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

Chapter: II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY

« Previous: I. INTRODUCTION
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." 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 2
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 3
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 4
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 5
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 6
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 7
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 8
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 9
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 10
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 11
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 12
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 13
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 14
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 15
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 16
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 17
Suggested Citation:"II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY." National Research Council. 1961. Organic Matter and the Moon, by Carl Sagan. Washington, DC: The National Academies Press. doi: 10.17226/18476.
×
Page 18

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II. PRODUCTION OF ORGANIC MATTER IN EARLY LUNAR HISTORY A. General Theory Investigations of the early history of the Solar System indi- cate that the Moon possessed a sequence of reducing gaseous en- velopes during and soon after its origin (Kuiper, 1951a, 1952; Urey. 1951, 1952). The evolution of the early lunar envelopes is discussed in section II B, where reasons are discussed for believing the envel- opes to have been composed largely of CRj, NHs, H2O, and H2, and to have been opaque to short- wavelength ultraviolet radiation. The effect of solar ultraviolet light (and electric discharge) on such an atmosphere is well known; organic molecules of fair complexity — up to molecular weight ~100 — are produced almost independently of the relative proportions of precursors. Amino and other organic acids, pyrroles, pyridines, and simple hydrocarbons and their polymers are among the synthesized molecules (Noyes and Leighton, 1941; Miller, 1955; Oparin, 1957; Groth and von Weyssenhoff, 1960). Because the molecular weight of these molecules and their in- termediates was greater than the mean molecular weight of the primitive lunar envelopes, they tended to diffuse to the surface under the influence of the lunar gravitational field. However, the newly- synthesized molecules, having greater ultraviolet absorp- tion cross sections than their precursors, were more readily photodissociated; a crucial datum is then the comparison between Id. the mean time for synthesized molecules to diffuse to atmos- pheric depths which are optically thick in the photodissociating ultraviolet, andTa, the mean time between successive absorptions of photodissociating photons. If we define a parameter p as the fraction of synthesized molecules which reach optically thick depths before being photodissociated, we have (3 = 1/2 whenTa= td, p~ 1 when ta » t^, and p * 0 when ta « t<j« Consider now ultraviolet radiation of intensity Q photons sec'1 in the synthetically effective wavelengths falling for t seconds on an opaque gaseous envelope surrounding the Moon, and producing molecules of mean molecular weight (j. with overall quantum yield 0. Let r be the distance from the center of the Moon such that all mole- cules of molecular weight ^ produced at distances less than r are

gravitationally captured, while those produced at distances greater than r will escape. The synthesized molecules will be distributed over a Moon of radius R. Assuming that 0 is independent of wave- length in the synthetically effective region of the spectrum (v. sec- tion IID) the mean surface density of synthesized material which es- capes photodissociation will be , , Q * r-2 n p t gm cm.2> {1) 4 NA R2 where NA. is Avogadro's number. In the following sections we discuss the numerical values appropriate for r, R, p, 0, Q and t. B. Historical Development of Lunar Gaseous Envelopes The picture of the early evolution of the Solar System in the neighborhood of the Moon which we present here was first developed by Kuiper and Urey in the early 1950's. Urey (1956a) has since suggested that alternative views might be better able to explain certain discordant facts, but for the sake of definiteness, and be- cause of the general success of the original picture, we adopt it here. It should be emphasized that the existence of a later second- ary reducing atmosphere for the Moon seems incontrovertible, in the light of the work of Suess (1949) and Brown (1949) on the under- abundance of rare gases in the terrestrial atmosphere when com- pared with the cosmic distribution of the elements. The Moon, along with other bodies of the Solar System, is believed to have formed some 4 to 5 x 10^ years ago from the solar nebula, a vast gas and dust cloud possessing a cosmic distribu- tion of the elements. The contraction timescale for the solar nebula was the Helmholtz-Kelvin period, approximately 108 years. At the end of this period, the Sun approached the main sequence in the Hertzsprung-Russell diagram, thermonuclear reactions were initiated, and solar electromagnetic and especially corpuscular radiation dissipated the nebula from around the protoplanets and their atmospheres. The dissipation timescale for the solar nebula is estimated by Kuiper (1953) as between 108 and 109 years. After the clearing out of interplanetary space, hot exospheres established in the protoplanetary atmospheres led to efficient evaporation of the planetary envelopes, a process aided by the long mean free paths in interplanetary space and the low escape velocities (due to smaller mass/radius ratios for the protoplanets than for the present planets). The time for the evaporation of the prototerrestrial atmosphere ap- pears to be roughly 108 years (Kuiper, 1951b). During the events just outlined, chemical compounds and con- dens ates were raining down on the protoplanetary surfaces, forming

