atoms are formed which cause in situ chemical changes.
4.1. Sources of ionizing radiation in extraterrestrial space Decays of radioactive elements and cosmic rays provide the energy for chemistry in space. An estimate of radiation energy deposited in an extraterrestrial dust particle depends on the assumptions on its chemical composition and its life, and can be made only in specific cases. Generally accepted facts on comets, however, make such estimates possible for a cometary nucleus (Tables 3 and 4). The interaction of high-energy protons with cometary matter is a very complex cascade process where other
4.2. Simulation experiments relevant to chemistry in cometary nuclei Frozen water, ammonia, methane, and their mixtures were among the first studied compounds relevant to cometary-ice chemistry. Radiations used were gamma, electrons, and protons at MeV energies; the temperatures were between 5 and 77 K (Berger, 1961; Glasel, 1962; Oro, 1963). Cometary dosimetry and chemistry are examined in some details (Draganic´ et al., 1984, 1987): the chemical
Table 3 Main radioactive contributors of energy to cometary nuclei Isotopes with long half-lives: 40 K, 235 U and 238 U, 232 Th, 244 Pu, 129 I, 247 Cm, 10 Be, and 237 NP. Total dose over 4600 million years of a comet life: about 2.8 MGy (Ryan and Draganic´, 1986). Short-lived radionuclides:a 26Al, half-life of 0.7 Ma: about 10.88 MGy (Draganic´ et al., 1984). a
26 Al was the most important energy supplier in the large variety of short-lived radionuclides produced by supernova explosions. Its energy was sufficient to maintain a liquid water in the core of cometary nuclei.
Table 4 The cosmic rays as energy source for chemistry in cometary nuclei The cosmic rays consist of: electromagnetic radiation, electrons, protons, and heavy ions with energies ranging from 1 keV to ten thousand million GeV (103–1019 eV). Dominating ionizing species: cosmic protons, about 95% of the cosmic rays flux. Most abundant are the protons which have about 2-GeV energy and a range in water of about 20 m. Dose absorbed in a 10-cm layer over the cometary nucleus life (Ryan and Draganic´, 1986): Surface: 3000 MGy 20-m depth: 3 MGy
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Table 5 Radiolysis of a multicomponent aqueous system relevant to the chemistry of a cometary nucleus (Draganic´ et al., 1985) Chemical composition Constituents taken in proportion in which they appear in a dense interstellar cloud: HCN, CH3OH, CH3CN, C2H5CN, HCOOH=1.0: 0.6: 0.2: 0.1: 0.05. The total amounts were adjusted to a carbon to nitrogen ratio of 1.8 and a water content of about 50%. The total amount of CN bearing compounds was taken to correspond to 0.4% of the cometary mass. Radiolytic products Experiments at 310 K: about forty compounds are identified, among them aldehydes, amino and carboxylic acids. Abundant polymeric material with molecular weights up to 80 000 daltons. Experiments at 77 K: the radiolytic products like at 310 K but with radiation–chemical yields lower by one or two orders of magnitude.
composition of the model system and the irradiation conditions were relevant both to processing of surface by cosmic rays and a possible existence of liquid core produced due to radiogenic heat of 26 Al. The radiation chemistry of a water-dominated multicomponent system is studied after irradiation (60Co gammas) at 77 and 310 K in the MGy dose range (Draganic´ et al., 1985). The model system was based on the assumption that comets originated in the outer regions of primordial solar nebula (Biermann and Mitchel, 1978) and that interstellar compounds are the chemical constituents of the nuclei (Mitchell et al., 1981). The chemical composition of the model system and other details are given in Table 5. The impressive inventory of organic compounds formed in simulation experiments is presented elsewhere (Draganic´ and Vujosˇ evic´, 1998). It is worth noting that the amounts of radiolytic products in the above experiment were measured at increasing doses and the data obtained were also used to consider radiation–chemical pathways leading to their origins. The basic aspect of the radiolysis of a liquid system is found to be present also at 77 K although with radiation–chemical yields lower by one to two orders of magnitude. It should be noted that the analyses of products formed in irradiated ices were usually made after thawing the samples and some unknown postirradiation chemistry may take place. Its understanding is important as shown in experiments relevant to the formation of organic molecules in astrophysical ices at cryogenic temperatures (Schutte et al., 1993). Abundant presence of water ices in the Solar System (Weissman et al., 1999) is of primary importance for exobiology and will stimulate radiation research activities (Draganic´, 2003). The presence of hydrogen peroxide and a tenuous oxygen atmosphere of the icy Galilean satellites make the studies of ices of Europa, Ganymede, and Callisto very attractive. An efficient experimental approach waits for more information on the chemical composition and ages of ices. This is true for future studies of other icy bodies in the outer Solar
System, which represent a promising field in extraterrestrial radiation chemistry.
