MIS35-01

Room:201A

Time:May 24 09:00-09:15

Impact history revealed by the cratering records of the Moon and planets
MOROTA, Tomokatsu1∗
1

Nagoya Univ.

It is thought that the supply process of water to the Earth is closely related to the dynamics of the early solar system. Unravelingthe impact history in the early Earth-Moon system is essential to understand the origin of water in the Earth. The cratering recordsof the Moon and planets preserve the impact history in the solar system during the past 4.5 Gyr, which is a key information tounderstand the source and the dynamical and collisional evolutions of small bodies.
In this talk, I would like to review the statistical studies of the cratering records of the Moon and planets and its findings. Also,
I will present findings on the impact history in the early Earth-Moon system revealed by the Japanese lunar explorer SELENE(Kaguya).
Keywords: impact, crater, cratering chronology, Moon, late heavy bombardment

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MIS35-02

Room:201A

Time:May 24 09:15-09:30

Solar system science by the Subaru Hyper Suprime-Cam survey
YOSHIDA, Fumi1∗
1

NAOJ

Hyper Suprime-Cam (HSC) survey has been started as a Subaru Strategic Program from March 2014, which has planned tosurvey about 1500 square degree on the sky. This is the first large field survey using Subaru telescope. Owing to a large apertureof Subaru telescope, it is expected to detect the small solar system bodies with the magnitude range of r’= 22-27 mag. It isdefinitely deepest survey for the small solar system bodies and absolutely an excellent opportunity to determine the feint end of
the size distribution of each small solar system bodies group.
This kind of observations that determine the feint end size distribution of small solar system bodies has been done by theSuprime-Cam, which is also mounted on the Subaru Telescope. However, the survey area was narrow (about <10 square degrees), it has been required to increase the determination accuracy of the size distribution with more samples.
Since the HSC survey can detect one order large samples than all of the survey done by Suprime-Cam so far, the size distribution of each main belt region (inner (mostly S-type asteroids) / center (S / C types) / outer (mostly C-types)) can be determined.
From that information, we are able to reveal a relationship between the collisional evolution and asteroid’s composition, therefore, aims to reveal the composition and internal structure of a whole asteroid belt. Another important expectation is the findingof main belt comet candidates by using the point spread function of object image, which is important bodies in order to clarify
the water distribution in the main belt.
Some of the main belt objects are known to evolve into near Earth objects (NEOs), they might supply a significant amountof water to the Earth. Therefore, the observational results collected by the HSC survey, which is the material composition/distribution of the entire main belt, together with the theoretical studies on planetary migration and the orbital evolutionof small bodies, will supply an important clue to reveal aspects of substance transportation into the Earth.
In the HSC survey, we can also detect enough samples of Jupiter trojans and TNOs. Therefore we have a plan to investigate
physical and chemical differences between the L4 and L5 swarms of Trojans and the size distribution difference between the cold
/ hot population of TNOs.
In my presentation, I also introduce results that has done by the Suprime-Cam so far and describe what we can do more sciencein the HSC survey.
Keywords: Small Solar System Bodies, Wide field survey, Photometry, Asteroids, Trojans, TNOs

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MIS35-03

Room:201A

Time:May 24 09:30-09:45

Exploring the origin of Earth’s water by the Hayabusa-2 near-infrared spectrometer
KITAZATO, Kohei1∗ ; IWATA, Takahiro2 ; ABE, Masanao2 ; OHTAKE, Makiko2 ; HIROI, Takahiro3 ;NAKAMURA, Tomoki4 ; KOMATSU, Mutsumi5 ; ARAI, Tomoko6 ; NAKATO, Aiko2 ; OHSAWA, Takahito7 ;NAKAUCHI, Yusuke5 ; WATANABE, Sei-ichiro8
1

University of Aizu, 2 JAXA/ISAS, 3 Brown University, 4 Tohoku University, 5 Graduate University for Advanced Studies, 6 Chiba
Institute of Technology, 7 JAEA, 8 Nagoya University

NIRS3, the near-infrared spectrometer onboard the Hayabusa-2 spacecraft, is a remote-sensing instrument to obtain the reflectance spectra including 3-µm absorption features due to structural OH ions and H2O molecules. In 2018-2019, we areplanning to perform proximity observations of a near-Earth C-type asteroid 1999JU3 using NIRS3 and to reveal the distribution of hydrated minerals on the asteroid surface. Recently, the results indicating the presence of internal water ice for C-type
asteroids were reported from ground-based observations. Hence, there is a possibility that the contribution of C-type asteroids
to formation of Earth’s ocean becomes larger than that predicted so far. It is required to understand the behavior of water in
the aqueous alteration to verify the internal water ice. Thus, NIRS3 aims at obtaining information about the aqueous alteration
discerning the secondary alteration effects such as thermal dehydration and space weathering from observations of the crater
formed by the artificial impact experiment. In this presentation, we introduce the expected outcome of NIRS3 based on thecurrent knowledge of C-type asteroids.
Keywords: Hayabusa-2, asteroids, water, near-infrared spectroscopy

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MIS35-04

Room:201A

Time:May 24 09:45-10:00

Dehydration and rehydration of hydrous carbonaceous chondrites
NAKAMURA, Tomoki1∗ ; MATSUOKA, Moe1 ; YAMASITA, Sayuri1 ; SATO, Yudai1 ; JOGO, Kaori1 ; AHN, Insu2 ;LEE, Jong-ik2 ; IMAE, Naoya3 ; YAMAGUCHI, Akira3 ; KOJIMA, Hideyasu3
1

Tohoku University, 2 Korean Polar Research Institute, 3 National Institute of Polar Research

