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Chapter 4 LOWER ATMOSPHERE INTRODUCTION The significant scientific questions about the lower atmosphere of Venus involve its physical structure, its chemical constitu- tion, the clouds, the levels of deposition of solar energy, and the planetary-scale flow patterns. While it is necessary to discuss these separately, it cannot be emphasized too strongly that each bears upon the others through complex cause-and-effect mechanisms. For example, the clouds, be they condensates or dust, must be produced by the circulation regimes, but by dic- tating the levels of absorption of sunlight they, in turn, pro- foundly modify the basic drive for atmospheric circulation. It may be necessary to unravel most of these interactions before one can reach a satisfactory understanding of anyone element. We recognize that it will take several missions to accom- plish the necessary measurements, and so we unreservedly en- dorse the concept of low-cost flights carefully designed to answer well-posed questions of scientific concern, with each mission as part of a coordinated program. An important feature of most of the planetary atmospheric problems identified is that they involve the three-dimensional distribution of various properties. Single-point measurements are inherently seve~ely limited in their ability to provide the requisite solutions. A key consideration is, therefore, the capability of simultaneous multiple entries into the at- mosphere at selected locations. One of the most difficult planetary measurements is the determination of wind velocity in a way that is useful to the theory of general planetary circulations. Radar techniques can measure the motion of an entry probe as it approaches the ground. This too, however, is a one-time measurement and does not lead to three-dimensional data. The dense atmosphere of Venus permits an alternative approach. It is technologically feasible to eject a number of small packages during entry which will inflate modest constant-level balloons to drift with the wind at several levels. These can be tracked from earth or from an orbiter and can have lifetimes of a month or more. 30
31 Why should we explore the atmosphere of Venus from the point of view of planetary dynamics? First, because the mas- sive and hot atmosphere is different from the other inner plan- ets, and knowledge of how Venus developed to its present state could provide an important clue to the understanding of the evolution of the solar system. Second, the very slow planetary rotation and the dense, ubiquitous cloud cover indicate a dif- ferent thermal and dynamical control of the planetary flow pat- tern. Thus we can present meteorological theory with a dif- ferent set of circumstances than those presented by the earth. Both must be explicable by a correct and general theory, and so our comprehension of our own planetary atmosphere can be improved in the process. It is instructive to compare the atmospheres of Mars, Venus, and the earth. On Mars the low atmospheric density, low water-vapor concentrations, and paucity of clouds all mean that the atmosphere is nearly in radiative equilibrium and that dynamical effects on the temperature distribution are of small consequence. This is a simple situation worthy of study in its own right. On Venus the very large densities and dense cloud cover act to minimize the effects of radiation so the temperature field is expected to be strongly controlled by dynamical wind systems. This is the opposite simplification from Mars. On earth, radiative and dynamical influences are more nearly of equal importance, resulting in a complex prob- lem. Clearly, we can expect fundamental gains by studying these simpler atmospheric systems provided by nature. The slow rotation of Venus means that Coriolis forces will be small, a situation found on earth in the tropics, where the significant part--the vertical component of the rotation vector--is also small. Thus the circulation of Venus may be fundamentally related to that in our own tropics, an area where our understanding is far less than for middle and high latitudes. Another potential relationship to terrestrial motions is to the part of oceanic circulation that is ther- mally driven. We know that the oceans absorb solar radiation close to the top boundary. We have reason to believe that much of the deposition of solar energy on Venus is in the upper part of the clouds, so the parallel to terrestrial oceans is apparent. The highly clouded state of the atmosphere of Venus as compared with that of the earth represents a case in which the absorption of solar energy and the absorption and re- emission of .infrared energy are strongly controlled by the clouds. Appropriate observation of Venus can add to our
32 understanding of this situation. Such understanding is poten- tially important to knowledge of the terrestrial atmosphere in which human activities are producing a progressive increase in turbidity. Any knowledge of the state represented by Venus may be overwhelmingly important. PHYSICAL STRUCTURE OF THE LOWER ATMOSPHERE Direct measurements of temperature and pressure, with simple and reliable instruments, are among the primary tasks that should be undertaken in any future investigation of the lower atsmosphere of Venus by spacecraft. These quantities are two of the most direct links existing between observation and the theory of atmospheric motion, and they are important in any attempt to understand the heating of the lower atmosphere. They are, moreover, often the final goal of complex methods of remote sensing. Our first knowledge of the physical structure of the at- mosphere of Venus came from the analysis of radio-astronomical observations. From studies of the variation of the intensity of radio emission with wavelength it was inferred that (1) the lapse rate in the lower atmosphere is adiabatic, and (2) the