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Chapter 3 PLANETARY ATMOSPHERES COMPOSITION Existing knowledge suggests that the compositions of Jupiter and Saturn are essentially the same as that of the sun, al- though the evidence for this is difficult to obtain from abundances measured in chemically differentiated regions of the planet. It is, however, conceivable that significant results may be obtained from abundance ratios of elements that would not be expected to have been separated by either differ- ential loss or concentration. Presently available evidence also strongly suggests that Jupiter and probably Saturn have retained all the elements in the same relative abundances as the sun. This is in marked con- trast to the inner planets, which have lost the lighter ele- ments during their formation and/or subsequent evolution. Evidently some other fractionation process has been active in the outermost regions of the solar system as well, since Uranus and Neptune are deficient in hydrogen with respect to the cosmic average and may be deficient in helium as well. In order to identify the causes of these abundance vari- ations, which must obviously be intimately related to the pro- cesses involved in the origin and evolution of the solar sys- tem, it is essential to have more quantitative data on the abundances of the light and heavy elements in the atmospheres of these planets to complement models giving the bulk com- position of their interiors. It is important to realize that the ratio of hydrogen to helium in these bodies has a significance that goes beyond the problems associated with an understanding of the solar systems. Rival theories for the origin of the universe have suggested that different amounts of helium will be produced. In the "big-bang" model, the abundance of helium relative to hydrogen presently observed in the sun is produced almost immediately as part of the initial expansion, whereas in other cosmologies, the helium is produced by nuclear synthesis in 25
26 stars. There is some question of whether the amount of helium observed in the sun is accurate. The values currently quoted have a large uncertainty but appear to be distinctly higher than values obtained from observations of old stars, a result that would be unacceptable to the big-bang cosmology. Thus verification is important, and a proper investigation of the Jupiter atmosphere would provide the necessary data. The abundance of the noble gases, especially neon and argon, which have particular significance, cannot be determined by remote sensing at planetary temperatures. This does not mean that the possibilities for extracting new information from data obtained from ground, balloons, or earth satellites have been exhausted. Indeed, modern instrumentation can provide important new data related to the abundance problem. However, it is related in such an inextricable fashion to the physical state variables, that no hope exists for solving both the abun- dance and structure problems from such data. There is another aspect of the abundance problem of great importance for under- standing the chronology of the solar system -- isotopic abun- dances, which are virtually unaffected by chemical reactions and are critical tests of astrophysical theories for element formation. A wealth of data related to the abundance of iso- topic nuclei in the earth's crust and the meteorites is avail- able, on the basis of which very precise determinations of the time of formation of the elements and of these bodies can be made. The nuclei used for this purpose are relatively heavy and rare and include strontium, rubidium, lead, and even fissionable species. For the sun, isotopic abundances can be estimated for only very few light nuclei, such as C/ C and D/ H. They are, nevertheless, of great importance, because variations in isotopic ratios among various bodies of the solar system imply the operation, at certain times and places, of high-energy processes capable of producing nuclear reactions in large numbers. Any information on the abundances of these nuclides in the planetary atmospheres may therefore be of great value for an understanding of this activity. On the basis of theories advanced to explain the observed abundance of certain nuclei in the earth, specific predictions have been made regarding isotopic abundances in other solar- system bodies. It has thus been predicted that the 2o/lH abun- dance ratio in Jupiter should be less than one hundredth that found in seawater. The verification of such a prediction for Jupiter would thus clarify our understanding of the composi- tion of the earth.
