Gas-rich meteorites: Probes for particle environment and dynamical processes in the inner solar system

The discovery in the early sixties of precompaction solar wind irradiation records in the gas-rich meteorites opened up the possibility of studying the solar activity at different epochs in the distant past. Subsequent studies in several meteorites have led to the discovery of the precompaction records of irradiation of constituent grains by solar wind, solar flare and galactic cosmic ray particles. There are also microcraters resulting from their collisions with interplanetary dust grains. Analyses of these records and their observed similarity with those found in the lunar samples led to the hypothesis that the precompaction records in individual components of these meteorites were imprinted while they were residing in the near surface region of their parent bodies, most probably the asteroids. Although the asteroids are the most plausible candidates for the parent bodies of gas-rich meteorites, there exist certain dynamical arguments which tend to favor a cometary origin in certain cases. Also, recent studies indicate that in the case of gas-rich carbonaceous chondrites solar flare irradiation of grains may have occurred prior to formation of the parent bodies.In this review we summarize the significant advances that have taken place in the multi-disciplinary studies (petrography, chemistry, and radiation effects) of the gas-rich meteorites and critically evaluate the present state of our knowledge regarding the origin and evolution of the gas-rich meteorites. The information on the spatial and temporal variations in the interplanetary radiation and particle fluxes, obtained from the analysis of precompaction irradiation records in these meteorites is presented and further studies in certain specific topics are suggested for resolving some of the unsolved problems.     

Water Reservoirs in Small Planetary Bodies: Meteorites,Asteroids, and Comets

Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed \(\leq 4\) Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter’s orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn’s orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources of the noble gases accreted by Earth and Venus. Possible constraints that these observations can place on models of giant planet formation and migration are explored.     

Mars Global Surveyor Measurements of the Martian Solar Wind Interaction

The solar wind at Mars interacts with the extended atmosphere and small-scale crustal magnetic fields. This interaction shares elements with a variety of solar system bodies, and has direct bearing on studies of the long-term evolution of the Martian atmosphere, the structure of the upper atmosphere, and fundamental plasma processes. The magnetometer (MAG) and electron reflectometer (ER) on Mars Global Surveyor (MGS) continue to make many contributions toward understanding the plasma environment, thanks in large part to a spacecraft orbit that had low periapsis, had good coverage of the interaction region, and has been long-lived in its mapping orbit. The crustal magnetic fields discovered using MGS data perturb plasma boundaries on timescales associated with Mars' rotation and enable a complex magnetic field topology near the planet. Every portion of the plasma environment has been sampled by MGS, confirming previous measurements and making new discoveries in each region. The entire system is highly variable, and responds to changes in solar EUV flux, upstream pressure, IMF direction, and the orientation of Mars with respect to the Sun and solar wind flow. New insights from MGS should come from future analysis of new and existing data, as well as multi-spacecraft observations.     

Core Formation and Mantle Differentiation on Mars

Geochemical investigation of Martian meteorites (SNC meteorites) yields important constraints on the chemical and geodynamical evolution of Mars. These samples may not be representative of the whole of Mars; however, they provide constraints on the early differentiation processes on Mars. The bulk composition of Martian samples implies the presence of a metallic core that formed concurrently as the planet accreted. The strong depletion of highly siderophile elements in the Martian mantle is only possible if Mars had a large scale magma ocean early in its history allowing efficient separation of a metallic melt from molten silicate. The solidification of the magma ocean created chemical heterogeneities whose ancient origin is manifested in the heterogeneous ¹⁴²Nd and ¹⁸²W abundances observed in different meteorite groups derived from Mars. The isotope anomalies measured in SNC meteorites imply major chemical fractionation within the Martian mantle during the life time of the short-lived isotopes ¹⁴⁶Sm and ¹⁸²Hf. The Hf-W data are consistent with very rapid accretion of Mars within a few million years or, alternatively, a more protracted accretion history involving several large impacts and incomplete metal-silicate equilibration during core formation. In contrast to Earth early-formed chemical heterogeneities are still preserved on Mars, albeit slightly modified by mixing processes. The preservation of such ancient chemical differences is only possible if Mars did not undergo efficient whole mantle convection or vigorous plate tectonic style processes after the first few tens of millions of years of its history.     

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ～10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide ¹⁰Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ～0.5 million year scale has remained constant within some ±10% during the past ～10 million years. The very long-lived radioactive nuclide ⁴⁰K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5–1 billion years within a factor of ～1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ～150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.     