temperature T and pressure P at the surface of the planet are elevated by terrestrial standards, with T 'V 700 K and P in the range from 25 to 200 atm. The in situ measurements of the Soviet space probes Venera 4, 5, and 6, in conjunction with the S-band occultation experiment of Mariner 5, have confirmed these atmospheric conditions in significant detail. Recent interferometric radio observations from earth indicate that horizontal temperature gradients at the planet's surface are small, even on the scale of planetary dimensions. Present knowledge, however, is too imprecise and limited in coverage to serve as the starting point for a theory of the general circulation of the atmosphere. For this purpose, si- multaneous measurements of the vertical profiles of tempera- ture and pressure are required over some grid covering the planet's surface. In its most rudimentary form, such a grid should consist of at least three or four widely separated points, located to yield information on the pole-to-equator side gradientsof P and T. and dark side-to-bright
33 TYPES OF MISSION The types of mission that are feasible for future planetary explorations of Venus with small vehicles are orbiter, entry probe, and balloon. Orbiters Infrared or radio-occultation experiments on an orbital probe can provide information about the upper 5 percent or less of the atmosphere, but this is inadequate for solving the major problems of the lower atmosphere of Venus. Entry Probes The instrumentation to p~rform measurements on .an entry probe is lightweight, modest in power consumption and telemetry re- quirements, and comparatively inexpensive. It is thus well suited to simultaneous deployment on several small entry probes launched from"a single bus. It is important that three small entry probes should ac- company a larger entry vehicle into the atmosphere of Venus. There are several factors, discussed later, which constrain the entry point of the main probe to lie within 35° of the subearth point. The small probes can then be independently targeted to lie 120° apart on the circumference of a circle not necessarily centered on the entry point of the main probe. We find it highly desirable from a scientific point of view that these small probes be separated by distances comparable to the planetary radius and that one probe lie within roughly 30° of either pole. It is then feasible and desirable for one of the remaining probes to enter at low latitude on the sunlit side of the planet and the other at low latitude on the dark side. If dual missions are flown, it may be desirable to se- lect the target locations for the second set of probes after the first set has actually entered the atmosphere. On both the main and small probes it is imperative that both temperature and pressure be measured down to the plane- tary surface. Some indication of actual or imminent contact of each probe with the surface is necessary in order to re- move the possibility that termination of data transmission is merely an indication of instrument failure under the severe
.;)'t environmental conditions encountered. It is reasonable to ex- pect that more accurate and frequent measurements will be pos- sible on the main probe than on the small probes because of its greater size and weight. Pressure measurements on the main probe should be made to an accuracy of at least 0.5 percent over the range from about 0.1 atm (or as determined by deployment) to 180 atm. Tempera- ture should be measured to an absolute accuracy of 1 K and, if possible, with a relative accuracy of 0.1 K; it should be mea- sured over the range 200-900 K. Both pressure and temperature should be measured and transmitted to earth at altitude inter- vals of at most 1 km. Doppler tracking of the vehicle will provide information about one wind component. A wind-drift radar will, if the probe rotates, give a vector wind measure- ment. On the small probes, temperature and pressure should be measured over the same range as on the main probe. Pressure should be measured to an absolute accuracy of at least 1 per- cent. Temperature should be measured to an absolute accuracy of 2 K and a relative accuracy of 0.2 K. Measurements need not be taken at equal altitude intervals~ but it is desirable that the intervals should not exceed 2km. With feasible te- lemetry systems, several hundred measurements of both pressure and temperature should be possible before impact with the ground. Balloons We see important advantages in the use of constant-density- level balloons situated at pressure levels of 50, 500~ and 1200 mbar. The balloons at the lower levels can carry in- struments to make global measurements of atmospheric tempera- ture and pressure. This kind of measurement will complement and reinforce the data obtained from entry probes. RADIATIVE HEAT BUDGET Venus is characterized by two features that provide a chal- lenge to our understanding of atmospheric science: high sur- face temperatures (700 K) and an extremely turbid atmosphere. These features are believed to be interrelated: Solar radia-
