Micrometeorite craters discovered on chondrule-like objects from kapoeta meteorite

Craters attributable to hypervelocity impacts of micrometeorites have been discovered on rare chondrule-like objects from the gas-rich meteorite Kapoeta. These chondrule-like objects, probably generated by impacts themselves, provide further evidence for the regolith origin of Kapoeta. The micrometeorite flux at the time of formation of the meteorites was probably an order of magnitude higher than the present flux, but the solar luminosity could not have been higher than 1.7 times its present value.     

The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets

Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn's moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth's volatiles.     

Electromagnetic heating in the early solar nebula and the formation of chondrules

Certain opaque inclusions within primitive meteorites exhibit textures that suggest chondrules formed during intense, short-duration radiative heating episodes in the early solar system. Experimental support for this interpretation is provided by the textures produced when chondrule-like assemblages are heated with visible laser light. Computer simulations of radiative heating provide additional evidence for the role of electromagnetic energy in heating nebular solids by offering an explanation for the size distributions of chondrules and the presence of dusty chondrule rims. Nebular lightning and magnetic reconnection flares are possible sources of electromagnetic energy for these transient heating events.     

Mining the apollo and amor asteroids

Earth-approaching asteroids could provide raw materials for space manufacturing. For certain asteroids the total energy per unit mass for the transfer of asteroidal resources to a manufacturing site in high Earth orbit is comparable to that for lunar materials. For logistical reasons the cost may be many times less. Optical studies suggest that these asteroids have compositions corresponding to those of carbonaceous and ordinary chondrites, with some containing large quantities of iron and nickel; others are thought to contain carbon, nitrogen, and hydrogen, elements that appear to be lacking on the moon. The prospect that several new candidate asteroids will be discovered over the next few years increases the likelihood that a variety of asteroidal resource materials can be retrieved on low-energy missions.     

Lunar anorthosites: rare-Earth and other elemental abundances

Elemental abundances of major (Ti, Al, Fe, and Ca), minor (Na, Mn, and Cr), and trace elements [14 rare-earth elements (REE), Y, In, Cd, Rb, Cs, Ba, Co, and Sc] in lunar anorthosites separated from Apollo 11 sample 10085 coarse fines have been determined by means of instrumental and radiochemical neutron activation analysis. The REE distribution pattern of lunar anorthosites, relative to ordinary chondrites, has a positive Eu anomaly. On the assumption that (i) the lunar composition is similar to that of ordinary chondritic meteorites low in total Fe ( approximately 13 percent); (ii) lunar anorthosites are derived from highland cratering events and are representative of the highlands; and (iii) the moon differentiated into olivine, hypersthene, and basaltic and anorthositic phases, and plagioclase crysstallization began after approximately 93 percent solidification, then mass balance calculations yield approximately 30-kilometer and approximately 10-kilometer thicknesses for the lunar highlands for the melting and chemical differentiation of the entire moon and of the upper 200 kilometers, respectively. Corresponding thicknesses of the basaltic basement rocks were approximately 5 kilometers and approximately 2 kilometers, respectively. Alternatively, if the anorthosites of this study are representative of the highlands and the onset of plagioclase crystallization occurred after approximately 50 percent solidification of the initially melted moon, calculations with REE and Ba partition coefficients suggest that the REE and Ba abundances in the primeval moon were similar to those observed in basaltic achondrites.     

Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites

The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid.     

92Nb-(92)Zr and the Early Differentiation History of Planetary Bodies

The niobium-92-zirconium-92 ((92)Nb-(92)Zr) extinct radioactive decay system (half-life of about 36 million years) can place new time constraints on early differentiation processes in the silicate portion of planets and meteorites. Zirconium isotope data show that Earth and the oldest lunar crust have the same relative abundances of (92)Zr as chondrites. (92)Zr deficits in calcium-aluminum-rich inclusions from the Allende meteorite constrain the minimum value for the initial (92)Nb/(93)Nb ratio of the solar system to 0.001. The absence of (92)Zr anomalies in terrestrial and lunar samples indicates that large silicate reservoirs on Earth and the moon (such as a magma ocean residue, a depleted mantle, or a crust) formed more than 50 million years after the oldest meteorites formed.     

