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Chapter 3 PLANETARY ATMOSPHERES Introduction The 1965 Woods Hole Study identified three principal objectives in the exploration of the solar system. The investigation of planetary atmospheres can contribute to these objectives; the relation of research on the inner planets to advances in terrestrial meteorology is particularly direct. Understanding of Man's Environment. Since the 1965 Study a number of theoretical investigations have been made of the lower atmosphere dynamics of Mars and Venus, and other studies are known to be under way. None of them is definitive, or even nearly so, but a remarkable relevance to terrestrial studies is already clear. Investigation of the Martian atmosphere focuses attention on a number of important aspects of the Earth's atmosphere that were not receiving the attention they deserved. The role of radiative transfer is greatly exaggerated on Mars but is also important on Earth. The lower boundary layer is essential for long-term weather forecasting. No theoretical model atmosphere that includes the boundary layer exists for Earth, but one is being developed for Mars. The low level diurnal jet has been treated as a minor feature of the Earth, but it is a planetary-scale phenomenon on Mars and is receiving increased attention. Diurnal tides also are of much greater importance on Mars. In the case of Venus, two questions are central. First, the circulation must closely resemble a Hadley cell, a type of circulation generally neglected on Earth, but of importance in the tropics. Models of Venus may be the first to give a modern numerical treatment. Second, we do not know how the Venus clouds couple to the atmospheric dynamics nor how a complete cloud cover can form. Why do the terrestrial oceans not evaporate and produce a hot, cloud- covered planet like Venus? We may discover that such a configuration was possible in previous geological eras. As we learn more about the structure of the atmospheres of Mars and Venus, we find that our understanding of the Earth's atmosphere is increased. Since atmospheric theories must now explain three planetary atmospheres, the new theories are more likely to be correct. There are examples in both meteorology and in upper atmosphere physics. The electron density in the ionospheres of Mars and Venus has now been measured. It has been surprising that these ionospheres do not have F2 regions as the Earth does. The physical processes that produce this high, dense ionization region on the Earth are being re-examined to understand not only why such a region does not form on Mars and Venus, but why it does occur on the Earth. The understanding of the thermal structure of the upper atmosphere is another such problem. Mars, Venus, and the Earth provide three atmospheric examples. Two of the planets have carbon dioxide atmospheres, the other an oxygen-nitrogen atmosphere. Venus rotates very slowly while Mars rotates with nearly the period of Earth. Now that exospheric temperatures of these planets are be- ing measured, the theory being developed to explain all three will increase our under- standing of the heating and cooling processes in the Earth's atmosphere. If Mercury has a few millibars' pressure of CC>2, its atmosphere may also be of meteorological interest. It is more likely, however, to be of most relevance to planetary aeronomy. The outer planets, and Jupiter in particular, are less similar to the Earth, but still of some importance. Stone's studies of symmetric instabilities extend the range of previous work on baroclinic stability and lead to a much better fundamental understanding of these phenomena. The Origin of Life. The determination of the composition of a planetary atmos- phere will show if the basic ingredients necessary for life, as we understand it, are present in the planetary environment. Compounds containing the atoms of carbon, oxy- gen, hydrogen, and nitrogen, and water in particular, are of central interest. -19-
-20- History and Origin of the Solar System. The atmosphere of an inner planet compris- es only a minute part of the whole planet. Nevertheless it is an indication of the re- cent surface history of the planet -- the first stage in understanding its planetology. The origin of the atmospheres can be better understood by comparing their compo- sition. The inner planets appear to have lost their original atmospheres. The Earth, we know, has obtained its present atmosphere from the outgassing of its interior. The amount of helium, argon, and nitrogen in planetary atmospheres such as Mars and Venus are indicators of the amount of degassing that has occurred. While Venus and Earth are about the same size and the same distance from the Sun and were probably formed from the same source at about the same time, their atmospheres have evolved quite differently. A major problem is the lack of water in the atmosphere of