5. Some remarks The radiolysis of waters on the primitive Earth and radiation–chemical processing of extraterrestrial ices are natural phenomena (Draganic´ and Vujosˇ evic´, 1993). Published work suggests that they deserve more attention than it has been given until now in chemical studies of the origin of life, as well as in the search for pristine matter of the Solar System. One of the benchmarks of Alexander I. Oparin’s idea on the origin of life was an anoxic environment of the primitive Earth. It is still the basic approach in prebiotic chemistry. The potential role of ionizing radiation as energy source in primitive ocean and underground waters of early Earth is quite neglected in prebiotic chemistry. Radiation–chemical studies suggest, however, that the generation of oxidizing species (free radicals, oxygen, and hydrogen peroxide) cannot be disregarded. The annual radiolytic generations of O2 and H2O2 were modest in the early ocean, on the picomolar and micromolar scales, respectively. Yet, it is important to note that it has occurred in a large volume of water and on a large geological time scale. In addition, it has occurred homogeneously, uninterrupted, and undisturbed because of the nature of the energy supply process, namely, the radioactive decay of 40K. The finding of an intrinsic oxidizing capacity of the early hydrosphere due to the radiolysis of water may be of interest to the studies of chemical evolution. This could have played a role in leading to the earliest forms of living matter as well as in shaping the environment where oxygen-tolerant life-forms evolved (Draganic´, 2000). Also, worth mentioning is a large number of chemical species observed in radiation–chemical experiments relevant to prebiotic chemistry. It involves dozens of
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free-radical intermediates and molecules, including polymers with molecular weights up to 80 000 a.m.u. (Draganic´ and Vujosˇ evic´, 1998). Cometary ice is considered as a pristine material of presolar nebula. However, radiation–chemical simulation experiments suggest that a complex chemistry takes place in the nucleus during millions of years of its life. It is certain that present space programs, directed to provide samples of cometary material, will clearly show that radiation–chemical processing cannot be neglected when considering its pristine nature (Draganic´, 2003). References Akaboshi, M., Fujii, N., Navarro-Gonzalez, R. (Eds.), 2000, The Role of Radiation in the Origin and Evolution of Life. Kyoto University Press, Kyoto, Japan. Berger, R., 1961. The proton irradiation of methane, ammonia, and water at 77 K. Proc. Natl. Acad. Sci. USA 47, 1434–1436. Biermann, L., Mitchel, K.W., 1978. On the origin of cometary nuclei in the presolar nebula. The Moon, the Planets 18, 447–464. Bjergbakke, E., Draganic´, Z.D., Sehested, K., Draganic´, I.G., 1989. Radiolytic products in waters. Part II: Computer simulation of some radiolytic processes in nature. Radiochim. Acta 48, 73–77. Brack, A., Raulin, F., 1991. L’e´volution chimique et les origines de la vie. Masson, Paris. Domingue, D., Allamandola, L., 2001. Introduction to the special section: photolysis and radiolysis of outer solar system ices (PROSSI). J. Geophys. Res. E 106, 33,273. Draganic´, I.G., Draganic´, Z.D., 1971. The Radiation Chemistry of Water. Academic Press, New York. Draganic´, I.G., Draganic´, Z.D., Altiparmakov, D., 1983. Natural nuclear reactors and ionizing radiation in the Precambrian. Precambrian Res. 20, 283–298. Draganic´, I.G., Draganic´, Z.D., Vujosˇ evic´, S., 1984. Some radiation-chemical aspects of chemistry in cometary nuclei. Icarus 60, 464–475. Draganic´, Z.D., Vujosˇ evic´, S.I., Negro´n-Mendoza, A., Azamar, J.A., Draganic´, I.G., 1985. Radiation chemistry of a multicomponent aqueous system relevant to chemistry of cometary nuclei. J. Mol. Evol. 22, 175–187. Draganic´, I.G., Ryan Jr., M.P., Draganic´, Z.D., 1987. Radiation dosimetry and chemistry of a cometary nucleus. Adv. Space Res. 7, 13–16. Draganic´, I.G., Bjergbakke, E., Draganic´, Z.D., Sehested, K., 1991. Decomposition of ocean waters by potassium40 radiation 3800 Ma ago as a source of oxygen and oxidizing species. Precambrian Res. 52, 337–345. Draganic´, I.G., Draganic´, Z.D., Adloff, J.P., 1993. Radiation and Radioactivity on Earth and Beyond, second ed. CRC Press, Boca Raton, FL.