Asteroidal water in hydrous C-complex asteroids is one of the possible source of Earth’s ocean. The hydrous C-complex
asteroids consist of hydrous carbonaceous chondrites and therefore the water came to the earth as such hydrated meteorites. CM
chondrites are the most abundant group of carbonaceous chondrites, composed mainly of hydrous minerals such as serpentine(cronstedtite) and tochilinite. They show 0.7-µm and 3-µm absorption bands in the reflectance spectra. The 0.7-µm band is aspectral feature characteristic of CM chondrites, because it is common in CM chondrites and rare in other hydrated carbonaceouschondrites (Cloutis et al. 2011). Recent investigation on spectral data of asteroids indicated that 30±5% of C-complex asteroids
shows 0.7-µm band (Rivkin, 2012), suggesting that CM materials are also common at main belt asteroids.
Some CM chondrites have been heated to temperature sufficient for dehydration of hydrous minerals (e.g., Akai, 1990: Nakato
et al. 2008) and are classified to heating stage I to IV based on the degree of heating (Nakamura, 2005). On the other hand, not a
few C-complex asteroids show reflectance spectra similar to dehydrated CM chondrites (Hiroi et al. 1993). In the present study,
we performed analyses of reflectance spectra, X-ray diffraction, and water contents of hydrated and dehydrated CM chondrite
samples. In addition, heating experiments were conducted in order to observe changes of reflectance spectra, X-ray diffraction,
and water content with increasing temperature. Murchison CM chondrite was heated for 50 hours at temperatures of 400, 600,
and 900 ℃ at IW oxygen buffer.
Heating experiments showed that tochilinite decomposes and serpentine partly becomes amorphous at 400 ℃, serpentine
completely decomposes and secondary olivine nucleates at 600 ℃, and olivine becomes well crystalline and metallic FeNi
generates at 900 ℃. The samples heated at 400, 600, and 900 ℃ reproduced the mineralogy of CM chondrites with heating stage
II, III, and IV, respectively. The 0.7-µm band disappears by heating at 400 ℃. The 3-µm band strength decreases with increasing
temperature, but does not disappear even at 900 ℃. Water contents of unheated and experimentally heated Murchison samples
were determined by the Karl Fischer titration method with stepped heating: 10.0, 6.6, 1.2, and 0.6 wt% of water recovered fromunheated, 400, 600, and 900 ℃ samples, respectively. The result clearly indicates that the dehydration proceeds with increasing
temperature. On the other hand, in the stepped heating analysis, most of the water was released below 600 and 900 ℃ from 600
and 900 ℃ heated samples, respectively. This indicates that water in the 600 and 900 ℃ samples (1.2, and 0.6wt%, respectively)
was acquired into samples by rehydration in the atmosphere after heating experiments. The rehydration water is tightly boundedto samples, because the largest release of water was detected at 400 ℃ from both 600 and 900 ℃ heated samples. If we omit
rehydration water from the total water contents, then dehydration is completed in Murchison by heating at 600 ℃ for 50 hours(heating stage III).
On the other hand, the results of reflectance spectra measurement of naturally heated CM chondrites reveals that even samplesof heating stage IV such as Dho735, B7904 and Y86720 show 3-µm absorption band, which suggests rehydration. Water-content
analysis of Dho735 confirms the rehydration: most of water was released at 300˜600 ℃. The water analysis concludes thatdehydrated CM chondrites are a strong water absorber. This suggests that dehydrated-CM materials on the surface of C-complex
asteroids would resorb the water released upon impacts of hydrous micrometeorites from other asteroids and comets.
Keywords: aqueous alteration, C-complex asteroids, reflectance spectra, water analysis

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MIS35-05

Room:201A

Time:May 24 10:00-10:15

Significance of incompleteness as an aqua planet: Earth’s climate with land-sea coexistence
YAMANAKA, Manabu D.1∗
1

JAMSTEC-Cooperative Devision, DP-GSS, Kobe University

Our planet Earth is in a habitable zone in the Solar System, but is not entirely covered by liquid water, because its crust has
separated into higher (continental) and lower (oceanic) parts. Such a planet of land-sea coexistence has a climate different fromso-called aqua-planets or land planets.
Firstly, because oceans cannot flow beyond lands, the zonal vortical motion dominant generally in rotating planetary fluids
must turn poleward near coasts in the oceans and contribute to meridional heat transport. Secondly, because of heat capacity
contrast between ocean and land, the periodical variability of solar irradiance on a planet with rotation and revolution induces
striking horizontal temperature gradient and wind in the bottom of atmosphere. Thirdly, the atmospheric mechanical forcing may
be balanced (interacted) with the oceanic thermal forcing, whereas the rapid maritime water vapor transport must be balancedwith the slow river flow on land. The coastline is just like a triple point among three geophysical phases of atmosphere, oceanand crust, maintained by an erosion-orogeny balance.
The land-sea coexistence is essentially important in equatorial climate, where the dominant motion is convective and governedby the temperature contrast and water budget between land and sea. Thus the longest coastlines in the Indonesian maritime continent (IMC) generate the largest rainfall on Earth. A super cloud cluster coupled with warm ocean water may circulate eastward
with an intraseasonal period along the equator on an aqua-planet, but must be modified by reflection of ocean water (waves) at
the coasts of IMC and American/African continents with an interannual time scale. Therefore, IMC is an important region inwhich we can study the climate of a land-sea planet based on observations.
The coastlines and rivers important climatologically as mentioned above are also zones in which human population, infrastructure and hence observations are concentrated. This is partly why we could start climatology even in an early history of thehuman beings. The leadership of climate observations in IMC is being transferred to the new G20 country, and we are entering
into a new era for truly global observation of our planet Earth.
Keywords: land-sea coexistence, coastline, climate, rotation and revolution, human beings, Indonesian maritime continent

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MIS35-06

Room:201A

Time:May 24 10:15-10:30

Water transport to the deep mantle and its effects on the mantle dynamics
NAKAKUKI, Tomoeki1∗ ; NAKAGAWA, Takashi2 ; IWAMORI, Hikaru3
1

Dept. Earth and Planetary Systems Science, Hiroshima Univ., 2 MAT, JAMSTEC, 3 Geochemical Evolution Research Program,
JAMSTEC
Numerical study for water transport under a volcanic arc revealed dynamics of the water processes inducing melt generation(Iwamori, 1998). Back-arc and intra-plate volcanisms also indicate water migration from a deeper section of the subduction zone.
Aiming to understand geodynamical processes of water derived and transported from the subducted slab in the deep subduction
zone, we developed a numerical model of water transport coupled dynamically with plate-mantle convection system with a wholemantle scale. We here focus on the mechanism of dehydration from stagnating or penetrating slab and water transport from the
mantle transition zone (MTZ). We also consider water transport to deeper mantle and the effects on the global distribution ofwater-compatible elements that is indicated by the independent component analysis (ICA) of isotope anomaly space (Iwamori
and Nakamura, 2012).
We assume that a viscous fluid in a 2-D rectangular box with an extended Boussinesq approximation represents the mantle
convection system with integrated lithospheric plates (Tagawa et al, 2007). We incorporate water transport and hydrous mineralphase diagram (Iwamori, 1998; 2007) into the numerical plate-mantle model. We assume that the water dehydrated from watersaturated minerals migrates upward instantaneously with porous flow that is much faster than mantle flow. We introduce reductionof the density and the viscosity due to the hydration into the density and rheology model according to experimental study (Karato
and Jung, 2003). We also consider viscous weakening of serpentine or chlorite that is important for water transport in shallow
subduction zone.
A serpentine layer generated by dehydration of the oceanic crust plays a key role to control water transport by the subductedslab shallower than about 150 km (Iwamori, 1998; 2007; Horiuchi, 2013). To continuously generate this layer, coupling between
the serpentine layer and the plate boundary fault is essential. After dehydration of serpentine, nominally anhydrous minerals(NAMs) (Iwamori, 2007) are a main veneer of the water. In this stage, water capacity of NAMs, which depends on the grainboundary storage as well as that of the hydrous minerals, is the primary factor to control the amount of transported water. This is
not so large as about 0.4 wt. % to maintain water-filled region under the arc. The water is carried without dehydration above the660 km boundary. When the water capacity of the NAMs is about 0.2 wt. %, the amount of the water transported to the mantle
transition zone is about 1 % of the basaltic crust mass. When the lower mantle water capacity is lower than the water capacity of
the NAMs, the water is expelled at the 660 km phase transition. While the water ascends with the porous flow, the medium rocks
descend with asthenospheric flow dragged by the downwelling slab. The repetition of these processes broadens the hydrous layer
at the 660 km boundary. A thin water-saturated layer is formed at the 660 km boundary around the penetrating slab. Because of
the buoyancy, this becomes unstable so that hydrous plumes are generated. The hydrated layer above the lower mantle slab isthickened by 4 to 5 time more than that in the upper mantle. This result in the increase of the water amount transported into thelower mantle by 4 to 5 times larger than that estimated from the ratio of the water capacity between the upper and lower mantles.
The water reaches the core-mantle boundary region with the subducted slab. The subducted slab sweep out the dense chemicallayer above the CMB, so that the dense materials forms a pile-like structure. The distributions of the hydrated materials become
exclusive to that of the chemical piles. Iwamori and Nakamura (2012) showed that water-compatible elements (IC2) and OIB
components (IC1) have divided distribution. This suggests that the dense chemical pile is not a source for the IC1.