27 Measurements are required of the abundances of the ele- ments, up to argon, with an accuracy of l0 ppm. We also need to know the abundance ratio of isotopes with special interest, such as ZE/1E, 3He/4He, 36A/40A, and l3C/l2C. The lower atmosphere by definition is well mixed, but it does not follow that it has a uniform composition. The abun- dance of molecules which may undergo phase changes or are affected by solar radiation will vary with height in the atmosphere and location on the planet. The only reliable procedure to obtain measurements re- lated to abundances and physical variables is by means of a direct probe-carrying mass spectrometers with suitably chosen sensitivity and sampling inlets, and pressure-temperature transducers. Because of its very nature, a direct probe samples a very small column in the atmosphere. An accurate direct measurement of the abundances, particulate content, and physical variables, at a given location, however, would provide the basis to unravel data obtained from the ground, satellites, and flyby missions. All data obtained through remote sensing have to be interpreted in terms of a model representing the physical conditions in the atmosphere. The verification of a model through direct measurements will pro- vide reliability to results obtained from models applied to other parts of the same atmosphere and even to other planets. It is for this reason that we recommend an early direct prob- ing of the Jupiter atmosphere, for which we have the most ex- tensive knowledge. The results gathered in such a probing will enable us to interpret observations obtained from flyby and orbiting missions, recommended for a later date, to planets other than Jupiter. The physical structure of the atmospheres of the major planets is directly determined, not by the abundances of the atomic species but rather by the abundance of the molecular compounds they can form. Only H2, NH3, and CH4 have been de- tected, but, undoubtedly, H20 and H2S are also present in large amounts at lower levels of the atmosphere. In addition, other more complex molecules are doubtless present in trace amounts, which are formed through photolytic processes in- duced by solar light. A manifestation of the presence of such complex molecules is probably the coloration observed in the cloud structures observed in Jupiter and Saturn, for which there is no concrete explanation available. Suggestions have been made that such colors arise by the presence of com-
28 plex molecules in the ammonia or methane crystals forming the clouds. Their actual nature, however, has been the subject of widely ranging speculation. At one extreme, purely inorganic compounds have been suggested; but at the other, the existence of organic polymers formed through nonequilibrium processes in the basic mixture has been hypothesized on the basis of some laboratory experiments. A special significance has been attach- ed to the possible formation of such complex organic molecules, because this formation is thought to be similar to the spon- taneous abiogenic chemistry that took place in the early his- tory of the earth and eventually lead to life. It is commonly assumed that conditions on the primitive earth were very different from those we presently experience. There is some disagreement over the details, but one school of thought has maintained that during this early period the ter- restrial atmosphere was highly reducing, consisting primarily of methane and ammonia. Chemical reactions in this atmosphere were stimulated by solar ultraviolet radiation, electrical dis- charges, and local sources of planetary thermal energy. The initial results of these reactions are assumed to be the com- plex organic molecules such as amino acids that are necessary precursers to the origin of life itself. Under the conditions that developed on earth, the progression of complexity continued to the level of the formation of primitive living organisms and ultimately the wide profusion of life we now observe. It is already known that conditions in the atmosphere of Jupiter are very similar to this hypothetical model for the primitive earth. A logical first step in the investigation of the outer planets from a biological point of view wouM be further investigation of their atmospheres to determine how favorable conditions are for the abiogenic formation of organic compounds. Are there warm regions in the lower atmospheres? Are electrical discharges present? What solvents are avail- able? What photochemical reactions are occurring in the up- per atmosphere? The next step in sophistication is a search for the com- plex organic substances themselves. It has already been sug- gested that some of the coloring matter observed in the Jovian cloud deck could be organic polymers dissolved in the cloud material. Laboratory experiments using mixtures of methane and ammonia subjected to electrical discharge have produced colored substances, thereby lending support to this interpre- tation. An unequivocal identification has not yet been achiev- ed , however.
29 To resolve these ambiguities, it is essential to probe the atmosphere of these planets and make in situ measurements. A primary objective of outer planet exploration is, therefore, the characterization of lower atmospheric environments in terms of biologically significant parameters and a search for and identification of abiogenically produced organic substances. Studies of upper atmospheres play an important role in a balanced program for exploration of planetary atmospheres. The upper atmosphere can be studied rather extensively from a flyby. Such observations have contributed significantly to our present understanding of Mars and Venus and would be ex- pected to be equally important for the outer planets. The atmospheric experiments which can be performed from a flyby with a suitable geometry are essentially of two basic kinds: radio occultation experiments and airglow observations. Radio occultation provides information on structure of the neutral atmosphere between pressures of l0 atm and l atm. It also provides information on the structure of the ionized component of the atmosphere, corresponding to neutral gas pres- sures of l0 atm down to essentially zero background pressure. Interpretation of the ionospheric measurements can elucidate the photochemical processes which modify atmospheric composi- tion at high levels. Thus Mariner IV and Mariner V showed, surprisingly, that C02 is not appreciably dissociated by ultra- violet radiation. In the outer planets, methane is expected to be photochemically modified by absorption of ultraviolet radiation. Present knowledge suggests that the detailed chem- ical processes lead to formation of complicated hydrocarbons, an exciting prospect which may have significant biological overtones. Occultation measurements of the ionosphere may clarify these qualitative ideas. Ionospheric studies can also allow one to infer the temperature in the exosphere of the planet. It is this parameter which in large measure con- trols the rate of gas escape from the planet and has corres- pondingly great significance for studies of the time evolution of the atmosphere. We feel that it would be unwise simply to assume that the exospheric temperature is low on all the outer planets as a consequence of the large distance from the sun. For example, recent work suggests that the exospheric tempera- ture of Neptune might be even higher than that of Mars. If so, significant hydrogen escape may be taking place, leading to enhancement in the H/He ratio on this planet. Upper-atmos- pheric measurements would clarify the role of escape in modify- ing composition of atmospheres.
