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. BIOLOGICAL CONTAMINATION OF THE MOON A. Possible Kinds of Biological Contamination There are four possible kinds of extraterrestrial biological contamination. For later discussion we categorize them, for the Moon, under the following headings: Biomixy The Moon may contain no indigenous living organisms, and may be incapable of supporting terrestrial organisms. Neverthe- less there may be relics of primitive indigenous organisms and deposited cosmobiota on or beneath the surface. Especially on a low-gravity, high-vacuum body such as the Moon, a vehicle im- pacting at or near escape velocity will distribute its contents over most of the lunar surface. Subsequent investigations might then be unable to distinguish among primitive indigenous organisms, cosmobiota, and terrestrial microbiological contamination. Sapromixy The Moon may contain no indigenous living organisms, and may be incapable of supporting terrestrial organisms. But sub- surface prebiological organic matter may exist which would be in- distinguishable from deposited terrestrial organic matter, whether biological or abiological in origin. Phagomixy The Moon may contain no indigenous living organisms, but may be capable of supporting some terrestrial organisms. This would require subsurface organic matter, moisture, and heat sources. The possibility then exists that a deposited terrestrial microorganism, in the absence of biological competitors or pre- dators, will multiply at a geometric rate limited only by the avail- ability of water and metabolities. Such a biological explosion might in a short time destroy large quantities of organic matter produced in the early history of the Moon. 34
Ecomixy The Moon may contain indigenous living organisms. There is then the possibility that deposited terrestrial microorganisms, by competition with or parasitism upon even one species of lunar organism, will completely disrupt the autochthonous ecology. We are interested in evaluating these possibilities in the light of the previous discussion. B. Distribution of Vehicle Impact Products Existing unfueled final stage carrier rockets of vehicles capable of lunar impact have masses of the order of 104 kg. If the carrier rockets are solid-fueled, then burning at very high temperatures occurs through most of the carrier interior, and very few of the contained microorganisms will survive. On the other hand, if the carrier rockets are liquid-fueled, no burning occurs in the fuel tanks, and many of the contained microorganisms should survive the powered phase. The final stage of Lunik II is believed to have been liquid-fueled. The energy of impact in a vehicle arriving at escape velocity is 2 x 1010 ergs gm"1, or about 0. 5 kcal gm" . Such energies applied over short time intervals are insufficient to kill most microorganisms, as the existence of shells and bombs for biological warfare proves. If the vehicle comes to a stop on the lunar sur- face after digging a crater one to ten meters deep, the mean ac- celeration will have been "10 to "10Â° g, applied over 1 to 0. 1 secs. Extrapolation of ultracentrifugation data on whole unlysed bacteria containing no large granular inclusions suggests that ac- celerations of these magnitudes and durations will not be lethal for many bacteria and viruses (Marr, 1961). Assume that a 10 kg liquid-fueled vehicle with a micro- organism population of 1010 per kg impacts the Moon at escape velocity (hard landing). It is easy to show that for a Maxwell- Boltzmann distribution of velocities, the fraction of particles moving with velocities less than some critical velocity, vc, is given by F = erf x - (Z/v/Tr) e-x2 x (16) where x = vc/vm, vm is the mean particle velocity, and x 2 erf x = (2/v^r) C e"V dy 35
is the error function. If half the energy of impact goes into the kinetic energy of the explosion products, vm = ve/ V2T where ve is the velocity of escape. From equation (16), with x = v27 the fraction of particles with velocities less than the velocity of escape after impact is seen to be F * 0. 74. Similarly, the fraction of particles with velocities less than the circular velocity after im- pact is F * 0.43. Hence the fraction of impact products with velocities between circular and escape velocity is 0. 31. Half of these particles will be moving in a downwards direction. The remaining half, or about fifteen percent of the particles, will be distributed approximately uniformly over the lunar surface. Since the impact will not kill the microorganisms contained in the im- pacting vehicle, the example we have chosen gives a mean surface density of about 0. 4 microorganisms per square meter of the Moon. Near the impact area, the surface density of microorga- nisms will be considerably greater. We have calculated that all but the small fraction of deposited microorganisms which is shielded from solar illumination will be killed by ultraviolet radiation in hours (section VI E). Therefore the mean surface density of viable microorganisms deposited in our example should be less than 0. 01 m"2. C. Evaluation of Contamination Possibilities This surface density of viable microorganisms is well below that detectable by existing biological techniques, such as plating. Lederberg (1959; v. also Davies and Comuntzis, 1959) believes that existing techniques can be immediately extended to detect one microorganism m"2, but considerable further refinement would be required to detect 10"2 m"2 where subsurface sample- gathering is also required. Cosmobiota and remnants of indigeneous lunar organisms, if such exist, would be sequestered almost ex- clusively at much greater depths below the surface than would deposited terrestrial microorganisms. We conclude that the probability of biomictic contamination of the Moon is very low. Since a typical bacterium has a mass of roughly 10"12 gm, the amount of organic matter deposited as microorganism in our example is 10"1" gm cm"2, an amount completely indetectable, and entirely negligible compared with the amount of indigenous organic matter which has probably survived from the early history of the Moon (section II G). A similar conclusion follows for organic matter arising from vehicle structural components, although it is clear that the use of such substances (e. g., shellac) should be minimized. We conclude that the probability of sapromictic contami- nation is negligible. 36