Formation and Processing of Organics in the Early Solar System

Until pristine samples can be returned from cometary nuclei, primitive meteorites represent our best source of information about organic chemistry in the early solar system. However, this material has been affected by secondary processing on asteroidal parent bodies which probably did not affect the material now present in cometary nuclei. Production of meteoritic organic matter apparently involved the following sequence of events: Molecule formation by a variety of reaction pathways in dense interstellar clouds; Condensation of those molecules onto refractory interstellar grains; Irradiation of organic-rich interstellar-grain mantles producing a range of molecular fragments and free radicals; Inclusion of those interstellar grains into the protosolar nebula with probable heating of at least some grain mantles during passage through the shock wave bounding the solar accretion disc; Agglomeration of residual interstellar grains and locally produced nebular condensates into asteroid-sized planetesimals; Heating of planetesimals by decay of extinct radionuclides; Melting of ice to produce liquid water within asteroidal bodies; Reaction of interstellar molecules, fragments and radicals with each other and with the aqueous environment, possibly catalysed by mineral grains; Loss of water and other volatiles to space yielding a partially hydrated lithology containing a complex suite of organic molecules; Heating of some of this organic matter to generate a kerogen-like complex; Mixing of heated and unheated material to yield the meteoritic material now observed. Properties of meteoritic organic matter believed to be consistent with this scenario include: Systematic decrease of abundance with increasing C number in homologous series of characterisable molecules; Complete structural diversity within homologous series; Predominance of branched-chain isomers; Considerable isotopic variability among characterisable molecules and within kerogen-like material; Substantial deuterium enrichment in all organic fractions; Some fractions significantly enriched in nitrogen-15; Modest excesses of L-enantiomers in some racemisation-resistant molecules but no general enantiomeric preference. Despite much speculation about the possible role of Fischer-Tropsch catalytic hydrogenation of CO in production of organic molecules in the solar nebula, no convincing evidence for such material has been found in meteorites. A similarity between some meteoritic organics and those produced by Miller-Urey discharge synthesis may reflect involvement of common intermediates rather than the operation of electric discharges in the early solar system. Meteoritic organic matter constitutes a useful, but not exact, guide to what we shall find with in situ analytical and sample-return missions to cometary nuclei. This revised version was published online in June 2006 with corrections to the Cover Date.     

On The 53Mn Heterogeneity In The Early Solar System

It is well established that the prolonged and thorough mixing of numerous nucleosynthetic components that constitutes the matter in the solar nebula resulted in an essential isotopic homogeneity of the solar system material. This may or may not be true for the short-lived radionuclides which were injected into or formed within the solar nebula just prior to or during solar system formation. Distinguishing between their heterogeneous or homogeneous distribution is important because the short- lived radionuclides are now widely used for the relative chronology of various objects and processes in the early solar system and as constraints for models of nucleosynthesis. The recent studies of the ⁵³Mn-⁵³Cr isotope system (half life of ⁵³Mn is 3.7 Ma) in various solar system objects have shown that the relative abundance of radiogenic ⁵³Cr is consistent with essentially homogeneous distribution of ⁵³Mn in the asteroid belt. Thus, the relative ⁵³Mn-⁵³Cr chronometer can be directly used for dating samples which originated in the asteroid belt. Importantly, however, all meteorite groups studied so far indicate a clear excess of ⁵³Cr as compared to Earth and to a lunar sample, which exhibits also a terrestrial ⁵³Cr/⁵²Cr ratio. The results from the Martian (SNC) meteorites show that their ⁵³Cr excesses are less than half of those found in the asteroid belt bodies. Thus, the characteristic ⁵³Cr/⁵²Cr ratio of Mars is intermediate between that of the Earth-Moon system and those of the other meteorites. If these ⁵³Cr variations are viewed as a function of the heliocentric distance, the radial dependence of the relative abundances of radiogenic ⁵³Cr is indicated. This observed gradient can be explained by either an early, volatility controlled, Mn/Cr fractionation within the nebula or by an initial radial heterogeneous distribution of ⁵³Mn. Although model calculations of the Mn/Cr ratios in the bulk terrestrial planets seem to be inconsistent with the volatility driven scenario, the precision of these calculations is inadequate for eliminating this possibility. In contrast, recent studies of the ⁵³Mn-⁵³Cr system in the enstatite chondrites indicate that, while their bulk Mn/Cr ratios are essentially the same as in ordinary chondrites, the ⁵³Cr excess in bulk enstatite chondrites is three times lower than that in the bulk ordinary chondrites. This difference cannot be explained by a Mn/Cr fractionation and, thus, strongly suggests that a radial heterogeneous distribution of ⁵³Mn must have existed in at least the early inner solar system. Using the observed gradient and the ⁵³Cr/⁵²Cr ratio of the bulk enstatite chondrites, their parent body(ies) formed at ∼1.4 AU or somewhat closer to the Sun. This revised version was published online in June 2006 with corrections to the Cover Date.     

