Solid Interior Research Articles

Internal structures of the Galilean satellites

New models for the interiors of Io, Ganymede, and Callisto are proposed. The model of Io consists of a thin, high-rigidity outer layer separated from a solid interior by a thin, molten or partially molten shell. The modulus of rigidity of the outer layer must be at least 100 times larger than that of the underlying partially molten shell. These layers have thicknesses of order 100 km or less. The near-surface partially molten layer was most likely produced early in Io's history as a consequence of accretional heating; enhanced tidal heating in the outer rigid layer has kept the underlying region partially molten to the present day. The model of Ganymede consists of an ice outer layer, a shell of undifferentiated, primordial ice-silicate mixture, and a rock core. Accretional heating is responsible for melting the ice in the outer layers of Ganymede's initially homogeneous ice-silicate interior. Most of the rock in this outer layer accumulates in a shell on top of Ganymede's early cold and rigid central region; the water in the outer layer quickly refreezes. Heating of the undifferentiated region by the decay of radioactive elements in the silicate fraction would gradually warm it and reduce its viscosity. The rock layer would become gravitationally unstable and sink through the undifferentiated materials to form a rock core. Callisto's heavily cratered surface strongly suggests that relatively little, if any, ice-rock differentiation has occured in its interior.

Convection in Mercury

In order to explain why Mercury is trapped in a resonant state of rotation with a period two thirds of its orbital period, it has been proposed that the planet departs from hydrostatic equilibrium. It is shown that its gravity field must include a second-degree harmonic term. It is argued that this must be caused by convection in the solid interior rather than an initial distortion retained by the finite strength of the interior. The presence of an iron core in Mercury poses an interesting question as to why a second-degree harmonic convection pattern in the silicate mantle is present and suggested solutions are discussed.

Global seismic effects of basin-forming impacts

Models of the thermal evolution of the moon and the terrestrial planets suggest that basin-forming impacts occurred when the planets had partially molten interiors overlain by thickening lithospheres, comparable in thickness to the basin radii. We are investigating the effects of large impacts on planetary surfaces using a Lagrangian computer program which treats shock wave propagation and includes the effects of material strength, elastic-plastic behavior and material failure. In this paper we describe the computer code and some physical details of our numerical techniques, and report the results of several initial calculations. We study the global seismic effects for cratering energies (10 24 and 10 25 J) intermediate between the Copernicus and Imbrium events on the moon, and compare the phenomenologies for assumed solid and molten planetary interiors. The principal results are as follows: 1. (1)|Far-field effects are largely independent of cratering mechanisms (e.g., simulated impact vs. buried explosion). 2. (2)|Antipodal seismic effects are significantly enhanced by focusing and are of substantial magnitude. Vertical ground motion may be on the order of kilometers, and accelerations approach one lunar gravity. 3. (3)|The most violent activity occurs at significant depth beneath the antipode, considerably after the passage of the initial compressive/rarefactive shock wave, and results from complex interations with the free surface. 4. (4)|Seismic effects are decidedly more pronounced for a molten planet than for a solid one. 5. (5)|Tensile failure may occur at depths of tens of kilometers beneath the antipode, and may also occur over the entire surface, although at shallower depths. These results support the suggestion of Schultz and Gault that the unusual terrains antipodal to large planetary basins may have been catastrophically modified by seismicity generated by the basin-forming impacts. We would further suggest that these impacts may in fact have pervasively and repeatedly brecciated the entire lithospheres of the terrestrial planets as these lithospheres formed and thickened.

Motional effects on the EPR spectrum of a perturbed H centre in neutron irradiated LiF

The gravitational force exerted by a central planet deforms its orbiting moons (tides) and periodically changes their rotation (librations). Tides and librations of large icy satellites rotating synchronously with their orbital motion are usually studied separately but we here show that tides strongly reduce the libration amplitude of large icy satellites with a subsurface ocean. Moreover, libration also slightly changes the tides. Periodic tides affect libration in three ways: first the periodic tidal bulges modify the gravitational torque exerted by the central planet on the satellite, secondly torques between the periodic tidal bulges of a solid layer and the static shape of another solid layer couple the libration of the ice shell and solid interior, and thirdly, the zonal tides periodically change the polar moment of the satellite, which acts as the inertia for rotational motion. We develop a formalism to include these elastic effects in the study of librations and calculate the amplitudes of the longitudinal librations at orbital period for the largest icy satellites in the Solar System: Europa, Ganymede, Callisto, and Titan. Without a subsurface ocean, elastic effects on libration are of the order of a few percent. However, if a subsurface ocean exists as indicated by several observations for the satellites considered, elastic tides have a crucial effect. If the solid layers of the satellites are considered rigid, the libration amplitude is at least one order of magnitude larger than for an entirely solid satellite. Periodic elastic tidal deformations of the layers, in particular of the ice shell, counterbalance this large increase and keep the libration amplitude at the level of the libration of the satellite without subsurface ocean. Besides gravitationally forced librations, seasonal variations in the atmosphere of Titan also cause the rotation of Titan to change on a seasonal timescale. We show that tidal elastic effects can strongly increase the amplitude of these long-term variations in the rotation rate by a factor of about four with respect to a rigid shell. We predict that these variations can have an amplitude above the detection limit of the Cassini radar.

