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The combination of the radio tracking of the MESSENGER spacecraft and Earth-based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density \rho, moment of inertia C, and moment of inertia of the outer solid shell Cm. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parametrized by the tidal potential Love number k2. We investigate the tidal response for sets of interior models that are compatible with the available constraints (\rho, C, and Cm). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell, and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain-size of 1~cm, we calculate that the tidal k2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

The heat transport and lithospheric dynamics of early Earth are currently explained by plate tectonic and vertical tectonic models, but these do not offer a global synthesis consistent with the geologic record. Here we use numerical simulations and comparison with the geologic record to explore a heat-pipe model in which volcanism dominates surface heat transport. These simulations indicate that a cold and thick lithosphere developed as a result of frequent volcanic eruptions that advected surface materials downwards. Declining heat sources over time led to an abrupt transition to plate tectonics. Consistent with model predictions, the geologic record shows rapid volcanic resurfacing, contractional deformation, a low geothermal gradient across the bulk of the lithosphere and a rapid decrease in heat- pipe volcanism after initiation of plate tectonics. The heat-pipe Earth model therefore offers a coherent geodynamic framework in which to explore the evolution of our planet before the onset of plate tectonics., The lithospheric dynamics of terrestrial planets is driven by the transport of heat from the interior to the surface (1). Terrestrial bodies with low heat flows (for example Mars ( [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.08.020 Byline: Jennifer Palguta (a), John D. Anderson (b), Gerald Schubert (a)(c), William B. Moore (a) Keywords: Satellites of Jupiter; Ganymede; Surfaces; satellite Abstract: Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions -- a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77[degrees] N, 333[degrees] W), (36[degrees] N, 0[degrees] W), (33[degrees] N, 130[degrees] W), and (7[degrees] N, 194[degrees] W). If the mass anomalies are located at the deep ice-rock interface 800 km below the surface, the regions' centers are approximately (65[degrees] N, 17[degrees] W), (32[degrees] N, 30[degrees] W), (37[degrees] N, 175[degrees] W), and (15[degrees] N, 211[degrees] W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10.sup.17 kg. Mass anomalies at the ice-rock interface are on average no more than an order of magnitude larger (10.sup.18 kg). There are two positive and two negative mass anomalies in both the surface and ice-rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice-rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice-rock interface. Author Affiliation: (a) Department of Earth and Space Sciences, University of California, 3806 Geology, Los Angeles, CA 90095-1567, USA (b) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA (c) Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA Article History: Received 19 April 2005; Revised 2 August 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.09.005 Byline: William B. Moore Keywords: Europa; Interiors; Thermal histories Abstract: Models of tidal-convective equilibrium for Europa's ice shell are computed using a laboratory-derived composite flow law for ice. Volume diffusion creep is found to dominate the flow law at equilibrium, and thus the thickness of the shell is strongly dependent on the poorly known grain size of the ice. This grain size is, however, constrained to be less than a few millimeters if equilibrium is achieved at the current eccentricity. Europa's ice shell cannot be thinner than about 16 km in equilibrium at present, since tidal dissipation cannot generate enough heat in such a thin shell to balance the heat transport. No conductive equilibria are found; this is likely due to the fact that most of a conductive shell must be cold if temperature gradients are to be large enough to carry the heat. A minimum eccentricity of about 0.0025 (about 1/4 the present value) below which there are no equilibria is also found. Author Affiliation: Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 3806 Geology Bldg., Box 951567, Los Angeles, CA 90095-1567, USA Article History: Received 19 May 2005; Revised 26 August 2005

Calculations of the tidal responses of Ganymede and Callisto reveal that tidal amplitudes on these bodies may be as large as a few meters if a liquid ocean exists to decouple the surface ice from the interior. Tides on Ganymede's surface can exceed 7 m peak-to-peak variation, while on Callisto the tidal amplitude can exceed 5 m in the presence of a liquid ocean. Without an ocean, tidal amplitudes are less than 0.5 m on Ganymede and less than 0.3 m on Callisto. An orbiting spacecraft using an altimeter for crossover analysis and Doppler tracking from Earth should be able to achieve sufficient accuracy to identify the tidal amplitude to within about a meter over the course of a few months (observing tens of tidal cycles).

