National Academies Press: OpenBook

Organic Matter and the Moon, by Carl Sagan (1961)

Chapter: VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON

« Previous: V. POSSIBILITY OF AN INDIGENEOUS LUNAR PARABIOLOGY
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
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Page 26
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
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Page 27
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 28
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 29
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 30
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 31
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 32
Suggested Citation:"VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 33

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VI. SURVIVAL OF CONTEMPORARY TERRESTRIAL MICROORGANISMS ON THE MOON We now turn to the problem of the survival of contemporary terrestrial organisms in the lunar environment. This problem is directly relevant to the question of biological contamination of the Moon; it also has a bearing on the panspermia or cosmobiota hypothesis, and on the possibility of survival to the present of lunar organisms or their remains, produced in the distant past. There seem to be three major hazards for survival of ter- restrial life on the Moon—temperature, corpuscular radiation, and solar electromagnetic radiation — which we discuss below. The probable absence of oxygen, water, and other substances from the lunar surface is not, of course, evidence against survival, particularly of dormant anaerobic microorganisms; but it does preclude their reproduction on the surface of the Moon. A. Temperature Lability Surface temperatures range from about + 100°C to about - 180°C during the course of a lunar day and night (Wesselink, 1948). However, just beneath the surface, temperatures are much more moderate; at a depth of less than half a meter, microwave radiation data indicate a temperature variation between 0° and - 70°C (Piddington and Minnett, 1949). It is well known that many microorganisms have extreme resistance to low temperatures, especially in the dried state and in vacuo. An especially relevant example is the experiments of Becquerel (1909, 1910) in which bacterial spores were kept at temperatures below - 180°C for periods greater than the length of the lunar night, and remained viable. Similarly, drying and evacuation greatly increase the tolerance of microorganisms to high temperatures. Even at temperatures ap- proaching 100°C survival of a significant fraction of the total number of vegetative bacterial cells and spores may be expected (Zamenhof, 1959, 1960). Still higher temperatures are required to inactivate desoxyribonucleic acid (Zamenhof, Alexander, and Leidy, 1953). 26

Thus, because of the dry vacuum conditions of the lunar surface, the temperature extremes appear not to be a serious hazard. Especially since it is likely that many of the deposited microorganisms will find themselves lodged just beneath the surface (section VI E below), the debilitating effects of the lunar surface temperatures can be neglected. B. Deflection of Incident Charged Particles by the Lunar Magnetic Field Cosmic rays, charged particles emitted by the Sun, and con- tinuous and discrete solar electromagnetic radiation are all in- cident on the Moon. Whether they arrive at the lunar surface, however, depends on the existence of a lunar magnetic field and a lunar atmosphere. The work of Biermann on the acceleration of comet tails indicates a flux of solar protons in the vicinity of the Moon of about 5 x 10 protons cm"2 sec"1, and a mean particle energy of about IKeV(v., e.g., Reiffel, 1959). Charged particles will be ex- cluded from regions where the magnetic energy density exceeds the particle kinetic energy density; i.e., for magnetic deflection the magnetic field strength must satisfy the condition B > (4 TT p)1/2 v, (14) where p is the mean density of incident particles, and v the mean particle velocity. Thus for the solar proton stream, the deflection condition on the lunar surface magnetic field strength is B > 10"2 gauss. (15) The mean density of the Moon is comparable with that of terrestrial surface material. This has always been understood as indicating the absence of an extensive liquid iron core, and pre- sumably the absence of an appreciable lunar magnetic field as well. Preliminary data from Lunik II indicate a surface field of < 3 x 10 gauss (Sedov, 1959); and in this case, solar protons would not be affected significantly by the lunar magnetic field. However, Neuge- bauer (1960) has called attention to the possible masking of the lunar field in the illuminated hemisphere by the solar protons them- selves. At the present writing, it is not known whether inequality (15) is satisfied. If the lunar magnetic field strength is much less than the terrestrial, then low-energy cosmic rays which are de- flected by the Earth's field will arrive in the vicinity of the Moon. 27