Draganic´, I.G., Vujosˇ evic´, S.I., 1993. Ionizing radiation and chemical processes of waters on early Earth. In: Ponnamperuma, C., Chela-Flores, J. (Eds.), Chemical Evolution: Origin of Life (Proceedings of the Trieste Conference on Chemical Evolution and the Origin of Life, 26–30 October 1992). A. Deepak Publishing, Hampton, VA. Draganic´, I.G., Vujosˇ evic´, S.I., 1998. Compounds observed in some radiation simulation experiments relevant to prebiotic chemistry. Viva Origino 26, 85–102. Draganic´, I.G., 2000. Radiation chemical approaches to chemical evolution processes on Earth and beyond. In: Akaboshi, M., Fujii, N., Navarro-Gonzalez, R. (Eds.), The Role of Radiation in the Origin and Evolution of Life. Kyoto University Press, Kyoto (Chapter 5). Draganic´, I.G., 2003. Radiation chemical processing of extraterrestrial ices. In: Celnikier, L.M., Tran Thanh Van, J. (Eds.), Frontiers of Life (Proceedings of the XIIth Rencontres de Blois, 25 June–1st July 2000). The Gioi Publishers, Hanoi, Vietnam. Dymek, R.F., Klein, C., 1988. Chemistry, petrology and origin of banded iron-formation lithologies from the 3800 Ma Isua supracrustal belt, West Greenland. Precambrian Res 39, 247–302. Ernst, W.G., 1983. The early Earth and the archean record. In: Schopf, J.W. (Ed.), Earth’s Earliest Biosphere. Its Origin and Evolution. Princeton University Press, Princeton. Ferradini, C., Jay-Gerin, J.-P., 1999. Radiolysis of water and aqueous solutions—History and present state of the science. Can. J. Chem. 77, 1542–1575. Glasel, J.A., 1962. The bombardment of solid D2O by lowenergy electrons under astronomical conditions. Proc. Natl. Acad. Sci. USA 48, 491–496. IAEA, 1975. The Oklo Phenomenon. International Atomic Energy Agency, Vienna. IAEA, 1978. The Natural Fission Reactors. International Atomic Energy Agency, Vienna. Miller, S.L., Orgel, L.E., 1974. The Origins of Life on the Earth. Prentice Hall, Englewood Cliffs, NJ. Mitchell, G.F., Prassad, S.S., Huntres, W.T., 1981. Chemical calculations of C2, C3, CH, CN, OH, and NH2 in cometary comae. Astrophys. J. 244, 1087–1093. Oro, J., 1963. Syntheses of organic compounds by high-energy electrons. Nature 197, 971–974. Ryan Jr., M.P., Draganic´, I.G., 1986. An estimate of the contribution of high energy cosmic-ray protons to the absorbed dose inventory of a cometary nucleus. Astrophys. Space Sci. 125, 49–67. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry. Wiley, New York. Schutte, W.A., Allamandola, L.J., Sandford, S.A., 1993. Formaldehyde and organic molecule production in astrophysical ices at cryogenic temperatures. Science 259, 1143–1145. Weissman, P.R., McFadden, L.-A., Johnson, T.V., 1999. Encyclopedia of the Solar System. Academic Press, San Diego, CA.
Radiation Physics and Chemistry 72 (2005) 181–186 www.elsevier.com/locate/radphyschem
Radiolysis of water: a look at its origin and occurrence in the nature Ivan G. Draganic´ Institute of Nuclear Sciences Vinc˘a, PO Box 522, 11001 Beograd, Serbia and Montenegro Received 5 September 2003; accepted 3 February 2004
Abstract The ubiquitous presence of water and ionizing radiation in the nature point out to the occurrence of radiolysis of water on Earth and beyond. Laboratory experiments and computer simulations of radiation-induced processes are reviewed: action of radioactive 40-K rays in early ocean water 3800 Ma ago and of natural nuclear reactor radiations in underground waters of the primitive Earth 3200 Ma ago. Also some findings relevant to the action of cosmic and radioactive rays in water ice of a cometary nucleus are presented. r 2004 Elsevier Ltd. All rights reserved. Keywords: Water radiolysis; Early ocean; Oklo phenomenon and underground waters; Cometary ice
1. Introduction Radiation-induced decomposition of water molecules, the radiolysis of water, is carefully studied from various reasons. Water is an important constituent of living matter. It has a significant role in nuclear technology. Physical chemists are attracted by a variety of shortliving reactive species in the radiolysis of water and aqueous systems. The radiolysis of water is a phenomenon known for almost one century and well understood (Ferradini and Jay-Gerin, 1999). However, the studies of the origin and occurrence of water radiolysis in the nature are somewhat neglected despite of the well-known fact of the ubiquitous presence of water and radiation in the nature. Such is the case with the studies of the origin of life and aqueous environment where the primitive living matter originated (Brack and Raulin, 1991). The situation is changing with an increasing interest on the role of ionizing radiation in the origin of life E-mail address: [email protected] (I.G. Draganic´).