35 tion is attenuated within the turbid atmosphere and heats the interior of the atmosphere. This heat energy is then rera- diated at infrared wavelengths. However, the clouds are be- lieved to trap the infrared radiation. This trapped infrared radiation has been assumed by some investigators to be solely responsible for heating the lower atmosphere of Venus. This mechanism is generally called the "greenhouse" effect. While this effect provides a qualitative explanation of some fea- tures of the temperature structure, quantitative estimates of the heat budget show that it is necessary to invoke dynamical as well as radiative considerations for any explanation of the thermal profile. One of the main purposes of the recommended atmospheric probes should be to provide the data required to understand these mechanisms. A quantitative measurement of the solar radiation flux . absorbed at various altitudes will be an essential part of the data required. Measurements of the penetration of solar ra- diation~ the emission of infrared, and the characteristics of the clouds as a function of altitude are made most definitively from probes that actually enter and penetrate the entire atmo- sphere. The radiative heating at each altitude is given by the divergence of. the net radiation flux. Entry probes can mea- sure this quantity directly from the difference between upward and downward directed fluxes as a function of altitude. Appro- priate flux sensors can be placed on the upper and lower sur- faces of a probe descending through the atmosphere. Such sen- sors have been employed extensively in terrestrial meteorology to measure fluxes to 2 percent accuracy. It would be desirable to obtain such measurements to better than 5 percent accuracy~ which can be realized if suitable care is taken in the design and testing of the solar-flux sensing system. The relative simplicity and compactness of solar-flux sensors renders it feasible to place such sensors on board all probes. Thermal flux plates have been employed in terrestrial meteorology to measure infrared cooling rates. These instru- ments are light and rugged, and they should also be employed on all probes. It has been our experience on earth that clouds in tem- perate and polar regions differ from those found in tropical regions. We have no prior knowledge about the latitude de- pendence of cloud structure on Venus. Distributing solar flux and infrared sensors on several probes will enable us to ascertain whether there are morphological differences in
36 the clouds which affect the radiative heat budget at different latitudes. If more than one instrument package is sent to Ve- nus, this opportunity should be taken to provide US with more global coverage op radiation measurements. ATMOSPHERIC CHEMISTRY Knowledge of the chemical and isotopic composition of the at- mosphere of Venus is essential to the understandipg of the conditions of origin of the terrestrial planets and the solar system in general. We can expect that the total abundances of certain volatile elements on Venus are very sensitively de- pendent on the temperature of that portion of the solar nebula in which proto-Venus accreted. Those volatile elements, such as nitrogen, mercury, and the rare gases, which would be in- capable of forming condensed compounds at the present surface temperature of Venus, would currently reside in the atmosphere. ~hus an analysis of the abundances of these elements in the atmosphere would contribute directly to an understanding of the origin of the entire planet and, by comparison with the atmospheres of the earth and Mars, might permit the develop- ment of consistent models for the origins of the terrestrial planets. In a very different way, knowledge of the atmospheric composition contributes to our understanding of the mechanisms by which planetary atmospheres are produced, maintained, and buffered. The chemical interaction between reactive gases and the surface rocks of Venus may well be a study in essen- tially pure and unperturbed chemical equilibrium. Understand- ing the emission of gases from terrestrial volcanoes and fuma- roles is made tremendously more complicated by the competing effects of equilibrium, kinetic barriers, temperature and pressure gradients, compositional variations in the rocks in contact with the gases, and, finally, contamination and oxi- dation by the oxygen-rich atmosphere. Venus may therefore be the ideal laboratory in which to isolate and study the effects of chemical equilibrium and thus open the way to the under- standing of the processes governing the initial generation of the earth's atmosphere and composition as a function of time. It is clear that the atmospheric composition also reflects the current mineralogy of the surfac.e. We thus may make
37 plausible medels fer the mineralegical cempesitien .of the sur- face frem atmDspheric cempesitiDnal data alene. In the absence .of precise temperature and pressure data en the surface .of the planet, it is even pessible teestimate these parameters by searching fer a limited temperature and pressure regime within which the .observed abundances .of atmespheric censtituents are cempatible with already-knewn buffer reactiens. This latter technique leads te seme ambiguity) because mere than .one pres- sure-temperature peint may satisfy all the available data. This leads te uncertainty cencerning the exact reactiens re- spensible fer the atmespheric cempesitien and thereby generates a list .of "plausible" minerals and mineral assemblages which may be present. This ambiguity may be remeved by applying seme additienal censtraints en the surface cenditiens, such as in situ~meas.urements .of the surf ace temperature and pressure. The cempesitiDn .of the atmDsphere reflects the equilibrium cDndi- tiens in that pDrtien .of the planet's surface which acts as a "celd trap,!! and thus we need a minimum temperature mere than just the mean Dr maximum temperature .of the surface. The use .of several lightly instrumented prDbes tD make direct determi- natiDns .of the surface temperature at widely separated pDints en the planet presents an ideal selutiD.n te this prDblem. A- mDng the regiDns which must be prDbed te answer these ques- tiDns, the pDlarand antisDlar regiDns are .ofparamDunt im- pertance. It can be seen that cempesitiDnal data .on the lcwer atmDsphere and surface temperatures at widely separated pcints are bath relevant te the surface mineralegy and petrclcgy, and that the results .of such measurements weuld be useful in the design .of future petrDlcgical, mineralDgical, and geechemical experiments. Current epinien .on the nature .of the cleuds .ofVenus fa- vars the view that the clauds are farmed by ccndensaticI'l.of gaseaus censtituents .of the atmesphere. (The pDssibility .of airborne dustcannDt~ hDwever~ be ruled cut, and a discussien .of this and related prablems is presented in the fcllawing sectien.) We can learn abaut the claud cempcsition and struc- ture frDm cDmpesitienal analyses .of the atmDsphere itself. The mDst direct applicatiDn .of the atmaspheric cDmpDsitien determinatiens is in the seart:h fer regiens in which the at- mDsphere is saturated with respect ta any cDnstituent Dr re- actien preduct fDrmable frDm the gas. Compesitienal discen- tinui ties shDuld occ.ur at levels where clcuds cendense ~ and adequate mDdels fer the cleud structure might fDI1DW frDm a simple thermDdynamic treatment in which the .only input param- eters are the atmDspheric cDmpesitien and the IDcal tempera-
