Click for next page ( 51


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 50
Chapter 5 PLANETOLOGY INTRODUCTION The long-range aim of planetological studies is to obtain basic knowledge concerning the nature of planetary bodies of various masses and chemical compositions, in order to infer the pro- cesses that determine their origin and evolution and to under- stand why different planets evolve in different ways. The most important development in this field in recent years has been the possibility of directly studying planetary bodies other than the earth, particularly the moon. The Mariner and Viking missions to Mars are extending our understanding to that plan- et, and future missions to Mercury and the major planets will also be important in this context. Venus, so similar to the earth in mass and radius, never- theless appears to have evolved quite differently. Thus its study promises to reveal new insights into planetary evolution. Because of its opaque atmosphere and the absence of satellites, our present knowledge of Venus is not comparable even with that of Mars; consequently there are numerous highly valuable plan- etological measurements to be made. Ideally, these would in- clude such things as the chemical composition and mineralogy of the surface'materials, the heat flux coming from the inte- rior, the presence or absence of an iron core, and the varia- tion of elastic-wave velocity with depth and density. The difficulties of making many of these necessary measurements under the conditions prevailing on Venus are severe; neverthe- less, a program of measurements on the scale proposed for the Planetary Explorers will permit some highly significant mea- surements to be made. These include measurements made from orbiters, probes, and landers. For example, surface eleva- tions can be measured with a radar altimeter on an orbiter, and some information regarding the distribution of mass in the planet can be obtained from perturbations of the orbit of the satellite. The internally generated magnetic field can be measured as a function of altitude, latitude, and lon- In addition, there are gitude from multiprobe missions. 50

OCR for page 50
51 measurements that can made from a probe which important will provide data needed the planning of lander missions. E~amples of these are miniature seismometers to be carried on probes to provide measurements of the seismic noise background subsequent to their impact and measurements of the near-surface opacity and illumination, which will be necessary in the plan- ning of lander missions which will involve television imaging. While it is questionable whether television should be carried on Explorer landers, it may prove useful in the next stage of exploration of Venus, and its feasibility should be evaluated. There are some important experiments which are sufficient- ly well defined and simple to be carried out by a lander on the surface of Venus within the framework of the Planetary E~plorer concept. These include passive and a<;:tiveseismic measurements and measurements of elemental composition using natural and in- duced gamma radiation. These are suggested for one or more lander missions following the early probe missions which should, in addition to their intrinsic importance, serve to define the problems of making scientific measurements in the deep atmo- sphere of Venus. These lander experiments will be discussed in more detail below. Beyond these proposed early experiments to be carried out on the surface of Venus, someday it will be desirable to carry out further geophysical and geochemical measurements of a more difficult nature. These will require experience in performing fairly complex manipulations on a remote planet. Similar re- quirements will arise in the study of Mars and future unmanned exploration of the moon, e.g., by an unmanned lunar roving vehicle. It seems wise to acquire such experience and assess the reliability of these techniques on a body such as the moon or Mars, where the physical environment is less rigorous, and where, in the case of the moon, the relevant quantities have already been studied at some sites, as a result of a program of manned explorations. Although it is difficult to see too far into the future, it does not seem that this more advanced stage of exploration of Venus should, at this time, be consi- dered in the framework of relatively inexpensive Planetary Explorers. This is particularly true because in many cases it will prove most to out a number of ent measurements on the same material at the same ,rather than one by one on a of missions to different sites. In order to preserve the the Planetary Explorer concept it would be to emphasize the highly able data obtainable on the more simple missions and leave the next stage of explora.tion of the surface and interior Venus open for future eva.luation.

