Ureilites

Ureilites are a rare group of five stony meteorites — feldsparless achondrites containing diamonds of preterrestrial origin the total weight of them being 315 carats. The whole carbon content in ureilites makes up 1.5–4.1%. Ureilites substantially differ from the other stony meteorites. In comparison with chondrites they are enriched in Mg but depleted in metal, troilite, alkaline elements. All ureilites are very similar by their structure. They contain elongated cavities generally stretched in the same direction. The structure of ureilites is an achondritic porphyric one. These meteorites consist of large olivine grains, there is less clinopyroxene (pigeonite). They contain kamacite (Ni content being 1.5–4%), troilite FeS, chromite. Carbon material is represented by diamond, graphite and organic material being present in a disequilibrium state. Two ureilites (North Haig and Dingo Pup Donga) were considerably oxidized during terrestrial weathering and contain secondary iron oxides.Diamonds are present in ureilites in thin intergrowths with graphite being disposed between silicate grains more or less evenly in the meteorite. The sizes of such black diamond-graphite aggregates are 0.3–0.9 mm. The sizes of the micromonocrystals of diamond and graphite are less than 1. The diamond-graphite aggregates contain dispersed particles of kamacite, troilite, chromite, nickelchrome, being present in very thin intergrowths with one another. Besides the usual diamond there is an admixture of lonsdaleite in the aggregates representing a hexagonal diamond with a würtzite-like structure. Lonsdaleite has been identified in the ureilites Novo Urei, Goalpara, North Haig. The diamonds of ureilites are characterized by inhomogeneities in the crystalline structure which are at least partly caused by the presence of donor nitrogen. According to the isotopic carbon composition the diamond in the meteorite Novo Urei has the value C¹³ = -5.7.The structure peculiarities of ureilites bear witness of the fact that these meteorites had been subject to the action of dynamic pressure about 300–600 kbar. The impact had taken place in cosmic space. All ureilites subdivide into two types: the first type are Novo Urei, Dyalpur, Dingo Pup Donga, the second type are Goalpara, North Haig according to the following signs: (1) olivine grains are finer in ureilites of the second type; (2) twinning is more typical of clinopyroxenes of ureilites belonging to the first type; (3) in ureilites of the first type a net-like iron distribution is observed, in ureilites of the second type kamacite plates are chiefly present between silicate grains; (4) the size of diamondgraphite intergrowths in ureilites of the first type does not exceed 0.3 mm, in ureilites of the second type it reaches 0.9 mm. Ureilites of the first type have undergone a less intensive impact than ureilites of the second type. Certain similarity of the material composition of ureilites and of the material composition of carbonaceous chondrites, the distinction of these two groups of meteorites from all other meteorites bear witness of the fact that ureilites have formed from carbonaceous chondrites during a collision of asteroid bodies in cosmic space, diamonds having been formed from the carbon material of carbonaceous chondrites.     

The Haverö meteorite

The meteorite Haverö, which fell on 2 August 1971 in Finland is a representative of a rare, but highly interesting group of stony meteorites-ureitites. Like other ureilites it is enriched in carbon (up to 2%) chiefly represented by diamond and graphite. The meteorite is strongly recrystallized; as a result its chief mineral component-olivine-has been transformed into a mosaic of small grains among which kamacite with an extremely low Ni content ( 1.7%) is distributed. Polysynthetic twinning is characteristic of pyroxene grains. The meteorite is penetrated by a system of cavities being stretched approximately in one direction. The cavities are sometimes occupied by kamacite plates containing 4% Ni. The peculiarities of the mineral and chemical composition of the meteorite indicate that its material had formed during a disequilibrium process. The content of cosmogenic isotopes witnesses to an unusual orbit of the meteorite.     