38 ture. It is clear that cloud-physics measurements in the ab- sence of such compositional data are incapable of identifying the chemical nature of cloud condensates. The atmospheric composition as a function of altitude, tied as it is to the problem of the location, mass, and opti- cal properties of cloud layers, assumes considerable importance when one attempts to understand the thermal structure and ther- mal balance of the atmosphere. The atmospheric opacity in the infrared regions is strongly dependent on the abundances of trace atmospheric constituents. Even as little as one part of 8°2 per 107 parts of CO2 may have a significant influence on the greenhouse effect, and the atmospher:lc opacity at wave- lengths less than about 3 cm may be largely due to trace con- stituents. One must therefore expect that knowledge of the atmospheric composition could help considerably in explaining the thermal structure of the atmosphere. Finally, detailed knowledge of the atmospheric composi- tion near the visible clouds permits investigation of photo- chemical processes and products whose existence might not be readily apparent to an outside observer. Mass spectroscopy is the most effective method of carry- ing out an adequate analysis of the atmosphere. We shall at- tempt to describe the performance requirements for such an analysis in light of the uses to which the results will be put. In order to permit the determination of isotopic abundance ratios for a variety of elements, it is essential that the resolution be no worse than one mass number. If cloud-produc- ing condensates are to be detectable down to the limits at which the clouds become vanishingly important, then the detec- tion limit should be 10-5 to 10-6 of the CO2 abundance. The mass range which must be covered by the analysis is not so easy to define specifically. It is certainly of great impor- tance to cover the range up to at least m/e = 44. This will ensure coverage of H2, He, H2O, HF, Ne, CO, N2' H2S, HCI, Ar, and CO2' It is unfortunate that some of the interesting con- stituents will not be sufficiently abundant relative to CO2 to be visible in the same mass spectrum. Of these, the rare gases might possibly be subjected to a separation step to re- move CO2 and N2, as well as species such as H35CI which may interfere at certain interesting mass numbers. For planeto- logical purposes one should not hastily dismiss the signifi- cance of the heavier rare gases. The relative abundances of fissiogenic and solar Kr and Xe and the possible existence of the extinct-radionuclide decay product of l29Xe would be of great interest, particularly in comparison with terrestrial
39 and meteoritic evidence. Thus the mass regions near m/e =80 and 125 are also of considerable value. In the immediate vi- cinity of the Kr and Xe masses are found the other volatile elements, bromine and selenium (near m/e = 80) and iodine and tellurium (near m/e = 125). Several other plausible atmospheric constituents are also of interest, such as the gases S02, cas, FeC12, Hg, and Hg ha- lides up to HgI2' We believe that coverage of the range from m/e = 1 to 140 is possible and highly desirable. The feasibi- lity and utility of a chromatographic column in conjunction ~ith the mass spectrometer should be studied with later and more sophisticated mission opportunities in mind. The sampling scheme for a mass spectral analysis should obey certain general constraints. The vertical sampling fre- quency should be sufficient so that at least two analyses could be conducted at widely separated altitudes above the visible cloud layer, at least two more between the 250 and 400 K levels, and at least one or two in the deep atmosphere. At least one analysis capable of giving the He, Ne, and Ar isotopic composition is essential. There are no clear reasons for preferring any particular area of the planet for conduct- ing these analyses. A bright-side entry of the large probe at low latitudes would be completely acceptable. Certain other data are essential for full utilization of the compositional information. These include pressure and temperature profile measurements at several widely separated points on the planet, nephelometer and condensimeter measure- ments as will be discussed in the next section, and some inde- pendent estimates of one very important parameter--the H2O vapor mixing ratio. A number of special-purpose instruments are possible candidates for measuring the water-vapor abun- dance. Our present remarks shall be confined to stressing one extremely important point: the water detector must be sensi- tive but highly specific. The presence of soluble hydrogen halides such as HCl and HF must be taken into account in the design and calibration of such detectors; otherwise the mea- surement would not be acceptable. Venus orbiters may also make a long-term contribution to the study of atmospheric structure above the critical re- fraction level (total pressure near 6 atm) via single- or dual-frequency occultation experiments. Attenuation and phase-shift data for the atmosphere over a wide range of la- titude and solar phase angle would be most useful in under- standing the structure of the uppet troposphere.
