Influence of the Interplanetary Magnetic Field on the Solar Wind Flow about Planetary Obstacles

The main effects caused by the interplanetary magnetic field (IMF) are analyzed in cases of supersonic solar wind flow around magnetized planets (like Earth) and nonmagnetized (like Venus) planets. The IMF has a relatively weak strength in the solar wind but it is enhanced considerably in the so-called plasma depletion layer or magnetic barrier in the vicinity of the streamlined obstacle (magnetopause of a magnetized planet, or ionopause of a nonmagnetized planet). For magnetized planets, the magnetic barrier is a source of free magnetic energy for magnetic reconnection in cases of large magnetic shear at the magnetopause. For nonmagnetized planets, mass loading of the ionospheric particles is very important. The new created ions are accelerated by the electric field related to the IMF, and thus they gain energy from the solar wind plasma. These ions form the boundary layer within the magnetic barrier. This mass loading process affects considerably the profiles of the magnetic field and plasma parameters in the flow region.     

Communication from Mars: Requirements and Limitations

The exploration of our nearest planets will require relaying large amounts of data to Earth for study and evaluation. However, our ability to communicate at interplanetary distances is limited. In this paper, an evaluation is made of our capability to communicate from the vicinity of Mars using the present S-band deep-space network, prospects for enhancing that performance, and limitations beyond which no additional improvement seems feasible. In addition, requirements for real-time television are evaluated and prospects for improving the communication rates by operating at higher microwave frequencies considered.     

Ion Acceleration and Outflow from Mars and Venus: An Overview

Solar wind forcing of Mars and Venus results in outflow and escape of ionospheric ions. Observations show that the replenishment of ionospheric ions starts in the dayside at low altitudes (??300?C800 km), ions moving at a low velocity (5?C10 km/s) in the direction of the external/ magnetosheath flow. At high altitudes, in the inner magnetosheath and in the central tail, ions may be accelerated up to keV energies. However, the dominating energization and outflow process, applicable for the inner magnetosphere of Mars and Venus, leads to outflow at energies ??5?C20 eV. The aim of this overview is to analyze ion acceleration processes associated with the outflow and escape of ionospheric ions from Mars and Venus. Qualitatively, ion acceleration may be divided in two categories:

1. Modest ion acceleration, leading to bulk outflow and/or return flow (circulation).
2. Acceleration to well over escape velocity, up into the keV range.

In the first category we find a processes denoted ??planetary wind??, the result of e.g. ambipolar diffusion, wave enhanced planetary wind, and mass-loaded ion pickup. In the second category we find ion pickup, current sheet acceleration, wave acceleration, and parallel electric fields, the latter above Martian crustal magnetic field regions. Both categories involve mass loading. Highly mass-loaded ion energization may lead to a low-velocity bulk flow??A consequence of energy and momentum conservation. It is therefore not self-evident what group, or what processes are connected with the low-energy outflow of ionospheric ions from Mars. Experimental and theoretical findings on ionospheric ion acceleration and outflow from Mars and Venus are discussed in this report.     

Solar Variability and Climate Impact on Terrestrial Planets

Some possible factors of climate changes and of long term climate evolution are discussed with regard of the three terrestrial planets, Earth, Venus and Mars. Two positive feedback mechanisms involving liquid water, i.e., the albedo mechanism and the greenhouse effect of water vapour, are described. These feedback mechanisms respond to small external forcings, such as resulting from solar or astronomical constants variability, which might thus result in large influences on climatic changes on Earth. On Venus, reactions of the atmosphere with surface minerals play an important role in the climate system, but the involved time scales are much larger. On Mars, climate is changing through variations of the polar axis inclination over time scales of ～10⁵–10⁶ years. Growing evidence also exists that a major climatic change happened on Mars some 3.5 to 3.8 Gigayears ago, leading to the disappearance of liquid water on the planet surface by eliminating most of the CO₂ atmosphere greenhouse power. This change might be due to a large surge of the solar wind, or to atmospheric erosion by large bodies impacts. Indeed, except for their thermospheric temperature response, there is currently little evidence for an effect of long-term solar variability on the climate of Venus and Mars. This fact is possibly due to the absence of liquid water on these terrestrial planets.     

