The Leonard Award Address Presented 1996 July 25, Berlin,Germany: The elemental composition of stony cosmic spherules

Abstract— Five hundred stony cosmic spherules collected from deep-sea sediments, polar ice, and the stratosphere have been analyzed for major and some minor element composition. Typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids. The samples are small and modified by atmospheric entry, but they are an important source of information on the composition of asteroids. The spherules in this study were all analyzed in an identical manner, and they provide a sampling of the solar system's asteroids that is both different and less biased than provided by studies of conventional meteorites. Volatile elements such as Na and S are depleted due to atmospheric heating, while siderophiles are depleted by less understood causes. The refractory nonsiderophile elements appear not to have been significantly disturbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the Earth. Typical spherules have CM-like composition that is distinctively different than ordinary chondrites and most other meteorite types. We assume that C-type asteroids are the primary origin of spherules with this composition. Type S asteroids should also be an important source of the spherules, and the analysis data provide constraints on their composition. A minor fraction of the spherules are melt products of precursor particles that did not have chondritic elemental compositions. The most common of these are particles that are dominated by olivine. The observed compositions of spherules are inconsistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry.     

The cluster abundance in cosmic string models for structure formation

We use the present observed number density of large X-ray clusters to constrain the amplitude of matter density perturbations induced by cosmic strings on the scale of 8  h ⁻¹ Mpc ( σ ₈), in both open cosmologies and flat models with a non-zero cosmological constant. We find a slightly lower value of σ ₈ than that obtained in the context of primordial Gaussian fluctuations generated during inflation. This lower normalization of σ ₈ results from the mild non-Gaussianity on cluster scales, where the one-point probability distribution function is well approximated by a χ ² distribution and thus has a longer tail than a Gaussian distribution. We also show that σ ₈ normalized using cluster abundance is consistent with the COBE normalization.     

Beryllium-10 and aluminum-26 in individual cosmic spherules from Antarctica

Abstract— We present data for the cosmogenic nuclides ¹⁰Be and ²⁶Al in a suite of 24 extraterrestrial spherules, collected from Antarctic moraines and deep sea sediments. All of the 10 large spherules collected in glacial till at Lewis Cliff are extraterrestrial. As in earlier work, the great majority of particles show prominent solar cosmic-ray (SCR) production of ²⁶Al, indicating bombardment ages on the order of 10⁶ years or even longer. These long ages are in direct contradiction to model ages for small particles in the inner Solar System and may require reconsideration of models of small particle lifetimes. A small fraction of the particles so far measured (6/42) possess cosmogenic radionuclide patterns consistent with predictions for meteoroid spall droplets. We believe that most of the spherules were bombarded in space primarily as bodies not much larger than their present size. The content of in situ produced ¹⁰Be and ²⁶Al in quartz pebbles in the same moraine suggests that these spherules may have on average a significant terrestrial age.     

Extraterrestrial water in micrometeorites and cosmic spherules from Antarctica: An ion microprobe study

Abstract— The D/H ratios and water contents were measured by ion microprobe analysis in 52 individual Antarctic micrometeorites (AMMs) and 10 Antarctic cosmic spherules (ACSs) containing nuggets of iron hydroxide (COPS phase). In AMMs, δD values vary from ?366 to +249%‰ and water contents lie between 0.4-3.7 wt%. The COPS nuggets in cosmic spherules have high water contents (2 to 8 wt%) and exhibit δD values from ?144 to +167%‰, which is indicative of an extraterrestrial origin of their constituent water. The silicate portion of ACSs also contain extraterrestrial H equivalent to ～0.l to 1.2 wt% water. Deuterium-exchange experiments were performed with isotopically spiked water. These experiments demonstrate that water in mineral phases of AMMs and ACSs is indigenous and does not result from contamination during residence in Antarctic ice. The frequency distribution of D/H ratios in AMMs allows us to further narrow the relationship between AMMs and carbonaceous chondrites to CM and CI chondrites but contrasts with that of stratospheric interplanetary dust particles (IDPs) of similar sizes (from ?10 to 50 μm). The relatively narrow range of D/H ratios measured in AMMs as well as in ACSs (which are more resistant and thus less susceptible to collection biases) suggests that D-rich IDP-like particles are very rare in our AMMs collections. This indicates that these D-rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in Antarctica would not be responsible for their strong depletion in the AMMs collections.     