We have concluded in section V that at a depth of a few tens of meters below dust-covered portions of the lunar surface there may well be large amounts of organic matter, some moisture, and constant temperatures which are near optimum for contemporary terrestrial organisms. A viable terrestrial microorganism in- troduced into such an environment might reproduce very rapidly. The extent of phagomictic contamination would depend on the degree to which concentrations of organic matter are in contact under the lunar surface, on possible self-limitation of the reproduction rate by accumulation of catabolites, and, of course, on the presence of the specific growth requirements for individual varie- ties of microorganisms. However, on the basis of the substances produced in terrestrial laboratory experiments simulating primi- tive conditions, and in the light of such phenomena as adaptive enzyme formation in microorganisms introduced into petroleum deposits (v., e.g., ZoBell, 1950), the presence of suitable metab- olites seems possible. It is very improbable that a given organism deposited near the surface would find its way to a depth of tens of meters, but when 10 microorganisms are deposited, the situa- tion is very different. Although the presence of appropriate temp- eratures, moisture and organic matter for terrestrial micro- biological multiplication remains to be rigorously demonstrated, at the present writing the likelihood of phagomictic contamination of the Moon is not negligible. We have also discussed in section V the possibility that in- digenous lunar organisms arose in primitive times and have sur- vived in a subsurface existence to the present. Although, of course, no final answer can now be given to this question, we have seen that the possibility of an extant lunar parabiology is by no means so implausible as has sometimes been assumed. Since we are con- sidering lunar organisms based on the same small molecules as are terrestrial organisms, confrontation of the two groups of organisms may well lead to ecological interaction. The chances of extra- terrestrial autochthons having a dextrorotary rather than a levorotary stereochemistry are probably one in two, but it should be remembered that some terrestrial microorganisms have the enzymatic capability of transforming dextrorotary to levorotary stereoisomers. Even if indigenous lunar organisms exist, the occurrence and extent of ecomictic contamination will depend on the detailed biochemistry and ecology of both the autochthons and the contaminants; but in our present ignorance the possibility of ecomixy cannot be excluded. Of all the kinds of extraterrestrial biological contamination, this would represent the greatest loss. 37
It is clear that a reliable estimate of the phagomictic and ecomictic possibilities must await further information. Sterilized soft-landing instrumented probes can provide this information. Details on the microstructure of the lunar surface in areas distant from maria and large craters are needed. Subsurface borings, as described in section XXI, are of the greatest importance. In addi- tion to the analytic chemistry recommended for boring cores, plating of samples from various depths is suggested. Automatic devices for the detection of live microorganisms by planetary probes are already under construction (Vishniac, 1959; Loderberg, 1960). Similar instrumentation should be included among the first soft-landing lunar probe to have subsurface boring capability. An additional consideration for the question of possible bio- mictic or ecomictic contamination of the Moon has recently been made by A. Turkevich (v. E. Anders, "The Moon as a Collector of Biological Material, " Enrico Fermi Institute for Nuclear Studies Report No. EFINS-61-8, University of Chicago, 1961; and Science, in press). He suggests that meteoritic impact on the Earth during geological time may have ejected terrestrial microorganisms to the Moon; and that samples of now extinct microorganisms may be found on the Moon. If large numbers of terrestrial microorganisms have been deposited on the Moon during geological time by this mechanism, then biological contamination may have already oc- curred. But recontamination by contemporary terrestrial micro- organisms would destroy unique opportunities in microbial paleon- tology. D. Decontamination Recommendations for Unmanned and for Manned Vehicles It is recommended that all future lunar probes be scrupulously decontaminated. Sterilization methods have received careful at- tention in recent months, and working methods have now been developed (Davies and Comuntzis, 1959; Philips and Hoffman, 1960). In general there are four stages of sterilization: sterile fabrication and assembly of components which might be damaged by subsequent heat, chemical, or radiation sterilization tech- niques; use of germicidal substances in vehicle construction; terminal sterilization by heat, radiation, and chemicals (especially ethylene oxide); and maintenance of sterilization by encasing the probe with a shroud containing a disinfectant atmosphere, the shroud being discarded only after passing through the Earth's atmosphere. 38
To date, two man-made objects have impacted the Moon, the instrument package and the 7700 kg carrier rocket of Lunik II. According to reports from the Soviet Union, both were steri- lized (Cause, 1959). From press reports it appears that a disin- fectant atmosphere was employed, but a detailed description of the decontamination procedures has yet to appear. Sterilization of all United States lunar and planetary probes which have a sub- stantial probability of impact has been announced as a firm ob- jective of the National Aeronautics and Space Administration. The microorganism population of a mammal may be as high as lO1^. Therefore no landing of animals or men should be at- tempted on the Moon until more information is available. If in the light of this information the possibilities of phagomictic or ecomictic contamination appear non-negligible, manned soft land- ings should be safeguarded by sophisticated decontamination tech- niques. Decontamination should be standard procedure during each air-lock operation, both on leaving and on re-entering the vehicle. Space suits must be designed to eliminate cracks and joints in which microorganisms might lodge inaccessible to de- contamination techniques. Finally precautions against explosive decompression and accidental hard landing should be even more rigorous than is indicated by a concern for human safety. 39