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Chapter 4 PLANETARY SURFACES Introduction Exploration of our solar system constitutes an intellectual endeavor in which the search for extraterrestrial life is only one of many fascinating elements. In- creased understanding of the origin and evolution of the solar system can be best accomplished by visiting a number of other planets. In particular, it is on the surfaces of these bodies that we must look for a record of their history. Hare we search for evidence of present or former life, and also study materials formed in diverse environments and over a long period of time. The interiors of most planets are largely inaccessible and, in any event, their present state tends to reflect only comparatively recent history. Planetary atmos- pheres are more readily studied but, here too, we examine the end product of a whole sequence of earlier processes. Only on a planet's surface do we find a written record of events that can lead us backward in time over a considerable fraction of the duration of the solar system. In the rocky surface of each of the minor planets is written a long record of planetary change through time. The early events of this record may be overprinted by later events, or even obliterated. Yet a planetary surface invariably preserves a clear sequence of many major events that have shaped its present form. At a partic- ular spot the last signature on the surface rock may be that of any one of dozens of different processes: the lava spewed forth from a hot interior; the accumulation of sediments and fossils on the floor of a former sea; the deposits left by a vanished glacier; the craters formed by infall of objects from outer space. Because planetary surfaces have been subjected to a great variety of modifying processes of both internal and external origin, we expect considerable local varia- tion. Some processes operated only in the distant past; others are still active. These processes can be recognized only when the surface is explored on the scale to which the processes succeed in modifying it. For example, large craters formed either by meteorite impact or by volcanic activity on the Moon or Mars are readily recognized in photographs having relatively poor resolution. Here recognition depends upon the distinctive and relatively simple shape of the feature. Events on the surface of the Moon can be recognized as being of internal origin in photographs with resolution of 1 km or better. In the absence of other information we may take this figure of 1 km as the upper limit of resolution needed to detect past volcanic activity on Mercury or Mars. Other processes require even better resolution. For example, Orbiter photo- graphs of the Moon with resolution of about 10 m show transported boulders with dis- tinctive skid and skip marks. Visual photographs may be supplemented by infrared images or, in the case of Venus, by radio emission plots which depict the thermal properties of the surface. This type of remote sensing also tends to distinguish between compact material (e.g., rock) and fine debris (e.g., sand) and may also provide evidence of recent volcanic activity. Imaging by radar seems the chief means by which the surface of Venus can be studied in view of its high surface temperature and extensive cloud cover. Non- specular radar tends to single out structure on the surface having comparable size to the radar wavelength and can penetrate thin coverings of dust to yield the pattern of the underlying rock. Specular radar echoes, while not forming images of high reso- lution, would reveal larger scale structure and would provide a measure of the dielec- tric constant and layering, such as might be produced by subsurface water. -27-
-28- From the recognition of physical processes revealed by large scale pictures of adequate resolution, one may proceed to the study of the particular by means of pic- tures with finer resolution, and eventually to local exploration by means of a landed vehicle. Photographic reconnaissance of representative portions of the surface is critically important for selection of best sites for landing missions. Without this background two dangers exist: first, that the lander may be directed to an unfavor- able site, and second, that the lander may return important information which is largely uninterpretable out of context. We single out Mars orbital imagery as the highest priority mission that is now possible in the area of planetary surfaces. It is also important to attempt at least an exploratory, preliminary examination of all the terrestrial planets. In practical terms, this means that we place consid- erable value upon a photographic reconnaissance of Mercury and radar examination of Venus. In the sections that follow, we outline what is currently known about the sur- faces of Mars, Venus, Mercury, and the moons of Jupiter. We discuss also the ques- tions we believe must be answered next and the strategy for arriving at the answers. Our evaluation is predicated on assumptions concerning the overall balance that the space program will have during the next seven years. Thus, we suppose that oppor- tunities to study the surface of Mars will far outnumber those for Venus and Mercury. Mars Both because of its proximity to Earth and its physical behavior Mars holds a favored position in plans for planetary exploration. In addition to providing a possible habitat for life, Mars has surface features and an atmosphere that can be easily and profitably studied. Our present knowledge of the Martian surface is at a critically favorable point. On one hand, we know that there are certain enigmatic but distinctive markings: dark and light