Planetary Magnetic Fields: Achievements and Prospects

The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of planetary interior properties and evolutions. Most planetary magnetic fields arise from dynamos, past or present, and planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or “snow”). Although planetary dynamos mostly appear to operate with an internal field that is not very different from (2ρΩ/σ)^1/2 in SI units where ρ is the fluid density, Ω is the planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of planetary magnetic fields depends as much on our understanding of the history and material properties of planets as it does on our understanding of the dynamo process. Future developments can be expected in our understanding of the criterion for a dynamo and on planetary properties, through a combination of theoretical work, numerical simulations, planetary missions (MESSENGER, Juno, etc.) and laboratory experiments.     

The Beginning: Swift and Violent

Radiometric dating shows that the earliest steps in forming the substance of meteorites and assembling it into planetesimals occurred in a very short interval of time, 1–2 Ma. This study shows that rapid formation is also dictated by the need to use short-lived ²⁶Al (half-life T _1/2=0.74 Ma) as a heat source to metamorphose and in some cases melt the meteorite parent bodies after they accreted. The earliest events in solar system history dated by cosmochemists, formation at high temperatures of the Ca,Al-rich inclusions that occur in chondritic meteorites, probably occurred during the most energetic stage of protosolar disk evolution, as the protosun neared its present mass and infall drew to a close. This revised version was published online in June 2006 with corrections to the Cover Date.     

The meteorite flux

Meteorites are being retrieved from the surface of our planet all the time, the number of located falls being at maximum in the range 0.3 to 1.0 yr^-1 (10⁶ km²)^-1. Using the known flux of meteorite parent bodies to the top of the atmosphere this paper reviews suggested ablation processes and predicts the actual flux of meteorites to the Earth. It is found that this is some 70 to 300 times what is observed.     

Are There Chemical Gradients in the Inner Solar System?

The solar system is apparently stratified with regard to the contents of volatile constituents, as judged from the rocky, volatile-poor inner solar system planets and meteorites and the huge volatile-rich outer planets. However, beyond this gross structure there is no evidence for a systematic increase of the volatiles' abundances with distance from the Sun. Although meteorites show comparatively large differences in volatile element contents they also differ in many other respects, such as Mg/Si-ratios, bulk Fe and refractory element contents. These variations reflect variations in the nebular environment from which meteorites formed. The various conditions of meteorite formation cannot, however, be related in a simple way to heliocentric distances. There are also no systematic variations in the chemistry of the inner planets Mercury, Venus, Earth, Moon, Mars, and including the fourth largest asteroid Vesta, that could be interpreted as a relationship between volatility and composition. Although Mars (as judged from the composition of Martian meteorites) is more oxidized and contains more volatile elements than Earth, this trend cannot be extrapolated to the dry volatile poor Vesta (sampled by HED meteorites) in the asteroid belt. If the Earth-Mars trend reflects global inner solar system gradients then Vesta must have formed inside Earth's orbit and moved out later to its present location. The quality of Mercury and Venus composition data is not sufficient to allow reliable extrapolation to distances closer to the Sun. Recent nebula models predict small temperature gradients in the inner solar system supporting the view that no large variations in volatile element contents of inner solar system materials are expected. This revised version was published online in June 2006 with corrections to the Cover Date.     

Induced Magnetic Fields in Solar System Bodies

Electromagnetic induction is a powerful technique to study the electrical conductivity of the interior of the Earth and other solar system bodies. Information about the electrical conductivity structure can provide strong constraints on the associated internal composition of planetary bodies. Here we give a review of the basic principles of the electromagnetic induction technique and discuss its application to various bodies of our solar system. We also show that the plasma environment, in which the bodies are embedded, generates in addition to the induced magnetic fields competing plasma magnetic fields. These fields need to be treated appropriately to reliably interpret magnetic field measurements in the vicinity of solar system bodies. Induction measurements are particularly important in the search for liquid water outside of Earth. Magnetic field measurements by the Galileo spacecraft provide strong evidence for a subsurface ocean on Europa and Callisto. The induction technique will provide additional important constraints on the possible subsurface water, when used on future Europa and Ganymede orbiters. It can also be applied to probe Enceladus and Titan with Cassini and future spacecraft.     