the outermost layers. After the evaporation of the atmospheres of the terrestrial protoplanets. internal heating must have vaporized much of the condensates, thereby forming secondary atmospheres of chemical composition similar to the initial protoatmospheres. The present Martian, Cytherean, and terrestrial atmospheres are believed to be ultimately of such secondary origin. Similarly, the Moon must have possessed a secondary atmosphere at one time, which, however, since has been lost to space because of the low lunar escape velocity. We now consider the penetration of solar ultraviolet radia- tion into the various gaseous envelopes which surrounded the Moon in its early history. The absorption cross section of ammonia, the most prominent nitrogen-containing molecule in cold cosmic gases, shortward of X.2400 is greater than 10"22 cm2. Hence, as long as the mean density of ammonia exceeded 10)9 molecules cm"^ between the Moon and the Sun in the solar nebula, solar ultraviolet light shortward of X.2400 did not reach the lunar vicinity. This ammonia density corresponds to a hydrogen number density of about l012 molecules cm"^ for cosmic abundances; i.e., about 3 x 10"^ gm cm"3. Interplanetary densities of this order were reduced rapidly (Kuiper, 1953), and we conclude that during most of the 10" to 1()9 years in which the solar nebula was being dissipated, solar radiation shortward of X.2400 was reaching the protoatmosphere of the Moon. Because the lunar protoatmosphere had not yet begun to escape, due to the short mean free paths within the solar nebula, the lunar protoatmosphere remained opaque to solar ultraviolet light during this period. After the dissipation of the solar nebula, the lunar protoatmosphere was opaque in the ultraviolet for most of its lifetime. In this same period, the Moon must have been situated within the protoatmosphere of the earth, and so the Moon's surface must have been protected from solar ultraviolet radiation by lunar and terrestrial protoatmospheres for almost 10)8 years. After the evaporation of these protoatmospheres, and the origin of the secondary lunar atmosphere, the secondary atmosphere was maintained for a period of time discussed in section II F, at suf- ficient density to absorb all incident solar radiation shortward of K2400. Because of the Moon's proximity to the more massive Earth, some material produced in the early envelopes near the Moon must nevertheless have been captured by the Earth. For purposes of computation with equation (1), we adopt as a mininum value of r, r = R; i. e., we neglect lunar gravitational capture of molecules produced outside a cylinder of lunar radius extending from the Moon to the Sun. This approximation is, of course, very nearly exact

for the secondary lunar atmosphere; but it gives only a lower limit to a during the times of the solar nebula and the original lunar protoatmosphere. C. Diffusion Times of Synthesized Molecules We have mentioned that newly synthesized molecules must rapidly diffuse to depths opaque in the photodissociating ultraviolet if they are to survive. It is very significant that the recent lab- oratory experiments of Groth (v. section II D) show that photons with wavelengths as long as X.2537 are synthetically as effective as photons with much shorter wavelengths, in the reducing atmos- pheres he chose. At the present time it is not clear to what extent the absorption at X.2537 in the absence of Hg sensitization is due to the weak predissociation continuum of ammonia and to what extent it is due to, for example, small quantities of aldehydes or aromatics possessing very large ultraviolet absorption cross-sections at these wavelengths, and maintained at some steady state concentra- tion by the ultraviolet light itself. Whatever the primary laboratory absorber turns out to be, the same molecule is expected to exist in the primitive lunar atmosphere, and the same quantum yields should be applicable. However, these two cases have somewhat different consequences for the question of diffusion, and we dis- tinguish them in the following discussion. Because so many more solar photons were available at X.2600 than at shorter wavelengths in the early lunar envelopes, most of the ultraviolet synthesis of organic molecules must have occurred in the vicinity of unit optical depth for X. > 2600 A. If the primary source of near ultraviolet absorption were ammonia, then T = 1 at X . = 2600 A implies T > 1 at X . < 2600 A, because the ammonia ab- sorption coefficient increases with decreasing wavelength. As a consequence, all newly synthesized molecules with photodissociation limits X.p < 2600 A will already be at depths opaque in the photo- dissociating ultraviolet. Subsequent diffusion and convection will carry them to even deeper levels in the atmosphere, and for such molecules, the fraction which avoids photolysis, p * 1. Many im- portant intermediates in the photoproduction of organic molecules are not photodissociated by X. > 2600 A. Among these molecules are the amines, the nitriles, the saturated hydrocarbons, and some un- saturated hydrocarbons. We now discuss the fate of those mole- cules which are dissociated by X. > 2600 A under the assumption of ammonia continuum absorption at these wavelengths. Such mole- cules include aliphatic aldehydes and ketones, aromatics, and some unsaturated hydrocarbons.