Space dust high-velocity particle influence on electronic equipment

Determination of failure causes of the control systems and navigation electronic elements of space flying vehicles (SFV), observed in the process of long-time flights, is of current importance. Failures of the control systems in the process of long-time flights considerably reduce their reliability and, consequently, raise expenses in this field. In particular, this requires an additional backup of satellites-retransmitters, which results in an increase of the number of SFV launches. At present, experts are actively elaborating the hypothesis that one of the most probable causes of the SFV electronic system failures are the flows of the space dust particles, moving in the near-Earth and interplanetary space, and possessing velocities from 7.8 km/sec to 16 km/sec. The flows of the high energy (Galactic) ions, for a number of cases, are considered to be the basic cause of the destruction of electronic control systems. This, in particular, makes it possible to explain the failures in flights to Mars of the autonomous space modules performed by the European and American space agencies. The optimum flight route to Mars passes through agglomerations of dust clouds, which necessitates a high probability of collision for the flying vehicles with dust clots. Therefore, the success of the American research module is quite logical, which was done with the highest deviation from the optimum route.     

The Formation of Mars: Building Blocks and Accretion Time Scale

In this review paper I address the current knowledge of the formation of Mars, focusing on its primary constituents, its formation time scale and its small mass compared to Earth and Venus. I argue that the small mass of Mars requires the terrestrial planets to have formed from a narrow annulus of material, rather than a disc extending to Jupiter. The truncation of the outer edge of the disc was most likely the result of giant planet migration, which kept Mars’ mass small. From cosmochemical constraints it is argued that Mars formed in a couple of million years and is essentially a planetary embryo that never grew to a full-fledged planet. This is in agreement with the latest dynamical models. Most of Mars’ building blocks consists of material that formed in the 2 AU to 3 AU region, and is thus more water-rich than that accreted by Earth and Venus. The putative Mars could have consisted of 0.1 % to 0.2 % by mass of water.     

Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites

Of the terrestrial planets, Earth and Mercury have self-sustained fields while Mars and Venus do not. Magnetic field data recorded at Ganymede have been interpreted as evidence of a self-generated magnetic field. The other icy Galilean satellites have magnetic fields induced in their subsurface oceans while Io and the Saturnian satellite Titan apparently are lacking magnetic fields of internal origin altogether. Parts of the lunar crust are remanently magnetized as are parts of the crust of Mars. While it is widely accepted that the magnetization of the Martian crust has been caused by an early magnetic field, for the Moon alternative explanations link the magnetization to plasma generated by large impacts. The necessary conditions for a dynamo in the terrestrial planets and satellites are the existence of an iron-rich core that is undergoing intense fluid motion. It is widely accepted that the fluid motion is caused by convection driven either by thermal buoyancy or by chemical buoyancy or by both. The chemical buoyancy is released upon the growth of an inner core. The latter requires a light alloying element in the core that is enriched in the outer core as the solid inner core grows. In most models, the light alloying element is assumed to be sulfur, but other elements such as, e.g., oxygen, silicon, and hydrogen are possible. The existence of cores in the terrestrial planets is either proven beyond reasonable doubt (Earth, Mars, and Mercury) or the case for a core is compelling as for Venus and the Moon. The Galilean satellites Io and Ganymede are likely to have cores judging from Galileo radio tracking data of the gravity fields of these satellites. The case is less clear cut for Europa. Callisto is widely taken as undifferentiated or only partially differentiated, thereby lacking an iron-rich core. Whether or not Titan has a core is not known at the present time. The terrestrial planets that do have magnetic fields either have a well-established inner core with known radius and density such as Earth or are widely agreed to have an inner core such as Mercury. The absence of an inner core in Venus, Mars, and the Moon (terrestrial bodies that lack fields) is not as well established although considered likely. The composition of the Martian core may be close to the Fe–FeS eutectic which would prevent an inner core to grow as long as the core has not cooled to temperatures around 1500 Kelvin. Venus may be on the verge of growing an inner core in which case a chemical dynamo may begin to operate in the geologically near future. The remanent magnetization of the Martian and the lunar crust is evidence for a dynamo in Mars’ and possibly the Moon’s early evolution and suggests that powerful thermally driven dynamos are possible. Both the thermally and the chemically driven dynamo require that the core is cooled at a sufficient rate by the mantle. For the thermally driven dynamo, the heat flow from the core into the mantle must by larger than the heat conducted along the core adiabat to allow a convecting core. This threshold is a few mW?m^?2 for small planets such as Mercury, Ganymede, and the Moon but can be as large as a few tens mW?m^?2 for Earth and Venus. The buoyancy for both dynamos must be sufficiently strong to overcome Ohmic dissipation. On Earth, plate tectonics and mantle convection cool the core efficiently. Stagnant lid convection on Mars and Venus are less efficient to cool the core but it is possible and has been suggested that Mars had plate tectonics in its early evolution and that Venus has experienced episodic resurfacing and mantle turnover. Both may have had profound implications for the evolution of the cores of these planets. It is even possible that inner cores started to grow in Mars and Venus but that the growth was frustrated as the mantles heated following the cessation of plate tectonics and resurfacing. The generation of Ganymede’s magnetic field is widely debated. Models range from magneto-hydrodynamic convection in which case the field will not be self-sustained to chemical and thermally-driven dynamos. The wide range of possible compositions for Ganymede’s core allows models with a completely liquid near eutectic Fe–FeS composition as well as models with Fe inner cores or cores in with iron snowfall.     