regions, bright polar caps, "canals," and craters. On the other hand, our present knowledge is so fragmentary that it is im- possible to test many of the models proposed to explain these features. A closer look at the planet by a Mariner-type orbital mission is certain to provide much of the information needed to guide further exploration. Present knowledge of the Martian surface comes from study of large scale varia- tions discernible with Earth-based telescopes and radar, and from Mariner 4 photo- graphs of a small part of the surface (less than one percent). Earth-based observa- tions show dark areas of irregular shape covering about one-third of the planet and featureless brighter areas covering the remainder. Polar caps, perhaps composed of ice frozen from either H20 or C02, appear seasonally. No agreement exists concerning topographic relations between bright and dark areas. Certain radar observations suggest that the dark areas are relatively high, but other observations indicate they are low. Mariner 4 photographs show a surface covered with craters having diameters from the resolution limit of 4 km to at least 120 km. The spectral and photometric properties of both the bright and dark areas are consistent with a surface composed primarily of silicates. The size-frequency distribution of particulate materials is unknown, but radar returns suggest a highly porous surface material. Two "waves of darkening" start alternately from the two polar caps at half- yearly intervals, cross the equator, and fade at about 22Â° latitude in the opposite hemisphere from which they began. One importance of these waves of darkening lies in their interest as a possible clue to the presence of life on Mars. This inter- pretation is challenged by other models involving inorganic changes affecting either the surface or the lower atmosphere. For example, gases seasonally generated by melting or subliming ice caps may systematically move light absorbent hazes towards
-29- the equatorial regions, causing clearing and color change. Alternately, subsurface seasonal changes in temperature may lead to melting of permafrost, thus changing the color and albedo of the soil. From present data no clear choice can be reached between various models. Scientific Priorities for Mars (a) Orbiters and Fly-bys. Mariner fly-by and orbital missions provide an ideal opportunity for a reconnaissance survey of the Martian surface by means of visual photographs and adjunct infrared-thermal and ultraviolet imagery. The orbiting spacecraft that fly during the 1971 opportunity will be able to image the southern hemisphere during the time of maximum change in contrast. Utilization of two space- craft will permit photography of two sets of critical phenomena. One spacecraft can monitor the area, recording its image at high Sun angles. These observations will allow determination of albedo difference and color contrast -- important for detection both of organic and inorganic processes. The second spacecraft could be launched at the appropriate time to obtain images at low Sun angles. These pictures contain the topographic detail that makes possible the interpretation of processes and their sequence in time. Study of the images obtained during the 1969 fly-by missions will assist in mission design; that is, in choosing the Sun angles that will make the orbital mis- sions most effective. Also, the results of the fly-by will influence camera design for subsequent missions. The importance of these considerations is indicated by the fact that only 5 of the 26 photographs taken during the Mariner 4 fly-by are high quality, appropriately exposed images. Study of the images of large portions of Mars obtained by the two orbital mis- sions will allow classification of those features of internal and external origin, and comparison with similar features on Moon and Earth. The crust of Mars can be subdivided into regional geological and environmental provinces. Predictions con- cerning the favorability and unfavorability of those provinces as hosts for bio- logical activity can be made. This study, then, assists the later direct search for life. A specific solution of the wave of darkening phenomenon may be indicated by topographic differentiation of light and dark areas, by textural variations between light and dark areas, or by secular changes in the boundary between light and dark areas. (b) Lander. It is reasonable to expect that much of the experimental pay- load on the first Mars lander will be devoted to experiments either investigating the ability of the surface environment to support life or seeking to detect life directly. Some of these experiments are also important, in characterizing the in- organic nature of the surface. For example, a photographic experiment, of critical importance in detecting large living forms, is also a first requirement for a de- tailed inorganic reconnaissance of the surface. It appears likely that the lander will be directed to regions thought to be large sedimentary basins -- a favorable kind of locality for the exobiological experiments. If, indeed, the region is underlaid by sediments a single panoramic photograph may confirm and amplify this presumption by showing stratification and other large scale sedimentary features. The panoramic photography should be followed by a limited and selective program of detailed photography to a lower limit of 1 mm coupled with 0.01-mm resolution microscopic photography of the sample. Rounded, frosted grains may bear witness to wind transport. Euhedral shapes may reflect a primary crystallization history in- volving either igneous processes or crystallization from low temperature media on the surface.