From the theory of gravitational diffusion of Chapman and Cowling (1939) the following expression can be derived for the time for a molecule of molecular mass m^ to gravitationally diffuse from a level characterized by subscript 1 to a level characterized by subscript 2: n, k T, V H2 '2 td = (2) 1.66x 1014g3/2( I +m/mi)1 (Nicolet, 1954). In equation (2), n and T are the number density and temperature, respectively, of the atmosphere at the two depths, m is the mean molecular mass of the atmosphere, g is the acceleration due to gravity, H = kT/mg is the scale height of the atmosphere at a given depth, dH/dz is the rate of change of scale height with altitude, and k is Boltzmann's constant. The numerical factor in the denominator arises from a mean collision cross-section of IT (3 x 10®) cm2; the precise value of this para- meter will not affect the conclusions below. Making the approximations 1/2 (dHldz) - 1 = - 1 and we also set n = r /aH. where T is the optical depth and a the absorption cross -section of the atmosphere at a specified wavelength and at the given atmos- pheric depth. We then have for the time for the synthesized mole- cule to diffuse from unit optical depth in the longest effective syn- thesizing wavelength (here~X.2600) to unit optical depth in the longest photodissociating wavelength (here X . > 2600 A), Tl/2 1/2 (s) (p) td=5.5xl0-U _ L * a. - a- . g a(s)

H is the mean molecular weight of the atmosphere, ^ is the mean molecular weight of the synthesized molecule, a(S) is the absorp- tion cross-section of the atmosphere at the longest wavelength which effectively synthesizes molecular species i, and a(P) is the absorp- tion cross-section of the atmosphere at the longest wavelength which photodissociates molecular species i. To determine (3, tj must be compared with the time between successive absorptions of photodissociating photons, ta, which at small optical depths at the photodissociation limit is given by ta = 1/(Q' ai), (4) where Q' is the photon flux shortward of the photodissociation limit, and aI is the absorption cross-section of the synthesized molecule of molecular species i at the photodissociation limit. Because of the rapid decline of the ammonia absorption cross- section longward of X.2600, a(S) »a(p) should hold for the synthesis of aromatics, aldehydes, etc. in the case that ammonia is the prin- cipal absorber at these wavelengths. We expect a^ » a(P) as well. Taking the most favorable case of a^/a(p)_=_ 10, and with ^ = 30, fjL = 10, T = 600° K, and g = 160 cm sec"2, we have from (3) and (4), ta/td * 10n/Q'. (5) Thus for the molecule to escape photolysis, the photon flux short- wards of X.p, where 2600 <X . < 3200 A, must have been less than about 1011 photons cm"2 sec"1. We will show in section II E that at no time in the history of the later solar nebula, the lunar proto- atmosphere, or the secondary lunar reducing atmosphere was the solar ultraviolet flux so low. Therefore under the above assump- tions molecules with photodissociation limits in excess of X.2600 must have been destroyed soon after they were synthesized. This conclusion is relatively insensitive to the particular numerical values chosen in the derivation of equation (5). In the alternate case that the principal absorbers at X. > 2600 A were such molecules as aldehydes, ketones, and aromatic com- pounds, an absorbing layer must have been established in the lunar atmosphere. The layer was populated photochemically and depopu- lated by photolysis, by convection and by gravitational diffusion. It differed from the present terrestrial ozone layer in that layer molecules which migrated to lower depths were not instantly des- troyed by chemical reaction. At the top of such an optically thick layer, depopulation occurs primarily by photolysis; at the bottom, primarily by convection and diffusion.