30 The most important airglow experiment is perhaps to mea- sure helium resonance radiation at X584, and we strongly en- dorse the flight of suitable instrumentation on Pioneers F and G. It should be recognized, however, that the probability that such measurements should provide an unambiguous determina- tion of the H/He ratio in the planetary atmosphere as a whole is not high. Above some level, namely, the turbopause, atmos- pheric gases are expected to separate gravitationally. A measurement of helium in the separation regime cannot readily be extrapolated to infer the helium mixing ratio in the lower atmosphere unless the location of the turbopause is specified by an independent experiment. Although possible in principle, we view this as unlikely and conclude therefore that a defini- tive helium measurement would require sophisticated instru- mentation on a deep atmospheric entry probe. We recommend measurements of (l) neutral composition; (2) ion composition; (3) electron densities; (4) temperature profiles, for electrons, ions, and neutrals; and (5) airglow emissions. Recommendations (3) and (5) may be performed from flybys. Recommendation (5) should emphasize the search for possible auroral emissions. Recommendations (l), (2), and (4) require entry probes and could be satisfied by unshielded entry probes or by more versatile probes to lower levels in the atmosphere. We therefore recommend that consideration be given to the development of small and relatively simple un- shielded entry probes capable of being carried on all outer- planet missions. THERMAL STRUCTURE AND DYNAMICS Studies of motions in the atmosphere of Jupiter have recently begun to achieve a quantitative status and are leading to new ideas about the behavior of rapidly rotating atmospheres and the interaction of cloud and planetary motions. While the atmosphere of Jupiter differs greatly from that of earth, the re-examination of fundamentals needed to embrace a different system should lead to ideas of importance to terrestrial mete- orology. Recent developments on Venus have led to a new under- standing of the circulation originally proposed by Hadley for the earth and current work on diurnal circulations on Mars is leading to a fresh appreciation of diurnal effects in terres- trial boundary layers. We may anticipate benefits of a simi- lar nature to meteorology and to other branches of atmos-
3I pheric physics from studies of the atmosphere of Jupiter and the other outer planets. The interpretation in terms of basic hydrodynamical pro- cesses of the horizontal circulation patterns at the visible surfaces of the major planets could lead in due course to infor- mation about the interiors of these bodies (e.g., lower atmos- pheric depths, angular momentum transfer, energy sources) that may be obtainable in no other way. The acquisition of this information is probably as important as the discovery and elu- cidation of the hydrodynamical motions themselves, which in turn will deepen our understanding of the terrestrial atmos- phere. As these motions are thermally driven, by solar radiation and by internal heat sources, the determination of horizontal and vertical temperature gradients will be of cardinal dynam- ical importance. The required information will not be obtain- able with ground-based observations and requires space missions to the planets. The central theoretical difficulty in all dynamical stud- ies of the motions of planetary atmsopheres is that of under- standing the interaction between motions on different scales. Owing to the great distances of the outer planets, we have (a) no knowledge of the scales of motion on Neptune; (b) indi- cations of planetary-scale banded structure which are vague in the case of Uranus, rather better for Saturn, and pronounced in the case of Jupiter; (c) evidence for strong equatorial jets on Saturn and Jupiter; and (d) a certain amount of infor- mation, going back a century or so, about large irregular mark- ings on Jupiter. (Whether Pluto possesses an atmosphere has not yet been ascertained.) These deficiencies in our knowledge of the scales of motions present and of the details of certain conspicuous features (edges of the strong equatorial jets on Jupiter and Saturn, Jupiter's Great Red Spot) will be remedied if high-resolution visual pictures required can be acquired during the contemplated missions to the major planets. Thermal Mapping and Vertical Temperature Structure In nonhomogeneous convective atmospheres, such as those of Jupiter and Saturn, upward mass motions occur which transport heat from hotter to cooler regions. At a sufficiently high level, this heat is radiated to the outside, and it can be measured remotely by an infrared sensor. If the receiver is
32 sufficiently sensitive, the surface of the planet can be scanned to provide a two-dimensional representation of the heat or temperature prevailing at a depth in the atmosphere determined by its transmission in the wavelength range admit- ted to the sensor. Infrared sensors available for deep-space missions are actually hopelessly insensitive for the pupose of obtaining thermal maps, comparable in scale and detail to a visual image. However, over restricted areas or regions and by means of improved detectors and taking advantage of the availability of the light at all thermal wavelengths, one should be able to study the temperature fields and their cor- relation with details visually observed. Studies have been made of the thermal emission of Jupiter in the l0-ym atmospheric window. Certain results indicate that at times little or no temperature contrast exists between visual fea- tures, such as the Great Red Spot and its surroundings, while at other times the brightness-temperature variations can be pronounc- ed. The Great Red Spot has appeared cooler on two widely separate occasions, once by ~2°. The shadows of satellites in the 8- to l4-ym range have been reported to be tens of degrees brighter than their surroundings; at other times no contrast is detectable. The average emission in the total range of wavelengths appears to be higher than the absorbed solar energy, indicating the existence of an internal heat source. A similar conclu- sion has been reached for Saturn. Near 5 vim, the thermal emission appears to be from lower and warmer levels than that for the l0-ym window. Preliminary, unpublished results indicate that most of the emission comes from tropical regions and that it has a complex structure. The methane band near 7 Pm shows anomalously high emis- sion at the band center. This has been interpreted to sug- gest the existence of a temperature inversion above the level of the clouds. A similar explanation has been invoked to explain an apparent reversal of lines in the l.25-cm NH-j microwave band. These remarks show that from ground-based observations little progress has been made in understanding the thermal structure of Jupiter, and there is even less progress on the other outer planets. On the theoretical side, a great step forward was made when the importance of pressure-induced hydrogen absorptions in the thermal emission spectrum was
