[105] Geophysical observations of planets consist of measurements of such quantities as gravity, topography, magnetic fields, heat flow, and seismic wave velocities. These data must then be interpreted in terms of physical models to derive information on the planet's thermal history, internal state, surface rheology, etc. Often geological and geophysical techniques are used in concert to suggest or constrain models. For example, photogeologic mapping and relative dating of features can be used together with gravity and topographic data in studies of tectonics. In this report we discuss planetary geophysics only in the most general terms, paying specific attention to the important interplay between this discipline and planetary geology.
5.1. Geological Constraints on Thermal Histories
Historically, moment of inertia and mean density measurements were among the first data to become available for the terrestrial planets. These measurements, coupled with photogeologic information on the nature of the planets' surfaces, and with models for planetary accretion, made it possible to construct early models of the internal structure and thermal histories of the terrestrial planets (Hsui and Toksoz, 1977; Johnston and Toksoz, 1977; Toksoz et al., 1973; Buck and Toksoz, 1980). In the absence of seismic and heat flow data (except for the Moon), these first models were not well constrained, and clues to additional boundary conditions were sought from the records preserved on the surfaces of these objects.
[106] Faulting and volcanism on a planetary surface can be closely related to the thermal evolution of the planet (Head and Solomon 1981). Interior warming leads to global expansion, surface extensional tectonics, and a crustal stress system that aids the extrusion of volcanic products. On the other hand, interior cooling leads to contraction, compressional tectonics, and crustal stresses that act to shut off surface volcanism. This relationship between thermal history and the global tectonic history of the Moon, Mercury, and Mars has been explored extensively (Solomon and Chaiken, 1976; Solomon, 1977, 1978). Time-dependent global stress fields have also been used in combination with lithospheric flexure theory to explain the spatial and temporal relationships of linear rifles and edges around lunar mare basins (Solomon and Head, 1979, 1980 Comer et al., 1979). A similar technique was used by Thurber and Toksoz (1978) to estimate the thickness of the elastic lithosphere in the Tharsis region of Mars.
5.2. Gravity and Topography Data
At present, gravity and topography provide virtually our only direct information on the state of the interior of the terrestrial planets (except in the cases of Earth and the Moon). In general, the height of topography places an upper bound on the finite strength of the lithosphere, as does the gravity load where it can be directly correlated to crustal features. If only broad resolution data are available, the correlation between the gravity and topography power spectra may be used to provide information of the global degree of compensation (be it static or dynamic) and can indicate whether significant lateral density inhomogeneities are present at depth within the planet.
Thurber and Solomon (1978) used a set of crustal models to examine the degree of lunar isostasy, the probable mode of compensation, and the thickness of the basalt fill in mare basins. Lambeck (1979) used power spectra of martian topography and gravity to show that long wavelength density anomalies on the planet are concentrated m a relatively thick lithosphere, in contrast to Earth where such anomalies originate in the deep mantle. He also found that the lithospheric thickness implied over most of the planet was consistent with predictions of existing thermal models, but that the wavelengths associated with the Tharsis plateau demanded either a much colder interior or some form of dynamic support. Phillips et [107] al. (1981) used similar techniques to analyze the support of the Aphrodite Terra region of Venus. They concluded that this feature was either quite young (less than 107 years) or was dynamically supported by convection. Either alternative implies a thermally active planet.
If reasonably high resolution data are available, as is the case for Mars, more detailed global modeling can be performed using observed surface tectonic features as an additional constraint. Banerdt et al. (1982) computed global stress fields for Mars using the observed gravity and topography as boundary conditions. The results of their more detailed computations agreed closely with Lambeck's (1979) conclusions. In addition, they found evidence of two distinct episodes in Tharsis' history, one of thermally driven construction, followed by a gradual loss of support and collapse. They also concluded that partial thermal support must exist at present. Given sufficient gravity and altimetry data, along with high-resolution radar imaging of the surface, the same types of analysis could profitably be performed for Venus.