Millennial Variations of the Geomagnetic Field: from Data Recovery to Field Reconstruction

Variations of the geomagnetic field over past millennia can be determined from archeomagnetic data and paleomagnetic sediment records. The resolution and validity of any field reconstruction depends on the reliability of such indirect measurements of past field values. Considerable effort is invested to ensure that the magnetic minerals carrying the ancient magnetization are good, if not ideal, recorders of the magnetic field. This is achieved by performing a wide array of rock magnetic and microscopy investigations, many of which are outlined here. In addition to data quality, the spatial and temporal distributions of archeomagnetic and sediment records play a significant role in the accuracy of past field reconstruction. Global field reconstructions enable studies of dynamic processes in Earth’s core. They rely on data compilations which ideally include information about the quality of a measurement and provide a useful archive for selecting data with the best characteristics. There is, however, a trade off between the total number of reliable data and the geographic or temporal coverage. In this review we describe the various types of paleomagnetic recorders, and the kind of measurements that are performed to gather reliable geomagnetic field information. We show which modeling strategies are most suitable, and the main features of the field that can be derived from the resulting models. Finally, we discuss prospects for progress in this kind of research.     

MHD processes in the outer heliosphere

Conclusion Much has been learned about the structure and dynamics of the outer heliosphere during the last decade as a result observations from the Voyager and Pioneer spacecraft. The large scale of the observations forces one to consider the heliosphere from a new perspective, to think of new dynamical processes, and to introduce new concepts. The early studies of isolated gas dynamic flows must be replaced by MHD dynamics of interacting flows and flow systems. The simple deterministic models that have been dominant in early studies of the solar wind are now seen to have limited applicability, and statistical approaches are being developed. New concepts that have been introduced, such as inverse cascades, filtering, entrainment, etc., must be further explored and clarified, to make them more precise and quantitative. MHD turbulence is probably very important in solar wind dynamics, but the subject is poorly developed from a theoretical point of view. The statistical analysis of solar wind parameters has scarcely begun, but it is clearly necessary for an understanding of complex, large-scale flows. The multitude of possible interactions among shocks and flows of various types needs to be explored systematically with observations, models and analytical theory. Voyagers 1 and 2 and Pioneers 10 and 11 are continuing to move through the outer heliosphere and gather data. The lengthy data reduction procedures require even more care in dealing with the low field strengths, densities and temperatures at large heliocentric distances, and the analysis of the complex flows and fields in the outer heliosphere becomes increasingly difficult. Thus one can expect continued growth of our knowledge of the heliosphere, but comprehensive understanding of the data will take some time. If this review stimulates the specialists in solar wind physics to think critically about the results presented and to remedy the deficiencies of current knowledge of the heliosphere, then it will have served its purpose. It is also hoped that this review will serve to encourage specialists in other fields to bring their talents to bear on heliospheric problems and to transfer results of heliospheric physics to their fields.     

Elemental and Isotopic Abundances of Carbon and Nitrogen in Meteorites

We have taken an inventory of the elemental and isotopic abundances of major carbon- and nitrogen-bearing components in different groups of meteorites. Primary phases, inherited from the solar nebula, are frequently isotopically heterogeneous, and surprisingly resistant to modification through parent body processing. Even melted and recrystallised meteorites retain primordial carbon and nitrogen isotopic signatures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Emerging Parameter Space Map of Magnetic Reconnection in Collisional and Kinetic Regimes

In large-scale systems of interest to solar physics, there is growing evidence that magnetic reconnection involves the formation of extended current sheets which are unstable to plasmoids (secondary magnetic islands). Recent results suggest that plasmoids may play a critical role in the evolution of reconnection, and have raised fundamental questions regarding the applicability of resistive MHD to various regimes. In collisional plasmas, where the thickness of all resistive layers remain larger than the ion gyroradius, simulations results indicate that plasmoids permit reconnection to proceed much faster than the slow Sweet-Parker scaling. However, it appears these rates are still a factor of ～10× slower than observed in kinetic regimes, where the diffusion region current sheet falls below the ion gyroradius and additional physics beyond MHD becomes crucially important. Over a broad range of interesting parameters, the formation of plasmoids may naturally induce a transition into these kinetic regimes. New insights into this scenario have emerged in recent years based on a combination of linear theory, fluid simulations and fully kinetic simulations which retain a Fokker-Planck collision operator to allow a rigorous treatment of Coulomb collisions as the reconnection electric field exceeds the runaway limit. Here, we present some new results from this approach for guide field reconnection. Based upon these results, a parameter space map is constructed that summarizes the present understanding of how reconnection proceeds in various regimes.     