(Akaboshi et al., 2000). Also, cometary chemistry in space programs, such as NASA’s Deep Impact Mission and similar ESA projects, is stimulating the studies of radiation chemistry of the outer Solar System ices (Domingue and Allamandola, 2001). The present work reviews some basic aspects of the origin and occurrence of water radiolysis on Earth and beyond; specific details of the radiation chemistry of water (Draganic´ and Draganic´, 1971) are outside the scope of this work.
2. Radioactive potassium (40-K) and early ocean water 2.1. Deposit of energy–the dose rate 40-K is a radioactive isotope, which decays (half-life: 1250 Ma) by emitting 1.46-MeV gamma (11%) and beta rays (89%), with an average energy 0.455 MeV.Its relative isotopic abundances on Earth, present and early Earth (4500 Ma ago), are 0.0117% and 0.145%, respectively.
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.09.012
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The amount of energy delivered over a year, the dose rate (eV kg1 a1), depends on the amount of radioactive potassium, i.e., on the composition of early ocean water and its content in the chemical element potassium. A reliable estimate was based on the Isua water-laid sediments of West Greenland, considered as representative of the state of Earth and very likely of ocean waters some 3800 Ma ago (Ernst, 1983; Dymek and Klein, 1988). They suggest that the salt content and the major chemical constituents were not essentially different from the contemporary ones. Consequently, the concentration of the chemical element potassium can be taken as 0.38 gkg1 of ocean water and that of 40-K as 0.38 mg kg–1. These data enable the calculation of the energy deposited annually by 40-K decay: 1:84 1015 eV kg1 a1 ð0:29 mGy a1 Þ: It should be noted that because of the long half-life of 40K, the above dose rate calculated for 3800 Ma ago would vary only by 75% for a shift of 7100 Ma. 2.2. Radiolysis of early ocean water (Draganic´ et al., 1991). The model system was ocean water with sodium, magnesium, calcium, strontium, and potassium as principal cationic, and chloride, bromide, sulphate, and bicarbonate as ionic constituents (Stumm and Morgan, 1981). The radiation-induced process was followed over a geologic time scale of one hundred million years (100 Ma). The beginning of the process is deliberately taken as 3800 Ma ago, the age of the Isua water-laid sediments of West Greenland. Computer modeling takes into account the radical– radical and radical–solute reactions in a process consisting of 87 reactions. Gaseous products (H2 and O2) accumulate with increasing dose but, unlike other radiolytic products, diffuse away and their liquid–gasphase concentration equilibrium was taken into account. It has been shown that the absence of minor constituents, or minor presence of new compounds, would not significantly affect the reaction mechanism because of the dominance of radiation–chemical reactivity of the chloride ion in the system considered. Also, the reaction mechanism is not sensitive to concentration changes if the concentration ratios of the major constituents are not affected, for example in the case of some local dilution or concentration (by freezing or evaporation). Analysis of computer simulation results shows that the annual generation of oxidizing radicals was modest, about 1011 mol dm–3 a–1. Depending on the presence of oxidizable substrates, these could have been more or less efficiently consumed leading to the syntheses of new compounds. A simple estimate
shows that about 1014 g of organic compound, with a molecular mass M p ¼ 75 a:m:u:; could have been synthesized in primitive oceans over 100 Ma. The total amount is significant even with a low-radiation chemical yield assumed in the above calculation: 0.15 molecules per 100 eV absorbed. The main issue in primitive ocean water is its decomposition with the formation of molecules of oxygen, hydrogen peroxide and hydrogen. When considering these data the total mass of ocean water was assumed to be M ocean ¼ 1:4 1024 g: Table 1 shows the total amounts of radiolytically generated radiolytic products obtained by the computer modeling. For comparison, the total oxygen content in the contemporary atmosphere is 1.2 1021 g. A significant amount of hydrogen peroxide is generated over 100 Ma. It is disappearing by reactions with primary free radicals (H, OH, e aq) and with oxidizible substrates present in the ocean water (ferrous ions, etc.); these reactions have been taken into account in the computer modeling. The generation of molecular hydrogen may also be worth noting: its amount is similar to that released by volcanic activity over the same period of time. Primitive hydrosphere contained very likely other constituents, inorganic and organic compounds, in addition to those taken into account in the reviewed computer simulations. Their origins were in various processes of different intensity and duration such as electric discharges, volcanic eruptions or cometary impacts (Miller and Orgel, 1974). It should be noted, however, that these chemical compounds were deposited locally and have been involved in chemistry before an efficient mixing occurred. Computer modeling may certainly provide a better insight also in such cases but this remains to be done.