33 recognized. On this basis, radiative-convective profiles have been computed for all the outer planets. Recent reassessment of the heat balance on Jupiter has indicated a more complex state of affairs in which thermal radiation mainly influences boundary conditions: The distri- bution of heat through the lower atmosphere will be dominated by fluid motions on a variety of scales, from that of the gen- eral circulation down to small-scale convection. Flybys and orbiters can be used, not only for thermal imaging, but for radio occultations and infrared and micro- wave thermal soundings. Radio occultations will give data down to ~l atm, just above the cloud level. It is unlikely that they can yield vertical temperature gradients of suffi- cient accuracy for dynamical theories (the required accuracy depends on the closeness of the gradient to adiabatic: if it is close, very high accuracy -- better than l0~ Fâ is needed). The horizontal resolution from occultation measurements is poor. Nevertheless, this type of measurement is so simple and reliable that it should be performed on all missions to the outer planets, and with the greatest precision possible. Infrared emission spectra can be obtained from flybys and orbiters. The techniques of temperature sounding from ir emission spectra are now well tested for terrestrial condi- tions, although further investigation is needed to discover whether a suitable methane band exists (ammonia bands are not so suitable because the gas may not have a constant mixing ratio in the region of the atmosphere under consideration). If a successful technique can be developed, temperature sound- ings in the two or three scale heights immediately above the cloud tops may be possible with significant spatial resolution. It is important that attention be given to the development of this technique. The microwave spectrum of ammonia can be used in a simi- lar way to give thermal soundings between the l20 and l80 K temperature levels. This technique has yet to be fully in- vestigated on earth, and it applies to a gas whose mixing ratio will vary in the vertical. It is difficult to avoid the conclusion that the verti- cal thermal structure inside the clouds, to the required accuracy for dynamical investigations, can only be discovered from entry probes. This alone might not justify a probe
34 mission, because only one geographical location can be investi- gated at a time. However, temperature measurements must also be made with accuracy in order to interpret the cloud layers which, in turn, interact thermodynamically with the motions. These measurements add justification to what appears to be the most important single mission concept for outer-planet atmos- pheric studies. VISUAL IMAGING Studying features of the visible surface of dense cloud over a broad range of horizontal scales requires photography of the planet beginning (in the case of a flyby) at a distance where the camera resolution is the same as that achievable from earth and continuing to a comparable distance after the instant of closest approach. In the case of Jupiter, for instance, with a miss distance of a few planetary radii, scales down to as little as a few kilometers would be resolved with a realiz- able camera system. During the l00 Jovian days occupied by the encounter, typical displacements of planetary-scale fea- tures in the equatorial zone relative to nonequatorial regions will be of under l00 deg of longitude (l00,000 km); the cor- responding relative displacements of features within the non- equatorial regions is about l000 km. Resolution of as little as a few kilometers (a fraction of the atmospheric scale height) at closest approach is needed for observations of the limb (including stellar occultations and optical aspects of the earth), which will lead to infor- mation on aerosol distribution and cloud stratification. De- terminations of the relative heights and motions of clouds could be made stereoscopically and possibly by measuring the evolution of the lengths of shadows cast by some of the clouds. The camera required must have high resolution, dynamic range, photometric accuracy, metric integrity, and be capable of rapid sequencing and fast movement from one target to another. CLOUDS Jupiter The most striking visual feature of Saturn and Jupiter is their banded and ever-changing appearance: we are clearly
35 observing atmospheric phenomena exclusively, and there is good reason to believe that, in the case of Jupiter, we are observ- ing a vast turbulent cloud layer composed principally of ammonia crystals. Spectroscopic data on the gaseous NH3 abundance above these clouds show that the amount of NH3 present is what one would calculate from the vapor-pressure equation of solid NH3 at the observed cloud-layer temperature, but no direct identi- fication of the composition of the clouds can presently be made from earth-based observations. The polarization data suggest that micron-sized cloud particles must be abundant in Jupiter's atmosphere but shed no light on the composition problem. Jupiter's clouds do differ from pure solid NH-j in respect to one readily observable property: color. Delicate shades of yellow, orange, brown, and even pink and very pale blue have been reported by numerous observers. It is often stated that these colors are due to small traces of intensely colored organic matter, produced by the action of solar ultraviolet light on the methane and ammonia in Jupiter's upper atmosphere. Finally, there have been some purely theoretical studies of the cloud-condensation process in atmospheres of solar com- position. These computations suggest strongly that Jupiter (and Saturn) may be shrouded by several distinct cloud layers of disparate composition. The topmost cloud layer is found to be solid NH-j, then a layer of NH/,.HS (ammonium hydrosulfide) clouds containing dissolved NH3, then NI^Cl (ammonium chloride) clouds near the 475 K level. The atmospheric pressure near the lowest of these cloud layers is roughly l00 atm. Whether this specific model is correct for Jupiter and Saturn cannot under any foreseeable circumstances be deter- mined from earth-based observations alone. Given a Jupiter flyby mission (such as Pioneer F and G) or an advanced orbiter, it would be possible to derive certain additional data concern- ing the clouds. Measurement of the planetary phase function, which is an effect produced by the clouds, gives data useful for other purposes but contributes little to our understanding of the chemistry and physics of the clouds. Photography of the cloud structures near the terminator may help in under- standing the scale of motions in the atmosphere near the cloud level. Such imagery could be conducted from either a flyby or an orbiter.