Planetary Magnetic Fields and Solar Forcing: Implications for Atmospheric Evolution

The solar wind and the solar XUV/EUV radiation constitute a permanent forcing of the upper atmosphere of the planets in our solar system, thereby affecting the habitability and chances for life to emerge on a planet. The forcing is essentially inversely proportional to the square of the distance to the Sun and, therefore, is most important for the innermost planets in our solar system—the Earth-like planets. The effect of these two forcing terms is to ionize, heat, chemically modify, and slowly erode the upper atmosphere throughout the lifetime of a planet. The closer to the Sun, the more efficient are these process. Atmospheric erosion is due to thermal and non-thermal escape. Gravity constitutes the major protection mechanism for thermal escape, while the non-thermal escape caused by the ionizing X-rays and EUV radiation and the solar wind require other means of protection. Ionospheric plasma energization and ion pickup represent two categories of non-thermal escape processes that may bring matter up to high velocities, well beyond escape velocity. These energization processes have now been studied by a number of plasma instruments orbiting Earth, Mars, and Venus for decades. Plasma measurement results therefore constitute the most useful empirical data basis for the subject under discussion. This does not imply that ionospheric plasma energization and ion pickup are the main processes for the atmospheric escape, but they remain processes that can be most easily tested against empirical data. Shielding the upper atmosphere of a planet against solar XUV, EUV, and solar wind forcing requires strong gravity and a strong intrinsic dipole magnetic field. For instance, the strong dipole magnetic field of the Earth provides a “magnetic umbrella”, fending of the solar wind at a distance of 10 Earth radii. Conversely, the lack of a strong intrinsic magnetic field at Mars and Venus means that the solar wind has more direct access to their topside atmosphere, the reason that Mars and Venus, planets lacking strong intrinsic magnetic fields, have so much less water than the Earth? Climatologic and atmospheric loss process over evolutionary timescales of planetary atmospheres can only be understood if one considers the fact that the radiation and plasma environment of the Sun has changed substantially with time. Standard stellar evolutionary models indicate that the Sun after its arrival at the Zero-Age Main Sequence (ZAMS) 4.5 Gyr ago had a total luminosity of ≈70% of the present Sun. This should have led to a much cooler Earth in the past, while geological and fossil evidence indicate otherwise. In addition, observations by various satellites and studies of solar proxies (Sun-like stars with different age) indicate that the young Sun was rotating more than 10 times its present rate and had correspondingly strong dynamo-driven high-energy emissions which resulted in strong X-ray and extreme ultraviolet (XUV) emissions, up to several 100 times stronger than the present Sun. Further, evidence of a much denser early solar wind and the mass loss rate of the young Sun can be determined from collision of ionized stellar winds of the solar proxies, with the partially ionized gas in the interstellar medium. Empirical correlations of stellar mass loss rates with X-ray surface flux values allows one to estimate the solar wind mass flux at earlier times, when the solar wind may have been more than 1000 times more massive. The main conclusions drawn on basis of the Sun-in-time-, and a time-dependent model of plasma energization/escape is that:

1. Solar forcing is effective in removing volatiles, primarily water, from planets,
2. planets orbiting close to the early Sun were subject to a heavy loss of water, the effect being most profound for Venus and Mars, and
3. a persistent planetary magnetic field, like the Earth’s dipole field, provides a shield against solar wind scavenging.