Table 1 Radiolysis of primitive ocean water 3800 Ma agoa Radiolytic product
Amount generated over 100 Ma
Molecular oxygen
3.0 1019 g O2 /Mocean/ 100 Ma 1.1 1018 g H2O2 /Mocean/ 100 Ma 3.8 1018 g H2 /Mocean/ 100 Ma
Hydrogen peroxide Molecular hydrogen a
Computer modeling concerns a process of 87 radical–radical and radical–solute reactions (Draganic´ et al., 1991). A total mass of ocean water is assumed to be Mocean=1.4 1024 g and its chemical composition is taken according to Ernst (1983) and Dymek and Klein (1988).
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3. Oklo phenomenon and radiolysis of underground waters of early Earth The phenomenon of Oklo concerns the old geological arrangements of water and uranium in which the fission chain process of uranium could be triggered. These particular nuclear fission zones are also known as natural nuclear reactors (IAEA, 1975, 1978). Underground waters had an important role in making of uranium deposits a natural nuclear reactor like in the 2000 Ma-old deposit in Oklo (Gabon, Africa). They contributed in propagating and controlling the fission chain process. At the same time the water was radiolyzed and the dissolved substances chemically altered. Underground water drain was of help to circulate efficiently the radiolytic products and take part in the chemistry on the primitive Earth. 3.1. Deposit of nuclear reactor radiation energy (Draganic´ et al., 1983) Natural nuclear reactors were sources of mixed radiations, charged particles (fission fragments, beta rays), neutrons, and gamma rays. Operating at a kilowatt-level power, their dose rates were similar to those available in small irradiation units in routine use in laboratories. Inpile dosimetry shows that within a reactor core with a diameter of 2 m, a natural nuclear reactor operating 3200 Ma ago at a power of 3 kW provided the dose rate of –1 8.1 1020 eV kg–1 h1 (1.3 kGy h ). 3.2. Radiolysis of sandstone underground water (Bjergbakke et al., 1989) The system used in the computer modeling was based on the Oklo phenomenon data: chemical composition of water, its high temperature and pressure, and a 3 kw-level power.
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Forty reactions were used in the computer simulation of radiation-induced processes in sandstone water. Bicarbonate ions dominate the radiolysis process. Main molecular radiolytic products are O2, H2O2, and H2 (Table 2). Data obtained concern the equilibrium concentrations in a closed radiation zone (gases cannot escape) after 1 month of operation. The results also show that small amounts of carbon dioxide, formate, and oxalate are formed. To estimate the total amounts produced over one reactor life the average operation time of Oklo reactors was taken as 0.7 Ma, with an average water flow-rate of 3 m3/ month. The data in Table 2 show the amounts produced during the operation time of one natural nuclear reactor only. A reliable estimate of the total production of oxidizing chemical species in underground water flowing through natural nuclear reactors on our planet remains to be made. It depends on the estimates of the potential number of reactors that could have been active between 4100 and 1800 Ma ago. The estimates vary from a conservative 103 to a more realistic value of 108 operating natural nuclear reactors (Draganic´ et al., 1993). Hence, the production of molecular oxygen could be up to 1015 g, and that of hydrogen peroxide up to 1013 g. It is worth noticing that these amounts of oxidizing species were generated, and contributed to chemistry, in underground waters of early Earth. One aspect of natural nuclear reactors as energy sources is also worth mentioning: during 1 year of criticality at a power of 3 kW, 6.9 1021 fission product radionuclides are produced in the reactor core. About 14% have half-lives longer than 1 year and could provide irradiation during reactor ‘‘shut-down’’ periods. Also, some of them would migrate (7% in the Oklo case) and continue to irradiate after their fixation in the environment or their uptake by the Precambrian biota.
Table 2 Radiolysis of sandstone underground water in Oklo natural nuclear reactors 3200 Ma agoa Radiolytic product
Molecular oxygen Hydrogen peroxide Molecular hydrogen
Amount After 1 month of reactor operation 1.5 104 mol/dm3 13.4 107 mol/dm3 3.2 104 mol/dm3
Total amounts produced over one reactor life The average operation time of Oklo reactors was assumed to be 0.7 Ma, with an average water flow-rate of 3m3/month. Molecular oxygen 1.2 108 gO2 Hydrogen peroxide 2.9 105 g H2O2 Molecular hydrogen 1.6 107 g H2 a Computer modeling concerns a process of 40 radical–radical and radical–solute reactions (Bjergbakke et al., 1989). It is based on information on the Oklo phenomenon: chemical composition of water, its high temperature and pressure, and a power of 3 kW.