Keywords: water transport, subduction, mantle convection, large-scale heterogeneity

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MIS35-07

Room:201A

Time:May 24 10:30-10:45

On the degassing of water vapor inferred from mantle convection simulations
NAKAGAWA, Takashi1∗
1

Department of Mathematical Science and Advanced Technology, JAMSTEC

The degassing process of volcanic activity would be influenced to the formation of surface environment of Earth, which has
been argued from simplified and theoretical model of co-evolution of planetary interior and surface [e.g. McGovern and Schubert,
1989; Tajika and Matsui, 1992]. Those models, however, used the parameterized convection model for heat transfer and volatilecirculation as well as degassing process. Recent progress of numerical modeling of mantle dynamics can trace the magmatic
activity and water circulation over the geologic time-scale [e.g. Nakagawa et al., submitted]. However, such an investigation was
not included for effects of melt-phase system such as expressed by the density structure of silicate melt. This effect would be
essential for reconciling the thermo-chemical state of early Earth’ interior [Labrosse et al., 2007; Lee et al., 2010]. In this study,
we attempt to construct global-scale water circulation model in thermo-chemical mantle convection simulations including meltphase system and degassing-regassing processes over the geologic time-scale. The melt-phase system is based on the density
structure of molten silicate found from Stixrude et al. [2009]. Preliminary results found from this study are suggested thathuge volcanic activity would be expected in early Earth when the density crossover between solid silicate and molten silicate is
assumed in the deep mantle, which the molten silicate is much denser than solid silicate. This also suggests that the degassing
rate of water vapor in the early Earth would be more efficient with the denser molten silicate in the deep mantle compared to theless dense case. More detailed results and discussion will be shown in the presentation.
Keywords: Melt-phase system, Water circulation, Mantle convection, Magmatism, Degassing

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MIS35-08

Room:201A

Time:May 24 11:00-11:15

Origin and evolution of water in the Earth inferred from geological evidences
KATAYAMA, Ikuo1∗ ; MATSUKAGE, Kyoko N.2 ; KIMURA, Jun-ichi3 ; KAWAMOTO, Tatsuhiko4
1

Department of Earth and Planetary Systems Science, Hiroshima University, 2 Department of Earth and Planetary Sciences,
Kobe University, 3 Japan Agency for Marine-Earth Scienc and Technology, 4 Department of Earth and Planetary Sciences, Kyoto
University
The persistent liquid ocean is a result of balance between input and output of water through the Earths mantle; however,
the mass of ocean could be markedly reduced since Archean and might be disappeared in future. In this project, we plane
to investigate the origin and evolution of water in the Earth, based mainly on D/H ratio of fluid inclusion, water content ofprimary magma and it time evolution, and water transportation into the mantle at subduction zone. Integrating these evidences,quantitative estimate of mass of liquid ocean through Earth history will be discussed.
Keywords: water differentiation in the Earth, water evolution in the Earth, water transportation into the Earth

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MIS35-09

Room:201A

Time:May 24 11:15-11:30

Water transportation into the earth’s interior - Oceanic plate and its evolution FUJIE, Gou1∗ ; KODAIRA, Shuichi1 ; KAIHO, Yuka1 ; SATO, Takeshi1 ; TAKAHASHI, Tsutomu1 ;TAKAHASHI, Narumi1 ; YAMAMOTO, Yojiro1 ; YAMADA, Tomoaki2
1

JAMSTEC, 2 ERI, Univ. of Tokyo

Plenty of liquid water exists at the Earth’s surface. If water is transported from the surface into the Earth’s interior, the water
affect various processes in the solid Earth, such as mantle convection, generation of earthquakes, and magmatism. The evolutionof the Earth cannot be explained without the transportation of water into the Earth’s interior from the surface. How is the watertransported into the interior? High pressure and high temperature in the deep interior prevent the penetration of water from thesurface. Instead, water is transported as hydrous minerals by the subduction of the oceanic plate as part of the global mantle
convection. Therefore, the amount of hydrous minerals within the oceanic plate just prior to subduction determine the amount ofwater transported into the Earth’s interior and have an impact on the evolution of the Earth.
Crustal hydration by the hydrothermal circulation at the mid-ocean ridge was formerly considered to be a first-order control
on the degree of oceanic plate hydration. However, recent several observations suggest that plate bending-related faults just priorsubduction may enhance the hydration of oceanic crust and mantle. If this hypothesis is correct, the amount of water transportedby the oceanic plate is much larger than foremerly expected because mantle have a potential to contain much larger amount ofwater than the crust.
In the last decade, to test this hypothesis, a number of structure studies have been conducted in the trench-outer rise region
around the world. We JAMSTEC also have conducted extensive active source seismic structure studies in the northwestern
Pacific. We have shown that seismic velocities gradually reduce toward the trench axis accompanied by the development of bendfaults. In addition, we have shown that the Poisson’s ratio (Vp/Vs ratio) increase toward the trench axis. These observation
indicates that water content within the oceanic plate increases toward the trench and suggests that the bend faults just prior to
subduction is one of keys to understand the evolutionary history of the Earth.
In this paper, we are going to show the results of our seismic structure studies as well as the other structure studies in the
trench-outer rise region. Then we will discuss the remaining issues such as quantitization and spatial inhomogenity in the wateramount.
Keywords: oceanic plate, outer rise, hydration, seismic survey, water transportation, Vp/Vs