Layer molecules will therefore be carried to the surface of the primitive Moon. At the present time, it is not clear whether windows will exist between X.2500 and X.2800 in such an atmosphere. In the experiments of Sagan and Miller (1960) in which molecules were synthesized in simulated primitive terrestrial atmospheres, no major absorbers in this wavelength interval were found. However, only a few cm-atm of such molecules as acetaldehyde will render this wavelength interval opaque. In the case that no window exists, all molecules at the bottom of the absorbing layer will be shielded, and the effective value of p ,... 1. In the case that such molecules as acetaldehyde are very rare, and a window between X.2500 and X.2800 exists, then molecules which absorb strongly in this wave- length interval will be rapidly destroyed, and for these molecules, p ~ 0. Some studies of the origin of life on Earth (Sagan, 1961) suggest that many aspects of the early evolution of life can be under- stood if a window between X.2500 and X.2800 did exist in the primitive terrestrial atmosphere. By analogy, a similar window would have existed in the primitive lunar atmosphere. But in either situation most simple gaseous amines, nitriles, and hydrocarbons would have survived, because of very low absorption coefficients in this spec- tral region. Even for molecules which do absorb between X.2500 and X.2800, it is easy to show that their contribution to atmospheric ab- sorption will generally make «^s' - a'P' small or negative, so that, by equation (3), photodissociation will be avoided. Thus, regardless of the nature of the principal absorbers in the near ultraviolet, it appears that for such molecules as amines, nitriles, saturated hydrocarbons, and some unsaturated hydro- carbons, photolysis was evaded in the early lunar envelopes. For molecules with ultraviolet absorption at longer ultraviolet wave- lengths, such as aldehydes, ketones, aromatics and some unsaturated hydrocarbons, a smaller fraction of those synthesized survived. If these molecules were also responsible for the absorption near X.2600, then p might be within a few orders of magnitude of unity; but if the principal absorption near X..2600 was due to ammonia, then p«10"2. Molecules with such small p will have been destroyed efficiently; more complex molecules will be among the dissociation products, but their overall quantum yield must be much less than the values for simpler molecules. Surviving molecules were then carried by diffusion and con- vection to the surface.

In the later stages of the evolution of the lunar atmosphere, open bodies of liquid water can be expected on the Moon (v. section II F). Solution of the synthesized molecules in water corresponds to the last step of contemporary laboratory experiments which pro- duce amino acids and other organic molecules from mixtures of re- ducing gases. We now proceed to discuss these experiments. D. Quantum Yields Recently a series of experiments on ultraviolet synthesis of organic molecules which permits quantitative conclusions has been performed by W. Groth in Bonn (Groth and von Weyssenhoff, 1959; 1960). Ethane, ammonia and water vapor were irradiated by the X.1470 and X.1295 lines of xenon in one set of runs, and by the K2537 line of mercury in another set of runs. The quantum yield for the production of amino acids alone was in both cases between 10"* and 10"5. With no sensitization by mercury atoms during X.2537 irradia- tion, the quantum yield was about 10"5, only a little less than with mercury sensitization. (Groth, 1959). The gases irradiated by the xenon lines were circulated over a water bath, while those irradiated by the mercury line were con- densed out with water. In the first case the interval between ir- radiation and immersion for a given molecule was about 0. 1 seconds; in the second case the interval between irradiation and condensa- tion was about 1 second (Groth, 1960). The emission in the xenon lines was about 10*" quanta sec" , and the distance from the source to the irradiated molecules was about 2 cm. Thus the flux was about 2 x 1014 quanta cm'2 sec'1. Even with an absorption coefficient as large as 10'1° cm2, only 0. 02 quanta are absorbed each second. Consequently, the interval between successive photon absorptions for a given molecule was much greater than the time between ir- radiation and immersion. A similar conclusion follows for the mercury line illumination with about 3 x 1Q18 quanta sec"1 and a source - molecule distance again of a few centimeters. We see that the quantum yields apply only to products which are removed from irradiation before absorbing another photon. In the primitive lunar envelopes this corresponds only to those molecules for which p > 1/2. In the primitive lunar envelopes, methane, not ethane, was the principal carbon molecule. The quantum yield for the photo- production of ethane from methane is about 10"1 at X.1470 (v., e.g.,