Geoid and topography observations are used to constrain the density structure of the venusian lithosphere under the assumptions that convective stresses are small and local isostasy prevails. Results are presented for 13 venusian highlands, including Ishtar Terra. In general, Venus has a thick (200-400 km) thermal lithosphere which is thinned beneath volcanic highlands by as much as 80%, inducing slopes of 0.05 to 0.18 in the base of the lithosphere. From scalings of geoid/topography ratio and lithospheric basal slope as a function of internal Rayleigh number and viscosity contrast, we estimate lower bounds to these parameters of 107 and [10.sup.s], respectively. Several lines of evidence point to values near these lower limits, which puts Venus in the sluggish-lid regime of convection. There also exist several accumulations of crustal material, most notably Ovda Regio, where there may be as much as 90 km of crust.

The boundary between the solar wind and the Earth's magnetosphere, the magnetopause (MP), is highly dynamic. Its location and shape depend on SW dynamic pressure and interplanetary magnetic field (IMF) orientation. We use a 3D kinetic Particle‐In‐Cell code (IAPIC) to simulate an event observed by THEMIS spacecraft on July 16, 2007. We investigate the impact of radial (θBx = 0°) and non‐radial (θBx = 50°) IMF on the shape and size of Earth's MP for a dipole tilt of 31° using maximum density gradient and pressure balance methods. Using the Shue model as a reference (MP at 10.3 RE), we find that for non‐radial IMF the MP expands by 1.4 and 1.7RE along the Sun‐Earth (OX) and tilted magnetic equatorial (Tilt) axes, respectively, and it expands by 0.5 and 1.6RE for radial IMF along the same respective axes. When the effect of backstreaming ions is removed from the bulk flow, the expansion ranges are 1.0 and 1.3RE and 0.2, and 1.2RE, respectively. It is found that the percentage of backstreaming to bulk flow ions are 16.5% and 20% for radial and non‐radial IMF. We also show that when the backstreaming ions are not identified, up to 40% of the observed expansion that is due to backstreaming particles can be inadvertently attributed to a change in the SW upstream properties. Finally, we quantified the temperature anisotropy in the magnetosheath, and observe a strong dawn‐dusk asymmetry in the MP location, being more extended on the duskside than on the dawnside. Plain Language Summary: The Earth's magnetopause (MP) is a sensitive region where the pressure of the Earth magnetic field balances the shocked solar wind (SW) ram and thermal pressures. Accurate space weather monitoring and forecast require an in‐depth knowledge of this region and of the physical processes that affect it. We investigate the effects of the orientation of the interplanetary magnetic field (IMF) on the MP size, location, and shape by using a numerical technique (3D kinetic simulation) that uses macro‐ions and macro‐electrons to mimic the real charged species and explicitly resolves their motion. We can extract the shape and location of the MP in three dimensions to estimate the sensitivity of the MP position to the ambient IMF and SW conditions, while our explicit tracking of ion motions allows us to investigate the role of ion species backscattered by the Earth's bow shock and magnetosheath. We find that when the IMF has a strong component in the Earth‐Sun direction (radial or non‐radial IMF), these backscattered ions play an important role in reducing the effective pressure of the incoming SW and allowing the MP to expand outward. Key Points: The MP expansion at the subsolar point for radial & non‐radial interplanetary magnetic field (IMF) is partly due to the reduction of the solar wind (SW) Pdyn caused by backstreamingionsAlthough backstreaming ions constitute only 16.5% and 20% of the SW bulk flow for radial and non‐radial IMF, they modify the dynamic pressureFor both radial and non‐radial IMF, the magnetopause is asymmetric, being more extended on the duskside than on the dawnside [ABSTRACT FROM AUTHOR]

The arcuate segments of chasmata and the gravity signal of coronae of the planet Venus indicate that there are two different types of coronae, formed by different processes. Small coronae with a diameter of 100 kilometers originating from diapiric mantle upwellings and coronae of different sizes formed from chasmata arcuate segments are the two types of coronae. The presence of the two types of coronae is revealed in the global spherical harmonic model of the gravity field of Venus.