The Mighei meteorite

The Mighei meteorite is generally considered to be unique amongst the group of stony meteorites known as the carbonaceous chondrites in a number of scientifically interesting aspects. The meteorite, which is related to the type II carbonaceous chondrites of Wiik's classification (or type C2 according to van Schmus and Wood), contains extraterrestrial organic compounds (general C content = 2.6%), and extraterrestrial water associated with iron-magnesium silicate crystals (general H₂O content=12%).The meteorite fall occurred in 1889, over a region in the Ukraine. In structure it was found to be a chondritic meteorite, having chondrules of order 0.5 mm in size. The composition of the meteorite is inhomogeneous. In mineralogical terms the meteorite is composed of two paragenetic associations, described as high and low temperature, which are generally distributed in equal proportions. The low temperature associations are a characteristic only of carbonaceous chondrites: the minerals involved are chlorites or the serpentine group, carbonates, free sulphur, sulphates and low temperature glass. In chemical terms the Mighei meteorite is somewhat enriched in the volatile elements S, C, H, N, O in comparison to the usual chondrites. These elements are found in different forms and the isotopic composition of the elements S, C, O, is different for different phases. The meteorite is also rich in a number of other fairly volatile element admixtures such as: B, F, Cl, Cu, Zn, Ga, Ge, Br, In, Te, I, Hg, Tl, Pb, Bi, and contains somewhat enhanced initial quantities of rare gases.The organic compounds are of an abiological nature in the meteorite and are located in finely dispersed distributions between the chondrules. They are present in the main, as polymerized organic compounds. Among these polymers there are gaseous hydrocarbons (saturated and non-saturated) and extractable organic compounds. In the latter condition the following organic compounds have been identified: aliphatic hydrocarbons, aromatic hydrocarbons, amino acids and others. The meteorite contains free organic radicals (10¹⁷ centres g^–1), uncoupled -electrons which are delocalized in the aromatic structure of the polymeric matter.The radiogenic age of the meteorite has been determined as from 2.4 to 3.2 × 10⁹ yr (by the K-Ar method) and up to 4.54 × 10⁹ yr (by the Rb-Sr method), while the radiation age is put at 0.5 to 2.4 × 10⁶ yr. Details of the meteorite structure give evidence of at least two processes in its formation; the accretion of the meteoritic matter, together with the simultaneous formation of organic compounds could have taken place at temperatures between 450 and 300 K.Reported on the XIV Meteoritic Conference, December 17, 1970, Moscow.     

The origin of tektites

The tektites called Muong Nong type by V. Barnes apparently represent the parent material from which other types are derived. In these tektites are found clues (coesite, angular voids) which indicate that they have not been substantially remelted since the event which detached them from the planet or satellite on which they were formed.From the nickel-iron spherules and the coesite it is deduced that the tektites were detached by meteorite impact. From the absence of cosmogenic isotopes and the distribution over the earth it is deduced that the source was either the earth or the moon. Calculations of rates of diffusion in silicates indicate that tektites could not have been produced from terrestrial sedimentary rocks; it has long been remarked that they are different from terrestrial igneous rocks. A lunar origin is therefore considered likely, in agreement with aerodynamic evidence. Contrary indications from the geochemical likeness of tektites to terrestrial materials, especially at the Ries Kessel and the Bosumtwi crater are noted, but these indications are considered to be outweighed by the difficulties of giving a physical account of a terrestrial origin.Interpreted as lunar materials, the tektites suggest that large portions of the lunar surface are covered with ash-flow tuff of a peculiar type, remarkably free of water and other volatiles. They also give evidence concerning the origin of the moon.     

