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The Author(s): Source: , New Series, Vol. 133, No. 3456 (Mar. 24, 1961), pp. 849-858 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1706530 . Accessed: 24/02/2015 13:54

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This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions 24 1961, Volume 133, Number 345-6

CURRENT PROBLEMS IN RESEARCH eventual manned expeditions to Venus must be exceedingly perplexed over whether to send along a paleobotanist, a mineralogist, a petroleum geologist, or a deep- diver. But new informa? tion has recently become available The Planet Venus which probably eliminates three of the four proposed surface environments; taken together with some of the earlier Recent observations shed light on the , data, it points the way to a consistent surface, and possible biology of the nearest planet. picture of the atmosphere and surface of Venus.

Carl Sagan Composition of the Atmosphere

Only the portions of the Cytherean The launching of the Soviet inter? thetical Carboniferous swamp was gen? atmosphere which are above the planetary vehicle toward Venus on 12 erally abandoned, to be replaced by an layer are accessible to spectroscopic in? February 1961 opens a new era in arid planetary desert, overlain by vestigation. Since the cloud layer may planetary studies. This article is an of dust from the wind-swept be situated tens of kilometers above the assessment of current knowletlge of surface (3) (Fig. 1). The arid surface surface (see the discussion below), the Venus at the of this era. also explained the great abundance of spectroscopic data are not necessarily The planet Venus is enshrouded by dioxide [which was accidentally directly applicable to the lower atmos? clouds which prevent telescopic exami? discovered (4) in a search for phere. It is possible that gases present nation of its surface. In the absence of vapor]; for, in the absence of water, the above the cloud layer in undetectable direct observations, reasons have been Urey equilibrium of carbon amounts are abundant in the lower at? adduced for proposing a variety of dioxide will not be established (5). mosphere. By laboratory intensity- differing and mutually iaconsistent sur? Hoyle (6) explained the lack of water matching of the Cytherean carbon di? face conditions. Since onty water clouds by assuming a great excess of hydro- oxide intercombination bands near were familiar to terrestrial observers, over water on primitive Venus, 8000 angstroms, the abundance of the apparent thickness of the Cytherean and subsequent oxidation of the hydro- carbon dioxide (above the atrnospheric (i) cloud layer seemed to argue for a carbons to carbon dioxide, until all the level at which an 8000-angstrom great abundance of water. From there water was depleted. He suggested that photon is efTectively reflected) is esti? it was only a step to the assertion (2) the surface is now covered with the re? mated to be 1 kilometer-atmosphere that "everything on Venus is dripping mainder of the hydrocarbons, and that (km-atm) (8). The only other pos? wet. . . . A very great part of the sur? the cloud layer is composed of smog. sibly identified atrnospheric constituent face of Venus is no doubt covered with Menzel and Whipple (7) replaced is water vapor, marginally detected on swamps. , . . The constantly uniform the wind-swept desert and the planetary Venus recently by high-altitude balloon climatic conditions which exist every- oil field with a global Seltzer ; spectroscopy. The abundance of water where result in an entire absence of they argued that if Venus were com? vapor (above the atrnospheric level at adaptation to changing exterior condi? pletely covered by water (because of which a 1.13-micron photon is effective- tions. low Only forms of life are there? the high atrnospheric content of carbon ly reflected) is estimated by Strong to be fore represented, mostly no doubt, be- dioxide, the water would, of course, be about 2 10-8 gm/cm2 (9). Kozyrev longing to the vegetable kingdom; and carbonated), the access of carbon di? reports observing several features in the the organisms are nearly of the same oxide to would be impaired, and of Venus corre? kind all over the planet." and for this reason the Urey equilib? sponding in wavelength to known emis? After many unsuccessful spectro? rium would not be established. The sion bands of N2, N2+, and CO+ (10). scopic attempts to discover water vapor state of our knowledge of Venus is From considerations of cosmic abun? in the Cytherean atmosphere, the hypo- amply illustrated by the fact that the dance and from terrestrial analogy, we Carboniferous the would to find N2?which has no The author is Miller research fellow at the swamp, wind-swept expect Institute for Basic Research in Science, the desert, the planetary oil field, and the permitted absorption spectrum in the Laboratory, and the Department Seltzer ocean each have their accessible of , University of at global presently wavelength region Berkeley. serious proponents, and those planning ?on Venus, and we would expect to 24 MARCH 1961 849

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions find carbon monoxide from the photo- observed; occasionally dark bands are features, the has a serrated dissociation of carbon dioxide. [It is a seen extending perpendicularly from appearance. The bright protruding fea? curious incidental fact that at one time the terminator onto the disk. The dif- tures are prominent near the apparent Kozyrev (II) believed the spectral ficulties encountered in making visual poles, especially near the southern pole; features discovered by him were due to observations of Venus are illustrated by they were also detected visually by microorganisms in the Cytherean at? the fact that the outstanding American such early observers as Schroter and mosphere.] Nevertheless, Newkirk (12) observer at the turn of the century, in Trouvelot, who explained them as found no evidence of many of the over a decade of regular observation enormous mountains, 60 or more kilo? strongest features reported by Kozyrev of Venus, was able to see distinct meters high. Ross proposed the more and found strong emission at a neigh- markings only once. On that occasion likely explanation that the protrusions boring wavelength (4505 A) not re? shading was evident, parallel to the are areas of atmospheric haze