Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles' size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.
Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray-produced (21)Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.
Based on the evidence derived from spectroscopic observation and meteorite analysis, some hydrous asteroids were heated and dehydrated for a certain period of time after aqueous alteration. In order to reproduce the dehydration processes, we experimentally heated Murchison CM chondrite at 600°C for 1 h (600°C/1 h), 600°C/96 h, 900°C/1 h, and 900°C/96 h under controlled oxygen partial pressures. The experimental products were compared with Belgica (B-)7904 CM chondrite, a meteorite from a dehydrated asteroid in terms of characteristic mineralogical and compositional properties. B-7904 shows properties intermediate between the two experimental products heated at 900°C/1 h and 900°C/96 h. In addition, the presence or the absence of some temperature-sensitive minerals in B-7904 suggests that it experienced heating at a temperature higher than 700°C but lower than 890°C. The duration of heating, based on the diffusion time needed to achieve the Fe-Mg zoning profile in olivine in B-7904, was estimated to be between 10 and 103 days at 700°C and between 1 to 102 h at 890°C. The obtained durations are much shorter than those expected from the internal heating model which requires prolonged heating over million years. Therefore, it is unlikely that the short-lived radionuclide of 26Al is a heat source for the dehydration of B-7904. Instead, short-duration local heating, such as that from impacts or solar radiation, is a more promising heat source.
Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.
The 3.2 Ga Dixon Island Formation in theCleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is ~350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identifi ed as an underground bypass zone for circulating hydrothermal fl uid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10-15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fi ne quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10-20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fi ne iron or iron-coated quartz pisolite within fi ne-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at ~500-2000 m paleodepth, and may have been on the slope of an immature island arc. Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3-2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15-0.45%) than that of the overlying laminated chert section (TOC = 0.02-0.15%) and the black chert vein (TOC = 0.1-0.13%), and the carbon isotope (δ 13 C) values of this lithology (−33‰ to ~-27‰) are also lighter than for the black chert veins (-29‰ to ~-26‰) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (−27‰ to ~-13‰). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of biogenic origin and were formed close to a lowtemperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicifi cation related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.
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