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Chapter 5 PLANETARY DYNAMICS AND INTERIORS Introduction This chapter discusses the orbital and rotational dynamics of the planets and their satellites(with the exception of the Earth-Moon system), asteroids and comets, and the composition and physical state of the interiors of these bodies. Of the three fundamen- tal scientific questions underlying solar system exploration, these matters pertain most strongly to that of the origin of the solar system: the locations and densities of the planets and other bodies provide the severest boundary conditions on the forma- tion of the planetary system. They also pertain to an understanding of the terrestrial environment in that models purporting to explain the mechanical and thermal history of the Earth must explain the history of the other terrestrial planets as well. Experiments that contribute to an understanding of planetary dynamics and interiors include: determination of orbits and rotations of natural bodies and of the orbits of artificial bodies; measurement of radii and topographic variations by radar and optical means; seismometry; surface chemistry by techniques such as alpha scattering and x-ray diffraction; and infrared and radio emission observations and magnetometry insofar as they indicate internal conductivities, temperatures, and density. Geological informa- tion obtained from pictures of surfaces is directly relevant to the problem of the evo- lution of the interiors. Spacecraft missions of interest to the study of planetary dynamics and interiors that appear practically feasible in the mid-1970's (listed in order of distance from the Sun) are: Mercury fly-by Venus orbiter Mars orbiter Mars lander Asteroid probe Short-period-comet probe Jupiter fly-by The plan of this chapter is to discuss each of the bodies to which these experiments pertain and then to take up the question of priorities. Mercury From the point of view of planetary dynamics, Mercury is perhaps the most important object in the soiar system. Being closest to the Sun, it is the most sensitive detector of departures from the laws proposed to account for planetary orbital motions. Its spin is also unusual, being coupled to its orbital motion in a three-halves resonance state. In view of its unusually high density, the interior of Mercury is also of special inter- est. A space-probe fly-by of Mercury could provide important information on both its dynamics and its interior. From photographs we may obtain the precise orientation of Mercury. Combined with similar pictures from later fly-bys, the vital knowledge of the direction of Mercury's spin axis and the fractional difference in its equatorial moments of inertia can be determined. Search for magnetic field strengths and determination of the electromagnetic radiation from Mercury's surface, as well as photographs of the sur- face, will provide important data on its interior, structure. The radius, mass, and hence, density, and the orbit can also be refined from the fly-by data. An orbiter is required to determine the detailed gravitational field of Mercury and a lander to study the in- terior by monitoring seismic activity. -35-
-36- Venus Venus is the planet most similar to Earth in size, density, and distance from the Sun. However, it differs significantly in having a much more massive atmosphere compos- ed mainly of carbon dioxide; a much higher surface temperature (700Â°K); a much slower, retrograde rotation (period, 243.1 Earth days); and no moon or oceans. The rotation period is within 0.1 day of a spin-orbit coupling to the Earth, which constrains the viscous decay time for a reasonable probability of capture into the coupling, and which constrains the moment-of-inertia difference, (B-A)/C, for stability. Of relevance to the planet's interior, Mariner 5 and Venus 4 did not detect a planetary magnetic field and ground-based radar has not been able to observe topographic height variations on a resolution scale of a few kilometers. The closeness of the mean density of Venus to that of the Earth (about 96 percent of the Earth's) compels us to presume, in the absence of evidence to the contrary, that the bulk chemical composition of the two planets is roughly the same. This similarity of composition would extend to the presence of the radioactive elements uranium, krypton, and thorium, which are believed to be the principal heat sources in the Earth. Hence Venus furnishes a valuable test of theories, both of the Earth's interior and of the origin of terrestrial planets: any acceptable theory must explain why Earth and Venus are different. Simple extrapolations from the Earth to Venus inevitably result in contradictions: a higher surface temperature indicates a hotter, weaker, and more rapidly creeping interior, which is consistent with the viscous decay time but inconsistent with the strength implied by the spin-orbit coupling. However, higher temperatures may imply not only lower viscosity but also greater dynamic imbalances --convection cells, for example --which can sustain departures from hydrostatic equilibrium. Yet if the in- terior were so active, we should also expect a far more irregular surface than is in- dicated by radar. We should also expect Venus to have a core and hence a magnetic field --unless the presence of a moon is necessary to provide precessional torques as a driving mechanism. The absence of a moon is the biggest problem of all in explaining the differences in the origin of the Earth and Venus. Plainly, more data about Venus are needed. It is also plain that chemical, seis- mic, or other measurements on the surface will be difficult if not impossible in the next decade. However, a Venus orbiter would yield valuable information. The deduct- ion that (B-A)/C exceeds 10 can be tested by measuring variations in the gravitational field, which are a function of the level of dynamic imbalance. A more sensitive mag- netometer measurement could lower the upper limit for an intrinsic magnetic field; such a limit would be relevant to discussion of the origin of magnetic fields driven by the action of precessional torques. Attempts