sur- ported by Kozyrev and corresponding terminator but not perpendicular to it rounding the planetary poles, such as to no known molecular emission. The (19). Danjon and Dollfus (20) have exist around . This, in turn, im? only features common to both observa? constructed planispheres from their plies an appreciable difference in tem? tions are unidentified emission bands at visual observations which show dark perature between the and the 4415 and 4435 angstroms (13). markings with little relative displace? poles. It would be of interest to deter? One might attribute the absence of, ment over periods of weeks. In some mine the composition of the polar for example, the N2+ first negative sys? drawings the markings tend to radiate haze. More recent ultraviolet photo? tem in Newkirk's spectra to low from the . Danjon and graphs show three bright and three magnetic activity at the time of his Dollfus interpret these dark markings dark bands, roughly perpendicular to observations, but the absence of the as surface features seen through stable the terminator and extending across the feature at 4505 angstroms in Kozyrev's gaps in the cloud layer. The apparent entire visible hemisphere of Venus. The spectra then remains puzzling. Unsuc- constancy in the position of the mark? inclination of these bands to the cessful searches for carbon monoxide ings suggested to the French astrono- of the Cytherean is estimated by absorption place the amount of carbon mers that the period of of Kuiper (22) at 32? and by Richardson monoxide above the relevant reflection Venus is equal to its period of revolu? (23) at 14?. The difference between level at less than 100 cm-atm (14). At tion, 225 days. However, Kuiper (21) these values emphasizes the observa? the present writing, it must be con? has pointed out that on a slowly ro- tional difficulties. cluded that there is no convincing direct tating planet, cloud patterns near the Ross attributed the presence of band evidence for nitrogen or carbon mon? terminator will bear an approximately structure to atmospheric circulation, as oxide on Venus. The present upper constant relation to the terminator, just has been suggested for the Jovian limit on in the high Cytherean as characteristic cloud patterns appear . If the explanation is correct, atmosphere is 100 cm-atm (3, 15); the at a given time of on the . then the speed of rotation at the equa? limit is 100 cm-atm for N2O, 4 cm-atm Photographic detail of Venus is most tor must exceed the speed of random for NHs, 20 cm-atm for CH*, 3 cm-atm evident in the near-ultraviolet region atmospheric winds (17), giving a maxi? for C2H4, and 1 cm-atm for CsHo (14). and least evident in the . It is mum period of rotation of a few weeks. None of these gases has been identified. impossible to explain this circumstance The minimum period is obtained from The last few abundances argue effec- by an inverse-power scattering law and the absence of a rotational Doppler The tively against the hypothesis that the the hypothesis that the observed detail shift at the planetary limb (24). cloud layer is composed of hydrocar- is below the visible cloud layer. Detail true period of rotation is probably be? bons (6, 16); at the observed tempera? on Mars becomes more evident toward tween 5 and 30 days. ture of the cloud layer, the vapor pres? the longer wavelengths. It is tempting A potentially effective method of sures of gaseous hydrocarbons are to suggest the presence around Venus studying the Cytherean eloud layer is greater by orders of than of high-altitude clouds which are the determination of the polarization the spectroscopic upper limits. opaque to ultra-violet light and more of reflected from Venus to transparent in the infrared. An alterna? Earth, as a function of the phase of tive explanation is that the cloud layer Venus. The polarization curve of Venus of the Cloud Layer is billowy, so that low-lying clouds ap? in integrated light was first obtained by pear darkened in the violet because of Lyot (25), who attempted to reproduce Visual observations of Venus gen? the increased scattering of light by the it in the laboratory, employing a wide for erally show very little detail, owing to overlying atmosphere (14). The classic range of substances. He found that the uniformly high of the cloud ultraviolet photographs by Ross (18) fine mists of water, decreasing the size layer. During telescopic obser? (Fig. 2) show time-variable bright and of the droplets caused the polarization of vations, when comparisons with terres? dark areas, sometimes stretching, band- curve increasingly to resemble that trial clouds can bs made, it is evident like, perpendicularly from the termina? Venus. However, before even a rough became that the planet is a pale lemon yellow. tor onto the disk. Generally there is a fit was obtained, the droplets colloi- The yellow coloration of Venus is also dark shading adjacent to the terminator, unstable. Lyot therefore prepared observed photometrically (17). Under as in visual observations. Perhaps the dal suspensions of bromonaphthalene, the best seeing conditions, faint dark most striking features of Ross's photo? which have the same differential index in and found markings can be perceived on the il? graphs are the departures of the crescent of refraction as water air, radius were luminated part of the disk. A broad from a symmetric form. Especially that droplets of 2-micron fit band of shade adjoining the terminator where there are bright features the plan? required to give an approximate sometimes appears brown (18). Other etary limb protrudes markedly. On the with the Venus observations. However, for certain markings, both bright and dark, can be other hand, when there are nearby dark this fit was rather poor; SCIENCE, VOL. 133