OCR for page 50
52 ANALYSIS OF SURFACE COMPOSITION Knowledge of the .elemental composition of the surface of a planet is fundamental to an understanding of the processes of geochemical differentiation which the planet has undergone, and these processes are strongly coupled with its thermal and tectonic history. Consequently ,.measurements of composition constitute a first-order problem. Ideally, such measurements should be made at a number of places on the planetary surface. However, as was the case for the Surveyor analyses of the moon, one or a small number of such analyses constitute a major ad- vance in knowledge. In accordance with the point of view expressed in the in- troductionto this Chapter, high priority is assigned only to those experiments that do not require complex manipulations. Consequently, it would be most desirable if a device could be developed which could be placed on the surface of Venus and, within a p.eriod of a few hours, would provide analytical data for at least the most abundant elements. Such measurements might be combined with the knowledge of the concentrations of condensible and permanent gases in the lower atmosphere to per- mit some understanding of the mineralogy of the surface, in addition to the directly measured elemental composition. Previous experience with unmanned chemical analysis of a planetary surface is limited to alpha backscatter measurements made by Surveyor on the moon. It is also reasonable to sup- pose that a simple device adequate for the moon or Mars could be based on x-ray fluorescence. However, the short range of alpha particles and of x radiation, combined with the high at- mospheric density at the surface of Venus probably preclude instruments of this kind. Their use would require detaching a sampl~ from the surface, transporting it to an air lock of some kind, and then transferring it to a low-pressure or vacuum environment within a pressure vessel. Such manipulations are in conflict \vfth the criteria proposed for these early studies. However, it does se.em likely that a device based on the much more penetrating nuclear gamma radiation could provide A simple sodium iodide gamma-ray spectrome- the data desired. ter placed on the surface would permit measurement of thena- An tural radioactivity of potassium, uranium, and thorium. operating lifetime of several hours should permit measurement of these elements even at the low concentrations found in chon- dritic meteorites. These elements are especially indicative of chondritic of processes that have fractionated material

OCR for page 50
53 composition to form rocks such as basalts and granites, con- taining characteristic enrichments of these elements. The gamma radi~tion will be sufficiently penetrating to be trans- mitted through the wall of the pressure vessel and through se- veral inches of thermal insulation. Therefore, it would prob~- bly be feasible to use an ordinary thallium-activated NaI crys- tal, although it is possible that some other scintillator, such as sodium-~ctivated Csl, would be preferable. The value of the data obtained would be greatly enhanced if, inste~d of using only the natural radioactivity of the planetary surface, a neutron source were used to irradiate the surface and thereby produce neutron-capture gamma radia- tion characteristic of particular elements. Devices of this general type have been developed at the Goddard Space Flight Center using as a source the intense flux of neutrons from the spontaneous fission of microgram quantities of 252Cf. With such a source, combined with the gamma-ray scintillation spec- trometer described above, such elements as Si, AI, Mg, Fe, Cu, Mn, and Ti could probably be determined in an operating time of 1 h. These neutron-capture gamma rays are more penetr~ting than those of the natur~l gamma emitters and consequently could penetrate considerably more thermal insulating material sur- rounding the detector. The neutron source could be operated at the high surface temperature. There are undoubtedly many design problems which would have to be overcome in modifying the existing instruments for operation on the surface of Venus. These appear to be sur- mountable. Studies would be necessary regarding such ques- tions as the need for including material for the moderation of the high-energy neutrons produced by the source or whether the surface material itself would provide adequate moderation. The necessary quantity of moderator and of shielding m~terial to isolate the detector from the source would be important factors in determining the weight of the instrument. It seems possible that the weight of the entire instrument, including these materials, could be held down to approximately 15 lb, although a much more detailed design study would be necessary to determine a reliable value for the weight. Inclusion of the neutron source would probably preclude the simultaneous measurement of U and Th, because of inter- ferences of the induced gamma radiation with the natural radia- tion, although measurement of K might still. be possible. In order to measure U and Th, it might prove feasible to jettison the neutron source following completion of the other measure- ments. At the risk of complicating the experiment, considera-