C. Attenuation of Incident Radiation by the Present Lunar Atmosphere From lunar occultations of cosmic radio sources, it can be estimated that the lunar atmosphere contains less than 10" mole- cules above each square centimeter of surface (Costain, Elsmore and Whitford, 1956; Edwards and Borst, 1958). Ultraviolet ab- sorption cross-sections for all molecules likely to be in the lunar atmosphere are generally less than 10" 16 cm at all wavelengths, except in the centers of resonance lines. Hence the optical depth in the ultraviolet is less than 10"^, and there is no attenuation of incident solar ultraviolet radiation by the lunar atmosphere. For the solar proton wind, a 1 KeV proton has a range of about 10"2 cm-atm, or, roughly 3 x 10*' molecules cm"2. Consequently, if the lunar magnetic field strength is less than about 10"2 gauss, the solar proton stream strikes the Moon's surface with negligible loss of energy due to its passage through the tenuous lunar atmosphere. A similar conclusion applies to the more energetic cosmic rays. D. Adopted Fluxes, Mean Lethal Doses, and Absorption Coefficients We now consider the effects of these radiations on terrestrial microorganisms deposited on the lunar surface. We consider micro- organisms because they are known to be much less radiosensitive than other life-forms (v.. e.g., Bacq and Alexander, 1955), at least in part because there is less which can go wrong in a simple or- ganism than in a complex one. In addition, the accidental deposi- tion of many microorganisms on the lunar surface is a much more likely contingency than the accidental deposition of large numbers of other life-forms, particularly for the immediate future. In the appendix, expressions are derived (eqs. A-7 and A-8) for the time in which a population of No organisms, having a mean lethal dose, D, for a given radiation, and characteristic dimensions, a, is reduced to N organisms by radiation of intensity I. In Table n, these lifetimes are tabulated for a number of values of N/N and a. The intensities are those appropriate to the lunar surface for negligible atmosphere and magnetic field strength, and so are equally appropriate to interplanetary space in the vicinity of the Earth-Moon system. Consequently the derived lifetimes are also those of an unprotected microorganism in free space, and so have a bearing on the panspermia or cosmobiota hypothesis (v., e.g., Oparin, 1957; Lederberg and Cowie, 1958). The X-ray emission in Table II is taken from a theoretical study of the solar corona 28

w m in i O DJO i— 1 C c CO •,rt -rH Ti " i i IM rH rH eo *-i C Q i oo ^- ^o rH rH rH I-H w cu o i^r o o o O O O O -c b SJJ — "^ U -.£ co £5 XXX XXX 4} (M (M <M CO (M (M in o ^f in m TJ< co ON OO 00 PH (M o 0 o o o 000 O o "x " XXX XXX "jj co -1 in m m m oo CO (M <sl ao c o o •o tf\ C ifi in in Tf co 0 O O 0 rH O 0 O O O o 0 , 75 CO 0 X XXX XXX X W ,— i ro co co co m CO c\l (M o 01 c in — lrH ^ ° ^ m uo ^ co O*- 00 00 CO 0 r-, O 0 O O O O O O o 0 g-- o X 3 XXX (M (M CO XXX X CO •O (M rH rH m g ^ ^ m in TJ< co oo r- i— CO -*-> m o 0 o o o o o o o •rH rd 1 a a o X j* XXX XXX X rt ^: -H 1— ( rH rH (M ON in in fj rH O O co J ^i CO M O O O oo r~ r- p— t o 2 o o o o SH rj O "x "x XXX XXX X •rH i-H rsj IM (M CO (M rH rH S In ' •o co -^ in co rh un O w CD . f < f*j CU 0) i i i i i i P J3 •rH CJ 3 0 O O 0 O O la i CO 0i cr - j .--• O rrj (VI cfl g. a m " a 00 "eg o 0 P Q, "H fi m CO IM *O ~~- g i 'o o o O <! <a. DJO 0 in -o CO D -» rvl (M i - fj i I • fH & d r- bo ,-, vO bJD ,H r- a o cu r- a, r- a 4-> T) g 0 rH g 0 r. S O Q) O CO a rH CO O rH CO O rH (H rH ^H rH $-( .b M ° — . CO -rH DJO ^ ^M rsl CM 1 1 CO (^ CO O O O 0 o rS -^ 'rH rH I-M O Q co" e 4-* 4-) o •" en r*l it Radiatioi ,2 S S S 4-> CO rt . c ^ C ODO O 3 O -r1 3 •i•* rHQrt! 3 1 CO .2 3 .jH P J-> rt S ° ° SH •" ° ° *J C 0 O cfl .S ° ° X 0 ll s" U o o «-§« -M **> CU r^ O CO (SI *3 C o o -r; o CM -H £> o x x O -r| 3 <«-. 1 -rH P a x x; m S co cn x cr 29