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4. Radioactive radiation, cosmic rays, and extraterrestrial ices Radiation-generated free radicals and radical ions are efficient promoters of chemical processes in space, occurring even in conditions considered hostile for chemistry known in the laboratory (Akaboshi et al., 2000; Domingue and Allamandola, 2001; Draganic´, 2003). Space projects have provided some general information on the chemical composition of cometary matter helping in reliably defining a model system for simulation experiments. Understanding of extraterrestrial radiation chemistry is presently based mainly on laboratory studies.
nuclides are involved (secondary, tertiary, etc.). An analytical formula is developed for the proton energy deposit and the dose-depth curve calculated down to 20 m (Ryan and Draganic´, 1986). The contribution of other cosmic rays (gamma, electrons, heavy particles) raises only the total dose within the first 1-meter layer (by as much as 100%); it decreases rapidly to become less than 1% already at a depth of 2 m. Radiation damage is also an important aspect of the interaction of cosmic ray-protons with cometary material. On its way through the solid icy matrix the energetic particle collides and knocks out the atoms encountered with formation of vacancies. The displaced atoms end in the interstitial space between atoms. In such events, also hot atoms are formed which cause in situ chemical changes.
4.1. Sources of ionizing radiation in extraterrestrial space Decays of radioactive elements and cosmic rays provide the energy for chemistry in space. An estimate of radiation energy deposited in an extraterrestrial dust particle depends on the assumptions on its chemical composition and its life, and can be made only in specific cases. Generally accepted facts on comets, however, make such estimates possible for a cometary nucleus (Tables 3 and 4). The interaction of high-energy protons with cometary matter is a very complex cascade process where other
4.2. Simulation experiments relevant to chemistry in cometary nuclei Frozen water, ammonia, methane, and their mixtures were among the first studied compounds relevant to cometary-ice chemistry. Radiations used were gamma, electrons, and protons at MeV energies; the temperatures were between 5 and 77 K (Berger, 1961; Glasel, 1962; Oro, 1963). Cometary dosimetry and chemistry are examined in some details (Draganic´ et al., 1984, 1987): the chemical
Table 3 Main radioactive contributors of energy to cometary nuclei Isotopes with long half-lives: 40 K, 235 U and 238 U, 232 Th, 244 Pu, 129 I, 247 Cm, 10 Be, and 237 NP. Total dose over 4600 million years of a comet life: about 2.8 MGy (Ryan and Draganic´, 1986). Short-lived radionuclides:a 26Al, half-life of 0.7 Ma: about 10.88 MGy (Draganic´ et al., 1984). a
26 Al was the most important energy supplier in the large variety of short-lived radionuclides produced by supernova explosions. Its energy was sufficient to maintain a liquid water in the core of cometary nuclei.
Table 4 The cosmic rays as energy source for chemistry in cometary nuclei The cosmic rays consist of: electromagnetic radiation, electrons, protons, and heavy ions with energies ranging from 1 keV to ten thousand million GeV (103–1019 eV). Dominating ionizing species: cosmic protons, about 95% of the cosmic rays flux. Most abundant are the protons which have about 2-GeV energy and a range in water of about 20 m. Dose absorbed in a 10-cm layer over the cometary nucleus life (Ryan and Draganic´, 1986): Surface: 3000 MGy 20-m depth: 3 MGy
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Table 5 Radiolysis of a multicomponent aqueous system relevant to the chemistry of a cometary nucleus (Draganic´ et al., 1985) Chemical composition Constituents taken in proportion in which they appear in a dense interstellar cloud: HCN, CH3OH, CH3CN, C2H5CN, HCOOH=1.0: 0.6: 0.2: 0.1: 0.05. The total amounts were adjusted to a carbon to nitrogen ratio of 1.8 and a water content of about 50%. The total amount of CN bearing compounds was taken to correspond to 0.4% of the cometary mass. Radiolytic products Experiments at 310 K: about forty compounds are identified, among them aldehydes, amino and carboxylic acids. Abundant polymeric material with molecular weights up to 80 000 daltons. Experiments at 77 K: the radiolytic products like at 310 K but with radiation–chemical yields lower by one or two orders of magnitude.
