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Stevenson, David J.

Publications and source records attributed to Stevenson, David J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Planetary Origin Evolution and Structure

This wide-ranging grant supported theoretical modeling on many aspects of the formation, evolution and structure of planets and satellites. Many topics were studied during this grant period, including the evolution of icy bodies; the origin of magnetic fields in Ganymede; the thermal histories of terrestrial planets; the nature of flow inside giant planets (especially the coupling to the magnetic field) and the dynamics of silicate/iron mixing during giant impacts and terrestrial planet core formation. Many of these activities are ongoing and have not reached completion. This is the nature of this kind of research.

Stevenson, David J.↗
NASA NTRS2002

Support for the 2001 Gordon Research Conference on the Origins of Solar Systems

This grant provided partial support for participants at the Gordon Research Conference on Origins of Solar Systems, held at Connecticut College, June 17 through June 22, 2001. This conference was chaired by the PI (David Stevenson) and Pat Cassell (NASA Ames) was Vice-Chair. In addition to the money provided by this grant ($15,000), the Gordon Research Conference organization provided about $22,000 (in large part from registration fees) and BPI provided $10,000 (through Director David Black); the latter targeted primarily for students and postdocs. Accordingly the grant money from NASA was used to cover the costs of registration and travel for about one half of the invited speakers (22 in total) and discussion leaders (about 10 in total).

Stevenson, David J.↗
NASA NTRS1992

Experimental investigation of carbonate formation on Mars

If Mars had an early dense CO2 atmosphere (greater than 1 bar), then the transition to the present low pressure (7 mbar) may require carbonate formation without liquid water (e.g., although the early greenhouse has been questioned on the basis that modeled CO2 condensation prevents buildup of such a pressure). Our experiments have focused on the kinetics of such 'dry' reactions. Booth and Kieffer suggested rates consistent with removal of greater than 1 bar of CO2 over geologic time, but their results were questionable since their reactions did not proceed beyond one monolayer of carbonate formed on powdered silicate under martian conditions. Theoretical considerations implied that if nonporous rinds formed, then the rate of carbonate formation would be limited by diffusion, not reaction. Therefore, our experiments optimize reaction conditions to test for continued carbonate production, as well as to verify previous conclusions for initial growth of carbonate.

Stephens, Stuart K.↗
NASA NTRS1992

Carbonate formation on Mars: History of the CO2 atmosphere from models of diffusion-limited growth in non-aqueous environments

We conducted preliminary experiments designed to measure the amount of CO2 reacted from a simulated Martian atmosphere to form carbonate on silicate grains. Warm experiments at constant T (300-350 K) and with no water (vapor or liquid) yielded no detectable reaction, suggesting the following result. If we are indeed operating in the thermodynamically-favorable regime (supported by Gooding (1978) for the gas-solid reaction), then the lack of a reaction at warm temperatures suggests that a reaction in the 200-300 K regime will be less favored if reaction kinetics dominate. The completely dry scenario is thus not favored. An additional experiment, with abundant water vapor and at T approx. 300 K (constant), yielded a negative result as well. However, this is not inconsistent with Booth's findings, since lower temperatures may be required for the absorption of a monolayer of water. We plan further (lower-T) experiments.

Stephens, Stuart K.↗
NASA NTRS1992

Melting and differentiation in Venus with a cold start: A mechanism of the thin crust formation

Recent works argue that the venusian crust is thin: less than 10-30 km. However, any convective model of Venus unavoidably predicts melting and a fast growth of the basaltic crust, up to its maximum thickness of about 70 km limited, by the gabbro-eclogite phase transition. The crust is highly buoyant due to both its composition and temperature and it is problematic to find a mechanism providing its effective recycling and thinning in the absence of plate tectonics. There are different ways to solve this contradiction. This study suggests that a thin crust can be produced during the entire evolution of Venus if Venus avoided giant impacts.