Plasma waves near saturn: initial results from voyager 1

The Voyager 1 plasma wave instrument detected many familiar types of plasma waves during the encounter with Saturn, including ion-acoustic waves and electron plasma oscillations upstream of the bow shock, an intense burst of electrostatic noise at the shock, and chorus, hiss, electrostatic electron cyclotron waves, and upper hybrid resonance emissions in the inner magnetosphere. A clocklike Saturn rotational control of low-frequency radio emissions was observed, and evidence was obtained of possible control by the moon Dione. Strong plasma wave emissions were detected at the Titan encounter indicating the presence of a turbulent sheath extending around Titan, and upper hybrid resonance measurements of the electron density show the existence of a dense plume of plasma being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn.     

Abundance and distribution of iron on the moon

The abundance and distribution of iron on the moon is derived from a near-global data set from Clementine. The determined iron content of the lunar highlands crust ( approximately 3 percent iron by weight) supports the hypothesis that much of the lunar crust was derived from a magma ocean. The iron content of lower crustal material exposed by the South Pole-Aitken impact basin on the lunar farside is higher ( approximately 7 to 8 percent by weight) and consistent with a basaltic composition. This composition supports earlier evidence that the lunar crust becomes more mafic with depth. The data also suggest that the bulk composition of the moon differs from that of the Earth's mantle. This difference excludes models for lunar origin that require the Earth and moon to have the same compositions, such as fission and coaccretion, and favors giant impact and capture.     

Chainpur-like Chondrites: Primitive Precursors of Ordinary Chondrites?

Chainpur and similar, apparently primitive, chondritic meteorites may be precursors of ordinary chondrites; a variety of evidence supports this working hypothesis. In general, carbonaceous chondrites seem to be related collaterally to this genetic sequence rather than being direct ancestors of ordinary chondrites. Metamorphic processes may be responsible for fractionations of elements such as indium and iodine, and type-II carbonaceous chondrites seem to be more primitive than types I or IIIA.     

Meteorites: optical activity in organic matter

A low- amplitude, positive, Cotton effect, centered at about 340 millimicrons, has been observed in organic extracts of samples from ordinary (noncarbonaceous) chondrites. Ancillary evidence renders it likely that this optical activity derived from contamination by biologic materials on Earth.     

A nebular origin for chondritic fine-grained phyllosilicates

Hydrated minerals occur in accretionary rims around chondrules in CM chondrites. Previous models suggested that these phyllosilicates did not form by gas-solid reactions in the canonical solar nebula. We propose that chondrule-forming shock waves in icy regions of the nebula produced conditions that allowed rapid mineral hydration. The time scales for phyllosilicate formation are similar to the time it takes for a shocked system to cool from the temperature of phyllosilicate stability to that of water ice condensation. This scenario allows for simultaneous formation of chondrules and their fine-grained accretionary rims.     

Mercury's Surface: Preliminary Description and Interpretation from Mariner 10 Pictures

The surface morphology and optical properties of Mercury resemble those of the moon in remarkable detail and record a very similar sequence of events. Chemical and mineralogical similarity of the outer layers of Mercury and the moon is implied; Mercury is probably a differentiated planet with a large iron-rich core. Differentiation is inferred to have occurred very early. No evidence of atmospheric modification of landforms has been found. Large-scale scarps and ridges unlike lunar or martian features may reflect a unique period of planetary compression near the end of heavy bombardment by small planetesimals.     