-30- The microscopic experiments will bear greatest dividends if coupled with a direct chemical-mineralogic experiment making use of a combined x-ray fluorescence and dif- fraction unit. So that the chemical analyses just mentioned will have optimal relevance, it is desirable to direct the lander toward an area representative of a large part of the Martian surface. Even then it is unreasonable to expect that the analyses will re- veal "the" chemistry and mineralogy of the entire Martian crust. But it is reason- able to expect that a careful reading of the results will indicate the major processes and products of crystallization and sedimentation on and perhaps below the surface. Venus Study of the surface of Venus may be more difficult and less rewarding than that of Mars. Recent evidence indicates surface temperatures between 600 and 700Â°K, and atmospheric pressure as high as 100 atmospheres. The combination of these properties renders it exceedingly difficult to devise instruments capable of operating on the surface for more than a short time. The dense atmosphere may prevent most imaging systems from observing the surface unless they are equipped with their own sources of illumination. From orbit one must contend both with the effects of the dense atmosphere and with extensive cloud cover. Despite these drawbacks the nature of the surface of Venus stimulates our scien- tific curiosity. What would happen to the surface of the Earth if it were raised to 700Â°K and blanketed by an atmosphere one hundred times as dense? Why should Venus, which is a near-twin of the Earth in size, mass, and location within the solar system have evolved so differently? Is Venus at a more primitive stage of evolution than the Earth? Ground-based radio and radar studies are the source of virtually all our knowl- edge of the surface of Venus. Radar ranging experiments when combined with orbital analysis yield a value for the radius of approximately 6050 km. The planet rotates in a retrograde manner with a period of 243.1 Earth days, with the result that the length of the solar day on Venus is about 117 Earth days and the Sun rises in the West and sets in the East. The 243.1-day rotation period implies capture of Venus' rotation by the Earth. Thus a stick placed on Venus to point to the Earth when Venus is closest would do so again at each succeeding inferior conjunction (i.e., at intervals of 19 months). This means that, as seen from Earth, Venus executes precisely four axial rotations between close approaches to our planet. The dynamical properties have something to say concerning the interior of Venus and seem to point to the existence of a liquid core. Radar distance measurements are capable of yielding information concerning the variation of surface height along a region close to the equator of Venus. The present measurements suggest that the surface of Venus is level to Â« Â± 2 km over horizontal scales of the order of 100 km. This is in contrast to the Earth and Mars where elevation differences of ~ 15 km are found between the tops of mountains and nearby valley (or ocean) floors. Mercury, the Moon, and Mars have similar radar reflectivity, but Venus reflects about twice as well. Thus the surface of Venus is more compact than that of the Moon, if the two are made of similar material. Comparison of the radar backscatter- ing properties of Venus and the Moon implies that Venus has a smoother and more gently undulating surface. Largely as a result of erosional processes, the surface of the Moon is smooth on a centimeter scale. One might therefore conclude that there is an extremely effective erosional mechanism on Venus.
-31- From the foregoing one might expect that the surface of Venus is extremely mon- otonous. Yet radar studies have defined about ten regions that are remarkably rough compared with their environs. These regions are several hundred kilometers across, close to the present level of resolution. There appears to be a clustering of these regions close to the equator, though observational selection cannot be ruled out as the cause. In the case of at least one feature the shape seems to resemble that of a large circular crater, suggesting an impact origin or possibly a major internal convective process. Radar maps of Venus made from Earth suffer from the inability to discriminate between the hemispheres unless multi-antenna systems are employed. Subject to this ambiguity, a small part of the disk of Venus has been mapped with a resolution of 50 km or less. By improving the capability of terrestrial radar systems, this limit might be reduced to a few kilometers, although still subject to the above ambiguity, which can be removed by an interferometer system. Scientific Priorities for Venus A step-by-step approach to the examination of the surface of Venus, as advo- cated for Mars, may not be possible within the present fiscal constraints. This suggests the advisability of making use of partial opportunities provided by other scientific missions. To be specific, visual imaging of Venus to detect discontinu- ities in the cloud cover seems a reasonable requirement for a Venus-Mercury fly-by, though it might not be accorded highest priority in a mission devoted entirely to Venus. Similarly, chemical analysis of the Venus surface at the termination of the descent phase of an atmospheric