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions phase angles, even the signs of the pears to be no consistent difference in because at the carbon dioxide laboratory and Cytherean polarizations polarization between the poles (28), of the cloud layer, temperatures of disagreed. Furthermore, the near-infra- while the bright areas near the apparent about 165?K are required for satura- red polarization curve of Venus differs south pole are usually more briliiant tion; even in the polar regions there is markedly from the theoretically pre? than those near the apparent no evidence for cloud temperatures so dicted curve for liquid water droplets pole. Indeed, comparison of visual and low as this. A condensable or sublim- (26). The polarimetric evidence argues polarimetric observations made on the able cloud layer is suggested by the against a liquid water cloud layer on same day shows no clear correlation, polar haze. Noncondensable substances Venus, but at the same time no sub? and it is possible that the particles re? which have sufficiently high reflectivities stance is known which provides a better sponsible for the polarization are differ? include quartz, AlaOs, CaO, MgCOa and fit to the polarization curve of Venus ent from those responsible for the a number of other geochemically less than do droplets of water. More work visual and photographic cloud layer. abundant materials, but it is improb- is needed, both on the of The problem of the composition of able that there is large-scale preferen- laboratory suspensions of liquids and the clouds has been approached in tial production of such substances on crystals at low temperatures and in pro? other ways. The visual albedo of Venus, Venus (29). A variety of molecules viding the Venus polarization as a func? corrected for yellow coloration, has has been suggested to explain the yel? tion of wavelength (27). been estimated to be 0.68 ? 0.04 (26). low coloration, among them polymer- The polarization varies over the disk The only common condensable or sub- ized carbon suboxide (26), ammonium of Venus. Regions of high (negative) limable substance which is known to nitrite (30), and nitrogen dioxide. Be? polarization are localized in the appar? have such high reflectivities at the tem? cause 4hese substances have low reflec? ent polar regions, and correlation of peratures of the Cytherean cloud layer tivities, it is unlikely that the clouds high polarization with the bright visual is water, either as droplets or as ice are composed primarily of any of them. and photographic features in the same crystals. Carbon dioxide sublimation is The presence of large quantities of regions is tempting. However, there ap- unlikely in the Cytherean atmosphere, polymerized carbon suboxide is further

Fig. 1. A painting by Chesley Bonestell showing possible surface conditions on Venus according to the dust-bowl hypothesis. [From a painting by C. Bonestell, in The Conquest of Space (Viking Press, New York, 1950)] 24 MARCH 1961 851

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This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions improbable because the monomer has Optical Temperatures oxygen) the temperature will remain a set of fairly strong absorption fea? roughly constant above the cloud layer tures extending to wavelengths just The radiation emitted from Venus until dissoeiation and ionization by short of 3350 angstroms, while all ultra? at various wavelengths has been ana? short-wavelength solar radiation be? violet spectra of Venus for this region lyzed, and corresponding temperatures come significant. have been negative; and also because have been obtained. However, the Carbon dioxide will begin to dissoei- in the photoproduction of each mon? radiation at a given wavelength may be ate above unit optical depth with light omer by the integration of emission from many of wavelengths of less than 1692 ang? altitudes in the Cytherean atmosphere, stroms, and the atomic oxygen produced 2CO C02 C302 + 02 + -fb-> and for this reason considerable caution will be ionized in the Schumann-Runge a molecule of oxygen is formed, while must be exercised in relating these continuum. The absorption cross-section searches for molecular oxygen have temperatures to specific atrnospheric of carbon dioxide just short of the pho- also been unsuccessful. On the other levels. Thermocouple observations in to-dissociation threshold is about 10~19 hand, small amounts of polymerized the 8- to 13-micron window in the square centimeters; therefore, the tem? carbon suboxide (Cs02)n must certain? terrestrial atmosphere yield mean tem? perature commences to increase, and ly be produced and may contribute to peratures for the disk of Venus of about the Cytherean begins at the the coloration of the clouds. The sur- 234?10?K (31, 36). The fact that level above which there is about 1 cm- prisingly low intensity of the carbon the 10-micron carbon dioxide bands atm of carbon dioxide. From observa? dioxide absorption bands in the 8- to are probably not opaque above the tions of the of Regulus by 13-micron region (31) suggests the cloud layer (32) implies that the cloud Venus (38) it is known that there is presence of a substance which is trans? temperature is close to the thermocou? about 1.7 cm-atm of atmosphere at an parent to the photographic infrared and ple temperature. The unilluminated altitude of some 70 kilometers above opaque to the thermocouple infrared; hemisphere has almost the same tem? the cloud layer. From the same ob? although it is possible that (Cs02)? can perature as the illuminated hemisphere; servations a scale height of 6.8 kilo? serve this function, the lack of unam- this circumstance has suggested (18) meters has been obtained for this level. biguous laboratory spectra of the pol? that the period of rotation must be Thus, if there is negligible photo- ymer (31) precludes a more definitive much less than the period of revolu? dissociation at this altitude, the ambient identification. Since polymerized car? tion?otherwise the bright side would temperature is about 300?K; if carbon bon suboxide absorbs in the near- grow much warmer than the dark side. dioxide is completely photodissociated, ultraviolet, it is also a likely candidate However, this conclusion is not neces- the temperature is about 150?K. The for the hypothesized high-altitude dark sarily valid if violent interhemispheric limb-darkening evidence that the atmos? clouds (32): Heyden, Kiess, and Kiess circulation exists, as indeed the banded phere is approximately isothermal (33) have called attention to a broad structure in ultraviolet photographs above the cloud layer suggests that the continuum in the near-ultraviolet spec? would seem to indicate. It is also pos? temperature 70 kilometers above the trum of Venus, which they attribute to sible that the temperature at a given cloud layer probably lies a few tens of N2O4. Since nitrogen tetroxide is readily atrnospheric depth is much lower on degrees below 230?K. An intermediate dissociated, at wavelengths