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Letters to Nature
Nature 215, 1259 - 1260 (16 September 1967); doi:10.1038/2151259a0

Life in the Clouds of Venus?

HAROLD MOROWITZ & CARL SAGAN

Department of Molecular Biophysics, Yale University, New Haven, Connecticut.
Harvard College Observatory, Smithsonian Astrophysical Observatory, Cambridge, Massachusetts.

WHILE the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether. As was pointed out some years ago1, water, carbon dioxide and sunlight—the prerequisites for photosynthesis—are plentiful in the vicinity of the clouds. Since then, good additional evidence has been provided that the clouds are composed of ice crystals at their tops2,3, and it seems likely that there are water droplets toward their bottoms4. Independent evidence for water vapour also exists5. The temperature at the cloud tops is about 210° K, and at the cloud bottoms is probably at least 260–280° K (refs. 4 and 6). Atmospheric pressure at this temperature level is about 1 atm.7. The observed planetary albedo falls steeply in the violet and ultra-violet8, which accounts for the pale lemon yellow colour of Venus. The albedo decline would not be expected for pure ice particles, and must therefore be caused by some contaminant. Dust, ozone, C3O2 and other gases may possibly explain these data but, whatever the explanation, the ultra-violet flux below the clouds is likely to be low. If small amounts of minerals are stirred up to the clouds from the surface, it is by no means difficult to imagine an indigenous biology in the clouds of Venus. What follows is one such speculation.

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References

1. Sagan, C., Science, 133, 849 (1961). | ISI |
2. Bottema, M., Plummer, W., Strong, J., and Zander, R., J. Geophys. Res., 70, 4401 (1965). | ISI | ChemPort |
3. Sagan, C., and Pollack, J. B., J. Geophys. Res., 72, 469 (1967). | ISI |
4. Pollack, J. B., and Sagan, C., Astrophys. J. (in the press; Sept. 1967 issue).
5. Dollfus, A., C.R. Acad. Sci., 256, 3250 (1963); Bottema, M., Plummer, W., and Strong, J., Ann. d' Astrophys., 28, 225 (1964); Belton, M. J. S., and Hunten, D. M., Astrophys. J., 146, 307 (1966); Spinrad, H., and Shawl, S. J., ibid., 146, 328 (1966).
6. Pollack, J. B., and Sagan, C., J. Geophys. Res., 70, 4403 (1965). | ISI |
7. Chamberlain, J. W., Astrophys. J., 136, 582 (1965).
8. Harris, D. L., in Planets and Satellites (edit. by Kuiper, G. P., and Middle-hurst, B. M.), 272 (University of Chicago Press, 1961).
9. Compare, Sagan, C., and Leonard, J. N., in Planets, 190 (Life Science Library, Time-Life Books, NY, 1966); Shklovskii, I. S., and Sagan, C., in Intelligent Life in the Universe, 329 (Holden-Day, San Francisco, 1966). The organisms postulated here are designed for the cloud levels of Jovian planets.
10. Gest, H., and Kamen, M. D., Handbuch der Pflanzenphysiologie (edit. by Ruhland, U.), 5, chap. 4 (Springer-Verlag, Berlin, 1958); Gest, H., in Proc. Intern. Symp. Enzyme Chem., 250 (Academic Press, NY, 1958).



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