Rattling iron concretions from the Waikato Coal Measures

Abstract Two iron-rich concretions from the Waikato Coal Measures are described. Each has a hard iron oxide shell with a weathered outer surface. In one case there is a solid siderite-rich interior; in the other the shell encloses a void which contains a clay rattle. Concentrations of Fe, AI, Ca, K, Si, P, S, Mn, Ti, Mo, Mg, and Cu, in various parts of the concretions have been measured and the dominant minerals in these parts have been determined. The concretions are considered to represent different stages in a process which begins when iron mobilised by decomposing organic matter is pre cipitated under reducing conditions as siderite in sediments. Subsequent oxidising conditions oroduce a series of “boxes” each with a dense ferric oxide shell. The shell thickens as the enclosed ferrous ions are oxidised and eventually a void is created within the concretion. The kaolin rattle represents the weathered remnant of the host sediment.

Release of microbial contamination from fractured solids.

Estimation of the probability of release of microbial contamination from the interior of solids upon fracture due to impact is essential to the formulation of planetary quarantine and spacecraft sterilization requirements. A model system was designed in which known concentrations of bacterial spores were incorporated in methyl methacrylate plastic. Pieces of plastic were fractured in a uniform manner exposing interior surface areas of consistent and measurable size. Known surface areas were incubated in sets of 20 culture tubes containing liquid growth medium. The subsequent occurrence of visible growth expressed as percent of tubes positive was interpreted as an estimate of the probability of release of at least one viable micro-organism.

Planetary Contamination I: The Problem and the Agreements

The sterility requirements for landed spacecraft tentatively adopted in the COSPAR resolution of 1964 are so severe as to pose a major obstacle to planetary exploration. This by itself would not justify modification of the re quirements, since preservation of the biological integrity of Mars is essential for proper exploration of the planet. However, when the physical and biological assumptions underlying the COSPAR recommendations are com pared with actual conditions on Mars, as established by recent observations, it becomes apparent that the COSPAR as sumptions are unrealistic in important respects. Specifically, the belief that eolian erosion on Mars can effect the release of spores trapped in the interior of solids in periods of time that are short compared with the time scale of the unmanned space program is unsup ported by either observation or theory. On the contrary, the analysis suggests that rates of eolian erosion on Mars are very low. Similarly, present knowledge of the Martian environment opposes the view that terrestrial microorganisms would readily contaminate the planet. The combination of dryness, lack of oxygen, and high ultraviolet flux makes the surface of Mars peculiarly unsuit able for the multiplication of terrestrial organisms. Recent studies give little sup port to the proposal that significant areas of geothermal activity exist on Mars. These various findings suggest that the COSPAR-recommended constraints could be substantially relaxed without compromising to any significant degree the biological condition of Mars. In particular, a distinction needs to be made between microorganisms trapped in solids and those on exposed sur faces of landed spacecraft. Surface sterility is an unconditional require ment, in the sense that it is imposed by considerations unrelated to the nature of the Martian environment. Sterilization of the interior of solids to the extreme level recommended by COSPAR, however, is based on the as sumption that entrapped organisms con stitute a substantial hazard to the ecology of Mars. This assumption now seems unjustified, and the need for a high degree of interior sterility is doubt ful. Current spacecraft-sterilization pol icies should be revised accordingly.

Measurement of Strain Waves in the Interior of Solids caused by Explosion

Measurement of Strain Waves in the Interior of Solids caused by Explosion

La Terre et la Lune: Forme extérieure et Structure interne

A BOOK which seeks to throw light on terrestrial evolution from a study of the moon must possess considerable interest to both geologists and astronomers. But we may prophesy that this book will be more welcomed by geologists than by astronomers, or, at any rate, by mathematicians. For M. Puiseux, disregarding somewhat lightly the weighty dynamical reasons which have been brought forward by Kelvin and Sir George Darwin, among others, in favour of a solid interior to the earth, throws in his lot with those geologists who support the theory of a thin crust surrounding a liquid interior. He bases his argument partly upon purely terrestrial phenomena—and here we must frankly admit that we do not find his reasons convincing—and partly upon analogy from the moon. M. Puiseux points out many interesting points of resemblance between the general configuration of the earth and some of the principal features of the moon's surface; and his argument that the moon's surface, having suffered but slightly from the action of water, can throw light on a stage of development through which the earth has passed has much to recommend it. But we fail to follow him in the further arguments that he brings forward to prove that it is only the reaction of a liquid interior on a gradually thickening crust which can have produced the effects which are now to be seen on the moon's surface. La Terre et la Lune: Forme extérieure et Structure interne. By P. Puiseux. Pp. 176. (Paris: Gauthier-Villars, 1908.) Price 9 francs.