40 CLOUD PHYSICS The essential questions that can be approached by cloud-physics measurements are: 1. Are the clouds condensation (reaction) products or wind-blown dusts? 2. Are the clouds cumuliform, stratiform, or haze? 3. What are the processes involved in cloud formation and dissipation? 4. How do the clouds affect the planet's heat budget? In addition, detailed knowledge of the physical characteristics of the clouds are important for the interpretation of earth- based and orbiter measurements. Because optical measurements from earth are necessarily limited to an examination of the upper regions of the atmo- sphere of Venus, the structure of the bulk of the clouds is unknown. Much larger particles and denser clouds are likely between the upper atmosphere and the planet's surface. Parti- cle concentration may well increase and the particle-size dis- tribution function broaden; however, large spatial variations in size and concentration should be expected. One may specu- late about the lower regions of the atmosphere, but the actual situation can only be revealed with certainty from in situ mea- surements. The measurement of the particle-size distribution is necessary to.establish the mechanisms by which the cloud particles form and are removed from the cloud. Scavenging by coagulation and agglomeration alone will lead to larger parti- cles and may ultimately cause sedimentation and some form of precipitation. If the particles are condensation products, particle-size information will provide important information on latent heat fluxes as well as insight into particle-growth regimes. In the event that the cloud particles are not condensa- tion products, the changes in the particle-size spectrum may provide clues to electrostatic processes active in agglomera- tion and coagulation that increase particle size. Carbon di- oxide is an excellent insulator, and frictionally produced electrification is a distinct possibility. Certain aspects of cloud particulates are easier to ex- plain if the cloud is formed by condensation from a relatively pure parent phase. Such a process in our terrestrial atmo- sphere is responsible for spherical water droplets and ice
41 crystals of hexagonal habit. On the other hand, the latent heat released by condensation drives vertical motions and tur- bulent mixing processes causing a high variability of cloud properties in space and time. Likewise, when the parent phase is impure, particles of irregular shape and nonuniform optical properties result; terrestrial pollution is an example. In view of the high surface temperature and pressure of Venus, a number of condensable vapors may exist and their particulate condensation products may thus be vertically layered. Whether cloud particles are condensation elements or dust scoured from the planet's surface has important ramifications in the atmo- sphere's dynamics and heat budget. The solar flux transported through the atmosphere depends on the absorption and scattering properties of the atmospheric constituents. The high albedo, A = 0.77, of the planet sug- gests that the scattered solar flux will be of marked signi- ficance. The solar flux is scattered predominantly by the clouds. The cloud particles most assuredly will affect the infrared cooling of the atmosphere. The radiation scattering and absorption characteristics of clouds depend on the size and shape of the particles and the material of which the par- ticles are composed. Instrumentation and Measurements Pa~tiaZe-Size Speat~omete~ To provide adequate size resolu- tion and dynamic range, multiranged instruments are desirable. Size measurements over a 2-500 !lm range, subdivided into 2-20, 10-100, and 50-500 !lmranges with ten size intervals, would pro- vide adequate size resolution with 50 percent overlap in range. Such an instrument is within the present state of the art. The instrument should be capable of making single-particle-size measurements at high concentrations (103 to 104 cm-3) and be relatively insensitive to particle orientation, shape, or refractive index, because particle morphology is completely unknown. These constra.ints make particle-size detection by imaging or extinction techniques more suitable than by scat- tering. However, detection of number density of submicron particles by scattering is both practical and desirable. It is desirable to transmit a complete size spectrum every 100- 300 m inside cloud layers. Au~eoZe Senso~ System Primary scattering by cloud particles will give rise to a halo or aureolearound the sun caused by