Fine-grained volatile components ubiquitous in solar nebula: Corroboration from scoriaceous cosmic spherules

The scoriaceous cosmic spherules (CSs) that make up to a few percent (for sizes >150 μm size) of total micrometeorite flux are ubiquitous and have remained enigmatic. The present work provides in-depth study of 81 scoriaceous CSs, from observed ~4000 CSs, collected from Antarctica (South Pole water well) and deep-sea sediments (Indian Ocean) that will allow us to analyze the nature of these particles. The fine-grained texture and the chemical composition of scoriaceous particles suggest that they are formed from matrix materials that are enriched in volatiles. The volatile components such as water, sulfide, Na, K, etc. have vanished due to partial evaporation and degassing during Earth's atmospheric entry leaving behind the vesicular features, yet largely preserving the elemental composition. The elemental ratios (Ca/Si, Mg/Si, Al/Si, Fe/Si, and Ni/Si) of interplanetary dust particles (IDPs) are compatible with the scoriaceous CSs, which in turn are indistinguishable from the matrices of CI and CM chondrites signifying similarities in the nature of the sources. Furthermore, the texture of cometary particles bears resemblance to the texture of the scoriaceous particles. The compilation of petrographic texture, chemical, and trace element composition of scoriaceous CSs presents a strong case for matrix components from hydrated and volatile-rich bodies, such as CI and CM chondrites, rather than chondrules. We conclude that the fine-grained scoriaceous CSs, the matrix materials of hydrated chondrites, IDPs, and cometary particles that overlap compositionally were widespread, indicating a dominant component in the early solar nebula.     

Transport phenomena and abundance anomalies in cosmic plasma

Hydrodynamical equations for a fully ionized hydrogen-helium plasma are derived by the Chapman-Enskog method. The electron and ion transport coefficients are found as the functions of electron and ion temperatures and number densities as well as of the magnetic field strength. The presented equations are needed for describing transport phenomena in laboratory and cosmic plasmas. It is shown that transport phenomena can produce abundance anomalies; e. g., a sound wave propagating through a homogeneous plasma may be accompanied by the oscillations of chemical composition. Various astrophysical consequences of the theory are discussed.     

Major and trace element geochemistry of S‐type cosmic spherules

Micrometeorites that pass through the Earth's atmosphere undergo changes in their chemical compositions, thereby making it difficult to understand if they are sourced from the matrix, chondrules, or calcium–aluminum‐rich inclusions (CAIs). These components have the potential to provide evidence toward the understanding of the early solar nebular evolution. The variations in the major element and trace element compositions of 155 different type (scoriaceous, relict bearing, porphyritic, barred, cryptocrystalline, and glass) of S‐type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma‐mass spectrometry. The S‐type cosmic spherules appear to show a systematic depletion in volatile element contents, but have preserved their refractory trace elements. The trends in their chemical compositions suggest that the S‐type spherules comprise of components from similar parent bodies, that is, carbonaceous chondrites. Large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites. Furthermore, four spherules (two of these spherules enclose spinels and one comprised entirely of a Ca‐Al‐rich plagioclase) show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules. The information on the chemical composition and rare earth elements (REEs) on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies. The Ce, Eu, and Tm anomalies found in the cosmic spherules have similar behavior as that of chondrites. Distinct correlations observed between different REEs and types of cosmic spherules reflect the inherited properties of the precursors.     