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MIS35-10

Room:201A

Time:May 24 11:30-11:45

Origin of the oceanic lithosphere inferred from Po/So waves
SHITO, Azusa1∗ ; SUETSUGU, Daisuke2 ; FURUMURA, Takashi3
1
3

Institute for Geothermal Sciences, Kyoto University, 2 Department of Deep Earth Structure and Dynamics Research, JAMSTEC,
Earthquake Research Institute,The University of Tokyo

It has long been recognized that the oceanic P and S waves (Po and So waves) have signal with high frequency, large amplitude,
and long duration and propagate for large distance up to 3000 km across the ocean. The Po/So waves are developed by multiple
forward scattering of P and S waves due to small-scale heterogeneities in the oceanic lithosphere and scattering and capturing of
P wave in seawater layer [e.g., Shito et al., 2013; Kennett and Furumura, 2013]. In order to study the origin of the small-scale
heterogeneities, the Po/So waves travelling in the Philippine Sea are analyzed.
The Philippine Sea is one of the marginal seas of the Pacific Ocean. It is fundamentally divided into two regions bounded by
the Kyushu-Palau Ridge, each is considered to be formed in different episodes of back-arc spreading and that western part (45-60
Ma) is older than eastern part (15-30 Ma) [e.g., Seno and Maruyama, 1984]. The comparison of the Po/So waves propagation in
the different ages of the oceanic lithosphere is expected to reveal the origin of the small-scale heterogeneities.
Seismological observations using BBOBSs was conducted in the Philippine Sea from 2005 to 2008 as a part of the Stagnant
Slab Project [Fukao et al., 2009], and high-quality Po/So waves from earthquakes in subducting Philippine Sea plate wererecorded very clearly. The findings from the observed Po/So waves in the Philippine Sea plate are summarized as follows [Shito
et al., 2014]. (1) The Po/So waves propagate even in youngest oceanic lithosphere (15 Ma) near the past spreading center of
the Shikoku Basin. (2) The Po/So waves propagate much more effectively in older western part than younger eastern part of thePhilippine Sea.
We investigate the mechanism of this propagation efficiency using numerical a Finite Difference Method simulations of 2-D
seismic wave propagation. The comparison of the observed and calculated Po/So waves indicates that the age-dependence can be
explained by the thickness of the heterogeneous lithosphere. The estimated thicknesses of the oceanic lithosphere are consistent
with those obtained by a previous study based on receiver function analysis [Kawakatsu et al., 2009]. The expected depth of thelithosphere asthenosphere boundary corresponds to the top of partial melting region calculated on the basis of the model definedby the water solubility of 1000 parts per million H2O [Mierdel et al. 2007].
The results suggest that the oceanic lithosphere including the small-scale heterogeneities thicken with age. These smallscale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via thesolidification of melts distributed in the asthenosphere.
Keywords: Po/So waves, Philippine Sea Plate, oceanic lithosphere

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MIS35-11

Room:201A

Time:May 24 11:45-12:00

Abyssal peridotites from the Central Indian Ridge: Implications for mantle heterogeneityand oceanic plate formation
MORISHITA, Tomoaki1∗ ; SENDA, Ryoko2 ; SODA, Yusuke1 ; ITO, Kazuya1 ; NAKAMURA, Kentaro3 ;KUMAGAI, Hidenori2 ; OKINO, Kyoko3 ; SATO, Hiroshi4
1

Kanazawa University, 2 JAMSTEC, 3 University of Tokyo, 4 Senshu University

It is generally accepted that abyssal peridotites are formed as residue after partial melting and melt extraction in the adiabatically upwelling mantle beneath the present mid-ocean ridge system. However, Os and some other isotopic characteristics ofsome abyssal peridotites suggest their ancient residual origin, which might be formed by ancient melting event(s) (Brandon etal., 2000; Standish et al., 2002; Harvey et al., 2006; Liu et al., 2008).
Indian MORBs are well known their distinctive isotopic compositions as compared to other oceans (e.g., Iwamori & Albarede,2008; Iwamori et al., 2010). Because abyssal peridotites are recovered from the Central Indian Ridge (CIR hereafter) (Hellebrand
et al., 2002; Seyler et al., 2003; Morishita et al., 2009, 2014; Zhou and Dick, 2013; Yi et al., 2014) where the spreading rate increases from north to south (DeMets et al., 2010), CIR peridotites would provide spatial information on mid-ocean ridge mantle.
Several JAMSTEC cruises succeeded to recover abyssal peridotites from the southern end of the CIR. Abyssal peridotites in thesouthern end of the CIR are characterized by residues after medium-degree of partial melting followed by chemical modifications from evolved melts, which form gabbroic veins (Morishita et al., 2015). These results combined with previous data from
northern part of the CIR suggest that relatively depleted residual peridotites are frequently outcropped in the CIR. Addition tothis, orthopyroxene-rich peridotites which have high in Os isotopic compositions are recovered from an oceanic hill on the ocean
ridge. These characteristics of the peridotites are expected for those from the mantle wedge. In conclusion, diverse peridotites,
which might be formed during or before the formation of the Pangea-Gondwanaland, are now incorporated into asthenosphericmantle beneath the CIR. If this is a case, some peridotites can upwell near the ocean floor without creating thick basaltic crust,
followed by faulting and hydration resulting in serpentinized peridotites as a part of oceanic plate. We need to reconsider theformation processes of the oceanic plate based on the heterogeneity of oceanic mantle.
Keywords: Central Indian Ridge, Abyssal peridotite, Mantle heterogeneity, Formation of Oceanic Plate

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MIS35-12

Room:201A

Time:May 24 12:00-12:15

Heterogeneous structure of the incoming Philippine Sea plate along the southwestern
Nankai Trough
NAKANISHI, Ayako1∗ ; YAMAMOTO, Yojiro1 ; YAMASHITA, Mikiya1 ; IWAMARU, Hikaru1 ; FUJIE, Gou1 ;MIURA, Seiichi1 ; KODAIRA, Shuichi1 ; KANEDA, Yoshiyuki1
1