of less than the dark than on the bright side but level of photodissoeiation is therefore 3000 angstroms, to the dioxide and to that the observer sees to greater depths indicated, as would be expected for atomic oxygen, it might be thought that in the unilluminated hemisphere be? such a transition region. (It should be cc a plentiful source of NO2 is available. cause of a sinking of the cloud layer, mentioned, however, that the T f However, the identification of the and so obtains misleadingly high com- law inferred from limb darkening can? tetroxide must be regarded as extremely posite temperatures. Evidence exists not apply high above the cloud layer tentative, since Kiess and Corliss (34) that the bright ultraviolet clouds in the and still be consistent with the occulta? have announced a very similar feature apparent polar regions are colder than tion data.) If the cloud layer is a few in the spectrum of , where so clouds in neighboring areas (31), as tens of kilometers above the surface, highly oxidized a molecule as N2O4 cer? would be expected if the clouds origi? then the Cytherean ionosphere begins tainly could not escape reduction at nate from the condensation or subli- at altitudes comparable to those of the all atrnospheric levels. mation of a gas. terrestrial ionosphere. If the surface temperature of Venus Thermocouple tracings of the disk of Molecular vibration bands show ro- is near 600?K (see the discussion be? Venus show distinct limb darkening, tational fine structure, which, if a Boltz- levels is low), the theory of the radiation bal? the intensity falling off approximately mann distribution of energy be used to derive tem? ance implies an ice-crystal cloud layer as (cos 0)i, where $ is the angle be? assumed, can some 30 or 40 kilometers above the tween the line of sight and the plane? peratures. If equipartition exists be? surface (35). The predicted cloud tem? tary vertical (31). From this law of tween rotational and translational ener? rotational so perature is equal to the thermocouple limb darkening, the vertical tempera? gies, the temperature will be the temperature, and the theoretical satura? ture distribution immediately above the derived appropriate gas tion abundance of water vapor above cloud layer can be computed; the at? kinetic temperature. On the basis of radiative transfer in an the clouds agrees with the balloon mosphere turns out to be nearly iso- an assumed abundances of water. The reflectivity thermal above the cloud layer, the optically thick Cytherean atmosphere a of such a cloud layer is high enough temperature decreasing only as the one- which scatters radiation isotropically, rotational of to explain the Cytherean albedo, but eighth power of the optical depth (37). composite temperature insufficient data are available on the The true lapse rate is only about half 285 ? 9?K has been derived from the dioxide bands in the polarization of ice crystals to make the adiabatic lapse rate. In the absence carbon 8000-ang- be From considera? comparisons with the Venus polariza? of an ozone layer (there should little strom region (39). tion curve. ozone in an atmosphere with no free tions of pressure broadening of the 854 SCIENCE, VOL. 133

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions spectral lines in an adiabatie atmos? case, the thermal emission might arise surface emissivity differs from unity. phere, the temperature at the bottom from free-free transitions in a Cytherean Molecular absorption and particle scat? of the layer emitting the 8000-angstrom ionosphere at an electron temperature tering would decrease the apparent features is estimated at 320?K (26). of 600?K. To explain the low tempera- temperatures in the millimeter region. The rotational temperatures must arise atures near 8 millimeters we must as- The 8-millimeter phase effect would from deeper levels than the thermo? sume that the ionosphere becomes then be attributable to a condensable couple temperatures, probably from be? transparent at a wavelength less than or sublimable cloud layer, which, if neath the visible cloud layer. This is about 1 centimeter, so that in the mil? analogous to terrestrial clouds, is trans? then evidence for the transparency of limeter region we are observing the parent at centimeter wavelengths but the visible cloud layer in the near infra? Cytherean surface at its temperature of has a nonzero opacity in the millimeter red; accordingly, observations at near- about 350?K. From the required opac- region. In the illuminated hemisphere it infrared wavelengths which lack mo? ity and temperature of the ionosphere must be supposed that cloud vaporiza- lecular absorption features should pro? it follows that the electron density must tion increases, and the attenuation of vide information on the lower Cyther? be at least 10? per cubic centimeter emission from the surface declines. ean atmosphere. (43), higher by a factor of 1000 than However, the existence of such high the electron density of the terrestrial surface temperatures must be explained ionosphere. If the electron density is before this model is acceptable. The Microwave Temperatures about ten times greater in the illumi? radiation temperature of an airless nated hemisphere, the phase effect can planet with the albedo and distance Since the 1956 inferior conjunetion, be accounted for exactly (43). from the of Venus is about 250 ?K, when Mayer, McCullough, and Sloan- Such high electron densities are very if the period of rotation is a few weeks. aker (40) made their first observations difficult to explain. Preliminary calcula? The high surface temperature must be at 3.15 centimeters, measurements have tions indicate that the ionization rates due to a very efficient greenhouse been made of the absolute intensity of are too low and the recombination rates effect: Visible radiation strikes the sur? the microwave radio emission from too high for such electron densities to face and increases its temperature, but Venus. If one assumes ad hoc that the be produced by solar ultraviolet radia? the infrared radiation emitted by the signal is due to black-body radiation, tion in a Cytherean atmosphere com? surface does not readily escape to a brightness temperature can be derived posed of carbon dioxide, nitrogen, and space, because of atrnospheric absorp? from the distribution. If the as? water vapor. If the electrons are to be tion. If the