Venus to correspond with Earth's oceans. The outer planets, notably Jupiter, appear to have their original atmospheres. Following these ideas, the Jovian atmosphere is a current-day example of what the material that formed the solar system was like. The composition and structure of the atmosphere of Jupiter may serve as criteria for understanding theories of the evolution of the solar system. There may be no sudden change of phase on Jupiter or Saturn and we may be able to understand a great deal about the bulk of the planet from a study of its atmosphere. Since these planets are in a quite different stage of evolution, and since they form the major part of the planetary system, they are vital to our understanding of the solar system. Measurements Required Venus The physics, chemistry, and dynamics of the lower atmosphere should be given high priority in investigations of Venus. The physics of the upper atmosphere is a desirable concomitant. Earth-based observations are still of value, but since the bulk of the Venus atmosphere is beneath the cloud tops, the atmospheric drop-probe will be the most important tool for investigating the lower atmosphere in the next decades. The following measurements are needed to test and develop existing theories: temperature-pressure profiles at subsolar and antisolar points and at the poles, with measurement of the point of impact; the vertical flux of solar radiation down to the surface; composition, number-density and size distribution of cloud particles; winds, turbulence and cloud dynamics; and detailed composition of the atmosphere, with partic- ular reference to condensible constituents. Molecules such as nitrogen, oxygen, and ozone can be measured by ultraviolet spectroscopy. The temperature of the upper atmos- phere and its variations can be determined from the ultraviolet radiation emitted by hydrogen atoms. The electron density in the ionosphere should be measured as a function of location about the planet by radio occultation techniques. A mass-spectrometer capability should be developed to measure the concentrations and isotopic compositions of the noble gases of Venus. Inventories of He^ and Ar^" in in the terrestrial atmosphere have been valuable indicators of the outgassing of radio- active rocks. These inventories will be similarly important for the other terrestrial planets. The kind and amount of the remaining rare gas isotopes in a planetary atmos- phere are functions of the original entrapment of those isotopes within the planet and of its subsequent outgassing history. As such, these determinations can set useful boundary conditions on theories of the planet's origin. Differences, if they exist, in isotopic composition between planetary matter and terrestrial matter are probably most easily detectable in the rare gases; and all the gases from neon through xenon can probably be included in the search because background interference in mass spec- trometer measurements tends to fall off more rapidly with increasing atomic mass than does the abundance of the rare gases. In the case of xenon, special anomalies in isotopic composition due to extinct radioactive 1^7 atuj pu2^* have been prominent in the meteorites and may be visible in Venus. If so, it may be possible to infer when Venus was formed.
-21- Mars The problems of the Martian atmosphere differ greatly from those of Venus; similar- ly, research vehicles and the types of measurement employed to resolve them will differ. Meteorological and aeronomical studies of the Martian atmosphere are concerned with the free atmosphere, above the surface boundary layer. This boundary layer will exist even over a perfectly smooth and uniform surface, will probably be from 10-m to 1-km thick, and will differ greatly from the overlying free atmosphere in its thermal and dynamical processes. A major difficulty in studying the boundary layer is that no one site can be considered truly representative. Except for compositional determinations, those atmospheric measurements made by an early Martian lander will be designed to support biological or surface studies rather than for atmospheric studies. Drop-probes are of secondary value to study the Martian free atmosphere; their main objective would be to make a detailed chemical analysis of the lower atmosphere. The most valuable vehicle is an orbiter. With the orbiter, radio occultation measure- ments can map the lower atmosphere temperature profile and the ionosphere. Airglow and resonance fluorescence experiments will give composition information. While the major constituent of the Martian atmosphere, carbon dioxide, has been identified, it is not known whether molecular nitrogen, molecular oxygen, or ozone are present and if so, in what amount. The diurnal temperature changes in the upper atmosphere should be measured. A CC>2-band infrared instrument could give crude temperature profiles. A radiometer could map