Thermal alteration of hydrated minerals during hypervelocity capture to silica aerogel at the flyby speed of Stardust

Abstract— Outside the Earth's atmosphere, silica aerogel is one of the best materials to capture finegrained extraterrestrial particles in impacts at hypervelocities. Because silica aerogel is a superior insulator, captured grains are inevitably influenced by frictional heat. Therefore, we performed laboratory simulations of hypervelocity capture by using light‐gas guns to impact into aerogels finegrained powders of serpentine, cronstedtite, and Murchison CM2 meteorite. The samples were shot at >6 km s^?1 similar to the flyby speed at comet P/Wild‐2 in the Stardust mission. We investigated mineralogical changes of each captured particle by using synchrotron radiation X‐ray diffraction (SR‐XRD), transmission electron microscope (TEM), and field emission scanning electron microscope (FE‐SEM). SR‐XRD of each grain showed that the majority of the bulk grains keep their original mineralogy. In particular, SR‐XRD and TEM investigations clearly exemplified the presence of tochilinite whose decomposition temperature is about 300 °C in the interior of the captured Murchison powder. However, TEM study of these grains also revealed that all the samples experienced melting and vesiculation on the surface. The cronstedtite and the Murchison meteorite powder show remarkable fracturing, disaggregation, melting, and vesiculation. Steep thermal gradients, about 2500 °C/μm were estimated near the surface of the grains (<2 μm thick) by TEM observation. Our data suggests that the interior of >4 μm across residual grains containing abundant materials that inhibit temperature rise would have not experienced >300 °C at the center.     

Under‐ice salinity and stable isotope distribution of Saroma‐ko Lagoon,Hokkaido, northern Japan

In winter, lakes and lagoons at high altitudes or high latitudes have interesting hydrological cycles that differ from those in other seasons or in other regions, because water surfaces are covered with ice. Hydrological balances of lakes and lagoons are complex dynamic systems, and to elucidate them, isotopic tracers of water have been used as effective tools along with observations of precipitation, evaporation, inflows, and outflows. Here, to understand hydrological processes during freezing periods in the brackish Saroma‐ko Lagoon, Hokkaido, northern Japan, we examined horizontal and vertical distributions of salinity and isotope compositions of lagoon water and ice in 2005 and 2006. Horizontal and vertical gradients of salinity and isotope compositions were observed from the river mouth to the sea channel, and factors determining these distributions were considered. The mixing of freshwater and seawater and a freezing effect were presumed to be factors in relationships between salinity and isotopes and in relationships between surface waters and ice just above the water. A simple box model for water balance was constructed based on these putative factors to reproduce the distributions of salinity and isotope compositions of surface waters and ice. An evaluation of the model revealed that this hydrological system is controlled primarily by horizontal advection of the epilimnion, freshwater influx, and the ice growth rate. Copyright © 2009 John Wiley & Sons, Ltd.     

Penetration into low-density media: In situ observation of penetration process of various projectiles

In order to understand the penetration process of projectiles into lower-density targets, we carry out hypervelocity impact experiments using low-density (60 mg cm^?3) aerogel targets and various types of projectiles, and observe the track formation process in the targets using a high-speed camera. A carrot shaped track, a bulbous, and a “hybrid” one consisting of bulbous and thin parts, are formed. The results of the high-speed camera observations reveal the similarity and differences on the temporal evolution of the penetration depth and maximum diameter of these tracks. At very early stages of an impact, independent of projectile type, the temporal penetration depth is described by hydrodynamic models for the original projectiles. Afterward, when the breakup of projectiles does not occur, intact projectiles continue to penetrate the aerogels. In the case of the breakup of projectiles, the track expands with a velocity of about a sound velocity of the aerogel at final stages. If there are large fragments, they penetrate deeper and the tracks become a hybrid type. The penetration of the large fragments is described by hydrodynamic models. Based on these results, we discuss the excavation near the impact point by shock waves.     

