Mercury’s Interior Structure, Rotation, and Tides

This review addresses the deep interior structure of Mercury. Mercury is thought to consist of similar chemical reservoirs (core, mantle, crust) as the other terrestrial planets, but with a relatively much larger core. Constraints on Mercury’s composition and internal structure are reviewed, and possible interior models are described. Large advances in our knowledge of Mercury’s interior are not only expected from imaging of characteristic surface features but particularly from geodetic observations of the gravity field, the rotation, and the tides of Mercury. The low-degree gravity field of Mercury gives information on the differences of the principal moments of inertia, which are a measure of the mass concentration toward the center of the planet. Mercury’s unique rotation presents several clues to the deep interior. From observations of the mean obliquity of Mercury and the low-degree gravity data, the moments of inertia can be obtained, and deviations from the mean rotation speed (librations) offer an exciting possibility to determine the moment of inertia of the mantle. Due to its proximity to the Sun, Mercury has the largest tides of the Solar System planets. Since tides are sensitive to the existence and location of liquid layers, tidal observations are ideally suited to study the physical state and size of the core of Mercury.     

Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars

Recent discoveries of planets orbiting several nearby solar-mass stars have focussed renewed attention on the frequency and evolution of planetary systems and their evolutionary precursors around both solar and intermediate (2 M/M 8) mass stars. As a result of a wealth of new observations at all wavelengths of the circumstellar material around the nearest of the young intermediate-mass stars, the so-called Herbig Ae/Be (HAeBe) stars, we are beginning to see how these systems are similar to the solar mass objects, and how they differ. A review of the recent literature is presented, including the evolutionary status of the stars, binary frequency and the star forming environment, the morphology of the circumstellar material, including the first direct detections of disks in Keplerian rotation around these objects, and mass loss and infall phenomena. Prospects for advances in this research area as a result of advances in instrumentation are reviewed.     

Background limited infrared and submillimeter instruments

The temperature and emissivity of infrared and submillimeter telescopes are basic parameters that drive the optical and thermal design of astronomical space projects. They determine also, among other parameters, the self-emission of the instrument and the photon noise produced by this radiation on the detectors. By comparing the telescope brightness with that of the sky in the 1m–1 cm wavelength range, general conditions for background limited photometry are derived. For <0.4 mm, temperature is the driving parameter, and for >0.4 mm, temperature and emissivity have equivalent importances. It can be shown on actual projects that these two regimes determine different optical and thermal concepts. Although based on a simplistic approach, this work intends to help designers to handle some basic system parameters of infrared and submillimeter instruments.     

Energetic particle transport coefficients in the heliosphere

It is the purpose of this review to summarize and discuss recent research done in the field of particle propagation in the heliosphere. Several lines of approach have been followed to treat this problem. As a starting point the different forms of the transport equation are discussed. Quasi-Linear Theory (QLT) relates the power contained in fluctuations of the Interplanetary Magnetic Field (IMF) to the transport coefficients of energetic particles, an outline of the basic results of this theory is presented followed by a discussion of subsequent corrections made to the original formulation with an emphasis in recent developments where the effects of wave polarization, its propagation respect to the solar wind and the dissipation of power at large frequencies have been taken into account. The numerical approach using test particle trajectory integrations to obtain transport coefficients based on in situ satellite measureents is also discussed. It is well known that the determination of the particles mean free path for solar particle events by alternative methods leads to conflicting results, corrections made to original QLT are attempts to bridge the gap. Determination of the transport parameters from different lines of approach in a comparative basis have been done recently by calculating power spectra of IMF measured at the time solar particles were detected on the same spaceprobe, and performing numerical simulations with equivalent IMF data. Some of the results of such studies point to the solution of the conflicting determinations of the mean free path which has existed for nearly 30 years. An assesment of the present situation in this respect is given. Numerical determinations of transport parameters in the outer heliosphere are also reviewed and its consequences for solar modulation of galactic cosmic rays discussed. Space Science Reviews 62: Printed in Belgium.     

Measurements of the 12C/13C Ratio in Planetary Nebulae and Implications for Stellar Evolution

We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebulae (PNe) by means of mm-wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition; the other group consists of 23 nebulae with rich molecular envelopes. We have determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have ratios well below the value predicted by standard evolutionary models ( 20), indicating that some extra-mixing process has occurred in these stars. We briefly discuss the implications of our results for standard and nonstandard stellar nucleosynthesis.     

Characteristics of the decay phase of proton fluxes in solar events as a function of observer’s heliolongitude

Events in energetic solar protons with the energy > 4 MeV at the stage of their decay are considered for the period from 1974 to 2001. It is shown that in the events with the exponential shape of decay for west flares (relative to the observation point), the characteristic decay time τ and the power index γ of the energy spectrum decrease with an increase in the angular distance between the observer and the source of the particles on the Sun, while this effect is absent for east flares.     

Seeing the Forest or the Trees? Shifting Categorical Effects in Map Memory

People use spatial and nonspatial information to structure memory for an environment. Two experiments explored interactions between spatial and social categories on map memory when mediated by retrieval (Experiment 1) and encoding (Experiment 2) demands. Participants studied a map depicting business locations (including proprietors' race). In Experiment 1, participants completed two memory tasks, one globally focused and the other locally focused. The global task compressed, while the local task expanded, within-category similarity. Furthermore, processing styles carried over to the subsequent task. Experiment 2 emphasized either the spatial or social category during encoding, which increased that category's weighting in memory. These results extend the work of Maddox, Rapp, Brion, and Taylor, suggesting that retrieval and encoding demands can shift how these categories affect spatial memory.     

Is Memory for Routes Enhanced by an Environment's Richness in Visual Landmarks?

We tested the hypothesis that a route's memorability is dependent on the frequency with which people are exposed to visual landmarks. Undergraduates learned either a route through an urban area lacking visually salient features, or a route in a neighborhood with many shops and urban objects. They were then asked to recall the learned route in the form of route directions and sketch maps. The results showed higher recall performance for the richer environment. When presented with photographs depicting scenes along the route, participants exposed to the richer environment had higher recognition scores and shorter response times than the others. The data confirm the functional role of landmarks in route memory and wayfinding.     

Physical Processes Related to Discharges in Planetary Atmospheres

This paper focuses on the rudimentary principles of discharge physics. The kinetic theory of electron transport in gases relevant to planetary atmospheres is examined and results of detailed Boltzmann kinetic calculations are presented for a range of applied electric fields. Comparisons against experimental swarm data are made. Both conventional breakdown and runaway breakdown are covered in detail. The phenomena of transient luminous events (TLEs), particularly sprites, and terrestrial gamma-ray flashes (TGFs) are discussed briefly as examples of discharges that occur in the terrestrial environment. The observations of terrestrial lightning that exist across the electromagnetic spectrum and presented throughout this volume fit well with the broader understanding of discharge physics that we present in this paper. We hope that this material provides the foundation on which explorations in search of discharge processes on other planets can be based and previous evidence confirmed or refuted.     

ODYSSEY, Orbit Determination Software for the Pioneer Data Analysis

The Pioneer anomaly refers to the difference between the computed trajectories of the Pioneer 10 and 11 spacecrafts and their actual trajectories as observed through Doppler tracking. This difference has been described by the Jet Propulsion Laboratory (JPL) as a constant anomalous acceleration. In order to perform an independent analysis, specific trajectography software, named ODYSSEY, has been developed. The paper will focus on the models implemented in this software and on the results obtained. The existence of a constant anomalous acceleration is confirmed with properties similar to those reported by JPL. Time dependent components of the anomaly are also found and discussed.