Solomatov, Viatcheslav S.↗
NASA NTRS1991

Planetary origin, evolution, and structure

Three areas of recent and ongoing research are presented. The first area is giant planet heatflows. Conventional wisdom attributes the heatflow of the giant planets to the gradual loss of primordial heat, except in the case of Saturn where helium separation is evidently occurring. There are two problems with this picture: (1) the observed helium abundance of Saturn's atmosphere is so low that Jupiter must also be differentiating helium since its internal entropy cannot be much higher than Saturn; and (2) the heatflow of Neptune (not to mention Uranus) is too high to be consistent with adiabatic cooling from an initial hot state. A self-consistent solution to these two problems is presented. The second area covered is that of the despinning protogiant planets. Modeling of the possible despinning of these protoplanets by hydromagnetic torques was performed and the model results are discussed. The third area covered is how Titan hides its ocean. Until recently, the favored picture of Titan's surface was a roughly kilometer-thick ethane/methane ocean, presumably global in extent with at most a few outcroppings of dry land. The depth of the ocean is well constrained by observed atmospheric properties, and the constraints on subaerial topography are obtained indirectly from tidal considerations. A different picture of Titan's surface was pursued which was motivated by the perspective that methane on Titan should more properly be considered as a magmatic fluid. In this picture, methane is stored subsurface in magma chambers fed from deep-seated sources of methane, most probably due to the high pressure breakdown of methane clathrate. Other aspects of this model of Titan are presented.

Stevenson, David J.↗
NASA NTRS1991

The search for brown dwarfs

The theory of brown dwarfs is summarized and observational findings regarding brown dwarfs are reviewed. The equation of state, the thermal properties, the interior transport properties, the boundary conditions, and the initial conditions are examined. Indirect observations, IR speckle interferometry, IR photometry, and field observations of brown dwarfs are discussed.

Stevenson, David J.↗
NASA NTRS1990

Rheology of solid methane and nitrogen - Applications of Triton

The existing information about the rheological properties of solid methane and nitrogen is reviewed. The main limitation is the absence of creep data for solid nitrogen, but estimated properties are derived on the basis of simple scaling arguments for van der Waals bonded materials. On Triton solid methane and most likely nitrogen are too soft to support any topography over billions of years. Topographic support by water ice-rich materials seems almost certain. Nitrogen and methane do not intermix by solid-state diffusion. The weakness (absence?) of the opposition effect on Triton can be explained by annealing of nitrogen grains, provided the latter have sizes characteristic of freshly deposited frost.

Eluszkiewicz, Janusz↗
NASA NTRS1990

Extensional and compressional instabilities in icy satellite lithospheres

The plausibility of invoking a lithospheric instability mechanism to account for the grooved terrains on Ganymede, Encedalus, and Miranda is presently evaluated in light of the combination of a simple mechanical model of planetary lithospheres and asthenospheres with recent experimental data for the brittle and ductile deformation of ice. For Ganymede, high surface gravity and warm temperatures render the achievement of an instability sufficiently great for the observed topographic relief virtually impossible; an instability of sufficient strength, however, may be able to develop on such smaller, colder bodies as Encedalus and Miranda.

Herrick, David L.↗
NASA NTRS1990

Chemical heterogeneity and imperfect mixing in the solar nebula

The 'standard' accretion-disk model of the solar nebula is examined together with its basic equations, which are solved for the steady-state case. It is shown that, for this model of the solar nebula, the efficiency of mixing the products of thermochemical processing from small to large disk radii depends not on the magnitude of the eddy diffusivity but only on the ratio k between the eddy diffusivity and the eddy viscosity. In most cases, this ratio k is probably of order of unity or less, implying that most of the solar nebula was not contaminated by the consequences of thermochemical equilibria that were established at 'small' disk radii. It is noted that, while this condition is almost certainly satisfied if the physical process responsible for viscosity is thermal convection or waves, it has uncertain validity during the early phases of disk evolution where accretion-induced shear instabilities may dominate.

Stevenson, David J.↗
NASA NTRS1989

Thermal state of an ice shell on Europa

The thickness of the ice shell presently hypothesized for Europa, which is decoupled from a silicate core by a liquid water layer, is calculated as a function of colatitude and longitude under suitable assumptions for two plausible ice rheology behaviors: that of Maxwell type, and that of generalized flow-law rheology. Due to the dissipation rate's pronounced temperature dependence, virtually all tidal dissipation is found to concentrate in the lowest few kilometers of the shell. While for some parameter choices an insulating regolith that raises the near-surface temperature by more than a few tens of degrees C may stabilize the shell against polar wander, a modest regolith may enhance the probability of such wander's occurrence through the reduction of the shell's retarding friction.