atmosphere is assumed to sumption of black-body emission is produced by solar corpuscular radia? be in convective equilibrium below the correct, the brightness temperature tion, the strength of the Cytherean effective reflecting layer in the 8000 should be independent of frequency. must be as low as 10~a angstrom bands, there are 18 km-atm Between 3 and 21 centimeters, the gauss for ionization by solar protons to of carbon dioxide above the surface; brightness temperature proves to be be effective; if the field strength is however, this is still insufrleient by constant within limits of experimental greater, the will be mag- many orders of magnitude for produc? error, and the Venus radio spectrum is netically deflected. If the ionization is ing the required greenhouse effect (35). inconsistent with known sources of produced by solar protons, there is still Absorption is required in the region nonthermal emission, such as cyclotron a difficulty in accounting for the low between 20 and 40 microns, and the or synchrotron radiation from charged recombination coefficients, but the dif? only likely molecule which absorbs in particles trapped in a Cytherean Van ficulty is not so severe as in the case of this wavelength interval is water. The Allen belt (35). The mean black-body ionization by ultraviolet radiation. In requisite total abundance of water temperature for these wavelengths is addition, there is evidence that the mag? vapor in the Cytherean atmosphere is ? approximately 600 50?K, possibly netic activity on the earth declines dur? between 1 and 10 grams per square depending somewhat on the phase of ing inferior conjunctions of Venus centimeter; saturation and ice-crystal Venus. On the other hand, observations (44); if this phenomenon is interpreted cloud formation occur at the thermo? near 8 millimeters appear to give as the deflection of solar protons by the couple temperature of the Cytherean significantly lower brightness tempera? Cytherean magnetic field, it follows cloud layer and give approximately the tures; at S.6 millimeters, observations that the field strength on Venus is of balloon water-vapor abundance above with the 10-foot radio of the the order of a few gauss, and that the the clouds (35). Despite an absolute Naval Research Laboratory give a Cytherean ionosphere cannot arise from water-vapor abundance of the same brightness temperature of 410 ? 160?K the solar wind. However, these meas? order as the earth's, the surface tem? (41), while observations at 8 milli? urements of the Cytherean magnetic perature is so high that the relative meters with the 22-meter telescope of field are very marginal. There is some humidity would be about 10~3 percent. the Lebedev Institute of yield radar evidence for the existence of a On the other hand, if the surface tem? a mean brightness temperature of dense Cytherean ionosphere (43), but perature were 350?K, a total abundance 350 ? 70?K (42). The Lebedev data it is also marginal. For these reasons of about 0.1 gram per square centi? also show a marked phase effect, in? it appears somewhat unlikely but not meter would be required for the green? from creasing about 315?K near con? entirely impossible that the 600?K effect; saturation and ice-crystal junction to about 430 ?K two emission arises from the ionosphere of cloud formation would occur at about later. Venus. 195?K, and it would follow that the There are two possible explanations The alternative explanation is that clouds are not composed of water, and of the thermal radiation at centimeter the surface of Venus is at 600?K, or that the balloon spectroscopy results wavelengths and two corresponding ex? perhaps at a somewhat higher tempera? (9) are incorrect. Thus if the visible planations of the lower temperatures ture if allowance is made for phase cloud layer is condensible or sublim? at millimeter wavelengths. In the first effects and for the possibility that the able, the ionosphere model of the origin 24 MARCH 1961 855

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions of the microwave emission becomes Early and Present At these temperatures and pressures untenable. Oniy with surface tempera? Surface Conditions there can be no global of water tures of about 600 ?K or greater can the (7) or of hydrocarbons (6, 16) to limit requisite greenhouse effect be explained If the surface temperature is 600 ?K the access to silicates of outgassed consistently. The Venus overcast is and the adiabatic lapse rate prevails in carbon dioxide. The essential point high, not because the cloud bank is very the troposphere, the surface pressure would seem to be that liquid water is thick, but because breaks in the clouds of carbon dioxide is about 3 atmos- required for the Urey equilibrium, both are very rare. There is no possibility pheres. From arguments on general as a catalyst and as a weathering of precipitation reaching the surface; abundance it appears that the value for agent to expose fresh silicates, and precipitation is always vaporized in the the partial pressure of nitrogen should that, because of the high temperatures, hot lower atmosphere, and ice crystal? be of the same order as the terrestrial there is no liquid water on Venus (45). lization occurs again at the cloud layer. value; the total surface pressure on This explanation was originally pro? From the equations of radiation Venus would then be approximately 4 posed by Urey (5) at a time when the balance it follows that 1 km-atm of . The partial pressure of surface temperatures were thought to carbon dioxide is sufficient to raise the carbon dioxide is so much greater be sufficiently low for liquid water to be ambient temperature some 30?K in the than the equilibrium partial pressure present but when all spectroscopic absence of other absorbing gases (35). (less than 10"5 atm) that it is clear that searches for water vapor had been Since 1 km-atm is the abundance of the Urey equilibrium fails on Venus. fruitless. It now appears that water is carbon dioxide above the effective re- flecting level in the 8000-angstrom bands, the temperature at that level should be raised about 30?K above the radiation temperature, or to approx? imately 280?K. This is in excellent agreement with the rotational tempera? ture for the same bands, 285 ? 