probe would be tremendously interesting even if not preceded by adequate reconnaissance missions. For study of the Venus surface, we accord highest priority to an orbital map- ping mission. Some form of radar imaging on a scale of about 1-km resolution, together with temperature measurements (albeit on a much coarser scale of about 100 km) may represent the two most worthwhile endeavors. The temperature mapping must be conducted at a wavelength that can completely penetrate the atmosphere with little or no absorption. This means a wavelength greater than 10 cm; the 12.6-cm communications wavelength would seem an excellent choice. The communica- tions antenna might be oriented toward the surface near periapsis. If the satel- lite is at an altitude of 1000 to 2000 km and the antenna diameter is about 2 m the required resolution could be achieved. The precession of periapsis would permit determination of temperature as a function of latitude and insolation. Large local temperature variations might indicate the existence of internal con- vection cells or volcanic areas. These, in turn, may be related to the rough regions. A finding of temperature variation with latitude would discredit that model which depends upon internal heat production, and would also provide a valu- able guide in the construction of models for the atmospheric circulation system for the planet. The radiometer developed for this measurement may be capable of a relative accuracy (day-to-day) of = 1 percent though the absolute accuracy may be poorer (e.g., 10 percent). Surface structure can be recognized by radar imaging. We propose that radar maps with resolution of about 1 km be prepared for parts of the surface. Perhaps this can be accomplished by using a bistatic radar system (e.g., illumination of Venus from the Earth). Relatively low weight requirements favor this experiment. However, it must be recognized that the feasibility of this technique remains to be demonstrated. An alternate spacecraft experiment would be a coherent side- looking radar. Unfortunately, it would probably require a large part of the total payload of a Mariner-class vehicle. Fly-by or probe data can provide some information on the Venus surface. The
-32- distribution of temperatures detectable by multiple probes will give indications of the extent to which the planet's high temperature is a result of internal or of ex- ternal heating. The atmospheric composition data may indicate the degree of degas- sing of its interior by past volcanic activity. Mercury Mercury is the smallest and most dense of the terrestrial planets. It is also the closest to the Sun. Albedo and radar reflection suggest a surface resembling the lunar maria. Only indistinct surface markings are visible from Earth. The great range in temperature between subsolar and midnight positions, the large solar radiation flux, and the probable lack of an atmosphere must all influence the nature of the surface in a major way. Yet the remoteness of Mercury from Earth has served to lessen its attractiveness to those interested in planetary exploration. Recently, however, an upsurge in interest has been sparked by the realization that in 1973 or 1975 a spacecraft launched by a modest-sized vehicle can take advantage of the Venus gravitational field to gain acceleration and thereby enter a trajectory that will send it past Mercury. Venus-Mercury Fly-by The Venus swing-by mission proposed for 1973 provides the first opportunity to examine Mercury. As a prime objective, we recommend photographing the planet with a resolution of about 2 km. Similar photography of Venus during the fly-by portion of the orbit may reveal cloud patterns indicative of the atmospheric circulation sys- tem. Additional experiments for the Mercury encounter should include a magnetometer to determine if Mercury has a magnetic field and some form of emission line photome- ter to determine if Mercury has an atmosphere. If the trajectory permits occulta- tion, a second test of the existence of an atmosphere can be achieved from the S-band radio links. If imagery of Mercury at 2-km resolution is indeed obtainable from a Pioneer- class Venus-Mercury fly-by, it becomes a most significant experiment from the view- point of planetary surfaces. The Sun may have a profound effect on its nearest neighbor so that an unusual balance of internal and external activity is evidenced by surface topography. The best way to underscore the scientific value of a limited imagery mission for Mercury is to recall the major changes in our thinking about Mars produced by the 4-km resolution Mariner 4 pictures. The short lead time, the low cost of a Pioneer mission, and the value to our national prestige of a planetary first are strong arguments supporting the basic scientific value of the mission. Jupiter and Its Moons The possibility of a "Grand Tour" in 1977-78 past the four large, low-density outer planets -- using their gravitational fields for assistance -- is an exciting prospect for it would permit the first complete reconnaissance of the major members of the solar system. The tour would provide much needed information on the outer planets which could be compared with our growing knowledge of the inner, more dense, terrestrial planets. The spacecraft would pass some satellites of the outer planets, objects of con- siderable interest in their own right. Jupiter, the most massive planet, has three satellites that are larger than our Moon and one that is larger than Mercury. They might be considered semiplanetary bodies whose properties bear on the general problem of the origin of the solar system. The surface features of these satellites provide