to derive the planetary heat flow would be frustrated by the atmosphere. A measurement that appears feasible with larger, more sensitive ground-based radars is a determination of the physical libration of Venus, which is a measure of (B-A)/C. The latter, in conjunction with orbiter measurements of (B-A)/MR2? would yield the moment of inertia. This important parameter would be very difficult to determine by any other method. Determination of these gravitational and rotational irregularities would not only contribute greatly to the question of the origin of the terrestrial planets, but would also stimulate better ideas about mechanical and thermal models of planetary in- teriors, which in turn would contribute to a better understanding of the Earth's interior. Mars With respect to its interior and dynamical properties, Mars is of no greater intrin- sic interest than Venus or Mercury, perhaps less. However, most of the appropriate
-37- measurements are much more easily made for Mars; we address ourselves to the determin- ation of the dynamics and interior of Mars with special cognizance of this fact and of the unique biological and gelogical interest in the planet. In rough order of priority, the principal questions are: (a) What is the internal mass distribution of Mars? (b) Does Martian surface physiography provide evidence, in addition to the pattern of impact craters detected by Mariner 4 photography, of present or past internal activity? (Belts of mountain building or major volcanism of the sort found on Earth would be seen without difficulty with higher resolution imagery.) (c) What is the present level of tectonic activity in Mars? (This activity would be reflected in the seismicity.) (d) What is the heat flow at the surface? (e) Does Mars possess even a feeble magnetic dipole moment? (Mariner 4 set an upper limit of about 3 x 10 of the Earth's dipole moment.) Mars Orbiters The answers to most of the above questions can be obtained by spacecraft orbiting Mars at rather high inclination. Observation of the spacecraft as they are occulted by the planet will give us the true geometric flattening of Mars, and thus resolve the present discrepancy between dynamical and optical values, which are 0.005 and 0.010 to 0.015 respectively. From these size and shape measurements, a better value of the mean density than the present 4.0 + 0.15 can be derived. Sustained observation of the spacecraft's orbit will yield the differences in moments of inertia of the planet and these data, coupled with values for density and flattening, provide significant in- formation on the internal mass distribution. The physiographic information will be provided by a program of high resolution television photography, such as is commonly included in orbiter planning. A more sensitive magnetometer in a magnetically clean spacecraft is technically feasible; a feeble dipole moment detected by this instrument might be taken to indicate the pres- ence of a small fluid core in Mars. Planetary heat flow is a difficult question, one that is unlikely to be answered by an early generation of Mars landers because of the weight and complexity of equip- ment needed to implant heat sensors beneath the surface. A crude measurement from orbiters may be possible. It will depend on surface mapping both by infrared radiomet- ry and by radio emission in the cm-wavelength range. From these maps we might hope to establish values of the thermal gradients at various points on the planet. In general, this problem is difficult to approach and any method must be subject to close scrutiny. Mars Landers The most important single instrument for study of the interior is a seismometer landed on the surface. Analysis of seismic signals permits a good first-order determin- ation of (a) the compressional wave velocity versus depth, (b) the density versus depth, and (c) the existence and locations of earthquakes. The first two are of direct interest in the study of planetary interiors and lead to certain inferences about the composition of the interior. Their determination depends heavily on the existence of earthquake-like disturbances. The third permits a few fairly firm inferences about how the internal dynamics of Mars compare with the Earth. The seismicity of Mars is a most informative measure of the planet's internal regime. Questions of interest here are: Is Mars highly seismic, aseismic, or slightly so? If it is seismic, do the epicenters cluster along narrow structural belts? The relevance of these questions stems from recent discoveries about the relation of the Earth's seismicity to its large-scale internal motions. It has been found that major "thin" (50 to 100 km) shells of the Earth's surface, with continental dimensions, are moving
-38- relative to each other at rates of about 2 to 20 cm/yr. These motions are apparently driven by heat generated in the mantle and produce nearly all the Earth's seismicity, which occurs along the boundaries between shells. Information on the seismicity of Mars can indicate the extent to which it shows the same tectonic style. This is of direct relevance to the question of the history of Mars, the history of its volatiles, and the amount of heat generated in its interior. Conditions required for meaningful results from a seismic experiment on Mars are: (a) some 10 to 20 earthquakes from various parts of Mars during the recording interval, (b) a three-axis seismometer with a bandwidth from 0 to 1 Hz, and (c) ability to re- solve signals in the presence of wind-generated noise, and to isolate the instrument from the direct vibration caused by wind. Whether these conditions can be met, we can state on the basis of current know- ledge: (a) One can only make educated guesses about the frequency of seismic events on Mars. We estimate that a recording period of 3 months is appropriate for 10 to 20 events. Useful, but incomplete internal models can be obtained from fewer signals, and the total absence of seismicity over this period would, as indicated above, be signif- icant. (b) The weight of the three-axis system is a matter of some concern. While existing systems for lunar and Earth studies