Ray T. Reynolds

Melting of Io by Tidal Dissipation

The dissipation of tidal energy in Jupiters satellite Io is likely to have melted a major fraction of the mass. Consequences of a largely molten interior may be evident in pictures of Ios surface returned by Voyager I.

The evolution of Enceladus

Saturns small icy moon Enceladus shows evidence of several episodes of geologic resurfacing and extensional tectonism spread over much of its history. Freezing of liquid in the interior caused global expansion and fracturing of the crust. Resurfacing took place by eruption of fresh material, perhaps containing NH3, to the surface. Solid-state convection could take place in Enceladus for a crustal thickness greater than about 30 km, assuming thermal properties like those of pure H2O ice. Melting in the interior may have been made more likely by the presence of NH3, as the H2ONH3 system has a eutectic point at 173°K. Tidal dissipation seems to be the only heating mechanism capable of melting Enceladus. For the thermal properties of pure H2O, the orbital eccentricity would have to be higher by a factor of 5–7 than the present value of 0.0044 to maintain a molten interior, and may have to be greater by as much as a factor of 20 to cause melting in an initially frozen body. If the thermal conductivity is diminished by inclusion of clathrate hydrates, a significant enhancement over the present eccentricity would still be required to initiate melting, but it might be possible to maintain a molten interior and allow geologic activity with the present eccentricity. Removal of eccentricity forcing would result in rapid eccentricity damping, freezing, and cessation of tectonic activity.

On the habitability of Europa

Abstract It has recently been suggested that tidal and radiogenic heating of Europa has led to formation and maintenance of a liquid water ocean overlain by a thin ice crust ( S. W. Squyres, R. T. Reynolds, P. M. Cassen, and S. J. Peale (1983). Nature 301, 225–226 ). The present work examines the environmental consequences of such a model with regard to the possible existence on Europa of regions that could satisfy the basic requirements for the survival of known organisms. Appropriate temperatures and long-term environmental stability are implied by the ocean model. The presence of necessary biogenic elements is assumed based on the expected origin of the ocean. The availability of biologically useful energy is assumed to be the principal limiting factor for life on Europa. Possible electrical, thermal, and chemical energy sources are discussed. Calculated resurfacing rates for the active crust model are used to estimate the quantity of photosynthetically active radiation that might reach the proposed ocean through crustal fractures. The amount of biomass that this energy could support, based on Antartic microorganism analogs, is estimated and discussed. Although these calculations cannot determine whether life forms exist or could exist on Europa, they do suggest that there may be regions on Europa, very limited on both space and time, with physical conditions that are within the range of adaptation of life on Earth.

Interactions of planetesimals with protoplanetary atmospheres

Abstract We compute the interaction of planetesimals with the envelopes of growing giant planets that form by the “core-instability” mechanism. According to this mechanism, a core grows by the accretion of solid bodies in the solar nebula, and the growing core becomes progressively more effective in gravitationally contracting gas from the surrounding solar nebula into an envelope, until a “runaway” accretion of gas occurs. We compute the two-body trajectories of planetesimals through this envelope, including the effects of gravitational and gas-drag forces for the envelopes surrounding cores of different masses ranging from 1.1 to 16.8 Earth masses. Curves are computed which show the depth of penetration of various-sized planetesimals through these envelopes, as are cross-sections for capture of icy and rocky planetesimals. The implications for the formation of the giant planets by this sort of mechanism and its influence on their composition are discussed.

Europa, tidally heated oceans, and habitable zones around giant planets.

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth.

Implications of Jupiter's early contraction history for the composition of the Galilean satellites

Abstract Graboske et al. (1973) have shown that Jupiters luminosity was orders of magnitude larger during its initial contraction phase than it is today. As a result, during Jupiters earliest contraction history, ices would have preferentially been prevented from condensing within the region containing the orbits of the inner satellites. The observed variation of the mean density of the Galilean satellites with distance from Jupiter implies that the satellite formation process was operative on a time scale of about five million years. Another consequence of the high luminosity phase is that water should be the only ice present in significant proportions in any of the Galilean satellites.

Tidal dissipation, orbital evolution, and the nature of Saturn's inner satellites

Abstract Estimates of tidal damping times of the orbital eccentricities of Saturns inner satellites place constraints on some satellite rigidities and dissipation functions Q . These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturns inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite.

On the thermal evolution of the terrestrial planets

Physical and chemical constraints for such different planetary objects as the Earth, the Moon and meteorite parent bodies can best be satisfied by thermal history models having high initial temperatures. On the basis of thermal calculations it is suggested that the evolution of the other terrestrial planets (Mars, Venus and Mercury) was also characterized by high initial temperatures. Under these conditions, melting and, consequently, fractionation would set in at an early stage. Because of the resulting redistribution of the long-lived radioactive heat sources and the concentration of these elements in the surface layers, large-scale differentiation could be achieved by partial melting.

Numerical experiments on the stability of preplanetary disks

Abstract Gravitational stability of gaseous protostellar disks is relevant to theories of planetary formation. Stable gas disks favor formation of planetesimals by the accumulation of solid material; unstable disks allow the possibility of direct condensation of gaseous protoplanets. We present the results of numerical experiments designed to test the stability of thin disks against large-scale, self-gravitational disruption. The disks are represented by a distribution of about 6 × 104 point masses on a two-dimensional (r, φ) grid. The motions of the particles in the self-consistent gravity field are calculated, and the evolving density distributions are examined for instabilities. Two parameters that have major influences on stability are varied: the initial temperature of the disk (represented by an imposed velocity dispersion), and the mass of the protostar relative to that of the disk. It is found that a disk as massive as 1M⊙, surrounding a 1M⊙ protostar, can be stable against long-wavelength gravitational disruption if its temperature is about 300°K or greater. Stability of a cooler disk requires that it be less massive, but even at 100°K a stable disk can have an appreciable fraction ( ∼ 1 3 ) of a solar mass.

Convection and lunar thermal history

The effect of solid convection on the thermal evolution of the Moon is explored for a variety of viscosities, radioactive differentiation efficiencies and initial temperature profiles. Convective heat flux in the models is calculated using an empirical relation derived from the results of laboratory experiments and numerical solutions of the Navier-Stokes equations. The method retains the spherically symmetric approximation and, therefore, greatly facilitates numerical calculations. Results show that even though solid convection may determine the thermal state of the lunar interior, it does not necessarily produce a quasi-steady thermal balance between heat sources and surface loss. An imbalance persists, due to the cooling and growth of the nonconvecting lithosphere. The state of the lithosphere is sensitive to the efficiency of heat source redistribution, while that of the convecting interior depends primarily on rheology. Convecting models have viscosities of 1021–1022 cm2s−1 in their interiors; the central temperature must be above 1100°C. Convection occurring within the first billion years after formation could have led to mare flooding by magma produced in hot zones of convection cells. However, it cannot be shown from model calculations alone that solid convection must have dominated lunar thermal history.