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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

III. REPORTS OF GAS CLOUDS ON THE LUNAR SURFACE From time to time there have been reports—primarily by British amateur astronomers with small telescopes—of low-lying gas clouds or 'mists' on the lunar surface (v., e.g., Moore, 1952). The clouds are said to be detected either by their high albedo, or by their obscuration of surface detail. Similar observations have in general not been made by professional astronomers with large instruments; e.g., Kuiper (1960), in a continuing program of lunar visual observations with the 82-inch reflector of McDonald Ob- servatory during the last decade has never observed cloud-like features on the surface of the Moon. Whittaker (1960) explains the mist reports as uncritical interpretations of the occasional absence of familiar fine detail because of poor seeing conditions. This ex- planation seems especially appropriate for the British Isles. Alter (1957) has obtained photographs of the region of the crater Alphonsus, which show less contrast in the photographic in- frared than in the visible. A gas cloud or haze would give a simi- lar effect. However, Alter's obscuration seems—at least in the published prints — not to be restricted to the floor of the crater, but appears to cover the entire photographed area. The possibility must be raised that the difference in contrast arises in the ter- restrial atmosphere rather than in a lunar gas cloud. In 1958, N. A. Kozyrev claimed to have made spectroscopic observations of a reddish cloud in Alphonsus, which indicated the presence of carbon compounds on the Moon. Because of the pe- culiar circumstances surrounding this observation, and its obvious importance if verified, it will pay to examine it in some detail. Kozyrev (1959a; v. also Alter, 1959) maintains the following: He was observing with the 48-inch reflector of the Crimean Astro- physical Observatory on November 3, 1958, with a spectrometric dispersion of 23 A/mm, and photographing the spectrum with Kodak 103 a - F emulsion plates. While guiding the slit of the spectrograph on the central peak of Alphonsus, he noticed a reddish cloud en- veloping the peak. He stopped the exposure he had been taking and inserted another plate. After a 30-minute exposure, the cloud ap- peared to have dissipated. Kozyrev then stopped this exposure and took a third spectrogram, this time with a ten minute exposure. 19

Development of the 30-minute spectrogram showed, in addition to the solar Fraunhofer absorption lines, emission features which were absent or only barely perceptible on the preceding and fol- lowing plates. The wavelengths of these features agreed with the wavelengths of the molecules G£ and €3 familiar in cometary spectra. He concluded that the molecules were outgassed from the lunar interior in a volcanic eruption, and exhibited emission bands due to the cascade of absorbed solar ultraviolet photons to lower molecular energy levels (fluorescence). Kozyrev delayed a week in announcing his discovery, so that when announcement was made Alphonsus was in the dark; two weeks were to pass before Alphonsus was again illuminated and corrobora- tory observations could be attempted. Then, a number of amateur observers (e.g., Poppendiek and Bond, 1959; Hole, 1959) claimed to have observed clouds in the vicinity of the central peak of Al- phonsus; Poppendiek and Bond, with a 6-inch reflector, saw a diffuse white cloud on November 18, and Hole, with a 24-inch re- flector, saw a small reddish cloud on November 26. On the other hand, Kuiper (1958) with the 82-inch McDonald reflector, and Focas with the superb seeing conditions at Pic du Midi, were unable to detect any unusual features in the vicinity of Alphonsus in the second half of November. Haas (1959) was observing Alphonsus at about the same time as Poppendiek and Bond and with a larger instrument; he observed no cloud. Hole thought he .had photographic evidence of an increased emissivity in the red for Alphonsus, but then found that the photographic redness of Alphonsus was present on old photo- graphs as well. The supplementary visual and photographic evi- dence for Kozyrev's observation does not seem very convincing. The published spectra (Kozyrev, 1959a) and glossy prints available in the United States from Soviet sources show a broad diffuse structureless feature extending for at least 800 A short- wards of the vicinity of X.4700. No other features besides the Fraunhofer lines are evident. The dominant features of C? in o a * cometary spectra are the Swan bands, B 7rg »-X TTU, of which the 1-0 vibrational transition has a band head at X.4737. The relative transition probability of the 1-0 transition (X.4737) is f = 0.36; for 1 - 1 (X.5129), f = 0. 6; for 1 - 2 (X.5585), f = 0. 4 (Phillips, 1957); and from the Franck-Condon principle it can be seen that all other transitions from v' = 1 have smaller f-values. Since the transition probabilities are atomic quantities uninfluenced by the pressure, temperature, and density of the gas, the 1-1 transition must occur about half again as often, and the 1-2 transition about as often, as the 1-0 transition, regardless of the mechanism of excitation of the B ^TT v' = 1 energy level. A 20

selective quenching of X ^TTU v' ' > 0 levels is impossible; in its absence, the X.5129 and X.5585 bands should be at least as intense as the X.4737 band. X.5585 is beyond the edge of the available spectra, and is in a region of low sensitivity of the 103 a-F emul- sion; but the X.5129 feature should appear strongly. Yet the region just short of the X.5184 Fraunhofer Mg triplet is remarkably devoid of detail. Kozyrev claims that the X.5129 feature appears on the original plates (Alter, 1959), but if it appears in the theoretically required strength on the plates, it must be easily visible on the prints, which it is not. This difficulty, first pointed out by Kuiper (1959b, 1959c), is a fundamental one. Unless some physically reasonable mechanism for the absence of the B -^ (v1 =1) - X TTU (v" = 1) line of C2 can be proposed, the identification of C2 above Alphonsus must be discounted. On the prints there is no sign of the C3 bands in the X.4000 region, which Kozyrev says can be seen 'clearly' (Alter, 1959), and the evidence for €3 seems non- existent. In addition, Opik (1960) points out that if an emitting gas were to expand adiabatically from the peak of Alphonsus for a period of thirty minutes, a spectrum taken during this interval should show emission from the area of the entire crater. Kozyrev's spectrum shows emission from the central peak only, and Opik finds it diffi- cult to understand how the observations can be explained as gas emission. However, if the gas were emitted from a fissure or volcanic cone at a temperature~104 °K, lunar escape velocity would be achieved, and the expanding cloud would have a conical envelope of small half-angle. But there is no evidence for such high temp- eratures. Because of these difficulties, some astronomers have pri- vately confessed doubts as to the authenticity of Kozyrev's observa- tions themselves. However, the observations were witnessed by several astronomers at the Crimean Astrophysical Observatory, and the developed plates were examined the following day by Boyarchuk, Orletzky, and Prokoffiev (Boyarchuk, 1960). It would therefore appear that something was occurring on or above the central peak of Alphonsus on the evening of November 3, 1958. Very recently the spectroscopist A. A. Kalinyak has supported Kozyrev's identification of C2 and C3 (v. N. Calder, New Scientist, 8: 1636, 1960), although the details have not yet been published. The conclusion that the observations were not of a carbon- compound gas cloud is reached with great reluctance, because the most likely precursors of C2 and €3 would be hydrocarbons and other organic molecules, as is probably the case for comets. A verified observation of C2 and C3 above the Moon would have been strong evidence for lunar subsurface organic matter. 21