     

Infrared Spectroscopy of Solar-System Planets

Most of our knowledge regarding planetary atmospheric composition and structure has been achieved by remote sensing spectroscopy. Planetary spectra strongly differ from one planet to another. CO₂ signatures dominate on Mars, and even more on Venus (where the thermal component is detectable down to 1 μm on the dark side). Spectroscopic monitoring of Venus, Earth and Mars allows us to map temperature fields, wind fields, clouds, aerosols, surface mineralogy (in the case of the Earth and Mars), and to study the planets’ seasonal cycles. Spectra of giant planets are dominated by H₂, CH₄ and other hydrocarbons, NH₃, PH₃ and traces of other minor compounds like CO, H₂O and CO₂. Measurements of the atmospheric composition of giant planets have been used to constrain their formation scenario.     

Modeling of Ground Deformation and Shallow Surface Waves Generated by Martian Dust Devils and Perspectives for Near-Surface Structure Inversion

We investigated the possible seismic signatures of dust devils on Mars, both at long and short period, based on the analysis of Earth data and on forward modeling for Mars. Seismic and meteorological data collected in the Mojave Desert, California, recorded the signals generated by dust devils. In the 10–100 s band, the quasi-static surface deformation triggered by pressure fluctuations resulted in detectable ground-tilt effects: these are in good agreement with our modeling based on Sorrells’ theory. In addition, high-frequency records also exhibit a significant excitation in correspondence to dust devil episodes. Besides wind noise, this signal includes shallow surface waves due to the atmosphere-surface coupling and is used for a preliminary inversion of the near-surface S-wave profile down to 50 m depth. In the case of Mars, we modeled the long-period signals generated by the pressure field resulting from turbulence-resolving Large-Eddy Simulations. For typical dust-devil-like vortices with pressure drops of a couple Pascals, the corresponding horizontal acceleration is of a few nm/s² for rocky subsurface models and reaches 10–20 nm/s² for weak regolith models. In both cases, this signal can be detected by the Very-Broad Band seismometers of the InSight/SEIS experiment up to a distance of a few hundred meters from the vortex, the amplitude of the signal decreasing as the inverse of the distance. Atmospheric vortices are thus expected to be detected at the InSight landing site; the analysis of their seismic and atmospheric signals could lead to additional constraints on the near-surface structure, more precisely on the ground compliance and possibly on the seismic velocities.     

Plasma populations in a simple open model magnetosphere

The acceleration of charged particles in the magnetic current sheets downstream from magnetic neutral lines is discussed and related to the plasma populations expected to be formed in a simple open model magnetosphere. A simple model of plasma acceleration in the dayside current sheet is set up, and it is shown that magnetospheric particles may take up a considerable fraction of the electromagnetic energy dissipated in the sheet even though they may represent only a small fraction of the total particle influx. The process should result in energetic ring current and ionospheric particles being found in boundary layers on either side of the magnetopause, and accelerated ionospheric particles in the plasma mantle. Acceleration of magnetosheath plasma in the dayside current sheet should result in enhanced flow speeds in these boundary layers, but the process may amount to little more than a return to the sheath plasma of energy previously extracted from it during its inflow on the dayside and stored in the compressed sheath field, due to the appreciable energy take-up from the current sheet by magnetospheric particles. The energy separation between ionospheric plasma and magnetosheath plasma on cusp field lines is shown to result in a spatial separation of polar wind and plasma mantle populations in the tail, the polar wind ions usually reaching out to only a few tens of R _E down-tail such that they usually remain on closed field lines, forming a wedge-shaped region within the mantle shadow-zone. Polar wind ions are then convected back towards the Earth and thus their major sink is via the dayside current sheet rather than outflow into the tail. The major source for the plasmasheet depends upon the location of the neutral line, but mantle ions may usually be dominant. However, with a near-Earth neutral line during disturbed periods ionospheric plasma will be the sole ring-current source. Under usual conditions with a more distant neutral line the spatial separation of the two plasma sources in the tail may result in an energy separation in the inner ring current, with ionospheric particles dominant up to 2 to 20 keV and mantle ions dominant at higher energies. Formation of the plasmasheet is discussed, and it is shown that a layer of ions unidirectionally streaming towards the Earth should be formed on its outer boundary, due to current sheet acceleration of lobe particles and inward convection of the field lines. A similar process leads to earthward flows on the inner layer of the dayside cusp. Finally, the region tailward of the nightside neutral line is discussed and it is shown that a thin tailward flowing two-stream plasma band should be formed across the centre plane of the tail. The slow-speed stream corresponds to incoming lobe ions, the faster stream to the current sheet accelerated ions.     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