36 But even combining the data from flyby and orbiter mis- sions with all the ground-based observations deemed possible through the l980 period, it seems certain that our ignorance about conditions beneath the topmost clouds will still be great. In order to investigate the structure and composition of the atmosphere and clouds below the very thin region accessible to outside observers, it is essential that an atmospheric entry probe be employed. It is necessary for cloud studies that a Jupiter entry probe capable of withstanding up to l00 atm pres- sure and instrumented with cloud-physics and cloud-chemistry experiments be developed for launch at the earliest possible opportunity. One instrument of central importance for analysis of the atmosphere is a mass spectrometer. Since the principal cloud layers on both Jupiter and Saturn are believed to be due to condensation of trace gases from the atmosphere, it appears that mass spectrometric analysis of the atmosphere combined with temperature and cloud-density measurements would permit determination of the cloud composition. At present, it appears that a mass spectrometer of dynamic range l0^:l would be cap- able of effecting a very detailed elemental and isotopic anal- yses for the elements H, He, 0, C, N, Ne, S, Ar, and Cl. It is of great interest to note that, in theoretical studies of the composition of Jupiter's deep atmosphere, these elements, which are the only ones expected to be present in meaningful amount (<l ppm) above the 500 K level, all have atomic weights less than 40. Thus an exhaustive analysis of the atmsophere and clouds at all points above this level is possible with a mass spectrometer of limited mass range. An essential complement of this analysis experiment is a direct measurement of the cloud optical density as a function of altitude. This is readily accomplished with a simple neph- elometer. In addition, it should be possible to measure the the cloud particle size distribution. Some essential knowledge of the depth of penetration of sunlight into the atmosphere may be obtained very simply from an omnidirectional solar radiometer. A method of detecting cloud layers and distinguishing hazy and clear regions inde- pendent of the nephelometer would be a downward-pointing thermal radiometer.
37 Finally, there remains the problem of the trace coloring matter in the clouds. The in situ detection and identifica- tion of such material by gas chromatography and mass spectrom- etrymight be an extremely difficult and uncertain undertaking if polymeric organic matter is responsible. On the other hand, theoretical studies of the cloud-forming process imply that (NH4)2S (ammonium sulfide) may be produced in substantial quantity. This material is a yellow-brown solid of high vola- tility, and analysis of its vapors would show only NH4 and H2S. Saturn Our present data on the structure and composition of Saturn show it to be similar to Jupiter in all important respects. Of the four giant planets, the most similar two are Jupiter and Saturn. Our knowledge of Saturn's clouds is severely limited by its smaller size and greater distance: the banded cloud structure is well established as is the presence of micron-sized particles. However, ammonia has not been detect- ed with certainty in Saturn's atmosphere, and the clouds are essentially devoid of color. Occasional reference is made to a "pale-lemon tint" on the planet. Theoretical models of the clouds and atmosphere of Saturn predict that its atmospheric structure should differ from Jupiter's only with respect to vertical scale. Thus the entire range of experiments detailed under the proposed Jupiter probe mission are directly applicable to Saturn. But herein lies a serious question of priorities: if funding is limited, can we then justify sending probes to the most similar pair of outer planets? A more appealing possibility appears to lie in sending the second such entry probe to Uranus or Neptune. Uranus and Neptune Is an entry probe mission to Uranus or Neptune justifiable in fact? From the point of view of atmsopheric and cloud composi- tion and structure, the answer must be a firm "yes." First, it is clear that, while Jupiter and Saturn do not deviate markedly from solar composition, Uranus and Neptune are strongly enriched in the heavier elements. This situation introduces the possibility of extremely complex multicomponent phase equilibria leading to fluid-fluid immiscibility in the deep interiors, with consequent drastic alteration of the
38 atmospheric composition from the average for the whole planet. Second, it appears that the uv-visible reflectivity of the disk of Uranus can be explained satisfactorily by a model in which only molecular scattering and absorption by hydrogen is considered: clouds may not be involved above approximately the l0-atm pressure level. Because of the formidable theoret- ical difficulties in constructing plausible physicochemical models of Uranus and Neptune and the near-complete lack of relevant high-pressure phase equilibrium data, it seems almost easier to probe the atmosphere than to attempt to predict its behavior. Certain useful contributions to the study of the clouds of Uranus and Neptune may be made by flyby vehicles. Visible and ir imaging of both planets, including study of the hori- zontal and vertical cloud structure near the terminator, and optical polarization measurements are all experimentally feasible and of potential value in our attempts to understand their cloud and atmospheric structures. It cannot be stressed sufficiently that earth-based observations of Uranus and Neptune are completely