Orientation of X-Ray Bright Points in the Quiet Sun

Thanks to the high-resolution images from the X-ray telescope (XRT) aboard the Hinode satellite, X-ray bright points (XBPs) in the quiet region of the Sun are resolved and can be seen to have complex loop-like structures. We measure the orientation of such loop structures for 488 XBPs picked up in 26 snapshot X-ray images near the disk center. The distribution of the orientation is slightly but clearly biased to the east – west direction: the random distribution is rejected with a significance level of 1% by the χ ²-test. The distribution is similar to the orientation distribution for the bipolar magnetic fields. The XBP orientation is, however, much more random than that of the bipolar magnetic fields with similar size. 24% of the XBPs are due to emerging bipoles, while the remaining 76% are due to chance encounters of opposite polarities.     

AKARI—Infrared Satellite Mission—Present Status and Early Results

AKARI, formerly known as ASTRO-F, is a satellite mission dedicated to infrared astronomy for the first time in Japan. It has a 685-mm aperture telescope with two focal-plane instruments cooled by liquid helium (LHe) and mechanical coolers on board for observations in the 2–180 μm infrared spectral range. AKARI was launched on 2006 February 21 (UT) into a sun-synchronous polar orbit and started observations in May, 2006. It carried 179 liter LHe that lasted for 550 days and observations with LHe were carried out for more than 15 months. During the LHe holding period, AKARI made all-sky survey observations with six bands from 9 to 160 μm, which surpass the IRAS all-sky survey data in the sensitivity, spatial resolution, and spectral coverage. Together with the all-sky observation, AKARI also made pointing observations for about 10 min at a given position of the sky to execute deep imaging and spectroscopy from near- to far-infrared. Both focal-plane instruments work successfully on orbit and more than 90% of the sky was observed in the all-sky survey. After LHe exhaustion, near-infrared observations are planned to continue. This paper reports the in-orbit performance of AKARI and its early observational results so far obtained.     

Intra-crater deposits of the Toga tuff ring, Oga Peninsula, NE Japan

The Toga tuff ring is a large, dissected tuff ring located on the modern shoreline of the Oga Peninsula, NE Japan. The crater measures 2 km by 2.4 km and the inner crater walls are inclined inward at 40–50° to form a funnel shape. Intra-crater beds are mainly composed of platy or blocky, non- to variably vesicular glass shards and pumice lapilli of K-rich rhyolite composition and dip inward at 10°–30° or less. A gravity model suggests they fill the downward-tapering conduit to a depth of 548 m below sea level. Fission-track dates from the intra-crater deposits indicate the age of the Toga tuff ring is ca. 420 ka, likely corresponding to a stage of global sea-level fall, MIS 12. Subsequent sea-level rise and marine transgression is inferred to have resulted in erosion of almost the entire outer tuff ring by post-eruptive wave action.The intra-crater deposit`s are exposed over a thickness of 50 m in the deeply incised crater floor. They comprise mainly monomictic tephra of phreatomagmatic origin and are similar in grain-size distribution and sedimentary structures to relatively high and low density turbidites, although the constituents, sparse block-sag structures, and multiple fluid-escape dikes suggest that they are the subaqueous equivalents of high- and low-density pyroclastic currents with similar grain-sizes and degree of grain-size sorting. Marine diatom frustules sparsely contained in the deposits suggest that the crater was likely open to the sea, enabling rapid access of seawater to the vent. Pyroclasts ejected through the water flowed back into the crater to form eruption-fed oscillatory or circular turbidity currents and were repeatedly recycled and variably abraded by subsequent explosions, while many juvenile pumice lapilli and ash grains were carried beyond the crater rim to form relatively dilute pyroclastic currents. The Toga example suggests that primary deposits emplaced in crater lakes are well sorted, graded and stratified with polymodal flow directions, sparse block-sags, and vesicular and fragile fragments that are more or less abraded by repeated explosions and recycling.     

Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface

Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An_95‐97), pyroxene (En_74‐75, Fs_21‐22,Wo_3‐4), and olivine (Fo_66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe³⁺/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H₂O steam/liquid or CO₂ gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact.     

Polycyclic aromatic hydrocarbons in the Jurassic–Cretaceous Tetori Group,central Japan

Primary data on the organic geochemistry of the Tetori Group provide basic information about depositional environments and thermal maturation of organic matter through two geological sections in the Hokuriku region, central Japan. The thermal maturity of organic matter was evaluated by the methylphenanthrene index‐1. The maturity progressively increases stratigraphically down through the Izumi section in Fukui Prefecture. The estimated vitrinite reflectance equivalent is <1.35% near the top of the section, gradually increasing to >2.0% near the base. The thermal maturation process can account for stratigraphic changes in abundance of PAHs through this section. However, the occasional occurrence of coronene in the middle of the Izumi section is attributed to possible paleo‐wildfires that supplied more coronene to the depositional site. The stratigraphic distribution of PAHs through the Tateyama section, Toyama Prefecture, also can be explained by the same scenario as envisaged for the Izumi section, but weathering and/or other secondary factors may have partly modified primary signals for this section. Polycyclic aromatic sulfur compounds were observed in nearly all samples from both sections, even in samples for which a freshwater paleoenvironment is surmised. Most plausibly, some reduced sulfur was re‐oxidized to elemental sulfur, which persisted in the sediments and may have contributed to the formation of aromatic sulfur compounds. Hence, polycyclic aromatic sulfur compounds do not provide an index to separate marine environments from the freshwater settings for the Tetori Group.     

Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics

The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (P_O2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (M_R), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe_R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe_R increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn_R also increased gradually similar to Fe_R but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe²⁺ and Mn²⁺ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled P_O2 variations during the Paleoproterozoic by applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols, which enabled us to derive an Fe²⁺ oxidation term referred to as ψ. Possible changes in temperature and P_CO2 during this time window and their effects on resulting models of P_O2 evolution have been also considered. We assumed four cases for the calculations of P_O2 variations between 2.5 and 2.0 Ga: no change in either temperature or P_CO2, long-term change in only P_CO2, long-term changes in both temperature and P_CO2, and short-term fluctuations of both temperature and P_CO2 during the possible, multiple global-scale glaciations. The calculations indicate that P_O2 increased gradually, linearly on the logarithmic scale, from <∼10⁻⁶ to >∼10⁻³ atm between 2.5 and 2.0 Ga. Our calculations show that the P_O2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.     

Molecular phylogeny and evolution of Scomber (Teleostei: Scombridae) based on mitochondrial and nuclear DNA sequences

A molecular phylogenetic analysis of the genus Scomber was conducted based on mitochondrial (COI, Cyt b and control region) and nuclear (5S rDNA) DNA sequence data in multigene perspective. A variety of phylogenetic analytic methods were used to clarify the current taxonomic Classification and to assess phylogenetic relationships and the evolutionary history of this genus. The present study produced a well-resolved phylogeny that strongly supported the monophyly of Scomber. We confirmed that S. japonicus and S. colias were genetically distinct. Although morphologically and ecologically similar to S. colias, the molecular data showed that S. japonicus has a greater molecular affinity with S. australasicus, which conflicts with the traditional taxonomy. This phylogenetic pattern was corroborated by the mtDNA data, but incompletely by the nuclear DNA data. Phylogenetic concordance between the mitochondrial and nuclear DNA regions for the basal nodes Supports an Atlantic origin for Scomber. The present-day geographic ranges of the species were compared with the resultant molecular phylogeny derived from partition Bayesian analyses of the combined data sets to evaluate possible dispersal routes of the genus. The present-day geographic distribution of Scomber species might be best ascribed to multiple dispersal events. In addition, our results suggest that phylogenies derived from multiple genes and long sequences exhibited improved phylogenetic resolution, from which we conclude that the phylogenetic reconstruction is a reliable representation of the evolutionary history of Scomber.