RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES 31 Fig.2. Dose distribution in one-sided (A) and two-sided (B) massive bone grafts. of irradiated object and the presence of bone chan- It is concluded that for massive bone fragments nels. For both dosimeters, the maximum dose was one-sided irradiation does not guarantee steriliza- equal to about 120% of the initial value, and the tion of the whole fragments, therefore two-sided dose distribution was very similar to radical con- irradiation should be applied as a routine in that centration distribution. case. RADIATION CHEMISTS VIEW ON PANSPERMIA HYPOTHESIS Zbigniew P. Zagórski The year 2006 was the last full year of my partici- The doses which decide about very low prob- pation as the member of Managing Committee in ability of panspermia are similar to doses applied the European action in chemistry COST D27 (Pre- in technology of radiation sterilization, widely used biotic chemistry and early evolution), as the only for killing all microorganisms in medical devices and radiation and radiochemist in the Committee. The medicaments [5]. The radiation should not damage field is closely connected to the so-called astro- the sterilized material, which survives the irradia- biology, which deals with life and prebiotic chemi- tion with a minimal loss of active compound or re- cal compounds present anywhere in the universe. mains with negligible degradation of material, usu- Problems involve transportation of that material ally a polymer. The same applies to celestial bodies: in any directions, what can be involved in the ori- if the sojourn in space is comparatively short, the gin of life on earth. only change, however important, is the killing of life, The very old [1] concept of panspermia hypoth- e.g. of spores which otherwise would be the seeds esis may be traced to Greek philosophers. Even now, of life transported to earth. As one can see from the idea that the transportation of spores from the the Clark table, the time of travelling in space with universe is responsible for the origin of life on resulting sterilization only, is astronomically speak- earth, is always widely accepted as long as it is not ing short. However, the real sojourn in radiation confronted with conditions in the universe. These fields lasts very much longer and the absorbed, ad- are characterized mainly by the existence of invis- ditive doses grow [6]. ible, but chemically and biologically active ionizing Clark’s table (in modified version see [4]) shows radiations. Their intensity is sufficient to kill highly more than when and in what object the spores will organized life and also primitive life, if the time of be inactivated. What happens during longer stays, interaction is long enough. We have presented the especially to objects of small size like cosmic dust, quantitative relationships on practically all work- easily penetrated not only by electromagnetic ion- shops organized during COST D27 meetings and izing radiations like gammas, but also by particles other conferences, e.g. on ISSOL 05 [2], stressing like protons and by heavier ions, present in galac- the chemical mechanism of the irreversible radia- tic cosmic rays? These objects are absorbing mam- tion damage to dry spores. The best description of moth doses of radiation energy, which causes many possibilities of spores to survive the transportation chemical reactions. The next to sterilization dose in the universe is the table published by Benton is the dose not very much higher, causing abstrac- Clark [3], then slightly modified [4]. Clarks paper tion of parts of biopolymers or parts of organic is seldom quoted; it is simply non-existent among compounds. There are many examples of such re- enthusiasts of panspermia. actions, the most general is the fact common in 32 RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES radiation chemistry, i.e. the abstraction of a group dramatically lower in comparison to those at am- which leaves in the molecule a site with unpaired bient temperatures. electron. The common reaction of abstraction of hydro- Another example is the destruction of chirality. gen, not limited to spores but of general occurrence Abstraction of a group, e.g. of ammonia from the [8], corroborates well with characteristic feature α-carbon in an amino acid, turns off the asymmetry of organic compounds, identified spectroscopically of the remaining radical. The free radical can attach in interstellar spaces of the universe. They are as a group, and the new compound will be chiral, if poor in hydrogen as possible, e.g. methyltriacetylene the group is different from the remaining three. (C7H4), methylcyanodiacetylene (C6NH3), cyano- Therefore, if the original amino acid was a pure allene (C4NH4), ketenimine (C2NH3), cyclopro- enantiomer, any products of the subsequent reac- penone (C6OH2) and many polyaromatics. The tion will keep chirality, but will be racemic, 1:1 D remaining hydrogen atoms are difficult to detach to L. Hypothetical homochirality of a compound