incapable of solving any of these problems. In almost every observable respect, the giant planets appear to fall into two fundamentally different classes, which we may refer to as Jovian and Uranian. It must be asserted that a knowledge of the composition and structure of both classes is necessary as a preliminary to the formulation of a satisfactory theory of the origin of the solar system. Al- most every piece of information essential to the solution of these problems must be derived from the results of entry probe missions. We therefore strongly recommend Jupiter and Uranus entry probe missions at the earliest possible date. GROUND-BASED OBSERVATIONS Optimum utilization of probes to the outer planets will require many supporting observations from ground-based, balloon-borne, aircraft, and earth-orbital observatories. We endorse NASA's sponsorship of the construction of three large optical telescopes for planetary studies (the l07-in. at the McDonald Observatory in Texas, the 88-in. at
39 the Mauna Kea Observatory in Hawaii, and the 6l-in. at the Catalina Observatory in Arizona) in addition to several 24-in. planetary patrol telescopes. We urge NASA's continuing sup- port of the construction of large optical telescopes for plan- etary observations. In particular, we recommend the construc- tion of a high-optical-quality telescope of the l00-in. class in the southern hemisphere (possibly at Cerro Tololo). This telescope will greatly facilitate spectroscopy, photometry, and imagery of Uranus and Neptune during the next four decades when these faint planets will both be at southern declinations. During the l980"s, Saturn will also be in the southern sky. One aspect of our knowledge of the major planets which could be substantially improved concerns the structure of their clouds. Different cloud models are characterized by different line profiles, and comparison of computed and mea- sured profiles of weak, medium, and strong lines of CIfy and NH3 over a range of wavelengths should allow one to establish the gross features of the upper Jovian cloud structure. A resolution of at least 0.l cm~^ will be required. Rotational temperatures and pressures determined from these measurements can be used to improve the initial model atmospheres and, consequently, the cloud models. The same is true of the relative abundances obtained from the measured equivalent widths and curves of growth computed for the cloud model. The line profiles can be efficiently measured with a Michelson interferometer at the focus of a large light col- lecting area of l0- to 20-m diameter having a resolution of a few seconds of arc. We therefore recommend that NASA sponsor the construction of a large light-collecting aperture in the l5-m class for very-high-resolution Fourier spectroscopy of the planets. This large light collector should be located at a very dry site and as near to the equator as is practical. At the same time, we recommend that NASA support further de- velopment of Fourier spectrometers so that these invaluable instruments may become more generally available and simpler to use. The development of other specialized equipment, e.g., multislit spectrometers, also requires support. New laboratory data will be required to analyze high- resolution infrared spectra of the outer planets. This is particularly true of the near-infrared combination bands of CH4 and NH3 for which line strengths, pressure-broadening coefficients, and J identifications are (with very few
40 exceptions) unavailable. A laboratory investigation of the collision-narrowing phenomena in the H2 quadrupole lines will also be required. We therefore recommend that a comprehensive laboratory program to measure the properties of the bands of CH4, NH3, H2, and other plausible molecules in the atmospheres of the outer planets be vigorously pursued. At the same time, we recommend that the complementary theoretical calculations of the structure of these bands be pursued with equal vigor. These laboratory and theoretical studies are essential to the understanding of the atmospheres of the major planets. An important parameter in any cloud model is the value of the continuum single scattering albedo. For a given cloud model, it can be determined from the measured brightness variation over the Jovian disk. Here measurements should be made in wavelength intervals specially selected to be free of absorption lines. Another important parameter is the scattering phase function of the cloud particles. In principle, it can be de- duced from the observed limb darkening if the cloud structure is uniform over the planetary disk. In the case of Jupiter, the cloud structure seems to vary considerably from the equa- tor to the poles. However, measurements of the equatorial limb darkening should provide information on the phase function near the equator. High spatial resolution is important in such measurements since most of the variation occurs near the limb of the planets. The 24-in. NASA planetary patrol telescopes, operating at f/75, should provide good material for analysis. Also, diffraction-limited photographs of Jupiter should soon be obtained with Stratoscope II, a 36-in. balloon-borne telescope. However, a precise determination of the scattering phase function of Jupiter's clouds will probably only come after analysis of the photometry of Jupiter planned for Pioneers F and G. It seems certain that detailed models of the Jovian clouds will require a variation in the cloud structure with Jovian latitude. Photographs of Jupiter obtained with an interference filter centered on a relatively strong CH^ band at 8850 A reveal that the methane absorption in this band is not uniform over the disk but occurs in "bands" at different latitudes. This nonuniform behavior can be surveyed with the aid of monochromatic images taken in other bands of CH4 and