Noyes and Leighton, 1941, p. 419). The number of solar photons available at X.1470 is about 104 times smaller than the number avail- able at X.2600, as follows from equations developed in the next sec- tion. Thus the effective quantum yield for the photoproduction of ethane from methane in terms of the quanta available at X.2600 is about 10"5. If 1/0} quanta are required to produce one ethane mole- cule from methane, and if 1/02 quanta are required to produce one amino acid molecule from ethane, ammonia and water, then the total number of quanta required to produce one amino acid molecule from methane, ammonia and water is 1/0 = 1/0I + l/02, or the net quantum yield for amino acid production from methane, ammonia and water is 0 = 0102/(01 + 02). (6) With our values of 01 and 02, we obtain 0 = 5 x 10"». To be con- servative, we adopt for later use 0 * 10"6. This computation should be checked experimentally. However, quantum yields (for the production of relatively simple molecules) > 10"6 are practically inevitable in the photochemistry of such gases. Note that since 02 is independent of wavelength between X.1470 and X.2537, and 01 is already adjusted for wavelength, 0 should be wavelength-independent in this same range, an assumption which was made in the deriva- tion of equation (1). From equation (4) and the photon fluxes derived in the fol- lowing section, it is easy to see that for all reasonable temperatures and densities at the synthetic level of the primitive lunar envelopes, the time between collisions is much shorter than the time between successive absorptions of quanta effective in either synthesis or dissociation. Since many collisions follow each synthesis, the dif- ference in pressures, temperatures, and densities between con- temporary laboratory and primitive lunar environments should not significantly affect the overall quantum yields. The preferential depletion of photochemically-produced aldehydes, ketones, aromatics, and some unsaturated hydrocarbons on the early Moon (Section IIC) will make the primitive lunar end- products differ from the contemporary laboratory end-products; but the overall quantum yields for organic matter should be unchanged. In the laboratory experiments in which corona discharges are used as the energy source, aldehydes have been shown to be inter- mediaries in the synthesis of amino acids (Miller, 1957). It is 10

not known whether aldehydes play a similar role in ultraviolet synthesis of amino acids. If they do, it is possible that the amino acid fraction of the primitive lunar organic matter was much less than the amino acid fraction in the laboratory. But it should be emphasized that 85% of the organic material produced in the corona discharge experiments is not amino acids and has not been identi- fied (Miller, 1957). A similar residue should be expected in the ultraviolet experiments. It is clear that much of the residue will not have aldehydes, etc. as precursors. For example, saturated higher hydrocarbons are known to be produced photochemically from a mixture of methane, ammonia, water, and hydrogen; the quantum yields are larger than 10"^ (v., e.g., Noyes and Leighton, 1941). It is difficult to predict which organic compounds were pro- duced on the primitive Moon, but it is very probable that the ap- propriate quantum yields were not smaller than 10"6. E. Ultraviolet Fluxes and Solar Evolution The number of quanta emitted between frequency v and c + di> each second by each square centimeter of a black body at tempera- ture T is given by the Planck distribution function, wF di> = 8 w 2 c2 ."""-1 -' «. P> where c is the velocity of light, and h and k are, respectively, Planck's and Boltzmann's constants. For solar photos pheric temperatures and ultraviolet frequencies, hv/kT » 1, and eq. (7) reduces to the Wien approximation, d, « (8) The photon flux emitted at all frequencies larger than some ref- erence frequency VQ is obtained by integrating eq. (8) from VQ to infinity: CO 1 m "> 1- it m (9) /IcT* " "he2 "o To find the photon flux shortward of wavelength \Q = c I VQ in the neighborhood of the Moon, IT F must be multiplied by a geometrical dilution factor which allows for the inverse square attenuation of intensity with distance. If the radius of the solar photosphere is R 11

and the mean distance between the Sun and the Moon is a, the geo- metrical dilution factor is W = R2/4 a2 . (10) In the case there is absorbing material between the Sun and the lunar atmosphere, an additional physical dilution factor less than unity must be included. Such a factor is appropriate only to the times of the early solar nebula before the clearing out of inter- planetary space by solar corpuscular radiation (see section II B), and we neglect it here. Combining equations (9) and (10), we find for the photon flux shortward of X.o at the top of the lunar atmos- phere at times when the equivalent solar black body temperature in the ultraviolet is T and the radius of the solar photosphere is R, Q = 2;r kT /^\2 e-hc/X.0kT hX.0a To apply equation (11), R and T must be known for previous epochs. During the period of the Sun's Helmholtz-Kelvin gravitational contraction, its evolutionary track in the Hertzsprung-Russell dia- gram was approximately along the line L R^- <° = const., where L is the solar bolometric luminosity (Henyey, Leve'e, and LeLevier, 1955). As the interior temperatures rose to the point where thermo- nuclear energy sources became competitive with gravitational energy sources, the evolutionary track dipped to lower luminosities, joining the main sequence tangentially. The solar nebula is believed to have existed during this period of pre -main sequence contraction. As- suming that the effective ultraviolet temperature is proportional to the mean bolometric temperature, a typical value of T during pre-main-sequence contraction can be read off the evolutionary track of Henyey et al. Using the subscript 0 to indicate present solar values, at a time when L * LQ, A log T = log T - log TQ » -0. 05. The present solar flux in the region of X.2600 is that of a black body of temperature about 5000° K (Tousey, 1955). Thus at a typical time during the early history of the solar nebula when the bolometric luminosity was approximately that of the present Sun, the tempera- ture near X.2600 was about 500° K less, T = 4500° K. Since the luminosity is proportional to R2 T4, the photospheric radius at this time is easily seen to be about 1. 2 RQ. With these values for R and T, and with X.o = 2600 A, equation (11) yields Q = 1. 3 x 1014 quanta cm"2 sec"l. At a much earlier time, near the beginning of the evolutionary track of Henyey et al., T - 4000° K, L * 0. 6 LQ, R ~ 1.2 RQ, and Q = 2.3 x 1013 quanta cm"2 sec "1. We see that, except for very early times in the history of the solar nebula, the 12