weigh 25 pounds or more, substantially lighter instruments can be constructed through effective utilization of modern methods of position sensing and signal treatment. The requirement of a three-axis, as opposed to a single axis, instrument is made to provide maximum information from the few recorded events anticipated. It would be far more difficult to unravel the structure of the planet from z-axis data alone, and the results would tend to be ambiguous. (c) The ability to resolve signals in the presence of wind-generated noise is an outstanding unsolved question. Theoretical and model study ought to be able to provide fairly good estimates of the noise produced by ground-coupling and direct instrument coupling from the wind. We may hope that orbital photography in an early Mars mission and numerical-theoretical work on the atmosphere will provide closer estimates of wind velocities near the surface. Jupiter Jupiter is obviously of great importance because of its huge mass and central location in the planetary system. A fly-by to Jupiter could be used to refine knowledge of its orbit and, perhaps, of its gravitational field, and could obtain information on its magnetosphere that might shed light on the nature of the planet's interior. However, ground-based radar observations of the Galilean satellites can provide more accurate orbital information and better determinations of Jupiter's gravitational field. The fly- by, on the other hand, can provide important data on Jupiter's heat balance and on the chemical composition of its atmosphere. Theoretical and laboratory studies based on such data offer great hope for improving our knowledge of Jupiter's interior. Comets and Asteroids While comets and asteroids are relatively low on priority lists, they are neverthe- less extremely interesting objects which potentially could provide significant insight into questions of solar system evolution. Next to eclipses of the Sun, comets are the celestial phenomena that have most aroused human curiosity since the earliest days of primitive man. We know relatively little about their structure and composition. Possibly they could give us information on the chemistry of objects from the outer fringes of the solar system, and hence indicate whether a chemical differentiation of the heavier elements occurred outward from the Sun. A probe passing through the coma of a comet and on into the tail might sample ions,
-39- neutral particles, electrons, and dust, using a mass spectrometer, an ion spectrometer, and various other sensing devices to measure composition. Interesting problems also exist relating to interactions between the solar wind and ions streaming away from comets. Most important would be an analysis of a cometary nucleus, but that is beyond our present capabilities. Asteroids are of particular interest because of their possible relation to meteor- ites, on which a vast amount of research has been performed in the last few years. How- ever, feasible experiments to achieve some such correlation are lacking. Probably the only practicable experiment would be one that determined mass from tracking data, and volume by imaging: from these the mass density could be calculated. In the case of both comets and asteroids the real need is for returned samples. To accomplish this seems beyond the realm of possibility in this decade. Related Investigations A number of scientific disciplines and techniques contribute to the solution of problems of planetary dynamics and interiors; only some of them are carried on within the framework of the space program. The optimal program of study would be one that attained the best balance of support among them. Some of the related areas of endeavor are: Celestial Mechanics. The main dynamical facts that must be explained by a satis- factory theory of the origin of the solar system, such as the distribution of angular momentum and mass, and various near-commensurabilities among orbits, have been known for some time. However, appreciable progress has been stimulated in recent years by new measurements such as the radar determination of the rotation of Mercury. These new data point to the various orbit-orbit and spin-orbit couplings as consequences of tidal friction effects. Some of the other near-commensurabilities may be the consequence of other types of energy transfer at an earlier stage in the history of the solar system. In recent years a better understanding has developed of hydromagnetic clouds and other phenomena, so that it is reasonable to expect improved dynamical models of solar system formation. Cosmochemistry. The dynamical model of solar system formation must provide the pressure and temperature environments necessary to account for the chemical differences between the Sun, the Earth's crust, the meteorites, and (soon to be learned) the Moon. These chemical restrictions are continually being refined. Geophysics. Most of our ideas about terrestrial planetary interiors are derived from studies of the Earth. Recent developments such as the evidence of sea-floor spreading have led to new notions about rheology and thermal conditions in the mantle, which in turn apply to the interiors of other planets. Stellar Evolution. A dominant effect in the origin of the planets may have been the over-luminous stage of formation of the Sun as it contracted onto the main sequence. The T Tauri stars appear to be stars in the final stage of contraction. Hence a satis- factory theory of solar system origin will explain many of the observed properties of T Tauri stars: their mass loss, enhanced radiation, nebulosity, etc; conversely, attainment of a better theory will be assisted by better observations of these stars. Planetary Astronomy. There is still much to be accomplished by ground-based astronomical techniques. Those that contribute most importantly to the study of planet- ary dynamics and interiors are radar measurements (which yield orbit and spin data as well as surface properties, including variations in topographic heights) and ir and thermal emission observations (which offer a possibility of obtaining the rate of heat flow from planetary interiors).