Cratering Chronology and the Evolution of Mars

Results by Neukum et al. (2001) and Ivanov (2001) are combined with crater counts to estimate ages of Martian surfaces. These results are combined with studies of Martian meteorites (Nyquist et al., 2001) to establish a rough chronology of Martian history. High crater densities in some areas, together with the existence of a 4.5 Gyr rock from Mars (ALH84001), which was weathered at about 4.0 Gyr, affirm that some of the oldest surfaces involve primordial crustal materials, degraded by various processes including megaregolith formation and cementing of debris. Small craters have been lost by these processes, as shown by comparison with Phobos and with the production function, and by crater morphology distributions. Crater loss rates and survival lifetimes are estimated as a measure of average depositional/erosional rate of activity.We use our results to date the Martian epochs defined by Tanaka (1986). The high crater densities of the Noachian confine the entire Noachian Period to before about 3.5 Gyr. The Hesperian/Amazonian boundary is estimated to be about 2.9 to 3.3 Gyr ago, but with less probability could range from 2.0 to 3.4 Gyr. Mid-age dates are less well constrained due to uncertainties in the Martian cratering rate. Comparison of our ages with resurfacing data of Tanaka et al. (1987) gives a strong indication that volcanic, fluvial, and periglacial resurfacing rates were all much higher in approximately the first third of Martian history. We estimate that the Late Amazonian Epoch began a few hundred Myr ago (formal solutions 300 to 600 Myr ago). Our work supports Mariner 9 era suggestions of very young lavas on Mars, and is consistent with meteorite evidence for Martian igneous rocks 1.3 and 0.2 – 0.3 Gyr old. The youngest detected Martian lava flows give formal crater retention ages of the order 10 Myr or less. We note also that certain Martian meteorites indicate fluvial activity younger than the rock themselves, 700 Myr in one case, and this is supported by evidence of youthful water seeps. The evidence of youthful volcanic and aqueous activity, from both crater-count and meteorite evidence, places important constraints on Martian geological evolution and suggests a more active, complex Mars than has been visualized by some researchers.     

Oxygen Isotopes in the Solar System

The oxygen three-isotope system has major advantages over the two-isotope systems of hydrogen, carbon, and nitrogen because different fractionation laws govern intraplanetary and interplanetary processes. This permits discriminating between solar nebular processes and parent-body processes. Oxygen isotopes also serve as a sensitive natural tracer for meteorite classification. This revised version was published online in August 2006 with corrections to the Cover Date.     

Extraterrestrial Organic Matter and the Detection of Life

A fundamental goal of a number of forthcoming space missions is the detection and characterization of organic matter on planetary surfaces. Successful interpretation of data generated by in situ experiments will require discrimination between abiogenic and biogenic organic compounds. Carbon-rich meteorites provide scientists with examples of authentic extraterrestrial organic matter generated in the absence of life. Outcomes of meteorite studies include clues to protocols that will enable the unequivocal identification of organic matter derived from life. In this chapter we summarize the diagnostic abiogenic features of key compound classes involved in life detection and discuss their implications for analytical instruments destined to fly on future spacecraft missions.     

Organic constituents of the carbonaceous chondrites

From a brief discussion of forms of meteorite carbon it is concluded that almost all the carbon in the carbonaceous chondrites is present as organic matter. Attempts to extract and identify this organic matter are then reviewed. It is shown that only 25 per cent has been extracted and only about 5 per cent chemically characterized. Of this 5 per cent most is a complex mixture of hydroxylated aromatic acids together with various hydrocarbons of the paraffin, naphthene and aromatic series. Small amounts of amino acids, sugars and fatty acids also are present. The possible chemical nature of the major fraction is discussed. It is suggested to be a mixture of high-molecular weight aromatic and hydrocarbon polymers.Possible sources of contamination of the meteorites are described and evidence indicating a general lack of organic contaminants is presented. It is concluded that most of the organic constituents are indigenous to the meteorites and are extra terrestial in origin. Synthetic processes for the compounds are mentioned and it is concluded that the organic material is probably of abiogenic origin.A brief review on studies of organized elements contained within the meteorites is presented. Difficulties of identification are discussed and photographs of some micro-structures of several carbonaceous chondrites are presented. No final conclusion about the nature of these objects is possible, but some appear to be various indigenous organic and mineral structures, while others are terrestrial contaminants.Contribution from the Chemistry Section, Space Science Division of Jet Propulsion Laboratory.     