flux of photons of wavelength shorter than X.2600 was always greater than the critical value of section II C, Q = 1011 quanta cm"2 sec'1, beyond which aldehydes, ketones, aromatics and some unsaturated hydrocarbons are photodissociated before reaching atmospheric layers thick enough to provide shielding. We are now interested in the radiation flux after the Sun's evolutionary track has joined the main sequence, and the dissipation of the solar nebula has been completed. At the junction with the main sequence some 5 x 1C)9 years ago, the luminosity was about half a bolometric magnitude less than at present, and the radius about 0. 87 RQ (Henyey, Levee, and LeLevier, 1955; Schwarzschild, Howard, and Harm, 1957; Hoyle, 1958). 10^ years later, about 4 x 109 years ago, the solar radius was about 0. 90R0. while the solar luminosity had increased from about 0. 69 LQ to about 0. 73 LQ (Hoyle, 1958). The effective ultraviolet temperatures at the two times were about the same (0. 975 TQ). From equation (11), the quiet solar ultraviolet fluxes at wavelength shortward of X.2600 in the vicinity of the Moon at both these times is computed to be Q ~4 x 1014 quanta cm"2 sec"1, at X. < 2400 K, Q ~ 9 x 1013 cm"2 sec"1; at X . < 2000 8., Q~ 2 x 1013 cm"2 sec"1. F. Lifetime of Secondary Lunar Atmosphere The time for the density of a planetary atmosphere to be re- duced to 1/e its initial value by escape to space is determined by the value of Tc, the temperature at the critical level in the at- mosphere above which a molecule moving outward with the velocity of escape is unlikely to encounter another molecule. If H = kTc/mg is the scale height at the critical level, and R is the planetary radius, the time of escape can be conveniently expressed as tj » 2.5 B (H/g)1/2 (1 + R/H)'1 eR/H. (12) A similar expression without the factor B was first derived by Jeans (1916). The correction factor B was introduced by Spitzer (1952) to allow for the non-isothermality of real planetary atmos- pheres. For the early lunar atmospheres, the appropriate values of Tc and B depend on the detailed structure of the atmospheres, and cannot be specified precisely. However, it is obvious that the value of tj is small compared with geological time. If, for example, we take Tc = 1000° K. , and the corresponding contemporary terrestrial value for B, B ~ 5 x 105 (Spitzer, 1952), the time for the lunar atmosphere to fall in density by a factor of 1/e is tj * 10 years. Other choices for Tc and B give similarly small results for tj. 13

