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NASA NTRS1983

Three Mars years - Viking Lander 1 imaging observations

The Mutch Memorial Station (Viking Lander 1) on Mars acquired imaging and meteorological data over a period of 2245 martian days (3.3 martian years). This article discusses the deposition and erosion of thin deposits (ten to hundreds of micrometers) of bright red dust associated with global dust storms, and the removal of centimeter amounts of material in selected areas during a dust storm late in the third winter. Atmospheric pressure data acquired during the period of intense erosion imply that baroclinic disturbances and strong diurnal solar tidal heating combined to produce strong winds. Erosion occurred principally in areas where soil cohesion was reduced by earlier surface sampler activities. Except for redistribution of thin layers of materials, the surface appears to be remarkably stable, perhaps because of cohension of the undisturbed surface material.

Arvidson, R. E.↗

NASA NTRS1983

Atmospheric tides on Venus. III - The planetary boundary layer

Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the bottom of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from about 5 to about 1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed.

Dobrovolskis, A. R.↗

NASA NTRS1983

Ethane ocean on Titan

Voyager I radio occultation data is employed to develop a qualitative model of an ethane ocean on Titan. It is suggested that the ocean contains 25 percent CH4 and that the ocean is in dynamic equilibrium with an N2 atmosphere. Previous models of a CH4 ocean are discounted due to photolysis rates of CH4 gas. Tidal damping of Titan's orbital eccentricity is taken as evidence for an ocean layer approximately 1 km deep, with the ocean floor being covered with a solid C2H2 layer 100 to 200 m thick. The photolytic process disrupting the CH4, if the estimates of the oceanic content of CH4 are correct, could continue for at least one billion years. Verification of the model is dependent on detecting CH4 clouds in the lower atmosphere, finding C2H6 saturation in the lower troposphere, or obtaining evidence of a global ocean.

Lunine, J. I.↗

NASA NTRS1984

Saturn ring particles as dynamic ephemeral bodies

Although Saturn's rings are within the Roche zone, the accretion of centimeter-sized particles into large aggregates many meters in diameter occurs readily, on a time scale of weeks. These aggregates are disrupted when tidal stresses exceed their very low strengths; thus most of the mass of the ring system is continually processed through a population of large 'dynamic ephemeral bodies', which are continually forming and disintegrating. These large aggregates are not at all like the idealized ice spheres often used in modeling Saturn's ring dynamics. Their coefficient of restitution is low, hence they form a monolayer in the ring plane. The optically observable characteristics of the rings are dominated by the swarm of centimeter-sized particles.

Davis, D. R.↗

NASA NTRS1984

The evolution of cataclysmic and low-mass X-ray binaries

The observational data for the 124 cataclysmic and low-mass X-ray binaries of known orbital period are compiled. It is found that the eruption properties are very well correlated with the orbital periods, in a manner that suggests that the mass transfer is the all-important determinant of evolution and of the eruptive behavior. Transfer rates of 10 to the -11th to 10 to the -7th solar mass/year are found, which are well correlated with orbital period. It is concluded that the mechanism which drives these systems is the magnetic braking of the secondary's rotation by its own stellar wind, coupled with the enforcement of synchronous rotation by tidal friction. This permits CVs to lose sufficient angular momentum to begin mass transfer in a reasonable time and drives the mass transfer at a high rate once it begins. An account of the long-lived phases of CV evolution is given.

Patterson, J.↗

NASA NTRS1984

The chaotic rotation of Hyperion

Under the assumption that the satellite is rotating about a principal axis that is normal to its orbit plane, a plot of spin rate-versus-orientation for Hyperion at the pericenter of its orbit has revealed a large, chaotic zone surrounding Hyperion's synchronous spin-orbit state. The chaotic zone is so large that it surrounds the 1/2 and 2 states, and libration in the 3/2 state is not possible. Rotation in the chaotic zone is also attitude-unstable. As tidal dissipation drives Hyperion's spin toward a nearly synchronous value, Hyperion necessarily enters the large chaotic zone, becoming attitude-unstable and tumbling. It is therefore predicted that Hyperion will be found to be tumbling chaotically.