-40- Conclusions and Recommendations The object of highest priority, balancing intrinsic importance and feasibility, is Venus. Venus is the planet most like the Earth, yet it has significant differences which, in the first place, must be explained by any theory of solar system origin, and in the second place, make Venus a test body for theories of the mechanical and thermal regime of the Earth. Because of its excessive surface temperature and thick atmosphere, however, the only useful measurements that appear feasible are determinations of the variations of the gravitational field from satellite orbit perturbations. Such determ- inations, preferably using orbiters of differing inclination, should yield significant boundary conditions on the mechanical and thermal state of the interior. The only space vehicle instrument required is the tracking transponder, so the three Venus orbiter missions proposed (see Table 3) can be relatively simple Pioneers and small orbiters. The exploration of Mars is of second priority, since it is of somewhat smaller size and should have a lower level of internal activity. Variations of Mars' gravitation- al field can also be determined by orbiter perturbations. However, the greatest im- provement in knowledge of Mars' interior will come from a seismometer placed on its surface. A three-axis seismometer can be made to weigh as little as 5 pounds; aside from determining seismicity, it might improve knowledge of the variation of density with depth. The surface chemistry of Mars should also be pertinent to considerations of the interior. Eventually it is hoped that some estimate of the heat flow may be obtainable from a combination of ir and radio-wavelength radiometry in orbit. Mercury is of particular interest as the end member of the sequence of terrestrial planets; some estimate of its internal activity may be obtainable from Mariner-type surface photography. Although there is little prospect that Mercury has its own mag- netic field, a measurement thereof should be attempted on a fly-by. Certainly a look should be taken at Mercury, if only to suggest the most appropriate questions to ask. Jupiter is of the greatest importance of all, relative to the question of the origin of the solar system. However, it is of low priority in the framework of this report because of the great difficulty of obtaining significant new information concern- ing its interior with the small spacecraft available in the time period under consider- ation. In addition to providing magnetic field measurements and heat balance information, a Jupiter probe could also improve our knowledge of Jupiter's orbit and mass. It should also carry a micrometeorite detector so we can learn something about the density of small particles in the asteroid belt. Comets and asteroids are of interest because they may contain material that has been relatively undisturbed since the origin of the solar system. The structure of comets may also indicate the manner in which material originally condensed in the solar nebula. With the simple probes available in the early and mid-1970's, however, it is difficult to find a useful measurement to make, other than perhaps to derive the mass density of an asteroid. Of the related investigative techniques, it is most strongly urged that support be given to the construction and operation of a more powerful radar system to study the variations in planetary rotations, orbits, and topography. Only in this manner can we hope, for example, to obtain the moments of inertia of Venus and Mercury. The topograph- ic variations of any planet are needed for any effective interpretation of its gravita- tional field. Table 3 summarizes the relevance to interior/dynamical questions of various planet- ary missions suggested for the early 1970's. Most of these experiments will yield con- straints or boundary conditions which are necessary but not sufficient to understand the actual state and history of the planetary bodies. Not until landed spacecraft permit the placement of seismometers can more direct information be obtained.
TABLE 3 OPTIMAL MISSIONS FOR PLANETARY DYNAMICS AND INTERIORS Capabilities of Interest Year Body Type Mission to Dynamics & Interiors 1969 1970 1971 1972 1973 1973 1975 Mars Venus Mars Venus Fly-by Orbiter Orbiter Orbiter 1972 Jupiter Fly-by (2) Mars Orbiter Mercury Fly-by (via Venus) Venus Orbiter Surface photography IR radiometry Tracking transponder (3 months in orbit) Photography IR radiometry Tracking, 3 mos. in orbit Tracking, 3 mos. in orbit; different inclination from '70 Magnetometer Micrometeorite detector Tracking Tracking, 3 mos. in orbit; different inclination from '71 Surface photography & photometry Magnetometer IR radiometer Tracking, 3 mos. in orbit; different inclination from '70 and '72. 1975 Mars Orbiter-lander Lander: 3-axis seismometer Alpha-scattering or x-ray diffract ion-fluorescence Orbiter: Tracking, 3 mos. in orbit; different inclination from '73 IR and 10-cra radiometry -41-