ground temperatures and compare them with topography. The same radiometer would give temperatures of clouds and the nightside. Winds, an important feature of the Martian atmosphere, are probably best measured indirectly by means of visual observations of clouds. Dust-devils and other local phenomena will probably also be visible if the resolution is sufficiently high. A floating balloon-sonde is not out of the question, but should probably be deferred until the results of cloud motion studies are known. Again, a mass-spectrometer capability is needed to measure the concentrations and isotopic compositions of the noble gases of Mars, for the same reasons as for Venus. It is likely that useful information can be obtained from orbiters. At one extreme, helium measurements should be possible in a spacecraft orbiting the planet at large distances. At the other extreme, a complete rare gas assay should be feasible from an orbiter of collapsing orbital radius, and certainly from a lander. Jupiter Jupiter, the nearest and most thoroughly studied of the major planets, presents interesting and important physical problems to planetary astronomers. Jupiter apparent- ly possesses an internal source of energy, and is thus of interest to stellar astronom- ers. Space probes to Jupiter are becoming technically feasible and they seem certain to be as fruitful as those already sent to Venus and Mars. One of the most important questions is the composition of Jupiter's atmosphere. A detailed knowledge of the composition will provide constraints on theories of the origin of the solar system. A better knowledge of atmospheric abundances will also permit improved models of the interior. It seems fairly certain that H£ and He make up the bulk of the atmosphere, but the H2/He ratio is quite uncertain. Although the evi- dence suggests that the abundance (by number) of He is comparable with H£, we cannot exclude a solar composition (H2/He 3) or a predominantly helium atmosphere/ ^2 <j 1\. Ground-based observations in the near infrared may provide the H2/He He ratio if the dominant source of opacity in this wavelength region is due to collision- induced transitions in H2- In this case, the sp'ectral distribution of the emitted radiation will depend on whether the H2 transitions are induced by He or self-induced. Although the results of these observations are not likely to be highly accurate, they
-22- are of great interest. With such limited information attainable from Earth, it seems fully justified to commit a significant portion of the instrumentation aboard a Jupiter space probe to determining the composition of the atmosphere. Radio occultation experiments, which have proved very successful on Venus and Mars, would yield an accurate value for the scale height. From this the mean molecular weight can be determined if the temperature at the occultation level is known. The radio occultation experiment is probably the only observation relating to atmospheric composition that is practical for the initial fly-bys. However, observations with a more advanced mission of the absorption of ultraviolet solar radiation by Jupiter's atmosphere may yield further results. In particular, the flux of the 584 A Hel chrom- ospheric emission line at Jupiter is of the order of 10^ photons/cm^ sec. It seems feasible to measure the helium abundance from the absorption of this line as the planet occults the Sun. Another important question to which a Jupiter space probe can contribute uniquely is the planet's energy balance. It seems plausible that the total energy emitted by the illuminated hemisphere could be measured quite well by the planned Convair-990 telescope. The major uncertainty in Jupiter's bolometric albedo, however, arises from the lack of knowledge of the phase function, since only a 12° variation in phase angle is observ- able from the Earth. This uncertainty can and should be removed by visual photometric measurements, ideally in several colors, during the Jupiter mission. Nevertheless, the I total energy emitted will not be known precisely until bolometric measurements are made of the dark hemisphere. Therefore, the spacecraft should also make sufficient measure- ments in the far infrared to establish the difference, if any, between the amount of radiation emitted from the light and dark hemispheres. Much information on Jupiter's atmosphere can be obtained from the absorption lines and bands found in the near infrared spectrum. However, the analysis of these lines and bands is greatly complicated by the fact that they are almost certainly formed in the clouds. A major question about Jupiter thus concerns its cloud structure. Is it a single layer or is it composed of several layers as is often the case in the Earth's atmosphere? Are the cloud tops highly irregular? Definitive answers will probably re- quire atmospheric drop-probes on more advanced Jupiter missions. One important