Ojakangas, Gregory W.↗
NASA NTRS1989

Polar wander of an ice shell on Europa

The present consideration of a hypothesized ice shell around Europa, which is decoupled from the silicate core by a liquid water layer and possesses a spatially varying thermal equilibrium thickness profile, proceeds through the development of equations for variations in the inertia tensor of a body when second-harmonic-degree topography is added to the crustal base. Attention is given to a realistic model in which the shell and ocean are assumed to undergo reorientations as a single entity independently of the core, but subject to viscous dissipation within the shell. Shell friction is in this case noted to preclude polar wander, unless a low conductivity regolith increases the near-surface temperature by a few tens of degrees C; the ice beneath the regolith would then behave viscously on the time-scale of polar wander.

Ojakangas, Gregory W.↗
NASA NTRS1989

The competition between thermal contraction and differentiation in the stress history of the moon

The stress history of the moon is discussed, taking into consideration the effects of thermal contraction and differentiation. The amount of expansion caused by extracting basalt from undifferentiated lunar material is estimated taking account of the uncertainty in the knowledge of the appropriate compositions, and the resulting estimate of the expansion is used to compare the relative importance of the thermal and differentiation effects in the moon's volumetric history. The results of calculations show that differentiation is likely to be of major importance and, thus, thermal expansion is not the sole possible contributor to evolutionary changes in the lunar radius.

Kirk, Randolph L.↗
NASA NTRS1989

Implications of the giant planets for the formation and evolution of planetary systems

The giant planet region in the solar system appears to be bounded inside by the limit of water condensation, suggesting that the most abundant astrophysical condensate plays an important role in giant planet formation. Indeed, Jupiter and Saturn exhibit evidence for rock and/or ice cores or central concentrations that probably accumulated first, acting as nuclei for subsequent gas accumulation. This is a 'planetary' accumulation process, distinct from the stellar formation process, even though most of Jupiter has a similar composition to the primordial sun. Uranus and Neptune appear to exhibit evidence of an important role for giant impacts in their structure and evolution. No simple picture emerges for the temperature structure of the solar nebula from observations alone. However, it seems likely that Jupiter is the key to the planetary system, and a similar planet could be expected for other systems. The data and inferences from these data are summarized for the entire known solar system beyond the asteroid belt.

Stevenson, David J.↗
NASA NTRS1988

Background heatflow on hotspot planets - Io and Venus

It is suggested that there is no simple relationship between lithospheric thickness and heatflow on planets where volcanism dominates the heatflow. This applies locally and globally, even away from regions of volcanic activity. This indicates that there is no basis for the assumption that the Io heatflow is as low as (or lower than) the hotspot component alone would suggest. A model is presented to describe the heatflow on hotspot planets. The model is applied to Io and Venus.

Stevenson, David J.↗
NASA NTRS1988

Gravitational instability in two-phase disks and the origin of the moon

Two-phase disks may be gravitationally unstable at temperatures or surface densities at which a disk composed of either single phase would be highly stable. It is argued that two-phase disks can achieve a marginally unstable state (in addition to a highly unstable state that leads to fragmentation), limited by the ability of the photosphere to radiate the energy dissipated in the disk. A self-consistent prescription for the viscosity induced by the slow instabilities is provided. Two-phase disks are more centrally condensed than single-phase disks, and their secular cooling time may be comparable to their spreading time. A circumterrestrial disk of sufficient mass to form the moon provides a detailed example of all the preceding points. Its stability, structure, and dynamical evolution are investigated, and it is concluded that its spreading time is short (about 100 yr); the moon is formed molten, or partially molten; the moon's initial orbit lies in the earth's equatorial plane; and only a small fraction of the disk mass is lost in a wind, although this may represent a substantial fraction of volatiles. Most of these conclusions are independent of how the disk was formed, e.g., from a giant impact.

Thompson, Christopher↗
NASA NTRS1988

Rapid formation of Jupiter by diffuse redistribution of water vapor in the solar nebula

In the present, water-vapor diffusive redistribution and condensation model of solid material abundance enhancement in the solar nebula's Jupiter-formation region, the assumed turbulent nebula temperatures decrease inversely with radial distance from the center, and time-scales are set by turbulent viscosities. The length scale for condensation of diffusively-transported water vapor is about 0.4 AU, and the surface density of ice in the Jupiter-formation zone undergoes enhancement by a factor of as much as 75; this surface density enhancement is enough to precipitate the rapid accretion of planetesimals into a solid core.

Stevenson, David J.↗