Plasma Morphology at Mars. Aspera-3 Observations

A total of about of 400 orbits during the first year of the ASPERA-3 operation onboard the Mars Express spacecraft were analyzed to obtain a statistical pattern of the main plasma domains in the Martian space environment. The environment is controlled by the direct interaction between the solar wind and the planetary exosphere/ionosphere which results in the formation of the magnetospheric cavity. Ionospheric plasma was traced by the characteristic “spectral lines” of photoelectrons that make it possible to detect an ionospheric component even far from the planet. Plasma of solar wind and planetary origin was distinguished by the ion mass spectrometry. Several different regions, namely, boundary layer/mantle, plasma sheet, region with ionospheric photoelectrons, ray-like structures near the wake boundary were identified. Upstream parameters like solar wind ram pressure and the direction of the interplanetary electric field were inferred as proxy from the Mars Global Surveyor magnetic field data at a reference point of the magnetic pile up region in the northern dayside hemisphere. It is shown that morphology and dynamics of the main plasma domains and their boundaries are governed by these factors as well as by local crustal magnetizations which add complexity and variability to the plasma and magnetic field environment.     

Ring Current Dynamics

This chapter reviews the current understanding of ring current dynamics. The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ∼10,000 to 60,000 km. Enhancements in this current are responsible for global decreases in the Earth’s surface magnetic field, which have been used to define geomagnetic storms. Intense geospace magnetic storms have severe effects on technological systems, such as disturbances or even permanent damage of telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the ring current are positive ions, with energies from ∼1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere into the inner magnetosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while active processes in geospace tend to increase the abundance (both absolute and relative) of O⁺ ions, which are of ionospheric origin. During intense geospace magnetic storms, the O⁺ abundance increases dramatically. This increase has been observed to occur concurrently with the rapid intensification of the ring current in the storm main phase and to result in O⁺ dominance around storm maximum. This compositional change can affect several dynamic processes, such as species-and energy-dependent charge-exchange and wave-particle scattering loss.     

Below One Earth: The Detection, Formation, and Properties of Subterrestrial Worlds

The Solar System includes two planets—Mercury and Mars—significantly less massive than Earth, and all evidence indicates that planets of similar size orbit many stars. In fact, one of the first exoplanets to be discovered is a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets have inspired new ideas about planet formation and evolution, and these “sub-Earths” should be no exception: they include planets with masses between Mars and Venus for which there are no Solar System analogs. Advances in astronomical instrumentation and recent space missions have opened the sub-Earth frontier for exploration: the Kepler mission has discovered dozens of confirmed or candidate sub-Earths transiting their host stars. It can detect Mars-size planets around its smallest stellar targets, as well as exomoons of comparable size. Although the application of the Doppler method is currently limited by instrument stability, future spectrographs may detect equivalent planets orbiting close to nearby bright stars. Future space-based microlensing missions should be able to probe the sub-Earth population on much wider orbits. A census of sub-Earths will complete the reconnaissance of the exoplanet mass spectrum and test predictions of planet formation models, including whether low-mass M dwarf stars preferentially host the smallest planets. The properties of sub-Earths may reflect their low gravity, diverse origins, and environment, but they will be elusive: Observations of eclipsing systems by the James Webb Space Telescope may give us our first clues to the properties of these small worlds.     