Hence the lifetime of the secondary lunar atmosphere depended entirely on the supply rate of gases from the lunar surface and in- terior. Although it is unlikely that the lunar craters are volcanic in origin, there is evidence of extensive igneous activity in the early history of the Moon. The maria are probably frozen lava flows (v., e.g., Baldwin, 1949; Urey, 1952; Kuiper, 1954). About twenty large features have been observed on the Moon which are classified as extinct volcanoes. They are of the order of 10 km in diameter; most have central calderae (v., e.g., Kuiper, 1959a). From pub- lished photographs and from visual inspection with large telescopes, there seems little doubt as to their volcanic nature. Associated with them are lower, larger objects which Kuiper (1959a) identifies as volcanic sinks. Reports of contemporary lunar volcanic activity will be critically discussed in section III, below. Outgassing of the lunar interior must have released material deposited there during the formation of the Moon. Since conditions at those times were highly reducing, the secondary lunar atmosphere resulting from outgassing must also have been reducing. Most of the products of contemporary terrestrial volcanic exhalations are oxidizing, but much of this material arises from recirculated ground water, and is not juvenile in origin. A further consideration of relevance is the lifetime of open bodies of water on the early Moon. The temperatures on the primi- tive Moon must have been much less extreme than on the contemp- orary Moon, because of a greenhouse effect initiated by such mole- cules as NH3, CH4, and I^O. If we assume that the mean tempera- ture was no higher than 20o C, the vapor pressure over liquid water was < 20 gm cm"2. The characteristic escape time of a water molecule was about 10^ years. Hence the average escape flux was < 10"9 gm cm"2 sec"1. If the Moon started with the present ter- restrial complement of liquid water, about 105 gm cm"2, bodies of water would have remained for > 3 x 10^ years. For this in- terval, at least, there would have been an appreciable atmosphere. Thus it is not impossible that the relevant lifetime of the secondary lunar atmosphere—during which organic molecules were produced in the atmosphere and dissolved at the surface—was as long as 10^ or 108 years. G. Surface Densities of Deposited Organic Matter We are now in a position to return to equation (1) and estimate the amount of organic matter synthesized in the primitive lunar 14

atmospheres, and deposited on the surface. For example, with r * R, p * 1, (i * 100, 0 = 10"6, Q = 4 x 1014 cm"2 sec'1, and t = 3 x l014 secs, we find <r = 5 gm cm"2 of amino acids. If such molecules as aldehydes or aromatic compounds were effective in near ultraviolet absorption, wavelengths longer than X.2600 would have produced organic matter; thus Q and <r would be larger. On the other hand, if only X. < 2000 A was effective, a would be ~ 0. 3 gm cm"2. Miller and Groth find efficient production of other sub- stances besides amino acids, some with greater quantum yields (especially formic and acetic acids) and many with lesser quantum yields. In the example the total organic matter deposition is prob- ably near 10 gm cm"2. In Table I, we have tabulated total organic matter depositions for the range of likely values of 0 and t, and with X.Q = 2600 R. Table I suggests that very considerable surface densities of organic molecules were produced from the solar nebula and lunar protoatmosphere (t = 10^ to 10^ years). However, most of this material rained down while the Moon was still being formed, and therefore must either be buried at great depths below the present lunar surface, or, more likely, was thermally dissociated in the outgassing processes which evolved the secondary lunar atmosphere. Organic matter produced in the secondary lunar atmosphere appears to have a much better chance of residing near the present lunar surface and having avoided dissociative processes (cf. section II H below). The overall deposition of organic matter after the Moon's formation may well have been as great as 10 gm cm"2. TABLE I Lunar Organic Matter Surface Densities in gm cm"2 t 0 in years 1D'5 io-6 1D'7 Envelope 104 105 10-1 10"2 10"1 10"3 10"2 Secondary lunar 1 iw 10 102 103 104 1 ^ atmosphere Lunar protoatmosphere I Solar nebula 10^ 108 109 10 102 103 ' j 10 / 102 15

H. Protection of Deposited Molecules and Present Location of Lunar Deposits of Organic Matter During the time of deposition, the lunar atmosphere would have inhibited thermo- and photo-dissociation of the deposited molecules. As the secondary lunar atmosphere gradually escaped to space, and outgassing declined, the rate of atmospheric organic synthesis decreased and the penetration of short wavelength radia- tion to the surface increased. In addition, the surface temperature gradually rose, due both to the loss of the insulating atmosphere, and to radioactive heating. The effect of heat and ultraviolet light on the molecules described above is most remarkable. Although the second law of thermodynamics is obeyed, a large fraction of the molecules, with activation energies supplied, partake in organic syntheses of a higher order of complexity. Polypeptides arise from amino acids, hydrocarbon dimers and trimers form long- chain polymers, and in general very complex organic molecules are constructed (v., e.g., Oparin, 1957; Fox, 1956). Finally, be- cause complex molecules are more resistant to heat and radiation than are simpler molecules (at least in part due to the Franck- Rabinowitch cage effect), the syntheses are biased towards the net production of the most complex organic molecules (Gordy, Ard, and Shields, 1955; Sagan, 1957). Although continued radiation and high temperatures would lead to the eventual destruction of all these molecules, we must remember that meteoritic matter was falling into the lunar at- mosphere throughout the period of organic synthesis. Whipple (1959) estimates that about 50 gm cm"2 of meteoritic matter falls on the Moon each 108 years at present rates of infall. In addition, it is almost certain that the rate of meteoritic infall on the Moon in primitive times was greater than it is today. As a consequence, the Moon's surface must have received a dust cover, probably composed primarily of silicates and ices, which can be identified, at least in part, with the present lunar surface material. The organic molecules would then be covered by a protective layer, insulating them from the extremes of lunar temp- erature and absorbing the incident solar radiation and subsequent meteoritic infall. With a temperature fluctuating mildly about 20° C., the thermostability halflives of many organic molecules are of the order of the age of the Solar System (Abelson, 1954). Tem- peratures of this order or lower are expected beneath a few centi- meters of surface (cf. section VI A below). Making the approximations that the rate of meteoritic infall has been constant in time at the present value, and that the mass of 16