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.     

The abundance of atomic 1H, 4He and 3He in the local interstellar cloud from pickup ion observations with SWICS on Ulysses

Pickup ions measured deep inside the heliosphere open a new way to determine the absolute atomic density of a number of elements and isotopes in the local interstellar cloud (LIC). We derive the atomic abundance of hydrogen and the two isotopes of helium from the velocity and spatial distributions of interstellar pickup protons and ionized helium measured with the Solar Wind Ion Composition Spectrometer (SWICS) on the Ulysses spacecraft between 2 and 5 AU. The atomic hydrogen density near the termination shock derived from interstellar pickup ion measurements is 0.115±0.025 cm^–3 and the atomic H/He ratio from these observations is found to be 7.7 ± 1.3 in the outer heliosphere. Comparing this value with the standard universal H/He ratio of 10 we conclude that filtration of hydrogen is small and that the ionization fraction of hydrogen in the LIC is low.     

Interplanetary shock waves generated by solar flares

Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or piston-driven shocks) will hopefully be combined with the study of the flare process itself.The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston).Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.This paper is a revised and updated version of an invited review originally presented at the IUGG XV General Assembly, Moscow, U.S.S.R., 2–14 August 1971.     

LISM structure — Fragmented superbubble shell?

Small scale structure in local interstellar matter (LISM) is considered. Overall morphology of the local cloud complex is inferred from Ca II absorption lines and observations of H I in white dwarf stars. Clouds with column densities ranging from 2–100 × 10¹⁷ cm^–2 are found within 20 pc of the Sun. Cold (50 K) dense (10⁵ cm^–3) small (5–10 au) clouds could be embedded and currently undetected in the upwind gas. The Sun appears to be embedded in a filament of gas with thickness 0.7 pc, and cross-wise column density 2 × 10¹⁷ cm^–2. The local magnetic field direction is parallel to the filament, suggesting that the physical process causing the filamentation is MHD related. Enhanced abundances of refractory elements and LISM kinematics indicate outflowing gas from the Scorpius-Centaurus Association. The local flow vector and Sco data are consistent with a 4,000,000 year old superbubble shell at –22 km s^–1, which is a shock front passing through preshock gas at –12 km s^–1, and yielding cooled postshock gas at –26 km s^–1in the upwind direction. A preshock magnetic field strength of 1.6 G, and postshock field strength of 5.2 G embedded in the superbubble shell, are consistent with the data.Abbreviations LISM Local ISM - SIC Surrounding Interstellar Cloud - LIC Local Interstellar Cloud     

HYDRODYNAMIC AND MHD EQUATIONS ACROSS THE BOW SHOCK AND ALONG THE SURFACES OF PLANETARY OBSTACLES

Examinations of the magnetohydrodynamic (MHD) equations across a bow shock are presented. These equations are written in the familiar Rankine–Hugoniot set, and an exact solution to this set is given which involves the upstream magnetosonic Mach number, plasma , polytropic index, and _B-v , as a function of position along the shock surface. The asymptotic Mach cone angle of the shock surface is also given as a function of the upstream parameters, as a set of transcendental equations. The standoff position of a detached bow shock from an obstacle is also reviewed. In addition, a detailed examination of the hydrodynamic equations along the boundary of the obstacle is performed. Lastly, the MHD relations along the obstacle surface are examined, for specific orientations of the upstream interplanetary magnetic field (IMF) in relation to the upstream flow velocity vector.     