OCR for page 50
54 tion should be given to the possibility of moving the source to two or more different distances (~l m) away from the detec- tor. If the instrument were made of hydrogen-free materials (e.g., if necessary, deuterated), this could permit measure- ment of the hydrogen concentration of the surface as well as of the surface density. The question of whether this surface analysis could be carried on the same lander as a seismic experiment must await a more detailed design study. It does not seem out of the question that this may be possible, provided that the weight of explosive used for an active seismic experiment is appro- priately limited. SEISMOLOGY Seismology represents the most potentially useful method of learning about the bulk properties of the interior of Venus. Information may be obtained from a variety of frequency bands, from 0.01 to 100 Hz, and for natural or artificial sources. This includes time-of-trave1 measurements for the rays and the dispersion characteristics of surface waveguide modes. The most important type of result is a global-scale determination-- involving several source-receiver distances in the 15-180° range. Clearly, such a program is possible only if enough sources of adequate magnItude are available. The quantitative meaning of "enough" or "adequate" depends critically on the shape and amplitude of the ambient ground-noise spectrum. In a first visit, any experiment that depends strongly on assumptions about the noise spectrum would be risky. The existence of natural seismic sources is of great intrinsic interest, in ad- dition to their importance as signal sources. Terrestrial earthquakes are known to be caused by the interactions of large, slowly advected plates of the lithosphere. Their frequency is a rough measure of the rate of thermally driven motion. A de- termination of seismicity on another planet, by comparison with the earth, provides a comparative measure of its thermal evolu- tion rate. For Venus, a lack of seismicity would be quite im- portant, since the vo1ume-to-surface ratio of Venus is so like that of the earth. The presence of seismic activity would be equally interesting and would provide sources for determining the interior structure.

OCR for page 50
55 TABLE 1 Number Qf Useful Seismic Events per 24 h Seismicityb ----------------------. Very 5 1 Noise Leve1a Small 1/10 --- 5-25 0 <1 Earth noise Earth si- luck tuation 10-500 m]J 1-5 required 0 'V1 5-25 Many 'V1-5 m]J <: Many <1 5-25 Many 'VO.1 m]J (Indeter- (Lunar minate) level) aNoise level is in millimicrons peak-peak in the band 0.3-5 Hz. bSeismicity is the number of quakes per 24 h compared with earth, assuming the same magnitude/number law. The situation may be summarized by the matrix of Table 1, in which the possible situations with respect to seismicity and noise level are used as coordinates. An educated guess of the noise level of Venus would put it roughly in the 1-5 m]J range; some natural sources could then be seen even if the seismicity were one tenth that on earth. An active source--from a bomb probe--is necessary to pro- vide any sensing of the interior, if the seismicity is too low. In any event, such a controlled source would be highly desir- able as a means of calibrating the signals from natural sources. It is highly desirable that the bomb probe be as far away from the receiver as signa1-to-noise considerations permit. This, of course, makes it desirable that as large as possible a coupling of energy into seismic waves be achieved. We, therefore, propose a three-stage strategy for the seismic exploration of Venus, based on the use of Planetary Explorer missions. 1. One of the early atmospheric probes should survive impact long enough (a few minutes) to transmit information on the magnitude of the seismic noise. This could be in the form of pressure data or one axis of accelerometer data. The noise

OCR for page 50
56 signals of interest at 5 Hz (~he band 1-10 Hz) would have: (rms) acceleration, 250 x 10- cm sec-2; displacement, 10-6 cm; pressure, 10-5 bar. Only mean rms power in some band would be required. This information would make it possible to choose the best distance for the bomb in stage 2. Unusually high noise level would reduce the prospective worth of the subse- quent missions. We hope that the entry-probe sensors can be designed with the aim of including information on turbu1ent- wind fluctuations as a function of height; this can also be used to estimate the magnitude of the seismic noise. 2. A Planetary Explorer seismology mission from a lander should be undertaken on the basis (if possible) of the noise data from the first probe. A passive three-axis seismo-acce1- erometer would go in the main lander probe. A bomb probe at a distance of 100-200 km would be used as an active source of signal. For passive operation and detection of natural seis- micity, the lifetime should be at least one day, but preferably longer. 3. Further exploration should be based on the knowledge gained at stage 2, and on engineering advances to that time. In particular, we foresee that, in the best of circumstances, future passive systems should have surface lifetimes of several months. Referring to Table 1, we emphasize our feeling that a fair strategy requires the seismic program to be given reduced priority if unfavorable conditions are found. This would occur after stage 1, if very high noise levels are found, or after stage 2, if no useful signals are obtained. LOW-FREQUENCY SEISMOLOGY If a stable low-noise vertical accelerometer based on the quartz fiber or LaCoste suspensicn can be operated on the sur- face of Venus for about 2 solar days, certain important new information on the bulk properciec of the planet may be col- lected. The e~:istence of a liquid core as large as the earth's would be detec~ab1e by a measurement of the solar gravity tide for 2 solar days. an undertaking is technologically out Such of the question at this time--because the appropriate instru- ment is not ypt ready for adaptation to planetary use and ttl~ long-lifetime technology is not available. Nevertheless, if