surface material escaping to space because of infall is much less than the mass accreted because of infall, we find that the layer of organic matter is localized at a depth of a few tens of meters. Pro- vided that no large-scale destructive events have occurred subse- quent to deposition, we may anticipate a mean surface density of organic matter in this layer of perhaps 10 gm cm" . if meteoritic infall causes appreciable mixing of the surface material, then the organic matter should be distributed through the upper lunar sur- face to a depth not exceeding a few tens of meters. The organic matter should be expected only in regions which have had no ex- tensive lava flows; the southern highland appears to be such a region, as does much of the far side of the Moon. The recent discovery of a dust belt around the Earth (A. R. Hibbs, J. Geophys. Res., 66: 371, 1961; F. L. Whipple, Nature, 189: 127, 1961) implies that not all the micrometeorites detected by impact with satellite and probe vehicles are on collision tra- jectories with the Earth. Consequently the rate of meteoritic deposi- tion on the Earth is less than previously thought, and the lunar meteoritic dust layer will be reduced in depth. If all impacting meteoritic debris remains on the Moon, if there is no appreciable stirring, and if the rate of infall is constant with time, then the layer of organic matter will be localized at a depth perhaps con- siderably less than several tens of meters. The mean tempera- tures at this depth will be lower, and the temperature variation will be greater, than has been estimated above. These considera- tions would make the survival of lunar prebiological organic matter or life-forms somewhat more unlikely. However, it should be re- called that the rate of meteoritic infall was probably far greater in primitive times than it is today. In addition, some studies of radio scattering from the Moon indicate that in certain areas the surface, dust layer may well be several tens of meters thick (K. M. Siegel, private communication, 1961). The resolution of this problem must await more definitive investigations of the lunar surface. I. Conclusions and Suggested Experiments We have concluded that at a depth of some tens of meters be- low the present lunar surface there may be localized a layer of or- ganic material deposited during the period in which the Moon pos- sessed a reducing atmosphere opaque in the ultraviolet. The value of the surface density is difficult to estimate, primarily due to the uncertainty in the lifetime of the secondary reducing atmosphere. But from Table I it is clear that instrumentally-detectable amounts should exist. A reasonable estimate would be one to ten gm cm"2. 17

A qualitative analysis of such organic matter would provide important information on the types of molecules produced in pre- biological organic syntheses on the Earth and elsewhere; it would furnish clues for the laboratory simulation of prebiological organic evolution, and for the reconstruction of pathways which lead to the origin of life. A quantitative analysis would supply evidence on the nature and lifetime of early lunar gaseous envelopes, on current theories of stellar evolution, and on hypotheses concerning the origin and early history of the Solar System. Instrumentation is required to recover boring cores from a depth of perhaps several tens of meters, perform simple quali- tative and quantitative analyses, and transmit the information back to Earth. The simplest analytic technique would be to determine the vapor pressure of cores from various depths as a function of temperature. This could identify many of the major categories of organic molecule, and could easily distinguish organic matter from silicates, irons, and residual ices. The temperature varia- tion might be provided by the lunar day-night cycle itself. For more detailed analysis the relative merits and feasibilities of re- mote gas and paper chromatography, remote spectroscopy, and remote reagent analytic chemistry should be investigated. There is an obvious advantage for the subsurface probing device to be in- corporated in a roving lunar surface vehicle; for example, mare and non-mare cores could be compared. Such instrumentation would have a wide range of non-lunar applications, the terrestrial sub-oceanic and Martian surface environments being the two most obvious. 18

Next: III. REPORTS OF GAS CLOUDS ON THE LUNAR SURFACE »
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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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