Cosmic-ray scintillations and dynamic processes in space

Cosmic-ray scintillations registered by ground-base observations reflect, as a rule, the action of a whole number of processes proceeding in interplanetary space and Earth's magnetosphere. The study of scintillation phenomena in cosmic rays, is, in fact, divided into a number of problems connected with the interaction of charged particles of cosmic radiation with the matter and fields which they encounter in the entire length of their propagation. The cosmic-ray scintillations established by different authors from the data of ground-base and high-altitude devices for quiet and disturbed periods, as well as the theoretical calculations of different models and mechanisms of the origin and development of cosmic-ray scintillations are analyzed. High-frequency scintillations of f 10^-5 Hz are shown to be precursors of an approaching shock wave, scintillations with periods of the order of 10–20 and 40–50 min being most sensitive to disturbances of interplanetary medium near the Earth. Since cosmic rays of different energies are sensitive to different processes in interplanetary space at different distances from the Earth, one can sound the conditions in interplanetary medium up to 10¹⁵ cm from the Earth by measuring particle fluxes at different energy ranges.     

Composition of anomalous cosmic rays and implications for the heliosphere

We use energy spectra of anomalous cosmic rays (ACRs) measured with the Cosmic Ray instrument on the Voyager 1 and 2 spacecraft during the period 1994/157-313 to determine several parameters of interest to heliospheric studies. We estimate that the strength of the solar wind termination shock is 2.42 (–0.08, +0.04). We determine the composition of ACRs by estimating their differential energy spectra at the shock and find the following abundance ratios: H/He = 5.6 (–0.5, ⁺0.6), C/He = 0.00048 ± 0.00011, N/He = 0.011 ± 0.001, O/He = 0.075 ± 0.006, and Ne/He = 0.0050 ± 0.0004. We correlate our observations with those of pickup ions to deduce that the long-term ionization rate of neutral nitrogen at 1 AU is 8.3 × 10^–7 s^–1 and that the charge-exchange cross section for neutral N and solar wind protons is 1.0 × 10^–15 cm² at 1.1 keV. We estimate that the neutral C/He ratio in the outer heliosphere is 1.8(–0.7, +0.9) × 10^–5. We also find that heavy ions are preferentially injected into the acceleration process at the termination shock.     

Shock acceleration of energetic particles in the heliosphere

The theory of shock acceleration of energetic particles is briefly discussed and reviewed with an emphasis on clarifying the apparent distinction between the V × B and Fermi mechanisms. Attention is restricted to those situations in which the energetic particles do not themselves influence the given shock structure. In particular, application of the theory to the acceleration of energetic particles in corotating interaction regions (CIR) in the solar wind is presented. Here particles are accelerated at the forward and reverse shocks which bound the CIR by being compressed between the shock fronts and magnetic irregularities upstream from the shocks, or by being compressed between upstream irregularities and those downstream from the shocks. Particles also suffer adiabatic deceleration in the expanding solar wind, an effect not included in previous shock models for acceleration in CIRs. The model is able to account for the observed exponential spectra at Earth, the observed behavior of the spectra with radial distance, the observed radial gradients in the intensity, and the observed differences in the intensity and spectra at the forward and reverse shocks.Calculations and resulting energy spectra are also presented for shock acceleration of energetic particles in large solar flare events. Based on the simplifying assumption that the shock evolves as a spherically symmetric Sedov blast wave, the calculation yields the time-integrated spectrum of particles initially injected at the shock which eventually escape ahead of the shock into interplanetary space. The spectra are similar to those observed at Earth. Finally further applications are suggested.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Heavy Elements and Age Determinations

The age of the universe, measured from the Big Bang to the present, is at the focus of cosmology. Its determination relies, however, on the use of stellar objects or their products. Stellar explosions, like type Ia supernovae serve as standard(izable) candles to measure the expansion of the universe. Hertzsprung––Russell diagrams of globular clusters can determine the age of such clusters and thus are lower limits of the age of the galaxy and therefore also the universe. Some nuclear isotopes with half–lives comparable to the age of galaxies (and the universe) can serve as clocks (chronometers) for the duration of nucleosynthesis. The isotopes ²³⁸U and ²³²Th with half–lives of 4.5×10⁹ and 1.4×10¹⁰ yr, decaying via alpha decay chains to Pb isotopes, are well suited to serve this purpose. They are products of the same nucleosynthesis process, the r-process. Therefore, the present paper aims at understanding the necessary environment conditions in the (stellar) production sites, the nuclear physics involved, the observational constraints for r-process nucleosynthesis, the results from nucleocosmochronology, and the remaining challenges and uncertainties which need to be overcome for a full understanding of the nature of the r-process.     