OCR for page 50
57 seismic methods are ineffective for reasons already discussed, the surface measurement of solar tides would be the best way to answer questions about a core. In addition, this instru- ment with response at periods near 1 cycle/h would be a detec- tor of free oscillations from large quakes. RADAR AND THERMAL MAPPING OF THE SURFACE OF VENUS Radar experiments are capable of precise measurements of range and velocity between an orbiting vehicle and a remote, passive planetary surface. Determination is also possible of surface scattering from which bulk electrical properties, surface structures at wavelength and larger size, and mean surface slopes may be inferred. Both monostatic- and bistatic-radar experiments specifically directed toward studies of surface electromagnetic properties and structure may be carried out from vehicles orbiting a planet. Radar measurements may be used to study Venus in a number of ways. The surface topography may be measured by direct al- timetry from an orbiter. Radar observations from earth may also determine surface topography. But, with the exception of a relatively narrow band of latitudes near the equator, the topographic accuracy available from these observations does not approach that given by relatively simple Venus-orbiting systems. If polar orbits are flown, altimetry from an orbiter of Venus can determine the planetary shape (flattening); from this the gravitational figure may be inferred. Bistatic radars are those employing a well-separated transmitter and receiver. Bistatic-radar observations may be carried out using transmissions between a ground station and an orbiting radar probe. A radar transmitter located on the earth is used to illuminate Venus with radio-frequency energy. Reflections from the planet's surface are detected by an on- board receiver; this permits observation of the forward-scat- tered energy. The reflected signals are distinguished from the much stronger, directly propagating wave, by a subs'tantia1 Doppler shift. The strength and polarization of the echo are related to the planet's surface material; the spectral proper- ties of the echo are related to surface roughness. Such ob- servations are extremely valuable in the construction of sur- face models and cannot be carried out from the ground alone.

OCR for page 50
58 More elaborate, orbiting, monostatic-radar systems can map the radar-scattering properties of the surface over a wide area and would yield maps similar in appearance and usefulness to photographic maps of the same region--if these were possible for Venus. Maps of radar-scattering properties may also be used to yield statistical information on local surface slopes, wavelength-sized roughness, and the bulk electrical properties of local regions of the surface. Here, however, great weight and complexity are required for an orbiting system with reso- lution comparable to that available from terrestrial, ground- based radars. Such imaging of Venus is directly competitive with ground-based observations and would provide similar data. These experiments would provide somewhat more coverage of the planet than ground-based radars can do in the next decade, but it is not yet clear whether the high cost of the additional information could be justified. Earth-Based Radar Studies Virtually all our present knowledge of the radius, rotation, and surface of Venus has been obtained using ground-based ra- dars. The general picture that emerges is of a planetary sur- face that is considerably smoother and denser than that of the moon. The surface dielectric constant of Venus as determined by both radiometric and radar techniques appears to lie between 4 and 5, substantially higher than that for any other solar- system target except the earth. On a lateral scale of hundreds of kilometers in the equa- torial region, Venus exhibits very little topographic relief. Only one region is known that departs from the mean radius by as much as 2 km, in contrast to Mars where peak-to-valley vari- ations up to 15 km are observed. Using delay-Doppler tech- niques, maps showing local departures from the mean scattering law have been prepared. These have been obtained at wave- lengths of 3.8, 12.5, and 70 em, with a linear surface reso- lution varying from 100 to 500 km, and are in remarkably good agreement. Perhaps a dozen permanent scattering anomalies have been shown to exist, although the precise nature of these anomalies has not been established. That they arise from lo- cal variations in surface roughness at a scale of the observing wavelength is clear, but whether they represent young moun- tainous regions, flows of lava, large meteoritic impact cra- ters, dune fields, or simply debris is not known. In the next decade, more sensitive observatories will come into existence. We may reasonably expect radar images