9?K Ionosphere (39). The argument also provides = strong evidence that the 8000-ang? 0 + to 0"+ e" strom bands from below the originate CO + to = C0++ e" visible cloud layer; otherwise the green? = + house effect would raise the cloud C02+ to CO 0 temperature to approximately 280?K. In Fig. 3, the data given in the fore- going discussion are collected, and the + = + H20 to OH H two alternative atmospheric models are presented. The altitudes of the cloud layer?12 and 37 km, respectively, for surface temperatures of 350? and 600 ?K?are based on an assumed adiabatic lapse rate of 10?K per kilo- meter in the Cytherean troposphere. Because of the difficulty in accounting for the ionospheric electron densities, the cloud temperatures, and the water- vapor abundance with a surface tem? perature of 350?K, the higher surface temperature is to be preferred at the present time. A definitive choice be? tween the two models could be made by comparing accurate phase-brightness temperature curves at 8 millimeters and at 3 centimeters with each other and with the occurrence of solar activ? ity, or by scanning the disk of Venus with a fly-by probe. In the latter ex? emission should periment, strong peaks Temperature in ?K be detected at the planetary limbs if Fig. 3. Preliminary temperature profile of the . Two altitude scales the radiation at centimeter wavelengths are given, corresponding to the two alternative sources of the 600?K microwave bright? is in ionospheric origin. An experiment ness temperatures. Troposphere pressures are computed from the 8000-A band to detect limb-brightening near 1 cm is intensities and the adiabatic gradient; the pressure at the base of the ionosphere is now scheduled by the National Aero? derived from occultation data. The altitude of the tropopause cloud layer is bracketed as but the of the clouds should be much less than is indicated. On the nautics and Space Administration for shown, depth model, the cloud layer could not be composed of water. If there the Mariner A Venus ionospheric tropopause fly-by probe, is nitrogen in the Cytherean atmosphere, nitrogen ionization should occur in the higher which will probably be launched in atmosphere in addition to the processes shown. The slope of the ionospheric temperature 1962. profile is schematic only. 856 SCIENCE, VOL. 133

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions present, but not in the liquid phase. be transparent in the near ultraviolet. not be solved, because the oxygen The total abundance of carbon dioxide Hot, arid, calm, and overcast, the sur? evolved in photosynthesis derives from on Venus is of the same order of face of Venus appears inhospitable for water, and the amount of carbon diox? magnitude as the total abundance of human habitation at the present time. ide in the Cytherean atmosphere is carbon dioxide in the of the more than 1000 times greater than the earth (35); hence, roughly equal amount of water vapor. An organism amounts of carbon dioxide have been Life is needed which can photosynthesize in outgassed from the interiors of the two the high atmosphere of Venus accord? planets during their history, but on the No known terrestrial microorganisms ing to the symbolic equation earth the carbon dioxide has been can survive more than a few minutes' COa + H20 -f- light -? (CH20) -f 02, sedimented, while on Venus it has re- exposure to 600?K; proteins are de- mained in the atmosphere. The ultimate natured, deoxyribonucleic acid is de- the oxygen arising from the water. In source of the oxygen in Cytherean polymerized, and even small organic time, the organisms would be carried carbon dioxide must be water (29). molecules are dissociated in short to lower atmospheric levels, where, be? Through diffusion of water vapor to periods of time. Temperatures at the cause of the higher temperatures, they high altitudes, its photodissociation by poles of Venus are probably not more would be roasted, decomposing ideally solar ultraviolet radiation, and the than 100?K cooler than the mean according to the symbolic equation escape of to interplanetary planetary temperature, and it appears (CH20) + heat -? C H20 space, enough oxygen has been released quite certain that terrestrial organisms + during the last 5x10? to account deposited on the surface of the planet Although the oxygen is derived from for the present abundance of Cytherean would quickly be killed. Consequently water, the over-all effect would be to carbon dioxide by the oxidation of re? there seems little danger of biological restore the water metabolized in photo? duced carbon compounds (29, 35). contamination of the surface of Venus synthesis to the atmosphere, and to The oxygen in terrestrial carbonates, (46). However, conditions are much dissociate carbon dioxide to carbon and atmospheric oxygen, and the oxygen in more favorable at higher altitudes, es? oxygen. water are similarly derived. If much pecially just beneath the cloud layer, Before such a scheme can be serious- of the primordial Cytherean water had and there is the distinct possibility of ly considered, much more information been present at one time on primitive biological contamination of the upper must be acquired about the composi? Venus, a very efficient greenhouse Cytherean atmosphere. At such high tion and of the Cytherean effect would have been quickly es? temperatures, and in the absence of atmosphere, and extensive laboratory tablished and the surface temperature liquid water, it appears very unlikely biological investigations must be per- would have risen, vaporizing the water. that there are indigenous surface formed. Nevertheless, some tentative Thus, it appears that extensive bodies organisms at the present time. If life specifications can be entertained at the of liquid water and low temperatures based upon carbon-hydrogen-oxygen- present time. In order to have appre? could not have occurred together for nitrogen chemistry ever developed in ciable photosynthesis before thermal any appreciable period in the history the early history of Venus, it must dissociation, the life form deposited of Venus. subsequently have evolved to sub- must be a microorganism. Since there Consequently, weathering by water surface or atrnospheric ecological is no liquid water anywhere on Venus, must not have occurred, and the niches. However, since, as has been the organism must be able to utilize present surface erosion on Venus must mentioned, there can have been no water vapor (from the atmosphere) or be due mainly to wind and high tem? appreciable