(Elwert, 1954) and is consistent with rocket observations at quiet Sun. The continuous ultraviolet intensities are computed from an integration of the Planck equation (eq. 11) for appropriate ultra- violet effective black body temperatures (Tousey, 1955). The cosmic ray flux is scaled from surface values to values expected outside the Earth's atmosphere. Discrete solar emission lines such as H Ly a, X.1215, and He II, X.304, are much less energetic than the continuous radiation, and are here neglected. It should be mentioned that the lethality times for cosmic rays listed in Table II are lower limits, because of overkilling. An average cosmic ray primary has an energy several orders of mag- nitude greater than that required to kill an average microorganism. Only when the particle is absorbed by a cluster of microorganisms, and the energy distributed among them will the killing times be as short as in Table n. For a given organism, the mean lethal dose in reps is approxi- mately invariant, under the same environmental conditions, for all ionizing radiation, corpuscular and electromagnetic. Viruses char- acteristically lie in the range D = 105 to 10^ rep (Luria, 1955); protozoa generally have the same range (Bacq and Alexander, 1955; Kimball, 1955). Bacteria and fungi usually have somewhat lower mean lethal doses, 103 to 104 rep for IS. coli. for example, and 104 to 105 rep for the spores of B_. mesentericus and A. niger (Zelle and Hollaender, 1955). However, there has been no systematic search for radio- resistant microorganisms, and it is possible that microorganisms having mean lethal doses as high as 10^ rep exist. In addition, D in general has some functional dependence upon such factors as the temperature, the oxygen tension, the time interval in which the killing dose is applied, and the presence of an external aqueous medium. The dependence is in different directions in different organisms, and the interaction of the various effects is quite com- plex; but the resulting variation in D is rarely as great as a factor of ten. Considering all these points, then, it appears that a con- servative estimate for an average mean lethal dose due to ionizing radiation is 107 rep. For the non-ionizing ultraviolet radiation, D has a strong functional dependence on wavelength, corresponding to the wave- length variation of molecular absorption cross-sections. There is an absorption maximum at roughly X.2600 due to the biochemically ubiquitous purines and pyrimidines, and another, more pronounced, maximum shortward of X.2300, due to simple diatomic functional 30

groups, such as N-H. Ultraviolet mean lethal doses are given in ergs cm"2, and are generally measured at X.2537. To obtain a mean value of D appropriate for a wide range of wavelengths we must know the wavelength variation of D. For common strains of E. coli, for example, D(X,3000) = 105 ergs cm"2, D(X.2537) a 104 ergs cm"2, and D(X.2300) • 10* ergs cm"2 (Zelle and Hollaender, 1955). Considering the decrease of D shortward of X.2300, a con- servative (i. e. , upper limit) mean value of D for the wavelength region X.3000 to X.2000 appears to be the value at X.2537; this should be roughly applicable for an ultraviolet black body spectrum with a Wien peak longward of X.3000. The mean lethal dose at X.2537 for the more radioresistant bacteria, such as B. subtilis spores, Sarcina lutea, and the B/r strain of E. coli, are approximately ergs cm"z (Zelle and Hollaender, 1955). An unusual case is the protozoon Paramecium multimicronucleatum, for which D(X.2537) = 10^ ergs cm"2 (Kimball, 1955). However, much lower doses serve to prevent this organism from surviving reproduction. Considering, finally, the environmental dependences of D mentioned in the pre- ceding paragraph, and the possibility of undiscovered microorganisms of extreme radio-resistance, we adopt as a mean value of D for ultraviolet radiation in the region X.3000 to X.2000, D = 107 ergs cm"2. For the region shortward of X.2000, D is certainly <10^ ergs cm"2. From the ranges of ionizing radiation in matter, the following mass absorption coefficients were adopted: 105 cm2 gm"l for 1 KeV protons, 103 cm2 gm"1 for 50 A soft X-rays, and 2. 5 x 10"^ cm2 gm"1 for cosmic rays. Because of the varying energies, especially for cosmic rays, these absorption coefficients are only approximate. It should again be emphasized that the mean lethal doses are purposely high to allow for anaerobiosis and drying. The resulting lifetimes should be upper limits, except for cosmic rays, and, perhaps, where p/n « pa for ionizing radia- tion, so the radiation does not penetrate to the interior of the organism. E. Survival Times Where the computed lifetimes are greater than a month, they have been divided by two — except for the cosmic ray lifetimes — to allow for the lunar night. For times shorter than a month, con- tinuous solar illumination has been assumed, but of course, all such times may be as long as a month if the organism is deposited in a region soon after the terminator has left the region. A 1 kg instrumented lunar package may easily contain 1010 microorganisms (Lederberg and Cowie, 1958); it is very unlikely 31