In addition to the global information provided by the long wavelength topography and gravity data, much can be learned by studying the signatures of smaller features. This type of analysis has been carried out for many features on the Moon (Dvorak and Phillips, 1979; Dvorak, 1979). Such studies indicate that the outer portions of the Moon are thick, cold, and rigid. Preliminary work has also been done on Mars (Sjogren, 1979; Sjogren and Wimberly, 1981) and Venus (Phillips et al., 1979; Sjogren et al., 1980; Cazenave and Dominh, 1981) and has yielded information on lithosphere strength and viscosity, crustal thickness, and mode of compensation. One drawback, however, has been that local topography data have been very limited. High-resolution topographic maps are now available for several important surface loads, including the Tharsis volcanoes on Mars, and accurate estimates of the lithosphere thickness can be made from the shape of the surface flexure and the gravity signature (Solomon and Head, 1982). In the past, such estimates were made without the topographic data by substituting observations of concentric fracturing around loads, a procedure involving much greater uncertainties in interpretation.
There are many features on the solid planets in the solar system that await to be analyzed in detail. Lunar farside basins, for example, could be studied to compare the farside crust with that of [108] the near side. Investigation of volcanic piles both on and off Tharsis could give estimates of lithospheric and crustal thicknesses at various places on Mars. The Caloris basin could be used to derive the crustal structure of Mercury. A host of landforms on Venus could be used to further our understanding of that planet's interior.
Loading of the lithosphere by the accumulation of volcanic material, or any mountain-building process, may lead to overall stress distributions that require compensation. If the history of such surface loads can be constrained independently by relative age dating, then estimates of the mantle viscosity and/or "paleoviscosity" may be made. If the effective viscosity is much larger than that of Earth's mantle, then the load response time is long with respect to the age of the planet and the "load" may be supported by the mechanical strength of the interior. Studies of lunar floor-fractured craters (Hall et al., 1981) and lunar impact basins (Solomon et al., 1982) have provided evidence for viscous relaxation of lunar topography, and suggest that differences in crustal viscosity exist over a planet's surface; these differences probably indicate variations m crustal temperature profiles. From the lateral extent (or spherical harmonic degree) and magnitude of the topographic and gravity anomaly supported, constraints on the thickness of a mechanical lithosphere can be derived. If the wavelengths of lithospheric flexure are relatively large, then the lithosphere may be assumed to be very rigid and probably very thick. One alternative to support by finite strength in thick lithospheres is that density anomalies occur due to deep (>500 km) convective circulation driven by thermal and/or chemical density imbalances that require times longer than 4.5 billion years to be dissipated. However, patterns and rates of convective circulation in deep planetary mantles may be more closely correlated with the rates and regional extent of planetary outgassing and volcanism than with regional density and topographic anomalies.
5.3. Shapes of Planets and Satellites
For Earth and the Moon, seismic information enables us to construct detailed models of the interiors that describe both the internal density distributions and the possible locations of solid-liquid interfaces. For other planets and satellites, this method is not available, and our information on the nature of the interiors, apart from a few clues from electromagnetic sounding (Hide, 1978), [109] comes from determinations of the mean density and measurements of the departure of the mean global shape from perfect sphericity.
Rotational and, in the case of the satellites, tidal forces distort bodies to a degree determined by the internal density distribution and by the strength, or state of relaxation, of the solid parts. The degree of distortion can be determined either by measurements of the gravitational moments (J2, J4, etc.) or by a measurement of the mean global shape. Measurement of J2 or of the shape of a body in hydrostatic equilibrium gives information equivalent to a knowledge of the body's moment of inertia once the rotational period is known (Cook, 1973).
The shapes of some of the inner satellites of Jupiter and Saturn are of particular interest in this context. Lewis (1971) and Consolmagno and Lewis (1976, 1977) have argued that some, but not all, of these satellites could be differentiated and could possess deep mantles of water and water-ice and rocky silicate cores. Dermott (1979) has shown that a wide range of internal density distributions have an appreciable effect on the shapes and gravitational moments of some of these satellites.