Non-equilibrium Condensation ina Primordial Solar Nebula: Formation of Refractory Metal Nuggets

We present a theoretical investigation of non-equilibrium condensation of refractory metalsinthe primordial solar nebula, in relation to the origin of “Fremdlinges” included inCAIs. Todescribe the nucleation process of grains from vapor, weadopted asemi-phenomenological modelmodified fromthe classical nucleation theoryby the introductionof the second virial coefficient of vapor. This modelachieves excellent agreement with nucleation rate experiment. However,the second virial coefficients are unknown for a vapor of refractory metals. To overcome this, weexpress the nucleation rate by theuse of the chemical potential of dimersinsteadof the second virial coefficient.On the basis of this new nucleation theory,we have performed numerical simulations ofnon-equilibrium condensation of refractory metals andfind thattheircondensation temperatures, T_c, decrease considerably in comparison withequilibrium condensation. Even if the characteristiccooling time scale is aslarge as 1×10⁵ years, the decrease in T_c isfrom 200 to 400 Kfor rare elements such as W, Re,and Os. This remarkablenon-equilibrium behaviormainly stemsfromthelow totalpressure in the primordial solar nebula. From our new modelwealso obtainthe typical size ofgrainsformed in condensation. We findthatthe cooling time should be ?1×10⁵ years for sub-micron-sized or largerrefractory metal nuggetsto form.     

Cases-97: Late-Morning Warming And Moistening Of The Convective Boundary Layer Over The Walnut River Watershed

Aircraft, radiosonde, surface-flux, and boundary-layer windprofiler data from the Cooperative Atmosphere Surface Exchange Study's 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layerpotential temperature []and mixing-ratio [Q] onthree nearly-cloudless days from 1000 CST to 1200CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency (`budget')equations for []and [Q]. We estimate three of the terms –local time rate of change, vertical flux divergence, andhorizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, buthorizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the -budget,vertical flux divergence accounts for most of the morningwarming, with horizontal advection of secondary importance.The residual in the -budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the []-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [];-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient andhence horizontal advection.     

Geomorphological study on a clastic accretionary prism: The Nankai Trough

Abstract The Nankai Trough, off southwest Japan, is one of the best sites for the study of geomorphic characteristics of a clastic accretionary prism. A recent multibeam survey over the central and eastern parts of the Nankai accretionary prism has revealed a large variation of the topography along the trough axis. Analysis of the bathymetric data suggests the existence of prism deformational features of different scales, such as depressions, embayment structures and cusps. These structures are the results of slope instability caused by basement relief of subducted oceanic plate. Unstable slopes recover by new accretion and development of a low angle thrust. Small-scale deformation due to the subduction of a small isolated seamount is then adjusted to the regional trend. By contrast, a 30 km indentation of the wedge observed in the eastern part of the Nankai Trough, the Tenryu Cusp, has seemed to retain its geometry. The subducted Philippine Sea plate has deformed greatly near the eastern end of the Nankai Trough, because of the collision between the Izu-Ogasawara (Bonin) arc and central Japan. Therefore, the indentation may be the result of the continuous subduction of a basement high, such as the Zenisu Ridge, which has been formed under north-south compression due to the arc-arc collision.     

Sr and Nd isotope compositions of atmospheric mineral dust at the summit of Mt. Sefuri, north Kyushu, southwest Japan: A marker of the dust provenance and seasonal variability

We analyzed Rb-Sr-Nd isotope ratios of mineral dust in total aerosol load collected with rainwater continuously from 1998 to 2006 at the summit of Mt. Sefuri, northern Kyushu, southwestern Japan. During this period, the total mass of the dust generally increased in late winter, peaked in early spring, and then decreased.⁸⁷Sr/⁸⁶Sr in atmospheric mineral dust varied from 0.7096 to 0.7180, and εNd(0)_CHUR from −19.9 to −3.5. During heavy deposition periods, the dust had high ⁸⁷Sr/⁸⁶Sr isotope ratios and low to middle εNd(0)_CHUR values, respectively. These compositions are comparable to those of sand and loess in arid areas of Northeast China, Takla Makan and Western Beijing. Such particles were transported by westerlies from those areas to northern Kyushu in winter and spring. In summer and autumn, the isotopic compositions of the dust varied greatly; however, during light deposition periods, the Sr isotope composition was low. In these seasons, the contributions to the dust from Japanese soils and volcanic ash, transported by southern winds, were relatively larger than in winter and spring because of decreased mineral dust particle transport from the continent. Nevertheless, fine sandy desert particles and loess in general accounted for most mineral dust deposition in northern Kyushu year-round, even in summer. Local soils derived from weathered granite and volcanic ash were minor components only.The net mass of water-insoluble inorganic matter in the collected mineral dust fluctuated from year-to-year; deposition on Mt. Sefuri was relatively large before 2001, decreased from 2002 to 2005, and increased greatly in spring 2006. These year-to-year differences probably reflected changes in the strength of the westerlies and in climate conditions in the arid source areas.     