that any packages for the immediate future will contain as many as l020 microorganisms. Accordingly, we see from Table II that all microorganisms deposited and exposed to the sun will be killed by ultraviolet light in a few hours. Similarly, fully illuminated microorganisms in cislunar space will also survive only a few hours. Hence the panspermia hypothesis is untenable for unpro- tected microorganisms of comparable radiosensitivity to terres- trial microorganisms. On the other hand, suppose some micro- organisms are deposited in a lunar crevasse or other depression, always shielded from solar radiation. Then, killing will be effect- ed only by cosmic radiation and by natural radioactivity. Because of secondary cascade, cosmic radiation reaches an intensity maxi- mum slightly greater than the surface value at a depth of about 10 cm on the moon, according to Filosofo (1958). It is reduced to 10"1 the surface flux at a depth of about one meter, and to ter- restrial surface values at a depth of a few meters. Hence, micro- organisms shielded from the Sun, but just beneath the lunar surface will not be killed by cosmic radiation for at least several hundred million years; microorganisms at greater depths will have even longer lifetimes. Similarly, cosmobiota imbedded in, for example, a meteorite would have lifetimes comparable to the age of the Solar System, and under these circumstances the panspermia hypothesis remains tenable. We now consider the possibility that microorganisms deposited on the Moon will actually be shielded. The nature of the lunar sur- face is a complex and much-debated problem (Baldwin, 1949; Urey, 1952, 1956a, 1956b; Kuiper, 1954, 1959; Gold, 1955, 1959; Whipple, 1959) which need not be reviewed here. But it is important to call attention to a few points. From eclipse temperature measurements, and polarimetric and radio observations, it is known that a dust covering exists on the Moon, possibly present in some areas, and not in others. Estimates of its depth range from millimeters to miles. However, Whipple (1959) has called attention to the experi- mental fact that dust, irradiated in a vacuum, will congeal, form- ing a low-density, semiporous matrix. If the lunar surface material has a similar sintered structure, it would appear very possible for microorganisms to be lodged in the interstices of the matrix, in such positions as to be shielded from the Sun's rays at all angles of insolation. Thus we may anticipate the survival for very great periods of time of perhaps a few percent of those dormant anaerobic microorganisms deposited at the lunar surface. A determination of the microstructure of the Moon's surface is of great importance to corroborate this conclusion. 32

F. Dissociation of Nonliving Organic Matter The killing of an organism, of course, does not necessarily involve its chemical dissociation, and long after death occurs, in an anhydrous aseptic environment, many aspects of the organism's characteristic biochemical structure will be maintained. After long periods of continued irradiation, enough bonds would be broken to destroy most of the long-chain biological polymers such as pro- teins and nucleic acids. The problem is complicated by the exist- ence of radiation protection devices (catalase, cytochromes, sulfhydryl compounds, photoreactivation mechanisms) in most con- temporary organisms. Because of the Franck-Rabinowitch cage effect, the collection of dissociated molecules arising from the original organism would tend to remain in close physical contact. Ionizing radiation is very much more efficient than non-ionizing radiation in depolymerizing and dissociating organic molecules. Breaking of all hydrogen molecular bonds and charring occurs at about 1010 rep (v., e.g., Reiffel, 1959). The last column of Table El gives the times for the various radiations to effect charring of all but 10" 15 of the exposed molecular aggregates. Charring by the solar proton wind occurs in from months to years, depending on the size of the dissociated organism. If, however, the lunar surface magnetic field exceeds 10"2 gauss and the proton wind does not penetrate to the surface, it may take as long as one hundred thousand years for charring to be induced by soft solar X rays. Thus the value of the lunar magnetic field strength has great relevance for the question of possible biochemical contam- ination of the moon. As dissociation advances, lunar temperature effects would become more important, small molecules being readily dissociated at 100°C. For example, the most thermostable amino acid, alanine, has a thermostability half-life at 100°C of approximately 10^ years (Abelson, 1954), with many other amino acids having half-lives not less than ten years. Molecules shielded from radiative dissocia- tion would be relatively unaffected by lunar temperatures, and if lodged beneath a few centimeters of insulating lunar surface ma- terial, would have lifetimes determined by the cosmic ray flux and natural radioactivity. 33

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The immediate future seems to hold both the promise and the responsibility of extensive contact between man-made objects and the Moon.

Current United States plans tentatively call for the soft landing on the Moon of instrumentation designed to detect indigenous organisms or organic matter, possibly in a roving vehicle, by 1964-67 in the Surveyor and Prospector Programs. The Soviet Union apparently has the capability of performing similar experiments at an earlier date. It is clear that positive results would give significant information on such problems as the early history of the Solar System, the chemical composition of matter in the remote past, the origin of life on Earth, and the distribution of life beyond the Earth. By the same token, biological contamination of the Moon would represent an unparalleled scientific disaster, eliminating possible approaches to these problems. Because of the Moon's unique situation as a large unweathered body at an intermediate distance from the Sun, scientific opportunities lost on the Moon may not be recoupable elsewhere.

This monograph is concerned with the possibility of finding indigenous lunar organisms or organic matter, and with the possibility of their contamination by deposited terrestrial organisms or organic matter.

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