4I NHo. Particular attention should be given to identifying time variations. Further studies should be made to determine the variation of the equivalent width of weak spectral lines (or bands) with latitude. These weak bands are probably formed deeper in the clouds than the strong bands and should there- fore reflect latitudinal variations of the deeper cloud levels. Another aspect of our knowledge of the major planets which should be improved before advanced space probes are sent is their energy budget. Although the present evidence strongly suggests that Jupiter has a substantial source of internal energy, several related questions could be answered by further observations (from the ground and from aircraft) of the far- infrared radiation emitted by Jupiter. First, does the total radiation of Jupiter vary with time? Monitoring the brightness temperature near 20 and 300 yra from the ground and from l0 to l00 ym from aircraft altitudes should settle this question. Second, does the brightness temperature vary with lati- tude as suggested by some measurements at 20 ym? Measurements with the 36-in. infrared telescope being constructed for the NASA aircraft should enable us to obtain some spatial resolu- tion for wavelengths up to 40 ym (where the diffraction limit is about l0 sec of arc). Higher spatial resolution is pos- sible in the 20-ym window with large ground-based telescopes. The nearly completed 88-in. telescope on Mauna Kea seems particularly well suited for this measurement. The large light- collecting aperture recommended above would be very advan- tageous at 300 ym. Third, how much east-west asymmetry, if any, is present in the emitted radiation? From the east-west asymmetry (or its upper limit), it should be possible to estimate the dif- ference in effective temperature between the day and night sides of Jupiter. Here measurements from the ground at 20 ym will probably provide the best data. Perhaps the single most important parameter in Jupiter's atmosphere is the H2/He ratio. Since the absorption coeffi- cient of the He-induced translational and rotational transi- tions in H2 has a different wavelength dependence than the self-induced transitions, it should be possible to estimate the He/H2 ratio by fitting the emission spectrum from l0 to l00 ym with model atmospheres. One drawback to the proce-
42 dure is the probable variation of effective temperature over the surface. However, the theoretical resolution of a 36-in. telescope is ~25 sec of arc at l00 ym allowing some resolution on Jupiter even at the longer wavelengths. In consideration of the above discussion, we recommend that the 36-in. NASA aircraft telescope be used extensively to measure the far-infrared emission spectrum of the major planets. We also recommend that the feasibility of operating the 36-in. at altitudes above 50,000 ft. be explored in order to reduce ⢠further the atmospheric attenuation in the l00-Mm region. Apparently the only possibility of directly studying the deep atmosphere of the major planets is by means of radio and radar observations at wavelengths between l0 and l00 cm. A large antenna array having a resolution of a few seconds of arc could observe structural detail in the lower (pressure >l atm) Jovian atmosphere. Of particular interest would be structure, which could be correlated with visibile surface features or with magnetic field structure as measured by the early flyby spacecraft. Although primarily designed for galactic and extragalactic observations, the large antenna arrays now being planned will be capable of making such planetary measurements. We there- fore recommend that the designs of the large radio antenna arrays now being planned include provisions for real-time pencil-beam observations of the planets. A number of important ground-based radar observations of the outer planets and their satellites will become possible with present radar systems or systems now being planned. It now seems most likely that the radar detection of some or all of the Galilean satellites of Jupiter will be within the capa- bility of several radar systems now in existence when they are improved as planned between now and l975. Jupiter occultations of these satellites occur frequently, and in each case the detailed manner of decay or rise of the radio signal can be studied in much the same way as during spacecraft occultation. From this, deductions can be made concerning the density and structure of the ionosphere of Jupiter, as well as the structure of the upper atmosphere and some indication of its chemical composition. This radar infor- mation becomes clearer and less ambiguous when results at more than one frequency are available, because of the different