JAMSTEC

The next large-thrust earthquake along the Nankai Trough, southwest Japan is concerned to occur within this century. First
break of historical large-thrust earthquakes along the Nankai Trough are known to be always located off the Cape Shiono. Nonvolcanic deep low-frequency tremors and earthquakes considered as one of indicators of the future large-thrust earthquakes areobserved around the down-dip limit of the coseismic rupture zone of the last Tonankai and Nankai earthquakes [Obara, 2002].
However the absence of Nonvolcanic deep low-frequency tremors and earthquakes is recognized between Shikoku Island and Kii
Peninsula. One of the causes of these low-frequency seismic phenomena is considered to be fluid generated by dehydration processes from the subducting slab. It is important to investigate structural variation in the incoming Philippine Sea plate, including
its fluid content to understand the generation of the low-frequency seismic phenomena as well as large-thrust earthquakes.
In 2014, we conducted the seismic refraction and reflection survey in the northern margin of the Shikoku Basin, where thePhilippine Sea plate is subducting beneath the Eurasia plate at the Nankai Trough. We conducted a 360km long seismic profile
about 50-60km seaward of the deformation front along the Nankai Trough. 35 OBSs were deployed along the profile with theinterval of 10km. A tuned airgun array shot with a total volume of 7800 cu. in. every 200m for OBSs, and 380 cu. in. every
37.5m for a 192-channel, 1.2km-long hydrophone streamer.
In the time-migrated reflection section, variation in the sedimentary layer and basement reflection can be recognized off
Shikoku, which may correspond with the boundary of the plate age proposed by magnetic lineation [Okino et al., 1999]. In thesouthwestern part of the profile, the basement reflection is not always clear, and shows smooth structure. Comparatively in thenortheastern part, basement changes in depth drastically with prominent reflection signals. Moreover result of first-arrival tomography based on the wide-angle OBS data shows dramatic change in P-wave velocity just beneath the basement corresponding
with the structural boundary observed along the reflection section as mentioned above.
We will show the structural variation of the oceanic crust and the uppermost mantle of the incoming plate, which may berelated to the formation of the Shikoku Basin as well as the generation of large-thrust earthquakes and low-frequency events, byusing OBS data.
This study is part of ‘Research project for compound disaster mitigation on the great earthquakes and tsunamis around the
Nankai Trough region’ funded by MEXT, Japan.

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MIS35-13

Room:201A

Time:May 24 12:15-12:30

Mantle wedge structure of the Japan Sea derived from Ocean Bottom Seismometer observation
NAKAHIGASHI, Kazuo1∗ ; YAMADA, Tomoaki2 ; UEHIRA, Kenji3 ; MOCHIZUKI, Kimihiro2 ; SAKAI, Shin’ichi2 ;SHIOBARA, Hajime2 ; SHINOHARA, Masanao2 ; KANAZAWA, Toshihiko3
1

Kobe Univ., 2 ERI, 3 NIED

Subducting hydrous oceanic plate carries water by hydrous minerals into the earth and contributes to a melt generation. Aqueous fluid dehydrated from the subducting oceanic plate plays an important role in magma generation. The Japanese Island arelocated at subduction zones where the Philippine Sea plate subducts from the southeast beneath the Eurasian plate and the Pacificplate descends from the east beneath the PHS and the Eurasian plates. To understand the water circulation and magmatism, a huge
number of seismic tomography studies have been conducted in the Japan Island. However, a regional tomography using the land
seismic station data could not reveal the deep seismic structure beneath the Japan Sea. The information of the deep mantle wedge
structure is important to understand transportation and circulation of water and melt generation in subduction zones. Therefore,
we conducted the repeating long-term seismic observations using Long-term ocean bottom seismometer(LT-OBS)s in the central
Japan Sea from 2001 to 2004 and from 2013 to 2014. We apply travel-time tomography method to the regional earthquake andteleseismic arrival-data recorded by LT-OBSs and land stations. We obtained the P and S wave tomographic images down toa depth of 300 km beneath the Japan Sea. The tomographic P-wave image has a high velocity anomaly in the mantle wedge
extending down to a depth of approximately 150 km beneath the Yamato Basin. In addition, the resulting tomographic image has
three low-velocity anomalies in the mantle wedge. First, an inclined low velocity anomaly approximately parallel to the Pacificslab within the mantle wedge is observed in the around 100 km upper part of the Pacific slab. Second, low velocity anomalies areimaged at a depth of 150 km beneath northeastern Japan and 250 km beneath central Japan. Third, a low velocity zone is imaged
from just above the subducting Pacific slab at a depth of 300 km. These low velocity anomalies are interpreted to be represented
melt production affected by the fluid dehydrated from Pacific slab. The depth of dehydration from subducting slab is consistent
with the results of numerical modeling studies. Our observations suggest that deep dehydration from the Pacific slab occurs at a
depth of approximately 300 km and the Pacific plate subduction drives a large-scale upwelling flow beneath the Japan Sea.

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MIS35-14

Room:201A

Time:May 24 12:30-12:45

Slab dehydration and melt generation in subduction zones
ISHII, Kazuhiko1∗
1

Graduate School of Sceiences, Osaka Prefecture University

Volcanic and seismic activities in subduction zones are the result of complex interaction of geophysical and geochemical
processes. I have investigated the hydration and dehydration and the generation and transportation of melt in subducting slab andadjacent mantle wedge using a numerical model. The model includes hydration and dehydration of the slab and mantle wedge,
melting and solidification of mantle peridotites, permeable flow of melt and aqueous fluids, and temperature-dependent solid flowof mantle peridotites with water- and melt-induced weakening. In addition to these processes, I will discuss in particular the effectof fractional melting on melt distribution and composition. The model shows the following features. 1) Lherzolite flows into
mantle wedge from back-arc side and harzburgite subducts with slab after segregation of melt under the arc. 2) Melt composition
and melt fraction vary spatially and are controlled by the degree of mantle depletion and the amount of water supplied by slab
dehydration. I will also discuss the effects of water content of the subducting oceanic crust and mantle on the melt distribution.
Keywords: subduction zones, slab dehydration, fractional melting

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MIS35-15

Room:201A

Time:May 24 14:15-14:30

Carbon-bearing saline fluids in the subduction channel and mantle wedge
KAWAMOTO, Tatsuhiko1∗
1