The succession of microbial community in the organic rich fish-farm sediment during bioremediation by introducing artificially mass-cultured colonies of a small polychaete, Capitella sp. I

We monitored seasonal changes of the abundance and composition of microorganisms in the fish-farm sediment in Kusuura Bay, Amakusa, Japan, using the quinone profiling technique, during bioremediation by introducing cultured colonies of polychaete, Capitella sp. I. In November 2004, approximately 9.2 million cultured worms were transferred to the fish-farm sediment, which increased rapidly, and reached 458.5 gWW/m(2) (528,000 indiv./m(2)) in March 2005. During this fast-increasing period of Capitella, the microbial quinone content of the surface sediment (0-2 cm) also increased markedly, and reached 237 micromol/m(2) in January 2005, although the water temperature decreased to the lowest levels in the year. Particularly, the mole fraction of ubiquinone-10 in total quinones in the sediment, indicating the presence of alpha subclass of Proteobacteria, increased by 9.3%. These facts suggest that the bacterial growth was enhanced markedly by the biological activities of worms in the sediment, and the bacteria played an important role in the decomposition of the organic matter in the sediment.     

The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo–Triassic metamorphic complexes in East Asia

The Higo terrane in west-central Kyushu Island, southwest Japan consists from north to south of the Manotani, Higo and Ryuhozan metamorphic complexes, which are intruded by the Higo plutonic complex (Miyanohara tonalite and Shiraishino granodiorite).The Higo and Manotani metamorphic complexes indicate an imbricate crustal section in which a sequence of metamorphic rocks with increasing metamorphic grade from high (northern part) to low (southern part) structural levels is exposed. The metamorphic rocks in these complexes can be divided into five metamorphic zones (zone A to zone E) from top to base (i.e., from north to south) on the basis of mineral parageneses of pelitic rocks. Greenschist-facies mineral assemblages in zone A (the Manotani metamorphic complex) give way to amphibolite-facies assemblages in zones B, C and D, which in turn are replaced by granulite-facies assemblages in zone E of the Higo metamorphic complex. The highest-grade part of the complex (zone E) indicates peak P–T conditions of ca. 720 MPa and ca. 870 °C. In addition highly aluminous Spr-bearing granulites and related high-temperature metamorphic rocks occur as blocks in peridotite intrusions and show UHT-metamorphic conditions of ca. 900 MPa and ca. 950 °C. The prograde and retrograde P–T evolution paths of the Higo and Manotani metamorphic complexes are estimated using reaction textures, mineral inclusion analyses and mineral chemistries, especially in zones A and D, which show a clockwise P–T path from Lws-including Pmp–Act field to Act–Chl–Epi field in zone A and St–Ky field to And field through Sil field in zone D.The Higo metamorphic complex has been traditionally considered to be the western-end of the Ryoke metamorphic belt in the Japanese Islands or part of the Kurosegawa–Paleo Ryoke terrane in south-west Japan. However, recent detailed studies including Permo–Triassic age (ca. 250 Ma) determinations from this complex indicate a close relationship with the high-grade metamorphic terranes in eastern-most Asia (e.g., north Dabie terrane) with similar metamorphic and igneous characteristics, protolith assembly, and metamorphic and igneous ages. The north Dabie high-grade terrane as a collisional metamorphic zone between the North China and the South China cratons could be extended to the N-NE along the transcurrent fault (Tan-Lu Fault) as the Sulu belt in Shandong Peninsula and the Imjingang belt in Korean Peninsula. The Higo and Manotani metamorphic complexes as well as the Hida–Oki terrane in Japan would also have belonged to this type of collisional terrane and then experienced a top-to-the-south displacement with forming a regional nappe structure before the intrusion of younger Shiraishino granodiorite (ca. 120 Ma).