Origin,age, and composition of meteorites

This paper attempts to bring together and evaluate all significant evidence on the origin of meteorites.The iron meteorites seem to have formed at low pressures. Laboratory evidence shows that the absence of a Widmanstätten pattern in meteorites with > 16% Ni cannot be attributed to high pressures, but to supercooling or an unusually fast cooling rate for these meteorites, which prevented the development of a pattern. The presence of tridymite in the Steinbach siderophyre provides further, direct proof that the Widmanstätten pattern can form at pressures less than 3 kb. Neither diamond, nor cliftonite, nor cohenite are reliable pressure indicators in meteorites. Diamonds were formed by shock while cliftonite may have been derived from a cubic carbide such as Fe₄C. Cohenite is apparently stabilized by kinetic rather than thermodynamic factors. Several lines of evidence suggest that the irons come from more than one parent body, perhaps as many as four.The frequency of pallasites is perfectly consistent with an origin in the transition zone between core and mantle of the parent body. Hybrid meteorites such as Brenham are not necessarily derived from the metal-silicate interface, but probably resulted from dendrite growth in the solidifying melt.Ordinary chondrites definitely are equilibrium assemblages rather than chance conglomerates. According to the best available evidence, Prior's rules seem to be valid. The metal particles in chondrites differentiated into kamacite and taenite in their present location, rather than in a remote earlier environment. Trace element abundances in ordinary and carbonaceous chondrites suggest that these meteorites accreted from two types of matter: an undepleted fraction that separated from its complement of gases at low temperatures, and a depleted fraction that lost its gases at high temperatures. These two fractions of primitive meteoritic matter are tentatively identified with the matrix and chondrules-plus-metal, respectively. New restrictive limits are placed on the iron-silicate fractionation in chondrites. No direct evolutionary path exists that connects the currently accepted solar abundances of Fe and Ni and the observed Fe/Si and Ni/Si ratios in chondrites. Apparently the solar abundance of iron is in error. The iron-silicate fractionation seems to have occurred while chondritic matter was in a more strongly reduced state than its present one.The U-He and K-Ar ages of hypersthene chondrites are systematically shorter than those of bronzite chondrites. Short ages are correlated with shock effects, and it seems that the hypersthene chondrites suffered reheating and partial-to-complete outgassing 0.4 AE ago. The cosmic-ray exposure ages of all classes of meteorites cluster distinctly, indicating that the meteorites were produced in a few discrete major collisions rather than by a quasi-continuum of smaller ones. The dates of the principal breakups are: irons, 0.6 and 0.9 AE; aubrites, 45 m.y.; bronzite chondrites, 4 m.y.; hypersthene chondrites, 0.025, 3, 7–13, and 16–31 m.y. All four clusters of hypersthene chondrites show evidence of severe outgassing 0.4 AE ago, which implies that most or all hypersthene chondrites come from the same parent body.As already noted by Signer and Suess, two distinct types of primordial gas occur in meteorites. Differentiated meteorites always contain unfractionated gas, while relatively undifferentiated meteorites contain fractionated gas. The former component is invariably associated with shock effects, and seems to have been derived from the solar wind. The latter component is correlated with other volatiles and seems to be a truly primitive constituent of meteoritic matter. Isotopic anomalies in the fractionated gas suggest that meteoritic matter was irradiated with 10¹⁷ protons/cm² at a very early stage of its history.There is very little doubt that most, if not all, meteorites come from the asteroid belt rather than from the moon. The orbits and geocentric velocities of stony meteorites resemble those of the Apollo asteroids (most of which are former members of the asteroid belt that have strayed into terrestrial space), but disagree strongly with the calculated orbits and velocities for lunar ejecta. Öpik's conclusions about the difficulty of accelerating lunar debris to escape velocity represent a further argument against a lunar origin of stony meteorites.The most likely parent bodies of the meteorites are the 34 asteroids which cross the orbit of Mars. Collisional debris from these objects will remain in Mars-crossing orbits, and perturbations by Mars will inject some fraction of this material into terrestrial space. Most of the Mars asteroids, comprising 98% of the mass and 92% of the cross-section, belong to three Hirayama families (Phocaea, Desiderata, and Aethra), and an additional, previously unrecognized family. These families were apparently produced by disruption of parent asteroids ca. 104, 105, and 46 km in diameter. The size distribution and light curves of asteroids indicate that the larger asteroids are original accretions, rather than collision fragments. There is no reason to believe that the meteorites ever resided in bodies larger than Ceres (d = 770 km).Various theories on the origin of the meteorites are critically reviewed in the light of the preceding evidence. Wood's theory, which postulates a high-temperature and a low-temperature variety of primordial matter, is in best accord with the evidence. Apparently the asteroids accreted from varying proportions of these two types of material, and were then heated by extinct radioactivity produced in the early irradiation.     