42 the forward diffraction lobe of the phase function. As the light penetrates more deeply into the atmosphere it is multiply scattered and the aureole broadens. The forward-scattered light is thus a function of the size and number of particles between the sun and the sensor and provides a relatively simple observation from which these parameters of the clouds may be inferred. An instrumental configuration on the main probe to measure the aureole would consist of collimated tubes which would sub- tend angles of 3 x 10-4 sr disposed within 20° of the center of the solar disk. The instrument package should have vanes so that the package spins as it descends through the atmosphere. It is desirable to determine the local uniformity of the cloud cover, for which purpose one simple collimated sensor can spin scan the cloud. One of the aureole sensing units could be monitored to meet this objective. Extination Measurement The intensity of solar radiance as a function of altitude can be related to the extinction coeffi- cient, which is a function of the number of particles, their size, and their composition. Although no unique inference of all these quantities is feasible with only a measurement of the extinction coefficient, the results obtained from all mea- surements obtained from the probe should be consistent with the measured extinction coefficient. The extinction of solar radiance could be measured by means of the cente~ sensor on the aureole sensor system. NepheZometer The simplest means of measuring the presence of clouds is to carry a light source on board the probe and to measure the light scattered back to the probe at a fixed angle by atmospheric particles. A rugged apparatus (nephelometer) based on this principle is feasible. Lasers, suitable for spaceflight have been produced, and solid-state light detec- tors are readily available. Television pictures of the sur- face of Venus may be of interest to planetology. A measure of the solar flux level at the planet's surface is a prereq- uisite to the design of such a television system. A nephe- lometer will allow the visibility near the planet's surface to be determined. Infrared CZoud Sensor Infrared sensors may be employed to derive data on the vertical distribution of cloud particles that emit the measured infrared radiances. A thermal radi- ometer should be mounted looking do,vnward. In a near
43 adiabatic atmosphere the difference between the brightness measured in a CO2 window at 7 flm and a CO2 absorp- tion at 10.4 wm is proportional to the transmission characteristics of the clouds. Small differences correspond to opaque clouds} large differences to thin or absent clouds. The trument thus provides a vertical profile of effective cloud density in a particularly simple way. Condensimete~-EVapopimete~ An essential datum in studying the clouds whether they are in equilibrium with the gas phase or 'Vlhethey are solid matter raised from the planet I s sur- face by the action of winds. The distinction between these two types of cloud is not necessarily complete. It is possible that condensed and gaseous phases are in equilibrium low in the atmosphere, but that at higher cooler levels the cloud parti- cles have such a low vapor pressure and lag-time constant that they are essentially equivalent to a dust cloud. A measurementto illuminatethis problemwould be development of a frost-point or dew-point hygrometer. A test surface or volume slightly belo,>" atmospheric temperature should increase in solid matter if the cloud is con- densing} i.e.} in equilibrium the vapor. If the tempera- ture is higher} on the other hand) the solid matter should disappear. A.device to cycle temperature above and below am- bient would show unmistakably \vhei:her the cloud is condensing. A of devicescould be considered. A mirror with variable temperature is one. A small cloud chamber} which can bo and compress the atmosphere, is another. Both ins have been developed for terrestrial investigations, and one should be installed on the main probe. Composition If the cloud particles are in equilibrium with the gas phase, it should possible to infer their composition by the methods described earlier. If they are dust, on the other hand, the problem is quite different. It not clear at this point whether the best way to proceed with dust particles to measure their chemical com- position directly, to determine the composition of the planet's surface from a lander, ot to do both. The instrumentation for particle collection and analysis is straightforward concep- tually, it is not yet involve a lengthy development program. It cannot, therefore, be placed on high
44 priority for an early mission. However, it is possible that there will be no acceptable alternative to this kind of mea- surement on one of the Planetary Explorers in the 1980's. Cloud Forms We do not know whether the Venus clouds are stratiform, cumu- liform, or more analogous to a terrestrial haze. The nature of the physical processes depends greatly upon which general cloud type prevails. We cannot hope to obtain this informa- tion for all cloud layers in a simple manner, but the upper- most cloud layer can be observed directly by visual and ther- mal imaging. A limited number of high-contrast images with a horizontal resolution of about 2 km or better (such as might be obtained by the Venus/Mercury flyby) are, therefore, a prime requirement for cloud-physics studies. Such imaging measurements can be made from an orbiter. The data rate requirements are not excessive because only an occasional image is required. The specification of a cloud- imaging experiment depends to some extent on the results of the Venus/Mercury flyby. It is possible that the visual images from the spacecraft, although very few in number, may be sufficient to answer some of the main questions about cloud morphology. On the other hand, if the spatial resolution of the system is insufficient, no definitive answer may be ob- tained from this mission. ATMOSPHERIC MOTIONS State of Knowledge There have been no direct measurements of atmospheric motions below the clouds, but there are two pieces of evidence that indicate movement. The most direct indication is given by the time variability of the undulations of the cloud surface as seen at the terminator. Less direct is the observation that the temperature variations over the cloud tops and over the surface are small. This can only be explained by a rapid transport of head by wind systems. Because Venus rotates slowly, Coriolis forces are much weaker than on earth and may be neglected to a first approxi-