Inst Geotherm Sci, Grad School Sci, Kyoto Univ

We find C-bearing saline fluids in the subduction channel and mantle wedge. Saline fluids are found with or without methanein jadeitites of serpentinite melanges located in Southwest Japan [Mori, Shigeno, Kawamoto, Nishiyama, in progress]. Carbon
dioxide-bearing saline fluid inclusions are also reported from sub-arc mantle peridotite xenoliths: 3.7 wt% NaCl in Ichinomegata
lherzolites, Northeast Japan arc [Kumagai et al., 2014] and 5.1 wt% NaCl in Pinatubo harzburgites, Luzon arc [Kawamoto et al.,2013]. These findings indicate that aqueous fluids in the subduction channel and mantle wedge can contain certain amounts of C
and Cl.
We suggested that separation of slab-derived supercritical fluids into aqueous fluids and melts plays an important role inelemental transfer from subducting slab to the mantle wedge [Kawamoto et al., 2012]. It is, therefore, important to determine theeffect of Cl on the trace element partitioning between aqueous fluids and melts. Synchrotron radiation X-ray fluorescence (XRF)
analysis is conducted to know Rb, Sr, and Pb partitioning between aqueous fluids and melts simultaneously at high-temperatureand high-pressure conditions. There is a positive correlation between partition coefficients and pressure, as well as salinity[Kawamoto et al., 2014]. Two slab-derived components, melt and fluid components, are suggested to explain trace element
characteristics of arc-basalts in the Mariana arc [Pearce et al., 2005]. The fluid component is characterized by enrichment ofalkali, alkali earth elements, and Pb. Such features can be explained if the fluid component is a saline fluid, because alkali earth
elements and Pb are much less mobile with Cl-free fluids than Cl-rich fluids [Kawamoto et al., 2014].
We suggest that slab-derived components have compositional features consistent with a saline fluid and a melt, which can beformed through a separation of a slab-derived supercritical fluid [Kawamoto et al., 2012, 2014]. Slab-derived supercritical fluids
contain Cl, and separated aqueous fluids inherit much of the Cl and some of the large-ion lithophile elements. Dissolution ofcarbon materials into aqueous fluids is enhanced by the salinity [Newton and Manning 2002] and their species can be controlledby oxygen fugacity.
Reference
Kawamoto T., Kanzaki M., Mibe K., Matsukage K. N., Ono S., 2012, Separation of supercritical slab-fluids to form aqueous
fluid and melt components in subduction zone magmatism. Proceedings of the National Academy of Sciences, U. S. A., 109,
18695-18700.
Kawamoto T., Yoshikawa M., Kumagai Y., Mirabueno M. H. T., Okuno M., Kobayashi T., 2013, Mantle wedge infiltrated withsaline fluids from dehydration and decarbonation of subducting slab. Proceedings of the National Academy of Sciences, U. S.
A., 110, 9663-9668.
Kawamoto T., Mibe K., Bureau H., Reguer S., Mocuta C., Kubsky S., Thiaudiere D., Ono S., Kogiso T., 2014, Large ion
lithophile elements delivered by saline fluids to the sub-arc mantle, Earth, Planets and Space, 66, 61.
Kumagai Y., Kawamoto T., Yamamoto J., 2014, Evolution of carbon dioxide bearing saline fluids in the mantle wedge beneaththe Northeast Japan arc, Contributions to Mineralogy and Petrology, 168, 1056.
Newton, R.C. and Manning, C.E., 2002, Experimental determination of calcite solubility in H2 O-NaCl solutions at deep
crust/upper mantle pressures and temperatures: implications for metasomatic processes in shear zones. American Mineralogist,
87, 1401-1409.
Pearce J. A., Stern R. J., Bloomer S. H., Fryer P., 2005, Geochemical mapping of the Mariana arc-basin system: Implications
for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems, 6, Q07006.

Keywords: aqueous fluid, fluid inclusion, arc magma

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MIS35-16

Room:201A

Time:May 24 14:30-14:45

Behaviour of subducted water and its role in magma genesis
KIMURA, Jun-ichi1∗ ; NAKAJIMA, Junichi2
1

JAMSTEC, 2 Tohoku University

Water at subduction zones is carried to mantle depths by the subducting oceanic plate and then released by dehydration. It
then migrates upwards and contributes to melting of the mantle wedge to form primary arc magma. The magma thus captures
and transfers water to the crust, or outgasses water to the atmosphere. Water, either in fluids or melts in both the slab and the
mantle, promotes the dissolution and mobilization of elements and affects the physical properties of the sub-arc slab, mantle,
and seismicity. In this paper, we present a coherent model to explain the geophysical and geochemical role of water beneath NE
Japan. We first investigate the seismic structures of the downgoing slab and sub-arc mantle and examine the role of subductedwater in forming these structures. We then use the Arc Basalt Simulator version 4, a petrological-geochemical model developed
to describe the geochemical behaviours of water and elements in the slab, mantle, and arc basalt. Parameters governing thesepetrogenetic processes are also estimated by the model and compared to geophysical observations. The combined approachshows that (1) subducted sediment and igneous oceanic crust are almost fully hydrated, whereas only partial hydration occurs in
the oceanic mantle; (2) this high slab water content leads to melting of the slab sediment and the uppermost basalt layer beneaththe arc; (3) the released water via slab liquid promotes 3?25% melting of the mantle wedge at a depth of 50?30 km at a mantle
temperature of 1250?1350(C); (4) virtually 89% of slab water is released, 22% of the water returns to the forearc, and 38% enters
the arc crust with the magma; and (5) 11% of the subducted water retained beyond a depth of 180 km is held in the slab, and 29%
in nominally anhydrous minerals in the wedge mantle.
Keywords: Subduction zone, Water, Magma

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MIS35-17

Room:201A

Time:May 24 14:45-15:00

Water carrier in the Earth’s deep upper mantle and residues under hydrous condition
MATSUKAGE, Kyoko N.1∗
1

Earth and Planetary Sciences, Kobe University

The chemical differentiations and homogenizations of the Earth’s mantle are largely controlled by melts and fluids (e.g., silicate melts, H2O fluid). To understand the influence of H2O to the chemical evolution of the Earth’s mantle, hydrous melting
experiments at temperatures up to 1900 K at pressure of 3-8 GPa were performed using the multi-anvil apparatus. In hydrous
conditions, the stability field of residual orthopyroxene expands relative to olivine above solidus, and the harzburgitic residue
contains large amounts of Mg-rich (Mg# >0.92) orthopyroxene at 4.5 to 6 GPa. The residual chemistry obtained from our experiments agrees well with the chemical variation of the continental cratonic garnet peridotites xenoliths, which was transported bykimberlite magmas. The observation indicates that the cratonic harzburgite with high orthopyroxene contents possibly reflects
formation by melt depletion under various water contents from almost anhydrous to 2 wt% in the upper mantle at depths of about
100 to 200 km. The orthopyroxene-rich harzburgite similar to the continental cratonic harzburgite may be formed at deep mantle
wedges in the present Earth because water is dragged into the deep mantle wedge by subducting slabs. In present, it is difficult
to obtain a petrological evidences for the formation of Si-rich and Mg-rich residues at the deep wedge mantle because a magma
upwelling from the deep mantle (>200 km) does not exist. However, the orthopyroxene-rich harzburgite may be detected byseismological observations because a jump of elastic wave velocities possibly occurs at 9-10 GPa (270-300 km in depth) in theorthopyroxene-rich harzburgite due to the orthorhombic to high-pressure monoclinic phase transition in (Mg, Fe)SiO3 pyroxene.
A small jump in seismic velocities at about 250-300 km in depth, the X discontinuity, has occasionally been observed in seismic
profiles from some subduction zone, southern Africa and southern Pacific. I consider that the phase transition of (Mg, Fe)SiO3
pyroxene in orthopyroxene-rich harzburgite may correspond to the X discontinuity.
Keywords: Hydrous melting experiments, high-pressure and high-temperature, wedge mantle, orthopyroxene, craton, X discontinuity