Potassium-argon ages of lunar rocks from mare tranquillitatis and oceanus procellarum

Crystalline rocks from Mare Tranquillitatis have yielded potassium-argon dates as old as 3.8 x 10(9) years. Crystalline rocks from Oceanus Procellarum give potassium-argon ages as old as 2.8 x 10(9) years. Evidently the maria are ancient features of the moon and were not formed contemporaneously, a conclusion that also has been verified by other methods. The potassium-argon ages of rocks from Oceanus Procellarum show much more loss of argon than the rocks from Mare Tranquillitatis, an indication that the rocks at Oceanus Procellarum have experienced more severe shock effects or longer cooling rates.     

Chemical Composition of the Lunar Surface in a Terra Region near the Crater Tycho

More precise and comprehensive analytical results for lunar surface material in a terra region have been derived from the data of the alpha-scattering experiment on Surveyor 7. The silicon content and the low sodium abundance are close to that of mare material. The abundances of titanium and iron are at least a factor of 2 lower, whereas the abundances of aluminum and calcium are significantly higher. The analytical results provide direct evidence for chemical differentiation in the moon and indicate a lunar crust of appreciably lower density than the whole moon and with lower density and higher albedo than lunar mare material.     

The formation of chondrules: petrologic tests of the shock wave model

Chondrules are millimeter-sized rounded igneous rocks within chondritic meteorites. Their textures and fractionated mineral chemistries suggest that they formed by repeated, localized, brief (minutes to hours) melting of cold aggregates of mineral dust in the protoplanetary nebula. Astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites. However, a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed. It also explains the volatile-rich nature of chondrule rims and the chondrite matrix.     

PLANETARY SCIENCE: Beating Up on a Young Earth, and Possibly Life

Lunar meteorite analyses reported on page 1754 of this issue reveal a burst of impacts on the moon 3.9 billion years ago and nothing before that; the cosmochemists conclude that the moon and Earth endured a storm of impacts 100 times heavier than anything immediately before or after. Such a lunar cataclysm would have scarred the moon with the great basins that now shape the man in the moon. On Earth, the same bombardment would have intervened in the evolution of life, perhaps forcing it to start all over again.     

Topographic-compositional units on the moon and the early evolution of the lunar crust

The distribution of elevations on the moon determined by Clementine deviates strongly from a normal distribution, suggesting that several geologic processes have influenced the topography. The hypsograms for the near side and far side of the moon are distinctly different, and these differences correlate with differences in composition as determined by Apollo orbital geochemistry, Clementine global multispectral imaging, and ground-based spectroscopy. The hypsograms and compositional data indicate the presence of at least five compositional-altimetric units. The lack of fill of the South Pole-Aitken Basin by mare basalts suggests poor production efficiency of mare basalt in the mantle of this area of the moon.     

The Earth as a planet: paradigms and paradoxes

The independent growth of the various branches of the earth sciences in the past two decades has led to a divergence of geophysical, geochemical, geological, and planetological models for the composition and evolution of a terrestrial planet. Evidence for differentiation and volcanism on small planets and a magma ocean on the moon contrasts with hypotheses for a mostly primitive, still undifferentiated, and homogeneous terrestrial mantle. In comparison with the moon, the earth has an extraordinarily thin crust. The geoid, which should reflect convection in the mantle, is apparently unrelated to the current distribution of continents and oceanic ridges. If the earth is deformable, the whole mantle should wander relative to the axis of rotation, but the implications of this are seldom discussed. The proposal of a mantle rich in olivine violates expectations based on evidence from extraterrestrial sources. These and other paradoxes force a reexamination of some long-held assumptions.     

Discovery of an unmelted h-chondrite inclusion in an iron meteorite

The link between H chondrites and silicate inclusions in group IIE iron meteorites has long been suspected, but direct evidence for a common parentage has remained elusive. The discovery of an unmelted chondritic inclusion in the Techado iron meteorite sheds light on the genetic relation between these two groups, providing clues on the origin of chondritic materials as inclusions in iron meteorites. It is proposed that the complex IIE iron meteorite breccias formed by collisions with several different bodies, followed by deep burial of metal and silicate fragments in the asteroidal megaregolith.