Thermal history models, historically used to understand Earth's geologic history, are being coupled to climate models to map conditions that allow planets to maintain life. However, the lack of structural uncertainty assessment has blurred guidelines for how thermal history models can be used toward this end. Structural uncertainty is intrinsic to the modeling process. Model structure refers to the cause and effect relations that define a model and are assumed to adequately represent a particular real world system. Intrinsic/structural uncertainty is different from input and parameter uncertainties (which are often evaluated for thermal history models). A full uncertainty assessment requires that input/parametric and intrinsic/structural uncertainty be evaluated (one is not a substitute for the other). We quantify the intrinsic uncertainty for several parameterized thermal history models (a subclass of planetary models).We use single perturbation analysis to determine the reactance time of different models. This provides a metric for how long it takes low-amplitude, unmodeled effects to decay or grow. Reactance time is shown to scale inversely with the strength of the dominant model feedback (negative or positive). A perturbed physics analysis is then used to determine uncertainty shadows for model outputs. This provides probability distributions for model predictions. It also tests the structural stability of a model (do model predictions remain qualitatively similar, and within assumed model limits, in the face of intrinsic uncertainty?). Once intrinsic uncertainty is accounted for, model outputs/predictions and comparisons to observational data should be treated in a probabilistic way. [ABSTRACT FROM AUTHOR]

Designing and developing the 1.7- to 2.1-MJ power conditioning system (PCS) that will power the flashlamps of the main and power amplifiers for the National Ignition Facility (NIF) lasers is one of several responsibilities assumed by Sandia National Laboratories (SNL) and Maxwell Physics International in support of the NIF Project. The NIF is currently being constructed at Lawrence Livermore National Laboratories (LLNL). The test facility that evolved over three years to satisfy the project requirements is called the First Article NIF Test Module (FANTM). It was built at SNL and operated for about 17 000 shots to demonstrate component performance expectations over the lifetime of NIF. A few modules similar to the one shown in Fig. 1 will be used initially in the amplifier test phase of the project. The final NIF system will require at least 192 modules in the four capacitor bays. This paper briefly summarizes the final design of the FANTM facility and compares its performance with the predictions of circuit simulations for both normal operation and fault-mode response. Applying both the measured and modeled power pulse waveforms as input to a LLNL amplifier gain code indicates that the 20-capacitor PCS can satisfy the NIF requirement for an average gain coefficient of 5.00%/cm and can exceed 5.20%/cm with 24 capacitors. Index Terms--Arc, capacitor, circuit simulation, electrode erosion, fault diagnosis, flashlamp, gain coefficient, inductor, power conditioning, power distribution cables, spark gap.

The tidal response of Europa to a time-varying potential is determined by solving the quasi-static equilibrium equations for a body composed of several uniform, Maxwell viscoelastic layers. Models in which Europa's surface ice extends all the way to the silicate mantle have weak tidal responses (~1 m vertical deflection) while models in which a liquid 'ocean' underlies the ice have about 30 m peak vertical deflection. The phase-lag of the tidal response is very small ([is less than] 2 [degrees]) in the case of an ocean, but may be large if there is no ocean and the ice has a viscosity around [10.sup.13] Pa s. For models with an ocean, the product of the thickness and strength of the ice determines the tidal response, making it difficult to determine the thickness of the ice by observing the tidal deflection of Europa. [C] 2000 Academic Press