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MIS35-18

Room:201A

Time:May 24 15:00-15:15

Water release from subducting slab inferred by upper mantle electrical resistivity structures beneath back-arc basins
SEAMA, Nobukazu1∗ ; MATSUNO, Tetsuo2
1

Department of Planetology, Kobe University, 2 Earthquake Research Institute, University of Tokyo

We present and compare four 2-D electrical resistivity structures of the upper mantle beneath three back-arc basins; the Lau
Basin, the southern Mariana Trough, and the central Mariana Trough. The magnetotelluric (MT) method is a base to estimate
upper mantle resistivity structures. The electromagnetic variation data on the ocean bottom were obtained from observations
using OBEM (Ocean Bottom Electro-Magnetometer) and OBM (Ocean Bottom Magnetometer). We carried out the time-series
data analysis is to estimate the MT responses and corrected topographic distortions in the MT responses. We have basically
performed a smooth model inversion analysis using the processed MT responses to estimate a resistivity structure with minimum
model smoothness, and also have considered a prior constraint in the inversion analysis for the subducting slab inferred from aseismic research. In the Lau back-arc basin, we obtained 12 months length data by 2 OBEMs and 7-9 months length data by 11
OBMs on the 2 observation lines across eastern Lau spreading center at 19.7 S and 21.3 S; the length of both observation lines
are 150 km. It is worth noting that it is the first experiment to use OBSMs (ocean bottom seismograph with magnetometer); thatis OBM attached to ocean bottom seismograph. Matsukura (2014) analyzed these data to derive 2-D upper mantle resistivity
structures beneath the two observation lines. In the southern Mariana Trough back-arc basin, we carried out an electromagnetic
observation along a 120 km length observation line across the spreading axis, and we obtained about 85 days length data by two
OBEMs and for about 60 days by six OBEMs. Shindo (2013) reported preliminary results from these data, and we reanalyzed
the data to derive a 2-D upper mantle resistivity structure beneath the observation line. We also compared these structures witha 2-D upper mantle resistivity structure beneath the central Mariana subduction system including the central Mariana Troughback-arc basin (Matsuno et al., 2010). All the 2-D upper mantle resistivity structures beneath three back-arc basins indicate thatthe mantle resistivity directly above the resistive subducting slab start decreasing at a characteristic depth; conductive region ofless than 50 ohm-m exists at the deeper region. The depth is 140 km except 60 km for the central Mariana Trough back-arc basin
where the subducting slab is so steep that the conductive region has ascended nearly vertically to the shallower depth or that thevertical resolution could not be good enough. This result suggests that the conductive region of less than 50 ohm-m is probably
due to water release from the subducting slab and that the water release becomes dominating in the mantle above the slab when
the slab reaches 140 km depth.
Keywords: water, subducting slab, upper mantle electrical resistivity structure, back-arc basin

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MIS35-19

Room:201A

Time:May 24 15:15-15:30

3-D electrical conductivity image of the upper mantle beneath the Society hotspot
TADA, Noriko1∗ ; TARITS, Pascal2 ; BABA, Kiyoshi3 ; KASAYA, Takafumi1 ; SUETSUGU, Daisuke1 ;UTADA, Hisashi3
1Japan Agency for Marine-Earth Science and Technolog, 2 Institut Universitaire Europeen de la Mer, 3 Earthquake Research Institute, The University of Tokyo
The mantle upwellings are one of the most important features for understanding the mantle dynamics. A large-scale mantle
upwelling beneath the French Polynesia region in the South Pacific has been suggested from seismic studies, which is called theSouth Pacific superplume. Nolasco et al. (1998) carried out magnetotelluric (MT) survey around the Society hotspot, which isone of the hotspots in the French Polynesia region, in order to estimate electrical conductivity structure beneath and the vicinityof the Society hotspot in two-dimension. This previous study is not enough to understand the geometry of the hotspot, because
the hotspot is tubed like form according to the results from the seismic study. Moreover, Suetsugu et al. (2009) suggested thatthe slow velocity anomaly continues from the lower mantle to the uppermost upper mantle just beneath the Society hotspot. The
geometry, temperature, and composition of the Society hotspot remain controversial, however, due to still insufficient accumulation of the geophysical data.
Then, we carried out the TIARES project that composed of multi-sensor stations that include broadband ocean bottom seismometers, ocean bottom electromagnetometers (OBEMs), and differential pressure gauges from 2009 to 2010. To obtain threedimensional (3-D) image of the upwelling of the Society hotspot in terms of electrical conductivity, we newly settled eleven
OBEMs and obtained MT responses at 20 sites totally. A 3-D marine MT inversion program, which can treat topographic change
distorting EM data, was applied to these MT responses to estimate 3-D electrical conductivity image. The result detected a conductive anomaly elongating from the mantle transition zone to the uppermost upper mantle just below the Society hotspot. Thisfeature is consistent with the slow velocity anomaly obtained from the surface wave tomography (Suetsugu et al., 2009). We calculated differences in temperature between the conductive anomaly and the surrounding mantle using conductivity-temperature
relationship for dry olivine based on laboratory measurements. The resultant temperature difference is about 400 K at the depthof around 100 km, which is much larger than that estimated from a past seismic study. These results might imply that the effectsof partial melt and/or volatiles are necessary in order to explain the high electrical conductivity anomaly beneath the hotspot.
Keywords: hotspot, marine magnetotellurics, electrical conductivity, three-dimensional inversion, the French Polynesia

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MIS35-20

Room:201A

Time:May 24 15:30-15:45

Quantitative interpretation of electrical conductivity structure of the mantle
BABA, Kiyoshi1∗
1

Earthquake Research Institute, The University of Tokyo

Studies on electrical conductivity of the mantle based on magnetotelluric (MT) survey frequently discuss the water distribution
in the mantle because the conductivity of the mantle minerals is thought to be highly sensitive to only small amount of water
dissolved in minerals. The electrical conductivity is also strongly dependent on the temperature and the fraction and connectivityof the melt if the mantle is partially molten. Thus, it is impossible to distinguish the impacts of these parameters from the electricalconductivity alone and use of other independent information is indispensable to distinguish the impacts of each parameter.
In this presentation, I introduce my recent work on quantitative interpretation of electrical conductivity of oceanic upper mantle
estimated from seafloor MT data. The keys are 1) selection of possible scenarios to reduce the model space to be searched, 2)
self-consistency between temperature and existence of melt. For oceanic upper mantle, it is reasonable to test a thermal model
like plate cooling or half-space cooling because these models explain the bathymetry subsidence and heat flow variation with thelithospheric age well. I here chose the model space of one-dimensional electrical conductivity associated with a thermal structure
which is described as the function of lithospheric age, thickness of thermally conductive plate, and potential temperature. The
effect of partial melting is taken into account by a self-consistent manner. I use the information about solidus temperature of
the mantle rock and incipient melting process with the solidus reduction due to dissolved water and carbon to check if the melt
is stable and how much melt is stable if so for given temperature and contents of water and carbon. The electrical conductivity
can be calculated applying laboratory experiment models for hydrous olivine and hydrous carbonated melt. The MT response
is produced from the conductivity structure by forward modeling and compared with the observed MT response in statisticalsense. This procedure enables me to detect possible range of the parameters to explain the data and the trade-off relation of eachparameter quantitatively. I show an example of application to real data obtained in the northwestern Pacific Ocean.
The future work must be a joint analysis with other observation, for example, seismic data to constrain each parameter morestrictly reducing the trade-off relation.
Keywords: oceanic upper mantle, magnetotellurics, electrical conductivity structure, H2O, CO2, partial melt

