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NASA NTRS1986

Modelling the pole tide and its effect on the earth's rotation

The pole tide is the response of the ocean to incremental centrifugal forces associated with the Chandler wobble. The tide has a potentially important effect on the period and damping of the wobble, but it is at present not well constrained by observations. Here, both analytical and numerical models for the pole tide are constructed. The analytical models consider the tide first in a global ocean and then in an enclosed basin on a beta-plane. The results are found to approach equilibrium linearly with decreasing frequency and inversely with increasing basin depth. The numerical models solve Laplace's tidal equations over the world's oceans using realistic continental boundaries and bottom topography. The results indicate that the effects of non-equilibrium portion of the deep ocean tide on the Chandler wobble period and damping are negligible.

Carton, J. A.↗
NASA NTRS1986

Masses of the satellites of Uranus

A marked dichotomy in the density distribution of the Uranian satellites has been observed. The densities of Ariel and Umbriel are similar to those of the inner satellites of Saturn, and are consistent with a composition of 40 percent rock and 60 percent water ice, while Titania and Oberon appears to have much higher densities. It is argued here that the problem posed by this density distribution arises from a misunderstanding of the orbital dynamics and may be imaginary. It is also shown that the observed eccentricities of the inner satellites, Ariel and Miranda, may be partly forced by the outer satellites, and that these forced eccentricities may be maintained despite strong tidal damping.

Dermott, S. F.↗
NASA NTRS1986

A collection of Galilean satellite eclipse observations, 1652-1983. II

The largest collection of Galilean satellite eclipse observations extent has been gathered from the published literature and from manuscript collections from 1652 to 1983. Many of the observations were thought to have been irretrievably lost for more than a century. The collection will be of great value in evaluating long-term effects on the Galilean satellites such as the possible existence of tidal dissipation on their periods.

Lieske, J. H.↗
NASA NTRS1986

On the distance measure for gravitational lenses

A discussion is presented of some of the physical elements that are important for geometric optics in curved space-time, with a view toward interpreting data on gravitational lenses. This is important because each gravitational lens provides geometrical information about an arc that crosses a substantial portion of the universe. The propagation of photons along long arcs is highly sensitive to the mean density in the beam, averaged along the arc. Fluctuations about this mean are not important unless their scale is comparable to that of the universe. Tidal distortion is important if only a few lumps are involved. The variation in the geometrical optics is considerably greater if there is a nonzero cosmological constant.

Alcock, C.↗
NASA NTRS1986

Lunar and Planetary Science Conference, 16th, Houston, TX, March 11-15, 1985, Proceedings. Part 2

The present conference presents papers on the criteria, data and implications of pristine lunar glasses, lunar granulities and their precursor anorthositic norites of the early lunar crust, characterization and evidence for early formation in the megaregolith of Apollo 16 regolith breccias, and anorthosite assimilation and the origin of the Mg/Fe-related bimodality of pristine moon rocks in support of the magmasphere hypothesis. Other topics include the mineralogy of Yamato 791073 with reference to crystal fractionation of the howardite parent body, the geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16, tidal dissipation in a viscoelastic planet, and cosmogenic neutron-capture-produced nuclides in stony meteorites. Also considered are the first results of hydrous alteration of amorphous silicate smokes, elemental analysis of a comet nucleus by passive gamma ray spectrometry from a penetrator, and uranium series dating of Allan Hills ice.

Ryder, G.↗
NASA NTRS1986

Gas in and toward the Magellanic Clouds

Attention is given to the Ca II K absorption spectra of 48 early-type supergiants in the SMC and LMC, with a view to the Clouds' possible fragmentary nature and the distance of the gas responsible for the absorption lines at 60 and 120 km/sec. The comparison of the spectra obtained with H I measurements toward the Clouds indicates that in both Clouds, the velocity components in the neutral hydrogen distribution lie at different distances along the line of sight. Good agreement is found between the radial velocity shown in absorbing material and the radial velocity spread predicted by Murai and Fujimoto's (1980) dynamical model of the formation of the Magellanic Stream by tidal stripping. It is speculated that both the SMC and LMC are extended and fragmented along the line of sight.

Songaila, A.↗
NASA NTRS1986

Lunar nodal tide and distance to the moon during the Precambrian

The first direct determination of the lunar distance in the Precambrian is presented. A 23.3 + or - 0.3 yr periodicity preserved in 2500 Myr BP Australian banded iron formation is interpreted as reflecting the climatic influence of the lunar nodal tide, which has been detected with its modern 18.6-yr periodicity in some modern climate records. The lunar distance at 2500 Myr BP would then have been about 52 earth radii. The implied history of precambrian tidal friction is in accord with both the more recent paleontological evidence and the long-term stability of the lunar orbit. The length of the Milankovitch cycles that modulate the ice ages today also evolve with the earth-moon system. Their detection in the Precambrian sedimentary record would then permit an independent determination of the lunar distance.

Walker, J. C. G.↗
NASA NTRS1985

Space telescope observations of globular clusters

Six proposed Space Telescope programs involving globular clusters are described. The projects appropriate for galactic clusters are: the detection of white dwarfs, the study of the faint end of the Population II luminosity function, the measurement of mass segregation, and the search for a cusp in the density distribution caused by core collapse or by a massive black hole. The two programs that involve extragalactic globular clusters are: the determination of the luminosity function of clusters around different galaxies and the measurement of tidal radii of clusters surrounding elliptical galaxies.