during absorption of ionizing radiation, because achieved (perhaps) far from the Earth will be lost they usually belong to structures, mainly polyun- during the travel. saturated and aromatic which possess the ability Moving to higher doses resulting from even to dissipate the absorbed ionization energy with- longer stays in the universe, we are turning into out ionization. the zone of complete degradation, and sometimes The question remains why the life on earth is even to disappearance of an organic compound. not destroyed after sufficiently long action of ion- The Scheme shows possible degradation reactions. izing radiation. The ionizing radiation which im- pregnates the universe is reaching the surface of earth in much reduced intensity, due to the shield of the atmosphere, equivalent to 3 meters of con- crete. Even the most penetrating galactic cosmic rays, which reach the earth in cascades of second- ary ionizing radiations are responsible for only small contributions to mutations forming in living organisms. Other ionizing radiations coming from outer space, like gamma bursts, proton beams from events in the Sun and other stars, are almost fully absorbed. Therefore, the contribution of space radiation to the radiation background on earth is Scheme. only similar to the present level supplied by radio- active material on earth. The deciding fact causing The final result can depend on the surrounding, not the inactivation of life on earth but, vice versa, whether oxidative presence of oxygen, or reducing, even some positive effects, is the interaction of or vacuum. In both two last named cases, the final radiation with living matter. Whereas the DNA in product can be nano-sized carbon dust, and such the living species, i.e. in aqueous suspensions, when reaction is called radiation-induced carbonization. damaged slightly by ionizing radiation can be re- Large objects staying for a long time in space paired, the DNA in dry spores is effectively, irre- obtain highest doses of ionizing radiation at the versibly damaged by the very first doses of radia- surface, gradually diminishing towards the inside. tion. The effect is additive, and even low doses are The depth dose can be estimated from the Clark damaging, after accumulation of elementary acts, table. The surface of the object is the most irradi- mainly of hydrogen abstraction, over ages. Repaired ated part and the best known object is the planet of damages to living populations do not accumulate. Mars [7]. Its regolith is not only completely steril- The by-product of low level radiation damage are ized but also relieved from organics. Perhaps part possible mutations, like many other factors in the of organics has been degraded to elementary car- environment. They occur only due to the action on bon, present, if so, as black dust moved with the living organisms, they do not occur when spores Martian thin CO2-wind. Anyway, chances for de- are irradiated, when irreversible changes take place. tection of organics on the surface of Mars are poor. The soon coming fifth anniversary of the COST Similar conclusions have been drawn by photo- action D27 asks for formulation of conclusions. In- chemists, assuming devastating chemical influence tensive, also experimental investigation of the frag- of deep UV reaching the surface of Mars. Their ar- ment of the action connected with panspermia and gument is poor, because supposed the Martian or- more, i.e. the action of ionizing radiation in the ganics show usually no absorption in UV, but even universe on objects travelling in space, demands in the presence of chromophoric groups the effect revision of some well established opinions. Theoreti- is of low importance, because the depth of penetra- cal considerations as well as experimental simula- tion is shallow. Therefore, the deciding influence of tions show that these objects are not only sterilized, ionizing radiation is more important. what demands comparatively low doses, but also The low temperature of space on the route of deprived of its homochirality, if any, and eventually moving objects is not a protecting factor, because even freed from organic compounds. Therefore, here radiation-induced reactions proceed with low ac- are examples of defined conclusions: tivation energy and presented chemical effects The very thoroughly investigated Murchison observed at liquid nitrogen temperatures are not meteorite, supposed to originate ca. 4 Giga years RADIATION CHEMISTRY AND PHYSICS, RADIATION TECHNOLOGIES 33 ago and bringing to the earth samples of organic years it was radiolysed down to inorganic remains. compounds, including amino acids from the begin- Therefore, there should be no hope to find organic ning of the solar system, could not be so old as matter whatsoever on Mars. And really, no traces claimed. Such a long sojourn in space is connected of organics have been found yet. Actual efforts to with the deamination of amino acids and related refine methods of investigation for organic matter loss of homochirality.