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Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 68
Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 69
Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 70
Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 72
Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 73
Suggested Citation:"PLANETARY INTERIORS." National Research Council. 1969. Outer Solar System: A Program for Exploration, Report of a Study. Washington, DC: The National Academies Press. doi: 10.17226/18530.
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Page 74

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Chapter 5 PLANETARY INTERIORS The fundamental importance of studying planetary interiors de- rives from the need to understand the total physics of each plan- et and the origin of the entire solar system. There is consid- erable interest in the outer planets (i.e., Jupiter, Saturn, Uranus, and Neptune) because of their size, low mean density, internal sources of heat (in Jupiter and Saturn), and because 99 percent of the mass of the entire planetary system resides in these four bodies. In particular Jupiter, the largest planet, accounts for most of the mass and angular momentum of the solar system and may be the most accessible sample of material whose chemical composition is similar to that out of which the system was formed. The study of planetary in- teriors is a particularly difficult task because of the need for relying on indirect observational evidence and often in- adequate theories. It should be stressed also that our views on planetary interiors affect rival interpretations of what is observable on planetary surfaces and in atmospheres. Various observational and theoretical aspects of in- vestigations of the interiors of the outer planets are out- lined below. THEORETICAL QUESTIONS Equations of State A basic problem which limits our understanding of the plan- etary interiors is the lack of knowledge of the pertinent equations of state of matter and of the various transport coefficients. As far as the interiors of Jupiter and Saturn are concerned, there is a need for a better understanding of the behavior of hydrogen, especially in its molecular form at high pressures and in particular in that range where the predicted phase change between the molecular and metallic forms occurs. Present values of the transition pressure are uncertain by perhaps 50 percent, which implies a comparable uncertainty of the depth of the phase boundary between the 67

68 two layers. Furthermore, no detailed study has been made of the effect of helium either on the metallic or molecular form of hydrogen at high pressures and temperatures. The problem of solubility of these two elements in each other and the question whether there is a miscibility gap has not yet been answered. This knowledge is essential for estimating the depth and composition of various layers in the interiors of Jupiter and Saturn and for drawing conclusions about their state of aggregation. It also may play an important role in evaluating one of the "floating raft" models of the Red Spot of Jupiter. Very little is known about the chemical compo- sition of Uranus and Neptune, but there too the equation of state of hydrogen as well as of denser terrestrial type materials is likely to be of importance. Experimental Studies Parallel with theoretical studies of the hydrogen-helium system should be experiments leading to verification of some of the predictions. Present-day technology of high pressures has progressed sufficiently to promise important results even though the phase transition pressure in solid hydrogen may not yet be attainable in laboratories. Transport Coefficients The problem of transport coefficients such as viscosity, diffusion, and thermal conductivity, including a radiative component, in planetary interiors is an extremely difficult one. Even an approximate evaluation of these quantities for at least the hydrogen-helium system would be of great value. The recent progress in the theory of dense fluids may be of help here. There should also be some consideration of the role of impurities. Such information is essential for bracketing the probable values of the heat transport and for evaluating the efficiency and kinetics of convective phenomena. The latter play a paramount role in determining the magnetic fields of the planets, their heat budgets, and in many instances even the motion and configuration of visible surface structures. Internal Energy Source Both Jupiter and Saturn are believed to emit more energy than they receive from the sun. Presumably this can be accounted

69 for by the gravitational self-energy and a progressive shrink- age of these planets. A detailed theoretical investigation of this process in terms of the best available equations of state is badly needed. HEAT FLUX AND BALANCES The heat "budgets" of Jupiter and Saturn are of great im- portance. These planets receive heat from the sun, the pre- cise amount being the solar constant appropriately diminished by the inverse square law and by "pure" reflection. The amount of pure reflection would be obtainable if one could measure, in spectral detail, the radiation coming from Jupiter for the entire ATT sr, and, of course, if one had some knowledge of how much of the radiation was reflected sunlight and how much was planetary radiation. In this context, it is usually assumed that radiation coming from Jupiter or Saturn, whose wavelength is less than 2.5 ymP, is reflected light; 4^ detection has to date not been possible for any planet. How- ever, if a planet's surface is spherical and homogeneous (statistically), 4ir detection could in principle be accomp- lished by varying the angle a subtended by the earth and sun at the planet by l80 . For the moon, a ranges over almost the required range; and for the inner planets, respectable ranges of a are achieved. For Jupiter, a cannot exceed l2°; and, for Saturn, a cannot exceed 6 . Moreover, for Saturn the reflection of the rings further complicates matters. Then too, both these planets, particularly Jupiter, have zonal structure parallel to their equators, and the reflection of sunlight perpendicular to their equators is possibly quite different from reflection in their equatorial planes. The total elastic cross section of a planet is TrR AR, where A is Bond or bolometric albedo. The best that can be done is little more than an evaluation of the back elastic scattering differential cross section of Jupiter(do/dft)=R2PB (which defines the geometric bolometric albedo Pg). A recent careful study (D. Taylor) of Jupiter gives Pg=0.28 with an error of l0 percent. Comparison with earlier data seems to indicate that Pg varies by perhaps 0.5 magnitude or by a factor of l.6 between its maximum and minimum values. Also, it has been noted recently that Jupiter's infrared emission is variable. Consequently, the possibility that Jupiter is functioning as a gigantic heat engine with intake and exhaust "strokes" is not out of the question. Clearly such an eventuality would need to be reckoned with in virtually all Jovian investigations.

