DEPARTM\IENT OF GEXETICS, STANFORD UNIVERWI'B SCHOOL OF MEDICINE, PALO ALTO, AND INSTITlrTE FOR BASIC RESEARCH IN SCIENCE, UNIVERSITT OF (`ALIFORNI.4, BERKELEY Conunwh-ated July 16, 1.962 `L'he present capability for space flight, gives t,imely urgency to discussions of problems of life on extraterrestrial bodies, especially Mars. Until the advent of these flights, t'here have been two approaches t,o the issue: the speculative construc- tion of models of the environment' and ecology of Mars, and direct telescopic searches for indications of lifr on that, planrt. These two approachrs ha\-{: pointed in opposit,e directions. The low average temperatures, low mean wat,cr vapor content,, absence of es- t,ensivt bodies of pure liquid water, low atmospheric pressure, absence of molecular oxygen, and possible high ultraviolet radiation flux are undeniable constraints on RIartian biology.*-5 Some terrest&l microorganisms survive in purported simu- lations of the average Mar&n cnvironment,.6r 7 Most' would fare poorly, and whether any can proliferat#e in an accurat,ely simulated environment is less clear. [n any went, how mcll the Mnrtinrb organisms hare learned to cope with the same constraints remains to be seen. The observational evidence falls into t,hree categories: (1) srasoual and swulal changes in the albedo, color, and delineation of the Mart,ian dark areas,?, 8, y (2) seasonal changes in t,he polarization of light, reflect,ed from the dark areas but not the bright areas, suggcst'ing periodic changes in the size and dist)ribut'ion of small scattering granules on t,he Alart,ian surface,`" and (3) spect,ral features in the in- frared reflection from the dark (but not, the bright) areas which have been inter- preted as absorption by CH and CHO. llj I2 There are obvious difficulties in detect- ing extraterrestrial life over interplanetary distances and none of t,hcsr observations can, by itself, be very convincing. Deliyueacent salt,s might darken and change color upon wetting, as might t,he polarimetric properties of mineral granules. The infrared observations arc rvidcncc for carbonawous matt,er on Alaw, not nrwssar- ily of life. Apart from these inconclusive observatSional arguments, how might WC construct a model for a life-bearing Mars? We should take account of t,hc severe constraitlts posed by t,hc areragr environment,al condit,ions! which have led some u>lthors to reject the possibilit,y of Martian life. An approach to this question has already been suggcsted.13 The Martian environment is het#erogeneous on a large scale both in spare and in timr. The surface trmpcrat,urr \-arirs from <2OOT< to 310K. Thr 1173 1474 ASI'RONOM Y: LEDEKHEKG dNI) SAGA A' PROC. N. A. s. seasonal changes in the polar ice caps imply a variation in the availabilit,y of atmos- pheric wat,er vapor. High resolution studies of the Martian topography show progressively finer and more variegated detail. We have much less insight into t,he small-scale variation of temperat,ure and humidity to specify the microenriron- ments in which organisms might) flourish. The average microwave temperature of Mars, int,egrated over the whole disk, is 211 * 28'K.14 As t'his temperature refers to a depth of some decimeters, it is un- likely t)o undergo much diurnal or seasonal variation. The low abundance of water in t,hr Liars atmosphere may be due to its condensat,ion as permafrost, at these low temperatures as well as to its escape from t,he atmosphere,15 as has already been suggested for the moon.lfi The permafrost hypothesis lends special importance to the dist)ribution of localized geothermal act#ivity---hot springs, fumarolcs, volcanoes. Such hot) spots should be accompanied by local out)gassing and by higher water vapor pressures in the overlying atmosphere, making t,hesc locales much more favorable mirroenvironmtnt,s for life. Recurrent clouds at specific locales-oft,en att,ributed t,o elevation differences---may be symptomatic of this local out,gassing of water vapor. Once the permafrost has been broken through, a continued supply of moisture and heat, should be available from deeper layers to maintain hot spots, the general circulatlion of gasses from t,he int#erior being impeded by the permafrost. JIcIAaughlin17 has proposed that, volcanism account,s for many of the major features of the Martian topography. His conclusions have been criticized18-21) and defendedzl on various grounds. Our proposal should be distinguished