45 mation. Theoretical arguments and laboratory experiments sug- gest that a simple convective circulation will be set up with air rising in warm regions and sinking in cold regions. The location of the heat sources and sinks is crucial in deter- mining the form of the circulation. Heat loss to space is by infrared cooling from the upper strata of the cloud deck. The altitude at which the solar energy is deposited in the atmo- sphere is not known; this is one of the most fundamental ques- tions in the dynamics of Venus's atmosphere. Enlightened guesses may be made, however. The extensive cloud and high albedo make it improbable that much of the sunlight incident on the atmosphere reaches the planetary surface. Detailed calculations on plausible clouds have indicated that most of the solar heating takes place in the clouds. The response time of the atmosphere of Venus to solar heating is long compared to a Venus day, up to a level approxi- mately coincident with the cloud top. The atmosphere below that level experiences an average heating like the earth, sym- metric about the rotation axis, highest at the equator, and vanishing at the poles. We may anticipate a Hadley circula- tion in which the flow is up over the equator, poleward at high levels, down over the poles, and equatorward at low levels. This circulation is not relevant to middle latitudes of the terrestrial atmosphere, but it does bear a relationship to the tropical atmosphere. Another possibility occurs if the solar energy is de- posited in a region in which the response time of the atmo- sphere of Venus is short compared to the Venus day. The cir- culation in that case would be expected to be up over the subsolar point, toward the antisolar point at high levels, down over the antisolar point, and back to the subsolar point at low levels. An analysis of a Hadley circulation, driven by solar heating and infrared cooling at the upper surface, has been performed. The model is perhaps primarily of heuristic value, as the numerical values used are quite uncertain. However, two important features emerge. First, the circulation will transport a sufficient amount of heat to reduce the tempera- ture contrast well below that of a model which neglects large- scale motions. This is in agreement with both the infrared measurements of cloud-top temperatures and with microwave surface-temperature measurements. Second, the deep circula- tion could be adiabatic, creating an adiabatic lapse rate. If this circulation extends nearly to the surface, it could produce the observed high surface temperature through compres-
46 sional heating, without requiring any solar radiation at the surface. Several features of this model must be established before it will be possible to consider more detailed questions. First, is the circulation of the equator-to-pole type? Second, how deep is the return layer? Third, how large are the velocities at the cloud top and in the ret~rn layer? Observations in blue and ultraviolet light have sometimes shown fain.t cloud markings that move around the planet in 4 days, in a direction opposite to that of rotation. Because they are seen at ultraviolet wavelengths, they are presumed to be above the visible surface. Some recent Doppler-shift measurements tend to support such motions. Such a circulation cannot be explained on the basis of the Hadley cell. An explanation has been put forward based on mo- mentum transport by thermally created eddy stresses. Labora- tory studies have shown that retrograde motions can be created, but further work must be done to demonstrate the relevance of this work to Venus. The existence and morphology of such disturbances needs to be established, along with such temperature measurements as will test the proposed explanation. Further, the relation- ship between the Hadley and the 4-day circulation, if any, needs observational and theoretical elucidation. Motions of planetary atmospheres result from horizontal and vertical density gradients. The questions raised above require that we measure spatial variation of temperature, pressure, solar, energy deposition, and motion. The number of measurements necessary to define these fields depends upon the questions posed. With our present knowledge, only the most fundamental questions may be realistically asked; i.e., where is the energy deposited, what is the stability of the atmosphere, what is the general form of the temperature dif- ferences and flow regimes? Measurements P~obes The major question concerning the lower atmosphere can only be answered by in situ measurements at several loca- tions. In particular, we need measurements of temperatures and pressures in the three regions of particular interest--the subsolar, the antisolar, and one of the poles--plus an inter- mediate region.