OCR for page 50
59 of Venus from these with a linear surface resolution comparable to present ground-based astronomical photographs of the moon km). Topographic maps with a height resolution of 0.1 to (3-5 0.2 km will be prepared for the region within about 10° north and south latitude in the vicinity of the subradar point at in- ferior conjunction and for a complete equatorial band of dimi- nished width elsewhere. Orbiter Radar Altimeter A relatively simple radar altimetry experiment from an orbiter of Venus can complement the equatorial topographic information available from earth. From a planetary orbiter, measurements of vertical relief averaged over a lateral extent of some 5 to 50 km may be made with relatively simple low-power systems. It is likely that such an instrument would achieve sufficient precision to permit a meaningful comparison of the geometric and gravitational figure of the planet and would be adequate for the detection and characterization of continent-size trends in elevation and the characterization of morphological surface features comparable in size to the crater Copernicus on the moon. If carried out. from a polar orbit, the variations in surface radius. could be measured over most of the planet with accuracies consistent with those that will be obtained from the earth for the equatorial region of Venus. The connection of the two sets of measurements, one (from the earth) taken along an equatorial swath and the other (from polar orbit) along displaced polar tracks, should permit the construction of a highly accurate map of the planet's geometric shape, from which the figure and smaller-scale topography can be extracted. Measurement of variations in the echo amplitude with plane- tary location will provide a large-scale, normal-incidence re- flectivity map for the same position of the planet, which may also be tied to the ground-based observations. These maps would be of considerable importance in the interpretation of the radar maps which will be prepared from the ground in the same period. It is essential to obtain both total echo power and some measure of surface dispersion effects. Bistatic Radar Measurements from an Orbiter Bistatic radar measurements provide a direct measurement of large-scale surface slopes and the bulk electrical properties.

OCR for page 50
60 Experiments comparable to those carried out on the moon, using the telemetry transmissions from orbiting spacecraft, can be carried out at Venus, provided that earth-based transmitters, with reception on the spacecraft, are employed. Measurements of both total echo power and the spectral broadening imparted by surface roughness are required. To be most useful, two orthogonal polarizations should be received. Bistatic radar measurements provide a unique capability for the determination of surface slopes in the region removed from the equator of Venus. Slope determinations for the moon, which are in good agreement with those determined by more direct (and laborious) optical means, photoclinometry, and photogrammetry, have been carried out using bistatic-radar observations on Explorer 35. These methods are presently undergoing further development and are expected to become highly reliable within the next few years. Bistatic-radar observations of small- (wavelength-) scale scattering phenomena have also been carried out on the moon. While the interpretation of the observations is at pre- sent uncertain, it is clear that such observations, when com- bined with monostatic (ground-based) observations strictly constrain the range of possible surface models. Planetologi- cally, such observations provide unique data on both large- and small-scale surface structures and bulk electrical properties. These quantities are in turn intimately related to such ques- tions as the evolution and erosion of surface topography, mountain building, weathering, and surface density. RADIOMETRIC MEASUREMENTS OF THE SURFACE OF VENUS The surface temperature of a major portion of Venus can be measured by observation of the thermal emission from the sur- face at (microwave) wavelengths for which the atmosphere is optically thin. Such an experiment by itself is currently of some general interest to both planetology and the study of the lower atmosphere. It would be possible to obtain thermal maps from a properly designed radar apparatus with only a small extra effort. The wavelength requirement that the atmosphere be opti- cally thin is compatible with the radar and implies frequen- cies lower than about 5 GHz. The temperature measurements, however; require a more elaborate antenna system than that Assuming a IO-cm wavelength and needed for the simple radar.