Bahcall, J. N.↗
NASA NTRS1986

Mesospheric gravity-wave climatology at Adelaide

The MF Adelaide partial-reflection radar has been operating continuously since November 1983. This has enabled a climatology of gravity-wave activity to be constructed for the mesosphere. The data have been analyzed for a medium-period range of 1 to 8 hr. and a longer period range between 8 and 24 hr. covering the inertio-period waves. The tidal motions have been filtered out prior to analysis. For the data analyses so far (Nov. 1983 to Dec. 1984), a number of interesting features emerged. Firstly, the wave activity at heights above 80 km shows a small seimannual variation with season with the activity being strongest in summer and winter. At heights below 80 km however, there is a similar but more marked variation with the weakest amplitudes occurring at the time of the changeovers in the prevailing circulation. If breaking gravity waves are responsible for much of the turbulence in the mesosphere, then the periods March to April and September to October might also be expected to be periods of weak turbulence. The wave field appears to be partially polarized. The meridional amplitudes are larger than the zonal amplitudes, especially in water. It is found that the degree of polarization is about 15% in summer and 30% in winter. The polarized component is found to propagate in the opposite direction to the background flow in the stratosphere, which suggests that the polarization arises through directional filtering of the waves as they propagate up from below.

Vincent, R. A.↗
NASA NTRS1986

A CS J = 2 1 survey of the galactic center region

A CS map of the galactic center region is presented consisting of 15,000 spectra covering -1 deg. less than 3. deg. 6 min., -0 deg.4 min. less than b less than 0 deg. 4 min., each having an rms noise of 0.15 K in 1 MHz filters. CS is a high-excitation molecule, meaning that it is excited into emission only when the ambient density is less than n much greater than or approx. 2 x 10 to the 4th power/cu cm CS emission in the inner 2 deg. of the galaxy is nearly as pervasive as CO emission, in stark contrast to the outer galaxy where CS emission is confined to cloud cores. Galactic center clouds are on average much more dense than outer Galaxy clouds. This can be understood as a necessary consequence of the strong tidal stresses in the inner galaxy.

Stark, A. A.↗
NASA NTRS1986

Some considerations on measuring the Newtonian gravitational constant G in an orbiting laboratory

A common suggestion for measuring the Newtonian gravitational constant G in a near-earth orbiting laboratory is simply to put two balls in orbit around each other and observe the resulting motion, thereby determining G. However, the radial variation with distance of the gravitational field of the earth is so large that tidal forces on the balls in near-earth orbit can be several times greater than the gravitational attraction between the two masses, leading some writers to assume that two objects will not stably orbit about each other and that this method of measuring G in low-earth is impossible, or at least impractical. Certain orbits were identified which are stable (at least over many periods of the spacecraft about the earth). In this case, the objects experience their gravitational interaction for a long time, and it becomes reasonable to consider such orbits as candidates for measurements of G.

Baker, Stephen D.↗
NASA NTRS1987

Geophysical parameters from the analysis of laser ranging to starlette

Starlette Satellite Laser Ranging (SLR) data were used, along with several other satellite data sets, for the solution of a preliminary gravity field model for TOPEX, PTGF1. A further improvement in the earth gravity model was accomplished using data collected by 12 satellites to solve another preliminary gravity model for TOPEX, designated PTGF2. The solution for the Earth Rotation Parameter (ERP) was derived from the analysis of SLR data to Starlette during the MERIT Campaign. Starlette orbits in 1976 and 1983 were analyzed for the mapping of the tidal response of the earth. Publications and conference presentations pertinent to research are listed.

Schutz, B. E.↗
NASA NTRS1987

Orbital dynamics of the Uranian satellites based on Voyager data

The satellites of Uranus all have significant non-zero eccentricities and in the case of Miranda a significant inclination as well. The Voyager 2 encounter with Uranus in January 1986, provided more accurate estimates of the masses and sizes of the satellites. Orbital history and possible tidal heating were reexamined by using the Voyager data. The relevant orbital dynamics equations are described.

Plescia, J. B.↗
NASA NTRS1987

Shapes of small satellites

The accurate measurement of limb coordinates on small satellite is the major technique for evaluating their sizes and shapes in the absence of good stereoscopic image coverage. The shapes of satellites may be clues to their internal structure if they are relaxed into ellipsoids. The distinction between irregular and ellipsoidal satellites can now be made quantitatively with the availability of many accurate limb profiles of satellites. The residuals from these fit ellipses provide a convenient and quantitative measure of the topography on the satellites. Departures from gravitational (and tidal) equipotentials would be greater, but on the average this is a good measure of the relaxed or nonrelaxed nature of the shape of the objects. The limb topography is also related to specific forms on the objects, and a systematic survey of these forms on small and larger objects is underway.

Thomas, P.↗
NASA NTRS1987

Investigations of the surfaces and interiors of outer planet satellites

Studies during 1985/86 include tidal heating and the structure and evolution of Io and Europa, crater relaxation on icy satellites using realistic non-Newtonian ice rheology, and convection through phase transition in the interiors of icy satellites. The abstracts of published papers on these subjects are reproduced.

Schubert, Gerald↗
NASA NTRS1987

A scaling law for accretion zone sizes

Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass).

Greenzweig, Yuval↗
NASA NTRS1987

Dynamics of satellites, asteroids, and rings

Work is reported on: (1) the shapes and the internal structures of satellites; (2) the tidal heating of Miranda; (3) the dynamics of arc-like rings; and (4) the structure of the zodiacal cloud that was revealed by the Infrared Astronomy Satellite. Significant progress was made in determining the shape and internal structure of Mimas and in understanding the dynamical evolution of Miranda's orbit.

Dermott, Stanley F.↗