70 Measurements of albedos and of phase functions of Jupiter and of other planets made over a suitable period of time thus have a basic importance from the point of view of the physics of the planetary interiors. These measurements would be done best from orbiters, although rough values could be obtained also from flybys. Data obtained on the dark hemispheres of the planets as well as across their terminators would be of particular interest for calculating their rates of cooling and heating. Measurements from earth or from an Orbiting Astronom- ical Observatory would also be of value in spite of the inherent limitations of the phase angles. PLANETARY MAGNETISM The discovery, just over a decade ago, and subsequent investi- gations of radio emissions from Jupiter at decimeter and deca- meter wavelengths have led to the inference that Jupiter pro- duces a poloidal magnetic field whose strength at the visible surface is tens of gauss. The configuration of the magnetic field inferred from the radio-astronomical data is more comp- licated than that of a centered axial dipole. The direct measurement of this configuration by means of an orbiter will settle certain points that are controversial at the present time and greatly extend the usefulness of the radio-astronomical data. The sources of decametric and decimetric radiation current- ly rotate about the axis of the planet with a period of 9 55m 29.7s that is 5 min less than the period of rotation of visible markings near the equator, but only l0 sec less than that of the Great Red Spot. The continued monitoring of these motions and the interpretation of the observations in terms of the dy- namics and magnetohydrodynamics of Jupiter's interior (at present a matter of controversy) will lead in due course to in- formation about the internal constitution of the planet that may be obtainable in no other way. If Jupiter's poloidal magnetic field is not primordial in origin, then a mechanism for maintaining the field against dissipative agencies has to be found. The atmosphere of Jupiter may be sufficiently deep (greater than l0 km) and electrically conducting (conductivity greater than about l0-^ fim in its lower reaches for fluid motions (not less than about l0~^ m sec ) there to be capable of producing, or at least modifying, the poloidal field. A concomitant of such a dynamo process might

7I be toroidal fields of l03 or l0^ G confined to the interior of the planet. Owing, among other things, to the rapid rotation of Jupiter, planetary-scale motions in the lower reaches of the atmosphere should be correlated with motions at higher levels, including the visible surface. It follows, therefore, that if the mag- netic field is produced in the lower atmosphere then features of the visible surface (e.g., the Great Red Spot) might be expected to show significant correlation with the magnetic- field pattern in the vicinity of that surface. As a corollary, no correlation is expected if the magnetic field is produced in a (hypothetical) fluid region well below the atmosphere. The other major planets — Saturn, Uranus, and Neptune — should contain extensive fluid layers that are sufficiently well stirred to produce magnetic fields by the aforementioned "dynamo mechanism," the process thought to be responsible for the magnetism of the two planets known for certain to produce magnetic fields of their own (earth and Jupiter). If Saturn, Uranus, and Neptune are indeed magnetic and possess radiation belts, then improvements in radio-astronomical techniques might ultimately lead to the detection of these magnetic fields. Until and if such investigations prove feasible, how- ever, serious consideration should be given to the design of appropriate orbiter and probe experiments. The best way to obtain significant data necessary to an- swer the questions concerning the interior of Jupiter is with an orbiter. A flyby could give some rough information, es- pecially about the planets beyond Jupiter. GRAVITATIONAL POTENTIALS Gravitational potentials of planets can be expanded in a series of spherical harmonics which indicate the distribution of density in the planet and the degree of deviation from sphericity. For a planet without a north-south asymmetry one obtains V(r) = - ^ [l - |j(|)2P2(cos 0 + yj K(|)4P4(cos 9)...],