measure- ment for these probes is cloud particle density as a function of depth. Closely re- lated are measurements of the upward and downward flux in visible wavelengths as a function of depth, indicative of the depth to which solar radiation penetrates. Direct measurements of pressure and temperature by a probe into Jupiter's atmos- phere would provide a direct test of atmospheric models and would require relatively simple instrumentation. Measurements with a mass spectrometer would provide an indepen- dent check on the atmospheric abundances, especially important in the case of the rare gases. . In addition to the observations suggested above, measurements of interplanetary magnetic fields and plasma and of solar and galactic cosmic rays to distances of 5 AU (further in case of a swing-by) would be of enormous importance to cosmic ray physicists. These measurements would decisively test theories of the solar modulation of galactic cosmic rays and provide a better knowledge of the low energy end of the cosmic ray spectrum. Extremely valuable measurements can also be made on Jupiter's immense radia- tion belts. These measurements include the energy spectra of the trapped particles and the strength of the magnetic field. In summary, because of its many diverse phenomena, Jupiter is probably the most interesting planet from a physical point of view. It is now technically feasible to send space probes to the vicinity of Jupiter. We therefore recommend that Jupiter
-23- missions be given high priority, and that two exploratory probes in the Pioneer class be launched in 1972 or 1973. Mercury Since it is doubtful that Mercury has any atmosphere, the planet is at present of minor interest for atmospheric investigations. Ground-Based and Near-Earth Studies of Planetary Atmospheres Observations from the surface of the Earth and from near-Earth orbit produce valuable information on the nearer planets, and for many years to come they will be our primary source of information on the more remote planets. Such observations are less expensive than planetary missions. Because our knowledge of the more remote plan- ets is very limited at present, every avenue of study from the vicinity of the Earth should be explored before we commit expensive probes to these objects. We note with satisfaction the assistance to ground-based planetary astronomy that NASA has been providing since the Woods Hole Study. An intensive ground-based program is obviously a sensible means to assure, at relatively low cost, that optimal use is made of deep-space exploratory missions. In the same sense, we urge that more attention be directed to the use of astronom- ical telescopes in Earth orbit for planetary observations. To date, the only space observations of planets are a few low resolution, ultraviolet spectra of the bright planets obtained from sounding rockets (which provide about 5 minutes' observing time above the atmosphere). (a) Earth-Based Observations. Important planetary observations have recently been made by conventional telescopes. One of the foremost are the excellent spectra of Venus, Mars, and Jupiter obtained by P. and J. Connes. The spectra were obtained with a resolution of approximately 0.1 cm but they covered only a limited part of the available infrared spectrum. This type of observation should be extended through- out the infrared and should specifically include the regions where molecular vibration, bending, and rotation are revealed. A resolution of at least 0.1 cm" appears to be a reasonable goal. At especially dry locations (total water vapor content of the atmosphere less than 2 mm) a sizable portion of this region can be reached from the ground. NASA is to be commended for sponsoring the construction of three large telescopes (the 105-inch telescope at the University of Texas, and the 88-inch telescope at the University of Hawaii, and the 61-inch telescope at the University of Arizona), and for up-dating for planetary use the facilities of two other large telescopes (the 60-inch at Mt. Wilson and the 82-inch McDonald telescope). Optical astronomy can furnish data that allow, or facilitate, interpretation of atmospheric data obtained from space missions and that guide the formulation of these missions. With respect to the structure of planetary atmospheres, ground-based meas- urements yield information on the following: (1) Composition of lower atmospheres, (2) Pressures and temperatures, (3) Wind velocities if clouds can be photographed and followed synoptically, (4) Properties of particulate matter in clouds, and (5) Varia- tions of minor atmospheric constituents (particularly water vapor) with season or other parameters. Another ground-based program supported by NASA is a planetary patrol to be carried out at 6 locations around the world for continuous diurnal photographic monitor- ing of the planets when they are favorably situated. This program goes far toward im- plementing item (3) above. We note, however, that immense quantities of data will be