43 frequency dependence of the effects of the ionized and neutral atmospheres. A resolution of better than one scale height is expected for Jupiter. Although attempts at detection have been made by the major U. S. planetary radar systems, there is no clear evi- dence that any radar signal from Jupiter has yet been detected. The difficulty of estimating the signal strength to be expected is great, since it is not known whether any surface of dis- continuity exists at any level in the atmosphere that the radio waves can reach. Two types of detectable signal might be searched for. First, there is the possibility that a surface representing a phase change or composition change does exist at an accessible level in the atmosphere. This may be an actual liquid or solid surface, or it may represent a well- defined level at which a constituent of the atmosphere changes its phase or concentration. A sharp boundary of clouds would come into this category. For these cases the reflection would occur because of a change in the dielectric constant, and the surface would be required to be normal to the radio beam to be detected. Long wavelengths would be preferable for this type of signal, both because they are expected to suffer less attenuation and scattering before reaching the level in ques- tion and because the surface of discontinuity does not need to be so sharp or smooth to give the specular type of return. The second type of radar echo that may be expected from Jupiter arises from incoherent scatter by cloud particles. Since these particles are almost certain to be small compared with any of the radar wavelengths used, the returned signal strength will depend on A~4. Thus the high-frequency radar systems would seem to be advantageous although the level at which large droplets or cloud particles exist may be low and atmospheric attenuation will disfavor the high frequencies. Either type of radar echo detection from Jupiter would give new information concerning the atmosphere: the level of a surface of discontinuity, the amount of change in the value of the dielectric constant, the attenuation above this dis- continuity, and the dependence of the attenuation on frequency all would be valuable for creating a model of the atmosphere. In the case of the detection of incoherent scatter from clouds, their height and particle size as well as windspeeds and over- lying atmospheric attenuation can be deduced from two-frequency radar observations.
44 The problems of radar observations of Saturn are similar to those of Jupiter, and the greater distance will require a much higher level of radar performance. The rings of Saturn, if composed of large particles, may, however, be good radar targets, as may the satellites, especially Titan. We expect that the development of radar systems in the next ten years will have the required capability. Ground-based radar beyond Saturn requires performance levels beyond those that can be discussed realistically at present. The above discussion on radar studies of the deep atmos- pheres of Jupiter and Saturn is also applicable to the poten- tialities of bistatic radar, where the same transmitting sys- tem is used on the earth but reception is at the spacecraft. This would provide an increase of sensitivity by several to many orders of magnitude, making it possible to study these atmospheres in more detail and to search for deep atmospheric characteristics of the more distant Uranian planets. Bistatic radar would also be an important method of studying the surfaces of the major satellites of the giant planets and the particles in the rings of Saturn. In consideration of the above discussion, we recommend that NASA support radar astronomy to the extent necessary to obtain direct measurements of the Jovian atmosphere and with the plan that the same ground-based facilities would be used with spacecraft for bistatic radar investigations. The rapid rotation of Jupiter plus a substantial magnetic field produce a complex magnetohydrodynamic situation. Some insight into this problem may come from systematic high-reso- lution imagery of the planet. Such synoptic data are expected from the NASA planetary patrol telescopes. Extraction of quan- titative results from these patrol data should be facilitated by the use of image-processing computer programs in searching for significant temporal and spatial correlations. Since Uranus has an angular diameter of near 4 sec of arc, some spatial resolution should be possible on photographs taken at times of good seeing. The relatively long exposure times on Uranus (compared with Jupiter) makes it much more difficult to catch intervals of superb seeing. Integrating television sys- tems are now becoming available which are about a factor of l0 faster than film after correction for their lower resolving
45 power. Utilizing the reduced exposure time possible in these systems on the NASA planetary patrol telescopes increases the probability of obtaining high-resolution photographs of Uranus. Image-processing techniques could be used to average the best photographs. Also, Stratoscope II is expected to obtain photo- graphs of Uranus with a resolution of 0.l sec of arc. Diffraction-limited imagery of the major planets with extended time sequences will be possible from the intermediate size (=l-m diameter), all-reflecting orbital telescopes planned for the mid-l970's. Ultraviolet spectroscopy from earth-orbiting telescopes can be expected to yield much valuable information on the atmospheres of the major planets. The University of Wisconsin OAO-A2 has already made a substantial number of planetary ob- servations. The potential for doing planetary astronomy with OAO's and other earth-orbiting telescopes now being planned is large. We urge that the designs of these telescopes, which are primarily for stellar and galactic astronomy, be sufficient- ly flexible to facilitate planetary observations. Special care should be taken to ensure that imagery and spectroscopy experiments which can be done from earth orbit are not included in planetary probes. We therefore recommend that the capabil- ities for high-resolution imagery and ultraviolet spectroscopy of the planets from earth-orbiting telescopes be fully exploit- ed, and we urge that these telescopes be designed to facilitate such observations.