composition of the model system and the irradiation conditions were relevant both to processing of surface by cosmic rays and a possible existence of liquid core produced due to radiogenic heat of 26 Al. The radiation chemistry of a water-dominated multicomponent system is studied after irradiation (60Co gammas) at 77 and 310 K in the MGy dose range (Draganic´ et al., 1985). The model system was based on the assumption that comets originated in the outer regions of primordial solar nebula (Biermann and Mitchel, 1978) and that interstellar compounds are the chemical constituents of the nuclei (Mitchell et al., 1981). The chemical composition of the model system and other details are given in Table 5. The impressive inventory of organic compounds formed in simulation experiments is presented elsewhere (Draganic´ and Vujosˇ evic´, 1998). It is worth noting that the amounts of radiolytic products in the above experiment were measured at increasing doses and the data obtained were also used to consider radiation–chemical pathways leading to their origins. The basic aspect of the radiolysis of a liquid system is found to be present also at 77 K although with radiation–chemical yields lower by one to two orders of magnitude. It should be noted that the analyses of products formed in irradiated ices were usually made after thawing the samples and some unknown postirradiation chemistry may take place. Its understanding is important as shown in experiments relevant to the formation of organic molecules in astrophysical ices at cryogenic temperatures (Schutte et al., 1993). Abundant presence of water ices in the Solar System (Weissman et al., 1999) is of primary importance for exobiology and will stimulate radiation research activities (Draganic´, 2003). The presence of hydrogen peroxide and a tenuous oxygen atmosphere of the icy Galilean satellites make the studies of ices of Europa, Ganymede, and Callisto very attractive. An efficient experimental approach waits for more information on the chemical composition and ages of ices. This is true for future studies of other icy bodies in the outer Solar
System, which represent a promising field in extraterrestrial radiation chemistry.
5. Some remarks The radiolysis of waters on the primitive Earth and radiation–chemical processing of extraterrestrial ices are natural phenomena (Draganic´ and Vujosˇ evic´, 1993). Published work suggests that they deserve more attention than it has been given until now in chemical studies of the origin of life, as well as in the search for pristine matter of the Solar System. One of the benchmarks of Alexander I. Oparin’s idea on the origin of life was an anoxic environment of the primitive Earth. It is still the basic approach in prebiotic chemistry. The potential role of ionizing radiation as energy source in primitive ocean and underground waters of early Earth is quite neglected in prebiotic chemistry. Radiation–chemical studies suggest, however, that the generation of oxidizing species (free radicals, oxygen, and hydrogen peroxide) cannot be disregarded. The annual radiolytic generations of O2 and H2O2 were modest in the early ocean, on the picomolar and micromolar scales, respectively. Yet, it is important to note that it has occurred in a large volume of water and on a large geological time scale. In addition, it has occurred homogeneously, uninterrupted, and undisturbed because of the nature of the energy supply process, namely, the radioactive decay of 40K. The finding of an intrinsic oxidizing capacity of the early hydrosphere due to the radiolysis of water may be of interest to the studies of chemical evolution. This could have played a role in leading to the earliest forms of living matter as well as in shaping the environment where oxygen-tolerant life-forms evolved (Draganic´, 2000). Also, worth mentioning is a large number of chemical species observed in radiation–chemical experiments relevant to prebiotic chemistry. It involves dozens of