OCR for page 50
61 a 5-ft parabolic antenna, the beamwidth would be 4.50 and pro- vide a planetary resolution of 1/13 of the spacecraft altitude. So large an antenna may not be justified for the radar measure- ments alone and would have to be considered as the cost of in- cluding the thermal measurements. Relative brightness tempera- ture measurements would have an accuracy of 12 K, based on an absolute accuracy of 11 K. If a spinning antenna system is em- ployed, the experiment will be limited in integration time by the motion of the antenna beam. Hence it is desirable to mini- mize the spin rate. Rates as low as five revolutions per min- ute can be achieved and are suitable for this kind of observa- tion. Radiometric measurements do not give actual surface tem- peratures unless the emissivity is known. This quantity can be inferred from the radar measurements, which therefore give valuable support to the radiometer. Current earth-based observations indicate that the broad- scale variation in the surface temperature is less than 20 K. Improvements in these observations can be expected during the 1970's. If such improvements are realized, and if in situ mea- surements confirm a small temperature variation over the plan- et, then the need for the microwave radiometric experiment is appreciably less and should be re-evaluated. The temperature of the lower atmosphere could be measured by using a wavelength less than 6 cm. The height range probed will depend on the wavelength chosen and could be estimated from model atmosphere computations based on the direct entry probe measurements. However, such an experiment could not make such convenient use of the radar receiver. More stringent con- straints on the mechanical properties of the antenna would also be required. Conclusions Given our present knowledge of Venus we believe that the de- termination of the topography and geometric figure of Venus, measured by a radar altimeter, is the most significant radar experiment concerning the solid planet which can be carried out from orbit. We also believe that bistatic-radar experi- ments) conjunction '\<1i ground-based observations, can pro- th vide a significant insight into the details of the surface structure and electromagnetic properties of Venus. Microwave thermal mapping of the planet from orbit offers a potentially powerful tool for high-resolution (10-40 km) temperature maps

OCR for page 50
62 of the surface... If such questions develop in the future, or if such measurements may be carried out easily in conjunction with other radio-frequency experiments, then they should be given sgrious consideration. In view of the rapidly improving of radar observatories capabilities on the earth to image Ve- nus, high-resolution, oblique, monostatic radar mapping of the planet from orbit seems relatively less important at the pres- ent time. GRAVITATIONAL FIELD The coefficients in the spherical-harmonic expansion .of the gravitational field may, in principle, be determined by anal- ysis of the perturbations of orbits of planetary orbiters. From a finite number of orbits and spacecraft, least-square fitt.ing to a. truncated set of low-order coefficients may be undertaken. In addition, local perturbations in orbiter ac- celeration may be determined directly from residuals in the Doppler tracking data. From this method comes a series of profiles of local variations in gravity. This information summarizes the shorter wavelengths in the field, which the low-order coefficients may not describe very well. The pos- sibility that useful numbers can come out of these undertak- ings depends on the orbits being quite low: fields of order n decay radially as r-n-l; and distant orbits have longer pe- riods, hence fewer orbits, as input data. The importance of the gravity coefficients may be summa- rized as follows: 1. The J2 coefficient reflects the distribution of mass in the rotational equatorial bulge. It dominates on the earth. with a value of approximately 3 x 10-3. The earth's high spin rate gives a precession, which, combined with J2, gives the moment of inertia. The low rate of rotation of Venus implies a J2 ~ 10-5 and a precessional period ~105 years. The latter appears unmeasurable, and the former is at the noise level defined by the other coefficients. 2. The full range of coefficients contains the informa- tion about the non-centro-symmetric mass moments--namely, those due to the departure of the planet from hydrostatic equilibrium. These tend to be of the order of 10-5 at low order and decay in magnitude with increasing order. The magnitude and decay rate of these coefficients are the primary outcome of the work and

OCR for page 50
63 differ according to the mechanical properties and history of the interior. The rotation of Venus appears to be coupled to the rela- tive orbital motion of the earth. This seems to require an asymmetrical equatorial bulge, which is worthy of special attention. 3. The local variations in gravity as determined by ac- celeration residuals in the tracking may be modeled in terms of near-surface masses and are of intrinsic interest along with other planetological imaging. Feasibility A good gravitational experiment would require orbiting tran- sponders in low «300 km), near-circular orbits, at several inclinations. Orbits with large apoapsis permit only a de- graded look at the gravitational field: it should be possi- ble to infer the second-order coefficients but not the spec- trum. Surface-mass concentrations corresponding to a horizon- tal scale of 400 km or more and a surface gravity variation of 100-1000 mg would be detectable during near-periapsis track- ing. This level of gravitational experiment is useful and in- teresting because it does not impact the spacecraft design. Too frequent orbital changes would degrade this experiment. Because of the great value of low, near-circular orbits, careful attention should be given to the possibility of using atmospheric drag to lower the apoapsis. The low periapsis re- quired for this purpose is useful for aeronomical studies. Once the apoapsis is low enough, the periapsis can be raised by a short burst of propulsion. MAGNETIC FIELDS Magnetic-field measurements are readily made from the orbiter, the bus, the probes, or the lander. We therefore discuss what of Venus. such measurements would reveal about the interior The Mariner 5 and Venera 4 observations enable an upper limit to be placed on the internally produced field of Venus of about 0.5 x 10-3 to 1 x 10-3 that of the earth. Magnetometer observations inside the anemopause are im- Some models of the portant for two quite distinct reasons.