72 where R Is the radius of the planet. Knowledge of coefficients J and K is essential for obtaining proper distribution of den- sity in the planet. The coefficients themselves are usually deduced from a study of orbits of satellites. The J values for Jupiter, Saturn and Neptune are known with precision de- creasing in that order. The K value for Jupiter quoted in the literature, 0.00ll]<K <0.00395, (standard error) is actually within the limits 0.00l74^K<0.00369 of any hydrostatic model of Jupiter with the correct J value and thus is of little significance. Higher precision in determining Jupiter's K and any additional information about gravitational potentials of other planets would be of great value. Furthermore, important information about the shape and stiffness of a planet as a whole can be obtained if odd har- monics in the series expansion of the gravitational potential do not vanish. Such is the case for earth as deduced from orbits of artificial satellites, and similar information could be obtained from Jupiter's orbiters provided their orbits were sufficiently inclined. Actually, a precise knowledge of the orbit of Amalthea, Jupiter's fifth moon, could perhaps give that information also, although its inclination is small. This is discussed more fully in Chapter 6. It thus appears that all efforts should be directed at obtaining as precise data as possible about the paths of flybys and about the orbits of orbiters. Furthermore, ground-based systematic observations of Amalthea's orbit should be made and corroborated by detailed study of the paths of Jupiter's orbiters as well as by imaging techniques. PLANETARY DENSITIES The family of outer planets has long been held to comprise two physically distinct genera, containing Jupiter/Saturn and Uranus/Neptune, respectively. The case for this dichotomy, re- flecting an alleged substantial difference in mean density be- tween the two genera, has now been somewhat weakened. While the masses of the four planets are known to a sufficient pre- cision, the mean densities of Uranus and Neptune (which are of basic importance in determining their composition) quoted in the current literature depend crucially on the adopted radius and oblateness of figure. A new radius for Neptune derived from the occultation of a star in l968 reduces the mean

73 density to l.65 g/cc, i.e., almost to that of Jupiter. Very likely the radius of Uranus too has been underestimated. Never- theless, the disparity between the two genera has not been oblit- erated. The relatively small masses of Uranus and Neptune, even if the mean densities are reduced, still imply the presence in the interior of a fair amount of elements heavier than hydro- gen. Ground-based observations of the diameters of Uranus, Nep- tune, and Pluto are unsatisfactory, as they require corrections for diffraction and physiological effects (contrast theory) compounded with limb darkening. Improved diameters of these bodies, which might be obtained during flyby, are a prerequisite for refined planetary models. The terrestrial observations of occultations of these bodies are so valuable and so rare that a determined long-range effort to predict occultation of fainter stars ought to be made so that astronomers will be better pre- pared for future opportunities. Nothing meaningful can yet be stated about the interior of Pluto. Almost any observation of Pluto's diameter at closer range would be a marked advance. In summary, the radii and oblateness of trans-Saturnian planets are of such fundamental importance that effort should be made to measure them with all available means: imaging from flybys, imaging from orbiters, and observation of occul- tations — either visual occultation of stars, radar occultation of satellites, or radio occultation of probes. GROUND-BASED RADAR OBSERVATIONS It is expected that important ground-based radar observations of the outer planets and their satellites will become possible with present radar systems or systems now being planned. From the point of view of the interiors of planets and the nature of their satellites the following data would be particularly valuable: (a) Radar reflectivity of the satel- lites of Jupiter and Saturn would indicate an approximate value of the dielectric constant of the surface material and thus place some restrictions on the nature of that material, (b) Occultation of satellites by parent planets would lead to better values of planetary radii, which are of fundamental

74 importance for estimating planetary densities. (c) Measurement of secular terms in motions of satellites would throw light on the tidal interaction with the parent planet. This information would be of particular significance for Jupiter. These same measurements can be conducted with higher sensitivity in the bistatic radar mode, for example, by using the same ground- based transmitter and a receiver in a spacecraft near the object of study.

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Outer Solar System: A Program for Exploration, Report of a Study Get This Book
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Outer Solar System proposes a program for the exploration of the outer reaches of the solar system in the years 1974 to 1980. Of course, the technological requirements of the many-year missions and the vast distances represent new and difficult challenges in many technological areas such as communication, reliability, and miniaturization. This report presents a substantive account of the major scientific objectives of flight missions to the outer planets, and discusses the technical requirements in typical missions.

This report complements the Space Science Board's 1968 study, Planetary Exploration: 1968-1975.

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