Paleomagnetic Records of Meteorites and Early Planetesimal Differentiation

The large-scale compositional structures of planets are primarily established during early global differentiation. Advances in analytical geochemistry, the increasing diversity of extraterrestrial samples, and new paleomagnetic data are driving major changes in our understanding of the nature and timing of these early melting processes. In particular, paleomagnetic studies of chondritic and small-body achondritic meteorites have revealed a diversity of magnetic field records. New, more sensitive and highly automated paleomagnetic instrumentation and an improved understanding of meteorite magnetic properties and the effects of shock, weathering, and other secondary processes are permitting primary and secondary magnetization components to be distinguished with increasing confidence. New constraints on the post-accretional histories of meteorite parent bodies now suggest that, contrary to early expectations, few if any meteorites have been definitively shown to retain records of early solar and protoplanetary nebula magnetic fields. However, recent studies of pristine samples coupled with new theoretical insights into the possibility of dynamo generation on small bodies indicate that some meteorites retain records of internally generated fields. These results indicate that some planetesimals formed metallic cores and early dynamos within just a few million years of solar system formation.     

Novel approach of crater detection by crater candidate region selection and matrix-pattern-oriented least squares support vector machine

Impacted craters are commonly found on the surface of planets, satellites, asteroids and other solar system bodies. In order to speed up the rate of constructing the database of craters, it is important to develop crater detection algorithms. This paper presents a novel approach to automatically detect craters on planetary surfaces. The approach contains two parts: crater candidate region selection and crater detection. In the first part, crater candidate region selection is achieved by Kanade-Lucas-Tomasi (KLT) detector. Matrix-pattern-oriented least squares support vector machine (MatLSSVM), as the matrixization version of least square support vector machine (SVM), inherits the advantages of least squares support vector machine (LSSVM), reduces storage space greatly and reserves spatial redundancies within each image matrix compared with general LSSVM. The second part of the approach employs MatLSSVM to design classifier for crater detection. Experimental results on the dataset which comprises 160 preprocessed image patches from Google Mars demonstrate that the accuracy rate of crater detection can be up to 88%. In addition, the outstanding feature of the approach introduced in this paper is that it takes resized crater candidate region as input pattern directly to finish crater detection. The results of the last experiment demonstrate that MatLSSVM-based classifier can detect crater regions effectively on the basis of KLT-based crater candidate region selection.     

Inside and outside of supernovae

A newly formed neutron star in a supernova finds itself in a dense environment, in which the gravitational energy of accreting matter can be lost to neutrinos. For the conditions in SN 1987A, 0.1M may have fallen back onto the central neutron star on a timescale of hours after the explosion, after which the accretion rate is expected to drop sharply. Radiation is trapped in the flow until the mass accretion rate drops to 2×10^–4 M yr^–1 at which point radiation can begin to escape from the shocked envelope at an Eddington limit luminosity. Between this neutrino limit and the Eddington limit, 3×10^–8 M yr^–1, there are no steady, spherical solutions for neutron star accretion. SN 1987A should have reached the neutrino limit within a year of the explosion; the current lack of an Eddington luminosity can be attributed to black hole formation or to a clearing of the neutron star envelope. There is no evidence for newly formed neutron stars in supernovae. Radio supernovae, which were initially interpreted as pulsar activity, probably involve circumstellar interaction; SN 1993J shows especially good evidence for outer shock phenomena.