47 In each of the regions we should like to measure the pressure at all levels to an accura.cy of 1 percent. To inte- grate the hydrostatic equation) a temperature measured to 1 percent accuracy is also desirable. To determine the lapse rate between successive determinations to within 3 percent of the adiabatic value, a relative accuracy of 0.2 K will be re- quired, but even 1 K is useful. Solar...fluxmeasurements should be made from as high as possible to the surfa.ce~ but at least a fe,vmeasurements should be made above the clouds. These mea- surements are easily within the capabilities of the state of art the weight and telemetry 1imita- probes. i,and-velocity measurement accuracy of a few meters per second is useful in the upper cloud levels. At lower , higher accuracy is desired, with a few centi- meters per second as the goal. This accuracy may be achieved from Doppler tracking data or, from a wind-drift radar on the probe itself. BaZZoons Balloons are useful primarily to examine the atmo- circulation of Venus by means of range and Doppler ents. A complete analys of the obtainable accuracy of measurement has yet to be made, but it is li~ely to be at least 1 .or 2 m sec-l, ,.;rhich precise enough for our is purposes. The discovery of scales of motion smaller than the planetary radius is quite possible and would offer importan.t clues to the details of the atmospheric circulation. As the balloons drift with the atmosphere of Venus,tem- perature, pressure, and solar and thermal flux can also be sampled. latter allow determination. of the variability cloud structure and thermal irregularities. a three-dimensional picture of the wind struc- , balloons should be placed at two or three levels. The , 500-, 1200-mbar pressure levels appear to be possible. Orbiter Bus ExperimentsGreat insight into the of the circulation of's atmosphere has been gained the planetwide coverage available from satellites. We may confidently expect the same for Venus. Two types of recommend themselves in particular. A high-resolution spin-scan camera can resolve features n 2 km an 600 km. This will tell there are small holes the cloud deck, or cumulus as have been suggested. On a large scale, the undula- the 'cloud surface probably reflect dynamic processes in the lower atmosphere. These will indicate the scale of
48 such motions and their direction. Such observations will give invaluable clues to lower-atmosphere motions and possible longitude variations of the type that is crucial to an under- standing of the earth's atmosphere. The payload of a mission to fly by Venus and Mercury in 1973 has not been settled, but it very likely will include a TV camera. The flyby distance is expected to allow a few thou- sand pictures, with resolution from 0.1 to 40 km during the time of passage. This will provide essential information on light levels and contrast of the cloud features of Venus, but it will pass by too rapidly to provide the coverage in time desired for wind measurements. A simple thermal radiometer, looking down in a window region of the spectrum, would allow temperature maps of the cloud surface to be drawn. For best results, a resolution much less than 100 km is desirable, and it should be bore- sighted with the TV camera. Measurements of Venus from earth have already shown a few degree temperature anomaly near the top of the clouds near the south pole. With a closer view, smaller and weaker anom- alies may be seen which will act as tracers of events that have been described as storms. The temperature structure above the clouds has been mea- sured twice when Mariner 5 passed behind the atmosphere of Venus. These measurements reached roughly the 5-bar level. Simple S-band tracking of an orbiter would allow this kind of determination to be made many times over. The obvious advan- tage of this 'is the need for no additional weight or power on the orbiter. On the other hand, there are only two determina- tions per orbit (~l7 h), and they will be made in very nearly the same place for many orbits in a row. The geographical coverage is limited. Vertical infrared sounding can also be performed, as it is done on the earth, with a multichannel radiometer looking at the emission of the l5-~m band of carbon dioxide. This has modest weight and power requirements and gives much more geographical coverage. However, analysis of downward verti- cal soundings yields only rather poor vertical resolution. Further, layers of particulates may modify the signal but not be detected. The technique of limb scanning overcomes some of these limitations. By looking along long paths through the edge of the atmosphere, much higher altitudes may be probed. It appears that temperatures at heights up to the base of the thermosphere of Venus may be determined. Much greater vertical
49 resolution is possible; 2 or 3 km has been obta~ned in simula- tions of the terrestrial atmosphere and seems easily obtainable in the atmosphere of Venus as well. Layered particulates, if sufficiently opaque, will be more easily located. The weight and power requirements are similar to those of a nadir-viewing multichannel radiometer. The geographical coverage can be ex- tended by scanning off the plane of the orbit. A final use of the orbiter is to obtain thermal maps from microwave images. We have reason to believe that there is no thermal boundary layer on Venus. There may even b~ a few kilo- meters of isothermal atmosphere. If this is so, the surface temperature will provide a measure of the atmospheric tempera- ture in the lowest one or two scale heights. The importance of such data cannot be overstated. We do not anticipate any large local differences of temperature, but it is possible that I K over several hundred kilometers will produce highly signi- ficant local circulations~ possibly of greater importance than the planetary-scale circulation. Such temperature contrasts may well come about if there is a significant internal heat source on the planet as has been postulated by some investi- gators. We cannot expect to measure all such local effects, if they exist, with probes. Some kind of remote thermometer is essential. The problem with a microwave radiometer is that of distinguishing between changes of emissivity and changes of temperature. Atmospheric investigations, by themselves, are unlikely to solve the problem; however, these data are also of interest for planetological purposes. It has been suggested that this imaging can be done almost as well from the ground as from an orbiter about Venus, and that development of the necessary radiometers for orbiters is not yet sufficiently advanced. Consequently, ground-based measurements are recom- mended for the immediate future. This conclusion must not, however) be taken to imply a lo\;.] priority for the thermal maps. By one means or anoth~r this mapping must be performed in order to study atmospheric dynamics. The allocation of time on large radio telescopes must be adequate for this purpose, and con- tinuing effort should be directed tm.]ard development of orbiter radiometers.