Recent Results from Titan��s Ionosphere

Titan has the most significant atmosphere of any moon in the solar system, with a pressure at the surface larger than the Earth??s. It also has a significant ionosphere, which is usually immersed in Saturn??s magnetosphere. Occasionally it exits into Saturn??s magnetosheath. In this paper we review several recent advances in our understanding of Titan??s ionosphere, and present some comparisons with the other unmagnetized objects Mars and Venus. We present aspects of the ionospheric structure, chemistry, electrodynamic coupling and transport processes. We also review observations of ionospheric photoelectrons at Titan, Mars and Venus. Where appropriate, we mention the effects on ionospheric escape.     

Electrical Activity and Dust Lifting on Earth, Mars, and Beyond

We review electrical activity in blowing sand and dusty phenomena on Earth, Mars, the Moon, and asteroids. On Earth and Mars, blowing sand and dusty phenomena such as dust devils and dust storms are important geological processes and the primary sources of atmospheric dust. Large electric fields have been measured in terrestrial dusty phenomena and are predicted to occur on Mars. We review the charging mechanisms that produce these electric fields and discuss the implications of electrical activity to dust lifting and atmospheric chemistry. In addition, we review theoretical ideas about electric discharges on Mars. Finally, we discuss the evidence that electrostatics is responsible for dust transport on the Moon and asteroids.     

HF-W Chronometry and Inner Solar System Accretion Rates

Models for the mechanisms of accretion of the terrestrial planets are re-examined using the experimental technique of high-precision isotope ratio mass spectrometry of tungsten (W). The decay of ¹⁸²Hf to ¹⁸²W (via ¹⁸²Ta) provides a new kind of radiometric chronometer of planet formation processes. Hafnium and W, the parent and daughter trace elements, are highly refractory; however, Hf is lithophile and strongly partitioned into the silicate portion of a planet, whereas W is moderately siderophile and preferentially partitioned into a coexisting metallic phase. More than 90% of terrestrial W has gone into the Earth's core during its formation. The residual silicate portion, the Earth's primitive mantle, has a Hf/W ratio in the range 10−40, an order of magnitude higher than chondritic (∼1.3). Tungsten isotopic data for the Earth and the Moon suggest that we can date a major event of planet formation: The Moon formed about 50 Myrs after the start of the solar system, providing strong support for the Giant Impact Theory of lunar origin. Recent simulations of this event imply that the Earth was probably only half formed at the time. From this we can deduce the planetary accretion rate. Tungsten isotope data for Mars provide evidence of a much shorter accretion interval, perhaps as little as 10 Myrs, but the rates for the Earth over the same time interval could have been comparable. The large W isotopic heterogeneities on Mars could only have been produced within the first 30 Myrs of the solar system. Large-scale mixing, e.g. from convective overturn, as is thought to drive the Earth's plates, must be absent from Mars. Limitations of the method such as 1) cosmogenic ¹⁸²Ta effects on lunar samples, 2) incomplete mixing of debris to cause W isotope heterogeneity on the Moon, and 3) initial ¹⁸²Hf/¹⁸⁰Hf heterogeneities of the early solar system are critically discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Radio science investigations with Voyager

The planned radio science investigations during the Voyager missions to the outer planets involve: (1) the use of the radio links to and from the spacecraft for occultation measurements of planetary and satellite atmospheres and ionospheres, the rings of Saturn, the solar corona, and the general-relativistic time delay for radiowave propagation through the Sun's gravity field; (2) radio link measurements of true or apparent spacecraft motion caused by the gravity fields of the planets, the masses of their larger satellites, and characteristics of the interplanetary medium; and (3) related measurements which could provide results in other areas, including the possible detection of long-wavelength gravitational radiation propagating through the Solar System. The measurements will be used to study: atmospheric and ionospheric structure, constituents, and dynamics; the sizes, radial distribution, total mass, and other characteristics of the particles in the rings of Saturn; interior models for the major planets and the mean density and bulk composition of a number of their satellites; the plasma density and dynamics of the solar corona and interplanetary medium; and certain fundamental questions involving gravitation and relativity. The instrumentation for these experiments is the same ground-based and spacecraft radio systems as will be used for tracking and communicating with the Voyager spacecraft, although several important features of these systems have been provided primarily for the radio science investigations.