-24- produced, and must be interpreted if they are to be of use. We commend NASA for supporting the planetary patrol and urge that data analysis be given particular emphasis. To be fully effective, ground-based observations should be made before, during, and following encounter of a specific planet by a space mission. Conditions found during encounter can thus be related to the large body of information acquired in the long history of telescopic examination. This procedure is important not only for photographic observations, but also for spectroscopic observations of variable minor constituents. (b) Planetary Observations from Near-Earth Orbit. The NASA planetary program should take account of the Earth-orbital telescopes that are being planned for the mid-1970's. These all-reflecting telescopes, having apertures of about 1 m are being designed for diffraction-limited performance at ^=5000 A and will therefore have a limiting resolution of 0.1 arc sec. High resolution imagery will be parctical in the wavelength interval from 2000 to 8000 A. Imagery below 2000 A is possible, but the relatively long exposure times required makes compensation for blurring due to plan- etary rotation and motion increasingly difficult. Imagery above 8000 A can also be performed, but special imaging sensors will be required. Long term observations of surface and atmospheric detail on the planets will be possible. The following table indicates the limits of resolution ^and its ratio to the diameter D of a planet at a favorable apparition: Planet* D (km) / (km) D/./ Venus 12 x 103 50 240 Mars 6 x 103 35 170 Jupiter 14 x 104 300 470 Saturn 12 x 104 650 180 Uranus 5 x 104 1200 40 Neptune 5 x 104 2000 25 For the mid-1970's, it appears that fly-bys and orbiters are practical for only the first three planets in the above table. In all these cases, a fly-by or orbiter is capable of at least 100 times more resolution than an Earth-orbital telescope, but over only a rather small fraction of the planet's surface at a given time. Therefore, the Earth-orbital telescopes should complement these detailed photographs by planet-wide photographs taken at appropriate intervals. High resolution photography of planets from the Earth's surface at periods of exceptionally good seeing should also be vigor- ously pursued. The largest contribution of Earth-orbital telescopes to planetary imagery would appear to be imagery of the major planets. With the exception of Jupiter, fly-bys of these planets cannot be expected until at least 1980. Furthermore, the relatively long exposures required for these objects, particularly Uranus and Neptune, increases the difficulty of obtaining ground-based photographs during intervals of good seeing. Many other solar system objects are photographically resolvable with a 40-inch diffraction-limited telescope. These include Pluto, about ten of the largest asteroids, *The planet Mercury is excluded because of the difficulties in arriving at a thermal design for an Earth-orbital telescope pointing within 20° of the Sun. These difficul- ties are unlikely to be solved in the next few years.
-25- Titan, and the Galilean satellites of Jupiter. The latter have angular diameters of about one arc sec and should exhibit surface features on photographs having a resolution of 0.1 arc sec. Ultraviolet spectroscopy from Earth-orbiting telescopes should also be used to complement spectrographic data obtained by planetary missions. In the next few years, three Orbiting Astronomical Observatories (OAO's) are scheduled to carry a variety of instrumentation for wide-band photometry and spectroscopy. Although primarily designed for stellar observations, each of these OAO's can give valuable supporting information on the planets. Attention should be directed toward the feasibility of specially modifying the Small Astronomical Satellite (SAS) for optical planetary observations. High resolution, 0.1 to 0.3 A» ultraviolet spectroscopy of the planets, utilizing the high spatial resolution of about 1 arc sec and a spectral resolution of about 1 A can be obtained by a 1-meter orbital telescope for all but the most distant planets. Instrumentation now being designed should allow such spectra to be obtained in the mid- 1970's. At present there are no active plans to perform infrared spectroscopy of the planets from Earth orbit. However, NASA is planning to equip a Convair-990 jet aircraft with a 36-inch ir telescope, stabilized to about one arc sec, and to be flown at about 40,000 feet. There appears to be very little water vapor above this level with the result that the infrared spectrum is nearly completely transparent below 25 V- and partially transparent above 25 M-. This new facility, scheduled to become operational in 1970 or 1971, should be vigorously utilized for obtaining ir spectra of the