-33- clues to the major events in their history. In all probability they will be cratered by meteorite impacts, but they may also provide some hints of the events associated with their separation from the gaseous material of their parent body, the extent of their internal activity, and the evolution of their own atmospheres. The Grand Tour will not have as its main focus an examination of the satellites, but insofar as trajectory and equipment limitations permit, any data gathered on satellite surface features will be of substantial interest. Comets and Asteroids Comets and asteroids may be the oldest members of the solar system; that is, they may represent those bodies least changed by subsequent events since their creation. It is possible that the surfaces of the Moon, Mars, Mercury, and Venus contain mate- rial not much older than the oldest on Earth (3.5 billion years). In this event, our efforts to study the origin and evolution of the solar system will be circumscribed unless we can examine cometary and asteroidal material. At the present time the im- mense difficulty of studying a comet or asteroid at close hand makes this an unprofit- able endeavor. The situation may change during the course of the next decade, how- ever; in that event, the question should be reopened. Principal Recommendations 1. Orbital Mapping of Mars The acquisition of orbital images of a significant portion of the Martian surface is among the highest immediate priorities of the planetary exploration program. The wide range of information to be derived from orbital images has bearing on almost all other aspects of Martian exploration. The success ratio of the Lunar Orbiter missions and the extent to which their data have modified both our view of the Moon's basic processes and the detail of much of our present lunar planning speaks eloquently for the need of similar data for Mars. We strongly recommend Mariner-class orbital map- ping missions of Mars in 1971 as justified by the large scientific return, by the early need for planning data, and on grounds that 1971 is an optimal year for orbit- ing of Mars. 2. Mariner Mapping of Venus By 1975 it is hoped that a Mariner-class mission to orbit Venus can be carried out. The prime objectives of this mission are mapping the thermal emission of the surface and radar imaging. The former of these objectives will require a radiometer operating at a wavelength of 10 cm, a low altitude periapsis, and a high inclina- tion orbit. A resolution of about 100 km is desired. Radar imaging could be carried out bistatically or by using side-looking radar, and should strive for a resolution of 1 km or less. By 1975 a substantial part of the surface may have been mapped at a resolution of about 2 km with Earth-based instruments. 3. Fly-by Mapping of Mercury If images of Mercury with 2-km resolution are indeed obtainable from a Pioneer- class Venus-Mercury fly-by this becomes a most significant experiment from the view- point of planetary surfaces and is recommended. The following table indicates those flights and instrument packages desirable for a meaningful planetary surface exploration program. Instruments are intentionally not described in strict engineering terms but rather in terms of desirable results. The results depend both on instrument design and flight configuration.
TABLE 2 Planet Mars Mars Desirable Missions for Planetary Surface Studies Flight Mode Year No. Payload Mariner Fly-by 1969 Mariner Orbiter 1971 Mars Mars Small Orbiter Planetary Orbiter Titan- Orbiter- Centaur Lander 1971 1973 Jupiter Pioneer Fly-by Venus- Mercury Venus Pioneer Fly-by 1972 or 1973 1973 Mariner Orbiter 1975 Visual imaging system producing several 100-m resolution photographs. IR radi- ometer designed to produce thermal maps of the surface. Visual imaging system producing several 100-m (0.2-km) resolution photographs of a large part of the southern hemi- sphere. One flight to obtain black-and- white images with maximum morphologic detail taken at low Sun angles from a high inclination orbit; the second, same resolution at several band passes that will produce color images taken at very high Sun angles from low inclination orbit. System should be designed to have a lifetime of a year. IR radiometer designed to produce ther- mal maps of the surface. A back-up system for the 1971 Mariner orbiter. Visual imaging system with =l-km resolution. Orbiter: Visual imaging system similar to that carried by the 1971 orbiter. IR radiometer designed to produce thermal maps of the surface. Lander: Panoramic photographic system and high resolution microscopic photo- graphic system (0.01-mm res.); x-ray fluorescence and diffraction; alpha backscatter. Photography of planet and satellites at =10-km resolution. Visual photography of part of Mercury with 2-km resolution and of Venus at =l-km resolution. Radar imaging of the surface with 1-km resolution effected either by bistatic or side-looking systems. Thermal map- ping of the surface in the 10-cm wave length range. -34-