[110] Specifically, such calculations suggest that if the shape factor, A-C (the difference of the Iongest and shortest axes of the biaxial ellipsoidal figure), can be measured to a precision of 1 or 2 km, then we will be able to determine whether the satellites lo, Mimas Enceladus, and Tethys are differentiated and estimate the sizes of the possible cores. The gravitational moment, G(B-A), can, in the best circumstances, be measured to a precision ~ 106 km5 sec-2 (Hubbard and Anderson, 1978), and it seems that such measurements could be useful in determining the degree of differentiation only in the case of Io and possibly Titan (Dermott, 1979). However, precise measurements of the shapes alone are capable of yielding information on the internal structures of Mimas, Enceladus, and Tethys and probably of other objects as well.
5.4. Magnetic Fields
Measurements of the external magnetic field also provide important clues as to whether or not convection is an efficient mechanism of heat transport in a planet's interior. For Earthlike planets strong magnetic fields indicate a molten state for a dense metallic core, and consequently a warm mantle. Since Earth has a strong magnetic field, it is clear that mantle convection is a relatively inefficient mechanism of heat removal. It is difficult to quantify how cool a planetary interior may become due to convection when a thick, immobile, nonsubducting lithosphere sits on top of the convecting layer. Accurate theoretical estimates of the interior temperature structure and evolution are crucial, however, for correlation with magnetic events, tectonic reconstructions, estimates of crustal chemical evolution, and lithospheric thickness. Recent convection calculations with temperature-dependent viscosity and spherical geometry show that the terrestrial planets equal to or larger in size than Mars may maintain both thick lithospheres and interiors hot enough to provide planet-wide volcanism over geologic time.
5.5. Outer Planet Satellite
Most of our discussion so far has emphasized the terrestrial planets, but there is little doubt that much geophysical effort during the coming decade will go into the study of the satellites of Jupiter and Saturn. The same geophysical principles used for studying the terrestrial planets can be extended to these bodies. Thermal models [111] of the icy satellites have grown increasingly sophisticated, attempting to take into account solid-state convection and other factors (Parmentier and Head, 1979a,b; Thurber et al., 1980). Peale et al. (1979) used thermal models to predict volcanism due to tidal heating on lo; similar ideas are now being applied to explain the enigmatic surface of Enceladus. Research is also being done to explain the origin of tectonic features on the Galilean satellites, including domes on Ganymede (Squyres, 1980) and multiringed structures on Ganymede and Callisto (McKinnon and Melosh, 1980).
The list of geophysical questions concerning the outer planet satellites that need answering is already long and will certainly grow as we learn to understand these objects better. For example, we do not really know how important accretional energy was as a heat source in the evolution of the larger satellites. We do not even know whether most objects really are differentiated. Nor do we know whether any of the satellites can support large density anomalies in their interiors. We do not know how thick lo's lithosphere is, or whether the high topography on some parts of the satellite is compensated. In the case of Saturn's satellites, we do not know whether ammonia played a significant role in lowering melting temperatures in the interiors.
5.6. Summary
Planetary geophysical observations traditionally include measurements of gravity, topography, and magnetic fields, as well as determinations of a planet's mass, mean radius, and density, and of its overall shape and figure. For Earth and the Moon, seismic and heat flow data are available and provide essential constraints on geophysical models. A major goal of planetary exploration should be to obtain similar data for other important solid bodies of our solar system.
Lacking seismic and heat flow data for most objects, information on gravity and topography, interpreted in the context of observed surface features, has been relied on to provide most of the constraints needed to develop models of how interior processes have affected the surface evolution of planets and satellites. Often, remote sensing techniques (photometry, spectrophotometry, and radar and thermal measurements) can provide valuable additional clues (chapter 8).
[112] Although mass, radius, and mean density data are well established for the terrestrial planets, these essential parameters remain poorly determined for many important objects in the asteroid belt and beyond. As the Voyager experience with the satellites of Jupiter and Saturn demonstrates, accurate mean densities require extremely precise determinations not only of the masses, but of the radii as well (chapter 6). A byproduct of accurate radii determinations is a measurement of the body's shape, which often contains important information on internal structure.