planets at moderate spatial resolution (1 to 2 arc sec). In light of these considerations, we recommend that the NASA planetary program planning be closely coordinated with the Earth-orbital telescopes being designed for the mid-1970's and with the infrared aircraft telescopes now under construction. Mission Priorities for Atmospheric Research The following attempts to order priorities for atmospheric research. It represents an assessment at one moment of time, and priorities should be reevaluated as results accrue from successive missions. Small Planetary Orbiters Small planetary orbiters of the Pioneer/IMP class should be a part of the planetary exploration program, whatever the level of funding allocated to it. While their value for planetary atmospheric research is not equal in every case to that of larger probes, the prospect of a relatively low-cost, ongoing, flexible program is extremely attractive. Instruments should include plasma probes and magnetometers for solar plasma measurements; aeronomy instrumentation in certain cases for use during orbit decay into planetary at- mospheres; radio occultation; imaging in the visible and infrared for meteorological studies, if the payload permits; microwave radiometry for Venus orbiters. Direct Impact Probes on Venus From the standpoint of planetary atmospheres, the study of the lower atmosphere of Venus is the most important single area in which rapid advance is possible at the pres- ent time. Critical measurements include physical and dynamical properties of the cloud- cover, thermal and dynamical characteristics at a number of locations above the planet's surface, the radiation environment, and the chemical composition of the atmosphere. Such measurements can be made only from probes that penetrate to the planet's surface. Since the probe must pass through the upper atmosphere, valuable aeronomical measure- ments can also be made during this phase of the mission.
-26- The most economical, convenient, and reliable mission employs a direct impact trajectory. Owing to the planet's very high surface temperatures, the sterilization problem can probably be overcome. A very elaborate experiment can be carried out using multiple probes on a Mariner-class mission, and it is possible that useful parts of the experiment can be accomplished using smaller vehicles. We recommend that in situ exploration of the lower atmosphere and cloud layer of Venus be one of the principal objectives of the space program of the 1970's. Mariner Orbiters to Mars We recognize the value of the planned 1971 Mars orbiter mission. Visual and infrared imaging are powerful tools for meteorological investigations. It is possible that wind velocities can be obtained from measurements of dust-devils, clouds, etc. Vertical temperature profiles can be measured from the 15-H C02~band emission. High resolution infrared spectroscopy may be used to map concentrations of polyatomic gases (including water vapor). Synoptic measurements of atmospheric pressure and temperature profiles, and ionospheric profiles, can be obtained from radio occultation. Plasma and airglow measurements can also be made. It is not clear to what extent these measurements can be made from small planetary orbiters or to what extent Mariner-class vehicles are required. The possibilities must be examined in order to plan missions after 1971. Jupiter - Small Fly-by Exploration of Jupiter must have a modest beginning. We recommend that this start be made during the early'70's. A small fly-by could perform a valuable occultation ex- periment, giving data on both the ionosphere and the neutral atmosphere. Measurements of plasmas and magnetic fields would be a prime objective of the flight. Mars Drop-Sonde Depending on the results of measurements from orbiters it may prove important to obtain in situ data on electron density and the ion species in the ionosphere, the compo- sition of the neutral atmosphere, and on atmospheric structure with particular reference to the presence of a turbopause. A small, hard-impact probe could perform these tasks. Mars Landers Because a surface station would be situated in a complex thermal and mechanical boundary layer the value of a lander for atmospheric studies is not great. We anticipate, however, that jsuch missions will take place in the 1970's and would wish to make use of them for accurate atmospheric analyses, particularly of the noble gases. To do so will require special instrumentation because the abundant gases should be gettered away before measurements are made. If weight is available on the lander during descent, the drop- sonde mission described above can be performed as well. Mercury Fly-by Atmospheric investigations of Mercury should not command a high priority in the near future. Fly-by missions to the planet may take place for other reasons, however. If so, radio-occultation and flourescence measurements would be of interest.