Wisdom, J.↗

NASA NTRS1984

Ring torque on Janus and the melting of Enceladus

The absence of craters noted on Voyager 2 images of Encedalus indicates geologically recent resurfacing, probably due to internal melting; heating mechanism calculations, however, yield heating rates too small to cause melting. If Janus, whose orbital mean motion is currently decreasing as Janus' orbit evolves outward due to resonant torques from Saturn's rings, were ever to become locked into a stable 2:1 orbital commensurability with Encedalus, the resulting angular momentum transfer could have sufficiently enhanced the eccentricity of Encedalus' orbit for the ensuing tidal heating to have melted Encedalus' interior. However, the predicted rapid time scale for ring evolution due to resonant torques from Saturn's inner moons remains a major problem.

Lissauer, J. J.↗

NASA NTRS1984

The generation and propagation of internal gravity waves in a rotating fluid

The present investigation is concerned with an extension of a study conducted bu Maxworthy (1979) on internal wave generation by barotropic tidal flow over bottom topography. A short series of experiments was carried out during a limited time period on a large (14-m diameter) rotating table. It was attempted to obtain, in particular, information regarding the plan form of the waves, the exact character of the flow over the obstacle, and the evolution of the waves. The main basin was a dammed section of a long free surface water tunnel. The obstacle was towed back and forth by a wire harness connected to an electronically controlled hydraulic piston, the stroke and period of which could be independently varied. Attention is given to the evolution of the wave crests, the formation of solitary wave groups the evolution of the three-dimensional wave field wave shapes, the wave amplitudes, and particle motion.

Maxworthy, T.↗

NASA NTRS1984

Determination of the mass of globular cluster X-ray sources

The precise positions of the luminous X-ray sources in eight globular clusters have been measured with the Einstein X-Ray Observatory. When combined with similarly precise measurements of the dynamical centers and core radii of the globular clusters, the distribution of the X-ray source mass is determined to be in the range 0.9-1.9 solar mass. The X-ray source positions and the detailed optical studies indicate that (1) the sources are probably all of similar mass, (2) the gravitational potentials in these high-central density clusters are relatively smooth and isothermal, and (3) the X-ray sources are compact binaries and are probably formed by tidal capture.

Grindlay, J. E.↗

NASA NTRS1984

The size of the lunar core

A 0.2 arc second phase shift in the 18.6 year forced precession of the lunar figure was inferred from analysis of lunar laser ranging data. The source of the phase shift is either viscous friction at a lunar core mantle boundary or solid friction caused by tidal flexing of the Moon by the Earth. Core mantle viscous coupling will explain the signature if the core radius R sub c approximately equals 330 km. On the other hand, solid friction can account for the signature only if the lunar solid friction 0 approximately equals 30 is abnormally small compared with that observed for, say, Mars (approximately equals 100 - 200). Although the inferred core radius is certainly within the limits imposed by the Apollo seismic experiment, it is significantly smaller than estimates of order 400-500 km from electromagnetic sounding. How accurate is the estimate of the R sub c derived from the phase shift? The effect of uncertainties in the frictional coupling mechanism, core density and ellipticity on the inferred core radius are discussed. The excitation of lunar free librations by core turbulence vis-a-vis other mechanisms (e.g., cometary or asteroidal impacts) and the influence of changes in lunar precession in the past on lunar dynamo generation are also examined.

Yoder, C. F.↗

NASA NTRS1984

Constraints on the origin of viscoelastic bodies

While the oceans undoubtly contribute to energy dissipation, it seems implausible that such a small amount (0.02%) of the Earth's mass could play such a dominant role in lunar orbital evolution. An alternate hypothesis assumes that most of the dissipation is associated with solid-body tides. This mechanism is also capable of varying significantly over time for viscoelastic bodies. In such an analysis of orbital evolution, the bodies are modeled as incompressible, Kelvin-Voight solids. While this material is simplistic, it does provide an analytical characterization of solid-body dissipation and the resulting tidal moment. The orbital history of a two body system is traced back in time to develop constraints on the satellite's origin. These constraints suggest the Moon appeared in a significantly inclined orbit at a distance greater than ten Earth radii. This distance is outside of the Roche limit and would appear to alleviate some difficulties associated with capture hypotheses. Small eccentricity at close approach suggests an origin by accretion.