by its stress on the significance of more localized geot#hcrmal activity such as fumaroles. h search for favorable microhabit,ats can be made from planet,ary flyby vehicles such as t,hc Mariner series.22 Energy limitations have rest,ricted the area1 resolu- Con of Earth-based spectroscopy and bolometry to >500 km; t,his should be im- proved by at least, an order of magnitude by early missions. Do the areas where organic matter is conccntrat,ed also show t,he greatest seasonal changes? hre there any local concentrat,ions of water vapor ? Do anomalous warm spots persist, during the hlartian night phase, which cannot be observed from Earth? Are these fea- tures correlated? If so, they mill point to t,he most plausible sites for landing ex- periments carrying monitors more explicitly directed to the det,ection of biological activit'y. A continuing series of flyby vehicles capable of video reconnaissance. infrared spectrometry, and bolometry with progressively higher topographical resolution-~-hopefully down to a few hundred met,ers- would be of great valur in directing our search for life on Mars. Summwy.-Deduct,ions as t,o the habitabilit,y of Mars, of great importance in planning for space explorations, must take account, of local variations, as well as the harsh, average features of the planet For example, substantial moisture may be frozen in the subsoil, moisture and warmth being available through localized geothermal activity. Models of this kind pose specific quest,ions for high-resolution reconnaissance in planet'ary flyby missions. * The work of the authors has been supported I),v grants N&81-60 and Pi& 126-61 from the National Aeronautics and Space Administration. f Currently at Harvard University and Smithsonian Astrophysical Observatory. 1 Kuiper, Ci. P., The Atmospheres of the Earth and Planets (Rev. ed.: Chicago: Universit>- of Chicago Press, 1952), ch. 12. ? Vaucouleurs, (+. de, Ph&cs of the Planet Mwrs (London: Faber and Faber, Ltd., 195-L). 3 Abelson, P. H., these PROCEEUINOS, 47, 575 (1961). 4 Kellogg, W. W., and C!. Sagan, The Atmosphwres o-f Mars and Venus, National Academy of Sciences-National Research Council, Puhl. 944 (Washington: 1962). 5 Salisbury, F. B., Science, 136, 17 (1962). 6 Fulton, J. D., Physics and Medicine of the Atmosphere and Space, ed. 0. (1. Benson and H. Strughold (New York: John Wiley and Sons, Inc., 1960). i Hawrylewicz, E., B. Gowdy, and R. Ehrlich, Xat7m, 193, 497 (1962). * Antoniadi, E. M., La Plan&e Mars (Paris: Lihrairie Scientifique Hermann, 1930). 9 Kuiper, G. P., Astrophys. J., 125, 807 (1957). lo Dollfus, A., Planets and Sate&&, ed. G. P. Kuiper and B. M. Middlehurst (Chicago: Uni- versity of Chicago Press, 1961), ch. 9. ii Sinton, W. M., Science, 130, 1234 (1959). iz Colthup, N. B., Science, 134, 529 (1961). 13 "I have been led to the opinion and conviction that the surface of the moon is not smooth, uniform, and precisely spherical as a great number of philosophers believe it (and the other heavenly hodies) t.o he, but is uneven, rough, and full of cavities and prominences, being not unlike the face of the earth, relieved by chains of mountains and deep valleys." Galileo, Sidereus .Tancius (1610); Discoveries and Opinions qf Galileo, Stillman Drake, translator (New York: Doubleday Anchor Books, 1957), p. 31. iI Giordmaine, J. A., L. E. Alsop, C. H. Townes, and C. H. hfaycr, Astronop)~. J., 64, 332 (1959). I5 Dnwvdow, V. W., I~eZtra?~mfahrt, 1, 14 (1961). iG Watson, K., B. C. Murray, and H. Brown, J. of Geophys. Res., 66, 3033 (1961). I7 McLaughlin, D. R., Pubis. Astrcmom. Sot. of the Pacijic, 66, 161 (1954); ibid., 66, 221 (1954); ibid., 68,211 (1956); Astronom. J., 60,261 (1955); Sci. Monthly, 83, 176 (1956); J, Roy. Astronom. Sot. of Canada, 50, 193 (1956). 18 Kuiper, G. P., Pubis. Astronom. Sot. of the Pacific, 6'7, 271 (1955); zbid., 68, 219 (1956). 19 Vaucouleurs, (:. de, Physics and Medicine of the $tmosphere end Space, ed. 0. 0. Benson and H. Strughold (New York: John Wiley and Sons, Inc., 1960), ch. 39. 2o Vladimirskiy, B. M., and K. A. Lyubarsky, Trudy Sektora Astrobotaniki. ;Ikademiya `\`auk Kazakhskoy SSR, 6 (1956). 21 Urey, H. C., P&s. Astronom. Sot. of the Pacific, 68, 220 (1956). 22 Newell, Homer E., NASA Space Science Program. U. S. Senate, Committee on Aeronautical and Space Sciences, NASA Authorization for Fiscal Year 1963 (Washington: 87th Congress, Srcond Session, H.R. 11737; USGPO 84468-0, 1962), pp. 190-331.