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free-radical intermediates and molecules, including polymers with molecular weights up to 80 000 a.m.u. (Draganic´ and Vujosˇ evic´, 1998). Cometary ice is considered as a pristine material of presolar nebula. However, radiation–chemical simulation experiments suggest that a complex chemistry takes place in the nucleus during millions of years of its life. It is certain that present space programs, directed to provide samples of cometary material, will clearly show that radiation–chemical processing cannot be neglected when considering its pristine nature (Draganic´, 2003). References Akaboshi, M., Fujii, N., Navarro-Gonzalez, R. (Eds.), 2000, The Role of Radiation in the Origin and Evolution of Life. Kyoto University Press, Kyoto, Japan. Berger, R., 1961. The proton irradiation of methane, ammonia, and water at 77 K. Proc. Natl. Acad. Sci. USA 47, 1434–1436. Biermann, L., Mitchel, K.W., 1978. On the origin of cometary nuclei in the presolar nebula. The Moon, the Planets 18, 447–464. Bjergbakke, E., Draganic´, Z.D., Sehested, K., Draganic´, I.G., 1989. Radiolytic products in waters. Part II: Computer simulation of some radiolytic processes in nature. Radiochim. Acta 48, 73–77. Brack, A., Raulin, F., 1991. L’e´volution chimique et les origines de la vie. Masson, Paris. Domingue, D., Allamandola, L., 2001. Introduction to the special section: photolysis and radiolysis of outer solar system ices (PROSSI). J. Geophys. Res. E 106, 33,273. Draganic´, I.G., Draganic´, Z.D., 1971. The Radiation Chemistry of Water. Academic Press, New York. Draganic´, I.G., Draganic´, Z.D., Altiparmakov, D., 1983. Natural nuclear reactors and ionizing radiation in the Precambrian. Precambrian Res. 20, 283–298. Draganic´, I.G., Draganic´, Z.D., Vujosˇ evic´, S., 1984. Some radiation-chemical aspects of chemistry in cometary nuclei. Icarus 60, 464–475. Draganic´, Z.D., Vujosˇ evic´, S.I., Negro´n-Mendoza, A., Azamar, J.A., Draganic´, I.G., 1985. Radiation chemistry of a multicomponent aqueous system relevant to chemistry of cometary nuclei. J. Mol. Evol. 22, 175–187. Draganic´, I.G., Ryan Jr., M.P., Draganic´, Z.D., 1987. Radiation dosimetry and chemistry of a cometary nucleus. Adv. Space Res. 7, 13–16. Draganic´, I.G., Bjergbakke, E., Draganic´, Z.D., Sehested, K., 1991. Decomposition of ocean waters by potassium40 radiation 3800 Ma ago as a source of oxygen and oxidizing species. Precambrian Res. 52, 337–345. Draganic´, I.G., Draganic´, Z.D., Adloff, J.P., 1993. Radiation and Radioactivity on Earth and Beyond, second ed. CRC Press, Boca Raton, FL.
Draganic´, I.G., Vujosˇ evic´, S.I., 1993. Ionizing radiation and chemical processes of waters on early Earth. In: Ponnamperuma, C., Chela-Flores, J. (Eds.), Chemical Evolution: Origin of Life (Proceedings of the Trieste Conference on Chemical Evolution and the Origin of Life, 26–30 October 1992). A. Deepak Publishing, Hampton, VA. Draganic´, I.G., Vujosˇ evic´, S.I., 1998. Compounds observed in some radiation simulation experiments relevant to prebiotic chemistry. Viva Origino 26, 85–102. Draganic´, I.G., 2000. Radiation chemical approaches to chemical evolution processes on Earth and beyond. In: Akaboshi, M., Fujii, N., Navarro-Gonzalez, R. (Eds.), The Role of Radiation in the Origin and Evolution of Life. Kyoto University Press, Kyoto (Chapter 5). Draganic´, I.G., 2003. Radiation chemical processing of extraterrestrial ices. In: Celnikier, L.M., Tran Thanh Van, J. (Eds.), Frontiers of Life (Proceedings of the XIIth Rencontres de Blois, 25 June–1st July 2000). The Gioi Publishers, Hanoi, Vietnam. Dymek, R.F., Klein, C., 1988. Chemistry, petrology and origin of banded iron-formation lithologies from the 3800 Ma Isua supracrustal belt, West Greenland. Precambrian Res 39, 247–302. Ernst, W.G., 1983. The early Earth and the archean record. In: Schopf, J.W. (Ed.), Earth’s Earliest Biosphere. Its Origin and Evolution. Princeton University Press, Princeton. Ferradini, C., Jay-Gerin, J.-P., 1999. Radiolysis of water and aqueous solutions—History and present state of the science. Can. J. Chem. 77, 1542–1575. Glasel, J.A., 1962. The bombardment of solid D2O by lowenergy electrons under astronomical conditions. Proc. Natl. Acad. Sci. USA 48, 491–496. IAEA, 1975. The Oklo Phenomenon. International Atomic Energy Agency, Vienna. IAEA, 1978. The Natural Fission Reactors. International Atomic Energy Agency, Vienna. Miller, S.L., Orgel, L.E., 1974. The Origins of Life on the Earth. Prentice Hall, Englewood Cliffs, NJ. Mitchell, G.F., Prassad, S.S., Huntres, W.T., 1981. Chemical calculations of C2, C3, CH, CN, OH, and NH2 in cometary comae. Astrophys. J. 244, 1087–1093. Oro, J., 1963. Syntheses of organic compounds by high-energy electrons. Nature 197, 971–974. Ryan Jr., M.P., Draganic´, I.G., 1986. An estimate of the contribution of high energy cosmic-ray protons to the absorbed dose inventory of a cometary nucleus. Astrophys. Space Sci. 125, 49–67. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry. Wiley, New York. Schutte, W.A., Allamandola, L.J., Sandford, S.A., 1993. Formaldehyde and organic molecule production in astrophysical ices at cryogenic temperatures. Science 259, 1143–1145. Weissman, P.R., McFadden, L.-A., Johnson, T.V., 1999. Encyclopedia of the Solar System. Academic Press, San Diego, CA.