periods of time when Venus ice crystals (from the cloud layer). peratures. In the greenhouse model, had both extensive bodies of water and The only known microorganisms which solar radiation absorbed during the surface temperatures below the boiling photosynthesize evolving molecular Cytherean day goes mainly into heating point of water, it is unlikely that life oxygen are the algae. It would be de? the massive atmosphere. As a result the ever arose on Venus. sirable to have an organism with re? troposphere should be less convective After the physical enviroment has sistance to extremes of temperature. than on Earth, and surface winds been thoroughly investigated and if, Blue-green algae are known to survive should be mild breezes. Such aeolian indeed, Venus proves to be without immersion in liquid nitrogen, and some erosion and the moderate thermal ero? life, there will exist the prospect of forms ordinarily live in hot springs at sion suggested by the microwave phase microbiological planetary engineering. 80?C. Since there is little likelihood data suggest that the surface of Venus To prepare Venus for comfortable hu? that the microorganisms would find closely resembles terrestrial desert man habitation, it is necessary to lower nitrogen in the form of nitrates or wastelands. The temperatures are too the surface temperature and to increase in the Cytherean atmosphere, high for the Carboniferous swamp, the the partial pressure of molecular oxy? they would have to be able to fix molec? planetary oil field, or the global Seltzer gen. Both ends could be accomplished if ular nitrogen from the atmosphere. the ocean, but, desert of St. John and a means were found to dissociate car? The only photosynthetic, nitrogen- Nicholson (3) is still roughly consistent bon dioxide to oxygen and elemental fixing, oxygen-evolving, temperature- with the data (See Fig. 1). From the carbon. The essential difference be? resistant aerial microorganisms are the surface of Venus we might see the sun tween the Cytherean and the terrestrial blue-green algae, primarily of the only dimly; the sky would be com? greenhouse effects is the great abun? Nostocaceae family. pletely overcast perhaps 90 percent of dance of carbon dioxide on Venus; the Extensive laboratory experiments the time (35), and the high-altitude quantities of water vapor in the two should be performed on the ecology of white clouds would sometimes appear atmospheres are approximately equal. the algae in simulated Cytherean en? reddened by dust in the lower atmos? Even if ordinary green plants could vironments. It is necessary to know phere. The Cytherean atmosphere may grow on the surface, the problem would whether the algae will be able to re- 24 MARCH 1961 857

This content downloaded from 128.103.149.52 on Tue, 24 Feb 2015 13:54:17 PM All use subject to JSTOR Terms and Conditions to thermal References and Notes and Planets, G. P. Kuiper, Ed. (Univ. of produce prior decomposi- Chicago Press, Chicago, rev. ed. 1952), chap. tion; whether a complete aerial ex? 1. In the literature one sometimes finds the ad- 12. Kuiper's maximum abundances have not istence is or if jective Venusian. This is incorrect; we do not been corrected for pressure-broadening. possible, they require say "Sunian," or "Moonian," or "Earthian." 15. See also T. Dunham, ibid., chap. 11. the cloud ice-crystals as a substratum; The appropriateadjective would be Venerean 16. Y. Mintz, "Temperature and Circulation of the Venus RAND whether a strain can be found which or Venereal, but at least the second of these Atmosphere," Corp. Rept. has been pre-empted by other areas of hu? No. P-2003 (1960). 17. for Msh /. will photosynthesize at low temper? man activity. The Greek goddess correspond? See, example, E. J. Opik, Astron. ing to Venus is , but the appropriate 4, 37 (1956). atures and high ultraviolet fluxes: adjective here, Aphrodisian or Aphrodisiac, 18. F. E. Ross, Astrophys. J. 68, 57 (1928). 19. E. E. Barnard, ibid. 5, 300 whether trace requirements can must be excluded for similar reasons. Since (1897). diseases and love-philterspreceded modern as? 20. A. Danjon, UAstronomie 57, 161 (1943); A. be supplied by meteoritic infall, or tronomy, we must be content with Cytherean, Dollfus, ibid. 68, 413 (1955). an which has been used 21. G. P. Kuiper, private communication (1960). whether metals must be provided arti- adjective by astron? omers for more than a century. Cythera is 22. -., Astrophys. J. 120, 603 (1954). ficially; and what the products of slow the Ionian island onto which Aphrodite is 23. R. S. Richardson, Publs. Astron. Soc. Pacific 67, 304 (1955). thermal be. But it said to have emerged from the sea. decomposition may 2. S, The Destinies the 24. V. M. Slipher, Lowell ObservatoryBull. No. Arrhenius, of 3 is conceivable that these problems can (Putman, New York, 1918), chap. 7. (1903). C. E. John 25. B. Lyot, Ann. ObservatoireMeudon> Univ. be and that the 3. St. and S. B. Nicholson, Astro- solved, microbiological phys. J. 56, 380 (1922). Paris 8, 66 (1927). 26. G. P. The Threshold M. re-engineering of Venus will become 4. W. S. Adams and T. Dunham, Publs. Astron. Kuiper, of Space, Soc. Pacific 44, 243 (1932). Zelikoff, Ed. (Pergamon, London, 1957), possible. Such a step should be taken 5. H. C. Urey, The Planets; Their Origin and pp. 78ff. 27. T. Gehrels (private communication) has re? only after the present Cytherean en? Development (Yale Univ. Press, New Haven, 1952). Urey proposed that on a planet having cently found that the Venus polarization vironment has been thoroughly ex- both surface silicates and surface water, the curve departs markedly from the curve of of CO2 is maintained Lyot at several different wavelengths. Lab? plored, to prevent the irreparable loss partial pressure by a sequence of equilibriumreactions, which can oratory duplication of these observations of unique scientific information. It be summarized by such over-all reactions as would represent a major step toward under? standing the Cytherean cloud layer. might be advisable to find suitable con? HaO 28. A. Dollfus, Ann. d*Astrophys. Suppl. No. 4 *" trols for the algae, because in the ab? MgSiOs+ COs MgCOa+ SiOa (1957). ^ 29. H. C. Urey, in Handbuch der Physik, S. sence of predators and competitors the and Flugge, Ed. (Springer, Berlin, 1959), vol. 52, p. 363. algae might reproduce at a geometric H2O >r 30. A. Dauvillier, UOrigin Photochimique de la rate and the entire conversion of carbon CaSiOs-f COs ^ CaCOs-f~ SiOs Vie (Masson, Paris, 1958). 