Vanarsdale, W. E.↗

NASA NTRS1984

Directional properties of circular Maria: Interpretation in the context of a testable gravitational capture model for lunar origin

Photogeologic study of circular lunar maria and mare-filled craters reveals that several of these features are either elliptical and/or asymmetrical. The ellipticity/asymmetry is interpreted as a directional property which may have genetic significance. Most maria indicate movement in easterly directions. The directional properties of these features are consistent with at least three models of mare basin formation: (1) mare basins were excavated by a swarm of fragments from a tidally disrupted planetary body impacting from a westerly direction; (2) mare basins were excavated by lunar satellites whose orbits decayed sufficiently to cause the satellites to impact onto the lunar surface from a westerly direction; and (3) basins were formed by the impact of large spheroids of lunar basalt onto the lunar surface from a westerly direction.

Malcuit, R. J.↗

NASA NTRS1984

Altimetry, Orbits and Tides

The nature of the orbit error and its effect on the sea surface heights calculated with satellite altimetry are explained. The elementary concepts of celestial mechanics required to follow a general discussion of the problem are included. Consideration of errors in the orbits of satellites with precisely repeating ground tracks (SEASAT, TOPEX, ERS-1, POSEIDON, amongst past and future altimeter satellites) are detailed. The theoretical conclusions are illustrated with the numerical results of computer simulations. The nature of the errors in this type of orbits is such that this error can be filtered out by using height differences along repeating (overlapping) passes. This makes them particularly valuable for the study and monitoring of changes in the sea surface, such as tides. Elements of tidal theory, showing how these principles can be combined with those pertinent to the orbit error to make direct maps of the tides using altimetry are presented.

Colombo, O. L.↗

NASA NTRS1984

Some Aspects of General Circulation and Tides in the Middle Atmosphere

Data from several sources suggest that between 100 and 200 km in the middle atmosphere, a layer of anomalous circulation must be situated, at least in middle latitudes. The mechanisms responsible for the circulation anomaly in this height region are discussed in terms of hydrodynamic equations. The current state of atmospheric tidal theory is also addressed.

Ivanovskiy, A. I.↗

NASA NTRS1984

Stability of the Galilean Satellites in Deep Orbital Resonance

The Galilean satellites are locked in the Laplace resonance, which is responsible for forcing their significant orbital eccentricites. The eccentricites in turn result in tidal energy dissipation within the satellites that is responsible for the great thermal activity on Io, including both volcanism observed by Voyager and heat flux measured from Earth. The resonance results from the nearly 2:1 commensurability of orbital periods (or equivalently of mean motions) taken by pairs. These geometrical relations enhance the mutual gravitational effects. Yoder pointed out that tides raised on Io tend to drive the system out of resonance (toward greater nu and correspondingly smaller eccentricities) while tides raised on Jupiter tend to drive the system towards deeper resonance (smaller nu and larger forced eccentricities). The possible behavior of the system in deep resonance is investigated.

Richard Greenburg↗

NASA NTRS1985

Giant Planets and Their Satellites: Cosmochemistry, Evolution and Structure

The following topics were discussed: surface state of Titan; statistical mechanical treatment of clathrate hydrates; planetary dynamos; formation of the giant planets; the e-ring; Uranian satellites and atmosphere; 10 sulfur lakes; episodic tidal evolution; resurfacing of Ganymede; and the origin of the Moon.

Stevenson, D. J.↗

NASA NTRS1985

Studies of Icy Bodies: Uranian Satellites and Cometary Nuclei

The evolution and structure of icy bodies of the solar system are summarized. The effect of tidal evolution, eccentricities, and decay time on Titania, Oberon, Miranda, Umbriel and Ariel is discussed. Observational measurements of the masses and radii of these satellites have recently become sufficiently reliable to use in investigation. Also considered is the problem of the sub-surface temperature distribution and heat transfer of icy comet nuclei.

Squyres, S. W.↗

NASA NTRS1985

Gravity Field of the Jovian System from Pioneer and Voyager Tracking Data

Data sets from the Voyager and Pioneer flybys of Jupiter and the Galilean satellites are employed to characterize the Jovian magnetic field and the effects of the Io torus on transmissions. Both optical and Doppler radio data are considered, except for periods when the Jovian radiation environment disturbed the oscillator stability of the radio transmitters. Account is taken of small accelerations of the spacecraft by tidal forces of a single rising satellite, density differences in the Great Red Spot producing a columnar gravitational change, and three unknown objects in the inner Jovian system. Correction parameters are developed for the effects on the S-band data induced by the Jovian plasmasphere inwards from the Io torus. Calculations are then made of the planet and satellite masses, gravity harmonic coefficients, and orientation of the rotational pole. Large reductions in the uncertainties in previous mass esimates are obtained.

Campbell, J. K.↗