The electron-proton abundance ratio in cosmic rays

An attempt has been made to understand the electron-proton abundance ratio in cosmic rays observed near the Earth. After correction for interplanetary and interstellar effects, the ratio has been obtained near the source boundary. A leaky source model which can describe consistently all components of the cosmic radiation was then used to obtain the abundance inside the source. Possible effects of injection and acceleration processes on the ratio are examined. From these considerations the most plausible mechanism seems to be injection of electrons and protons by hot gas, and their acceleration by a mixture of Fermi and betatron processes; this is followed by leakage of particles into interstellar space in a rigidity dependent fashion.     

Interior textures,chemical compositions,and noble gas signatures of Antarctic cosmic spherules: Possible sources of spherules with long exposure ages

Abstract– The interior texture and chemical and noble gas composition of 99 cosmic spherules collected from the meteorite ice field around the Yamato Mountains in Antarctica were investigated. Their textures were used to classify the spherules into six different types reflecting the degree of heating: 13 were cryptocrystalline, 40 were barred olivine, 3 were porphyritic A, 24 were porphyritic B, 9 were porphyritic C, and 10 were partially melted spherules. While a correlation exists between the type of spherule and its noble gas content, there is no significant correlation between its chemical composition and noble gas content. Fifteen of the spherules still had detectable amounts of extraterrestrial He, and the majority of them had ³He/⁴He ratios that were close to that of solar wind (SW). The Ne isotopic composition of 28 of the spherules clustered between implantation‐fractionated SW and air. Extraterrestrial Ar, confirmed to be present because it had a ⁴⁰Ar/³⁶Ar ratio lower than that of terrestrial atmosphere, was found in 35 of the spherules. An enigmatic spherule, labeled M240410, had an extremely high concentration of cosmogenic nuclides. Assuming 4π exposure to galactic and solar cosmic rays as a micrometeoroid and no exposure on the parent body, the cosmic‐ray exposure (CRE) age of 393 Myr could be computed using cosmogenic ²¹Ne. Under these model assumptions, the inferred age suggests that the particle might have been an Edgeworth‐Kuiper Belt object. Alternatively, if exposure near the surface of its parent body was dominant, the CRE age of 382 Myr can be estimated from the cosmogenic ³⁸Ar using the production rate of the 2π exposure geometry, and implies that the particle may have originated in the mature regolith of an asteroid.     

Numbers,types, and compositions of an unbiased collection of cosmic spherules

Abstract— Micrometeorites collected from the bottom of the South Pole water well (SPWW) may represent a complete, well‐preserved sample of the cosmic dust that accreted on Earth from 1100–1500 A.D. We classified 1588 cosmic spherules in the size range 50–800 μm. The collection has 41% barred olivine spherules, 17% glass spheres, 12% cryptocrystalline spherules, 11% porphyritic olivine spherules, 12% relicgrain‐bearing spherules, 3% scoriaceous spherules, 2% I‐type spherules, 1% Ca‐AI‐Ti‐rich (CAT) spherules, and 1% G‐type spherules. We also found bubbly glass spherules, spherules with glass caps, and ones with sulfide coatings—particles that are absent from other collections. A classification sequence of the stony spherules (scoriaceous, relic‐grain‐bearing, porphyritic, barred olivine, cryptocrystalline, glass, and CAT) is consistent with progressive heating and evaporation of Fe from chondritic materials. The modern‐day accretion rate and size distribution measured at the SPWW can account for the stony spherules present in deep‐sea collection through preferential dissolution of glass and small stony spherules. However, weathering alone cannot account for the high accretion rate of I‐type spherules determined for two deep‐sea collections. The SPWW collection provides data to constrain models of atmospheric‐entry heating and to assess the effects of terrestrial weathering.     