OCR for page 50
64 anemopause structure and mechanisms require an ionospheric magnetic field, and some exclude it. The extent to which the interplanetary magnetic field can diffuse into the ionosphere, the possibility that ionospheric gas can be caught up in the solar-wind flow because of the interchange instability, and the possibility of field-line reconnection across the anemo- pause all require study. Venus may also have a small magnetic field of its own. If so, its strength, its relation to the axis of rotation, and its spatial distribution need to be determined. Because there are no natural satellites, the moment of inertia factor of Venus is unknown: density and radius provide only an insecure basis for a model of its physical constitution. Thus the size of a possible iron core of Venus cannot be estimated. Also the theory of the geomagnetic dynamo is still in many respects unsatisfactory. It is probable that the dominance of the ro- tation on the core motions is essential to the generation of a field. Thus the very low rotation rate of Venus may be un- favorable to dynamo action, but any quantitative statement is lacking. The discovery at Venus of a small magnetic field of internal dynamo origin would, therefore, be of fundamental interest, both from the viewpoint of its physical constitution and also for the theory of planetary magnetic fields. Other magnetic fields of the order of 10-100y with inter- nal sources occur in the earth: steady fields from permanent magnetization of the crust and varying ones induced by varying external fields. At the high surface temperature of Venus, most minerals. will be near or above their Curie points, and magnetic anomalies may be much weaker than on earth. There is a possibility of strong magnetization remaining from a strong planetary field in the past, but it seems unlikely. The variations in conductivity, height, flow patterns, and other properties of the ionosphere should be fixed in position relative to the sun. Were the ionosphere sufficient- ly conducting and firmly fixed in place, the short-period (of the order of 1 min to 1 h) fluctuations in the solar wind would be screened out and not observed near the surface. If the ionosphere is such that short-period variations are pres- ent, they would induce electric currents in the upper parts of the planet, and these would contribute a measurable frac- tion of the field observed below the ionosphere. Although magnetometer observations near the surface pro- vide information on a variety of interior and exterior char- acteristics, conclusions will in most cases be very ambiguous unless the effects of interior and exterior ~ources can be

OCR for page 50
65 separated and unless some information can be obtained on the structure of these sources. This requires simultaneous obser- vations at more than one point, preferably enough observations to make a harmonic analysis as for the earth. In principle, probes similar to the miniprobes of themultiprobe mission could be used. Each would have a slow spin about the vertical, controlled by fins or grooves, and three samples per revolu- tion by an inclined single-axis magnetometer would determine a static field. The characterization of the sources is greatly improved if an adequately long vertical profile can be measured by each probe. A careful analysis should be made to determine how many probes are needed to obtain significant results. If four are enough, a serious effort should be made to include these magnetometers on the firs.t probe mission. It is likely that no sound analysis can be made until magnetic observations have been made down to 150 or 200 km elevation by the bus for the first probe mission, and that the more ambitious study is more appropriate for one of the later missions when it can be designed with better knowledge of the requirements and condi- tions. Magnetometers should also be given very serious considera- tion for the balloon missions because these will give both the simultaneous ~easurements at widely spaced positions and the observations of temporal variations needed to determine the conductivities of the ionosphere and surface layers as well as the extent to which the anemopause (ionopause) and ionosphere screen out the effects of changes in the interplanetary magne- tic field and in solar-wind pressure.