Active Region Dynamics

New methods of local helioseismology and uninterrupted time series of solar oscillation data from the Solar and Heliospheric Observatory (SOHO) have led to a major advance in our understanding of the structure and dynamics of active regions in the subsurface layers. The initial results show that large active regions are formed by repeated magnetic flux emergence from the deep interior, and that their roots are at least 50 Mm deep. The active regions change the temperature structure and flow dynamics of the upper convection zone, forming large circulation cells of converging flows. The helioseismic observations also indicate that the processes of magnetic energy release, flares and coronal mass ejections, might be associated with strong (1–2 km/s) shearing flows, 4–6 Mm below the surface.     

The Canyon Diablo meteorite

The giant diamond-containing Canyon Diablo meteorite is in composition a typical representative of the widespread group of iron meteorites — the coarse octahedrites. But it is unique in a number of scientifically important aspects. When it fell, it formed the Arizona (Barringer meteorite) crater (1220 m in diam), which is of the explosive type. The investigations of the shock recrystallization of the crater rocks and the meteoritic material are of importance for planetology, and in particular for the eludication of matter recrystallization conditions during the collisions of large cosmic bodies. The study of the diamonds contained in the Canyon Diablo meteorite is of importance to various branches of carbon cosmochemistry.The Canyon Diablo meteorite fell in Arizona, U.S.A., some tens of thousands years ago. However, the Arizona meteorite crater is well preserved owing to the semi-arid climatic conditions. Signs of shock recrystallization of the rocks — shatter cones, impactites, dense and super-dense silica modifications were revealed in the Arizona meteorite crater. Around the crater many samples of the Canyon Diablo iron meteorite have been found (at distances of up to 9 km from the crater), together with a great amount of finely dispersed meteorite matter. The total weight of the material attributed to the meteorite is put at 30 tons. A number of meteorite fragments have been oxidized to different degrees during terrestrial weathering.Typical samples of the Canyon Diablo meteorite are represented by coarse octahedrite matter with kamacite band widths of 1.8–2.2 mm. In many meteorite fragments, especially the fragments found on the crater rim, the Widmanstätten pattern has been destroyed as a result of the explosion which occurred when the meteorite hit the Earth. The meteorite is rich in accessory minerals (cohenite, schreibersite, troilite.). The Ni content is, in typical samples of the meteorite, about 7.16%, in rare, atypical, medium octahedrite samples the Ni content reached 8.2 %. In the content of trace elements the meteorite may be classified with the I Ga-Ge group. In the content of stable isotopes of elements there is no substantial difference between the Canyon Diablo meteorite and other octahedrites. Radioactive cosmogenic isotopes are represented by isotopes with a large half-life.The diamonds in the Canyon Diablo meteorite are unevenly distributed and are found inside the highly recrystallized meteorite fragments at the rim of the crater. Diamonds are present in the form of intergrowths of microcrystals, crystallite sizes are < 1, the sizes of the intergrowths reach 2–5 mm. Admixtures of graphite and the hexagonal diamond lonsdaleite are present in the intergrowths.From the evidence of shock recrystallization of the meteorite matter it would seem that the diamond containing fragments of the Canyon Diablo meteorite have undergone shock pressures of from 400 kbar to 1 mgbar at impact, at these pressures the diamonds would crystallise. The diamond-containing sample of the Canyon Diablo meteorite investigated by the author has experienced a shock pressure of up to 1 mgbar during the explosion.     

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N₂:CH₄ atmospheres (such as Titan, Triton, and the early Earth).