Crumbs from the crust of Vesta: Achondritic cosmic spherules from the South Pole water well

Abstract— Ten glass cosmic spherules (CS) from the South Pole water well collection were analyzed by electron microprobe. Nine of them have Fe/Mn and Fe/Mg ratios in the range typical of chondrites. One of them (SP37‐3), along with up to six other previously analyzed CS, have nonchondritic Fe/Mn and Fe/Mg ratios that agree well with values typical of either (basaltic) howardite, eucrite, and diogenite (HED) meteorites or Martian basalts, but not of lunar samples. SP37‐3 also contains an anorthite relic grain. Anorthite has not previously been reported in cosmic spherules, but is well known in HED meteorites. The much greater frequency of HEDs among hand‐sized meteorites suggests but does not prove that HED precursors are more likely for the nonchondritic spherules. We estimate that HED‐like micrometeorites constitute ～0.5 ± 0.4% of the total population of micrometeorites in the South Pole water well, a fraction that translates to a flux of 1.6 ± 0.3 × 10^?8g HED micrometeorites/m²‐y. The ratio of HED‐like objects to carbonaceous objects is about 100 times less in micrometeorites than among hand‐size specimens. We infer that the comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation.     

Enhancing the relative Fe-to-proton abundance in ultra-high-energy cosmic rays

We study a generic class of models for ultra-high energy cosmic ray (UHECR) phenomenology, in which the sources accelerate protons and nuclei with a power-law spectrum having the same index, but with different values for the maximum proton energies, distributed according to a power-law. We show that, for energies sufficiently lower than the maximum proton energy, such models are equivalent to single-type source models, with a larger effective power law index and a heavier composition at the source. We calculate the resulting enhancement of the abundance of nuclei, and find typical values of a factor 2-10 for Fe nuclei. At the highest energies, the heavy nuclei enhancement ratios become larger, and the granularity of the sources must also be taken into account. We conclude that the effect of a distribution of maximum energies among sources must be considered in order to understand both the energy spectrum and the composition of UHECRs, as measured on Earth.     

Elemental abundance enhancement in cosmic rays and their relation to interstellar depletion

We have shown that a correlation exists between the enhancement of abundance of heavy nuclei in cosmic rays and their depletion in interstellar space. A correlation also exists between the abundance enhancement and condensation temperature. We suggest that these correlations imply that much of the heavy nuclei content of cosmic rays may come from grains. A possible model is that grains in star-cloud complexes are accelerated to injection energies by radiation pressure in a supernova explosion and, subsequently, the grain debris is accelerated by shocks to cosmic ray energies.     

Observations on the abundance of nitrogen in the primary cosmic radiation

New measurements of the intensity and spectrum of cosmic ray nitrogen nuclei made by instruments flown on balloons and on the Pioneer-8 space probe are reported. The nitrogen spectrum is found to be identical with that of the other medium nuclei, carbon and oxygen, over the range of measurement from 100 MeV/nuc to > 22 GeV/nuc. The ratio of N to all M nuclei is found to be =0.125, constant to within 10% over this energy range. This ratio is extrapolated to the cosmic-ray source using the most recently obtained abundances of oxygen and heavier nuclei and fragmentation parameters for the production of nitrogen from these nuclei. Taking an average material path length of 4 g/cm² of hydrogen constant with energy, as required to make the abundance of L nuclei 0 at the cosmic-ray source, the resulting N/M source ratio is 0.03. In other words, to the same degree that the so-called L nuclei are absent in the cosmic-ray sources, N nuclei are also absent. This nitrogen abundance is therefore different from the estimated solar atmospheric abundance of 0.10 for the N/M ratio which is believed to represent the integrated effects of nucleo-synthesis in the galaxy at the time of the formation of the sun. Nevertheless under certain conditions in the CNO bi-cycle that operates for the production of nitrogen in stellar objects a negligible production of nitrogen might be expected. It is suggested that these conditions exist in the cosmic-ray sources. The C/O ratio of 0.9 deduced for cosmic-ray sources is compatible with the observed low nitrogen abundance arising in this CNO bi-cycle.NRC-NASA Resident Research Associate at Goddard Space Flight Center.     