31. W. M. Sinton and J. Strong, Astrophys. J. dioxide would then be accomplished in If, at equilibrium,the abundance of CO2 in? 131, 470 (1960). creases (for example, through volcanism), relatively short periods of time. 32. L. Kaplan (private communication) has re? the rate of carbonate sedimentation also in? cently shown from new laboratory data that Ideally, we can envisage the seeding creases; if the abundance of CO2 decreases, the observed intensity of the Cytherean far the reactions and of the reverse, deposition infrared carbon dioxide bands is approxi? upper Cytherean atmosphere increases. The partial pressures of CO2 on the mately the intensity to be expected above the with appropriate strains of Nostocaceae earth are within about an order of magnitude cloud layer. The need to hypothesize a high of the equilibrium pressures, at room tem? level infrared absorbing layer above the visi? after exhaustive studies have been per? perature, resulting from the to dolo- ble cloud layer is thereby diminished.Weaker mite above. formed on the existing environment of given bands of carbon dioxide, such as those in the 6. F. Hoyle, Frontiers of Astronomy (Harper, 8000-angstromregion, must come from greater Venus. As the carbon dioxide content New York, 1955), pp. 68-72. depths, probably from beneath the visible of the the 7. D. H. Menzel and F. L. Whipple, Publs. cloud layer. atmosphere fails, greenhouse Astron. Soc. Pacific 67, 161 (1955). 33. F. J. Heyden, C. C. Kiess, H. K. Kiess, effect is rendered less efflcient and the 8. G. Herzberg,in Atmospheresof the Earth and Science 130, 1195 (1959). Planets, G. P. Kuiper, Ed. (Univ. of Chicago 34. C. C. Kiess and C. H. Corliss, Sci. News surface temperature fails. After the Press, Chicago, rev. ed. 1952), chap. 13. One Letter 75, 229 (1959). atrnospheric temperatures decline suf- centimeter-atmosphere(cm-atm) is the amount 35. C. Sagan, "The Radiation Balance of Venus," of gas correspondingto a path 1 cm long at Calif. Inst. Technol. Jet Propulsion Lab. ficiently, the decreasing rate of algal atmospheric pressure and room temperature. Tech. Rept. No. 32-34 (1960). 36. E. Petit and S. B. Publs. Astron, decomposition will reduce the water Centimeter-atmospherescan be converted to Nicholson, molecules per square centimeter by multipli- Soc. Pacific 67, 293 (1955). abundance slightly and permit the sur? cation with Losehmidt's number. One kilo- 37. 3. I. F. King, "Probe Observationsof Venus face to cool below the of meter-atmosphere(km-atm) equals 10s cm-atm. at Close Range," Johns Hopkins Univ. Rept., boiling point 9. J. Strong, private communication (1960). J. Strong, Ed. (1960), appendix3. water. At this time, the original mech? The value of 20 microns of precipitablewater 38. D. H. Menzel and G. deVaucouleurs,Astron. is derived for one atmosphere pressure. Be? J. 65, 351 (1960); private communication. anism becomes inoperative, because the cause of lower pressure broadening, if the 39. J. W. Chamberlain and G. P. Kuiper, algae are no longer thermally decom- pressure at the relevant Cytherean reflecting Astrophys. J. 124, 399 (1956). level is, for example, 0.2 atmospheres, the 40. C. H. Mayer, T. P. McCullough, R. M. posed, but now surface photosynthesis water vapor abundance would be about Sloanaker, ibid. 127, 1 (1958). in Paris becomes possible. At somewhat lower 4 X 10-2 gm/cm3. This corresponds to the 41. 3. E. Gibson and R. J. McEwan, saturation vapor pressure of water above Symposium on Radio Astronomy, R. N. temperatures, rain will reach the sur? a convective cloud layer at about 240?K. Bracewell, Ed. (Stanford Univ. Press, Stan? and the will be However, it should be noted that the probable ford, Calif., 1959), p. 50. face, Urey equilibrium error of the balloon measurements exceeded 42. A. D. Kuz'min and A. W. Salomonovich, initiated, further reducing the atrnos? the absolute value of the results; and that Astron. Zhur. 37, 297 (1960). there was no astronomical calibration?for 43. C. Sagan, K. Siegel, and D. Jones, in prepara? pheric content of carbon dioxide to example, against the ?so the possibility tion. terrestrial values. With a few centi? remains that some of the water detected was 44. J. Houtgast, Nature 175, 678 (1955). in the terrestrial atmosphere. 45. Even if the Urey equilibriumdid apply in the meters of precipitable water in the air, 10. N. A. Kozyrev, Bull. Crimean Astrophys. absence of liquid water at 600?K, the partial surface temperatures somewhere near Observatory12, 169 (1954); H. C. Urey and pressure of CO2 would be less than 3 atm A. W. Brewer, Proc, Roy. Soc. (London) by several orders of magnitude. room temperature, a breathable atmos? A241, 37 (1957). B. Warner, Monthly Notices 46. See, for example, C. Sagan, Proc. Natl. Acad. Astron. Soc. 279 has shown phere, and terrestrial microfiora await- Roy. 121, (1960) Sci. U.S. 46, 396 (1960). that the agreement in wavelength of many of 47. This study was supported in part by the ing the next ecological succession, Ve? Kozyrev's features with known bands of Na, California Institute of Technology Jet Pro? is than would be ex? and nus will have become a much less for- N2+, O, and 0+ greater pulsion Laboratory,National Aeronautics pected by chance. However, many inconsisten- Space Administration, under subcontract No. bidding environment than it appears to cies remain. NASw-6. Part of the work was performed 11. N. A. Kozyrev, Akad. Nauk S.S.S.R. Astron. at the Yerkes Observatory, University of be at present. Hopefully, by that time Circ. 175, 26 (1956). Chicago. I am indebted to Drs. J. W. Cham? we will know with more certainty 12. G. Newkirk, Planetary Space Sci. 1, 32 (1959). berlain, D. Jones, G. P. Kuiper, J. Lederberg, 13. It may not be irrelevant to mention that 5. Scher, R. Wildt, and to my fellow mem? whether to send a paleobotanist, a min? perhaps the outstanding unidentified inter? bers of the Planetary Atmospheres Study National Acad? eralogist, a petroleum geologist, or a stellar absorption feature is one about 40 A Group, Space Science Board, wide, centered at 4430 A. emy of Sciences, for stimulating discussions deep-sea diver (47). 14. G. P. Kuiper, in Atmospheres of the Earth on many aspects of this subject.

858 SCIENCE, VOL. 133

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