The crystalline lunar spherules: Their formation and implications for the origin of meteoritic chondrules

Abstract— Crystalline lunar spherules (CLS) from three thin sections of Apollo 14 regolith breccias (14318,6; 14318,48 and 14315,20) have been examined. The objects have been classified and their abundances, size distributions, bulk compositions, and (where possible) plagioclase compositions determined. By number, 64% consist predominantly of very fine-grained equant plagioclase grains but can also contain larger (～50 μm) feldspar crystals (type X), while 22% contain plagioclase lathes in a fine-grained mafic mesostasis (type Y). Plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue CL of the normal feldspar of the breccias. Type Z spherules (5%) contain feldspar with blue CL and minor amounts of olivine and pyroxene. Type Q spherules (4%) contain feldspar with yellow CL but in a luminescent mesostasis (of quartz or feldspar?). A few spherules are mixtures of type Y and type X textures. Most type X spherules, and a few type Y spherules, have fine-grained opaque rims. Compound objects were also found and consist of two or more CLS that appear to have collided while still plastic or molten. The CLS are thought to be impact spherules that crystallized in free flight, their coarse textures suggesting fairly slow cooling rates (～ <1 °C/s). The abundance of the CLS resembles that of chondrules in the CM chondrite Murchison, and their cumulative size-frequency distributions are very similar to those of the chondrules in several meteorite classes. The bulk compositions of the CLS do not resemble regoliths at any of the Apollo sites, including Apollo 14, or any of the common impact glasses, but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain. The Apollo 14 site is located on a region containing Imbrium ejecta, and we suggest that the CLS derive from the Imbrium impact. Ballistic calculations indicate that only impact events of this size on the Moon are capable of producing melt spherules with the required free flight times and slow cooling rates. Smaller impacts produce glassy spherules and agglutinates. As has been pointed out many times, the CLS have many properties in common with meteoritic chondrules. While much remains unclear, difficulties with a nebular origin and new developments in chondrule chronology, studies of asteroid surfaces and impact ejecta behavior, and the present observations indicate that meteoritic chondrules could have formed by impact.     

UH cosmic rays and solar system material: The elements just beyond iron

The origin of the elements from Cu to As in the UH (ultra-heavy) cosmic rays is investigated and related to current concepts of the nucleosynthesis of solar system material. The charge spectrum of the UH cosmic rays in the interval 29Z60 is studied via a fully developed propagation calculation for source abundances given by solar system material, ther-process, the massive-star core helium-burnings-process, and explosive carbon burning. None of these sources considered individually can explain the cosmic ray observations. However a combination of material produced in ther-process, the core helium-burnings-process and in explosive carbon burning provides a good representation of the experimental data. The cosmic-rayr-process is found to differ from solar systemr-process events by an underproduction of the low-massr-process peaks relative to theA195 peak. The large cosmicray abundance forZ=40–44 may be due to anr-process fission component, but this explanation is by no means certain. Improved cosmic-ray data, especially for Zn–Sr, can provide limits to the various source contributions. The model described here gives a consistent picture for the origin of both the cosmic rays and the solar system elements just beyond iron, and adds additional evidence for the importance of massive stars as a site of nucleosynthesis and the birthplace of the cosmic rays.Enrico Fermi Institute.     

The Os-Pt-Hg abundance peak in Ap stars and the problem of very heavy cosmic rays

Relative abundances in the region 74Z83 (W to Bi) are determined for 73 Dra, HR 4072, and some other Ap stars. Abundance peaks occur at atomic massesA=191±2 on 73 Dra, atA=201±3 on HR 4072, atA=199±5 on other main group Ap stars, and atA=201±2 on Mn stars. Pb has a relatively low abundance on Ap stars and also in cosmic rays which have an abundance peak atA=193±3. The abundance peaks on main group Ap stars are due to the cyclicr-process which occurred in explosions of former companion stars. Fission products of transuranic elements are recycled by further rapid neutron captures. At the end of ther-process, the high neutron flux decreases gradually so that the final -decays take place in a neutron-rich environment; superheavy elements (Z110) formed in ther-process may be partly destroyed by neutron-induced fission. The pulsar remnants of the explosions accelerater-process elements to cosmic-ray energies. The peak atA 201 on Mn stars is discussed briefly.