1/1

MIS35-P01

Room:Convention Hall

Time:May 24 18:15-19:30

A resistivity model of the back arc region in the NE Japan arc based on marine and island
MT data
ICHIHARA, Hiroshi1∗ ; TADA, Noriko1 ; BABA, Kiyoshi2 ; KASAYA, Takafumi1 ; ICHIKI, Masahiro3 ;KAIDA, Toshiki3 ; OGAWA, Yasuo4
1

Japan Agency for Marine-Earth Science and Technology, 2 Earthquake Research Institute, the University of Tokyo, 3 ResearchCenter for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, 4 Volcanic Fluid Research Center, Tokyo Institute of Technology
Distribution of physical properties in the back-arc in NE Japan subduction zone have not been understood because the area ismostly located beneath the sea floor. In this study, we estimated electrical resistivity distribution in this area based on electromagnetic data obtained on the seafloor and islands in the eastern part of Japan Sea. The ocean bottom EM data were obtained with 6
ocean bottom electro-magnetometers (OBEMs) between April and August 2013 by MR13-02A and NT13-18 JAMSTEC scientific cruises. The island data were acquired in the 3 islands in the Japan Sea (Tobishima, Awashima and Sado islands) between
April and October 2013. These recorded time-series data were converted to a frequency-domain impedance tensor based on the
BIRRP program (Chave and Thomson, 2004). As results, high-quality MT responses and geomagnetic tippers in both the trenchand back-arc areas. The phase tensor ellipses (Caldwell et al. 2004) indicates high Φmax (>65 degrees) and Φmin (>50 degrees)in the long periods (>8000 seconds) implying conductive zone in the deep area. The ellipses in the short period show strong
contrast between western part (Yamato basin) and eastern part of study area, which indicate heterogeneity in crustal structure.
Then we also inverted the MT impedances into resistivity distribution based on the 3-D inversion code (Tada et al., 2012) after
the correction local topographic effect. The inversion result shows a significant conductor above the subducting Pacific plate.
A surface conductor is also estimated beneath the Yamato Basin. These features are consistent to the phase tensors discussed
above. The deep conductor may be related to dehydration in subducting Pacific Plate and convention in the mantle wedge. The
surface conductor may reflect sediments rocks formed during back-arc opening in Miocene.
Keywords: NE Japan arc, Japan Sea, Back arc, magnetotelurics, resisitivity, Geofluid

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MIS35-P02

Room:Convention Hall

Time:May 24 18:15-19:30

Water content of small body surfaces: Effects of mutual collisions
NAKAMURA, Akiko1∗
1

Graduate School of Science, Kobe University

Small bodies in the Solar System transported water from outer region beyond the snowline to the Earth and other terrestrial
planets. These small bodies experienced mutual collisions since their formation through their evolution. Consequently, craters
were formed on the surfaces, rubble-pile bodies, i.e., once collisionally disrupted and re-accumulated bodies, such as asteroid
Itokawa were formed, and asteroid families, groups of asteroids with similar orbital elements, were created.
In order to understand the evolution of small bodies from the early Solar System to the present, with focusing on the role ofsmall bodies as the transporter of water, the present orbital and size distributions of dry and wet small bodies are ones of key
clues. The presence of hydrated minerals and water ices are detected by visible and infrared spectra observations.
In this presentation, we discuss on the relationship between water content of small body surfaces and their mutual collisions.
Even if hydrated minerals or water ices were lost from the surfaces due to some processes, the interior of the bodies may be
still wet. Impact-induced dehydration is one of possible processes that release water from the surface. On the contrary, erosive
collision may expose fresh wet materials from the interior. On the other hand, collision brings wet materials on the surfaces ofdry small bodies.
Impact-induced dehydration rate has been studied by analysis of shock-recovered hydrated minerals or detection of impactinduced water vapor (e.g., Sekine et al., 2015). The dehydration ratio is dependent on not only peak shock pressure but also
the initial porosity. Meteorite texture was found to be comminuted due to vapor released from hydrated minerals in porous
carbonaceous chondrite materials in shock recovery experiments, and fine fragments were expected to be explosively dispersed
from hydrated, porous asteroids (Tomeoka et al., 2003). We prepared dunite and serpentinite blocks as dry and wet targets andgypsum and hemihydrate blocks as porous samples for impact experiments. Projectiles accelerated to the velocity of about 5
km/s, which is the average collision velocity in the asteroid belt, were impacted to the targets. No clear difference was observedin the ejecta patterns, i.e., fragment velocity fields, for non-porous dunite and serpentinite targets. The similar cone-shaped ejecta
were observed for the hemihydrate targets, while hemispherical vapor clouds were observed from gypsum targets. How thedehydration, comminution and ejection of fragments depend on impact angle and porosity and water content of targets have notbeen fully addressed.
On the other hand, impact-delivery of exogenic materials on the surface of small bodies was visualized by the discovery ofcarbonaceous materials on the surface of asteroid Vesta by Dawn space mission. We performed impact experiments of projectilesof rocks, meteorites and porous ceramic projectiles into targets of various materials in order to investigate the degree of disruptionof projectiles and penetration depth of projectiles into the targets. We found that fragments of rock projectiles are mixed andconsolidated with simulated regolith particles. We also showed that impactors with large porosity can survive with larger fraction
owing to local microscopic collapse of pores that inhibits growth of overall fracture of the impactors and thus can penetrate
deeper into targets. We will discuss on the effect of volatile materials in impactors based on the results of impact experiments
conducted using plastic projectiles.

Keywords: impact, dehydration, ejecta, fragments, porosity, small bodies

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MIS35-P03

Room:Convention Hall

Time:May 24 18:15-19:30

Transportation of NEOs from their source region and its dependence on collisional disruption
ITO, Takashi1∗
1

CfCA, NAOJ, NINSThe dynamical origin and evolution of Near-Earth Objects (NEOs) still have many issues yet to be solved. Recent discovery
of NEO families has raised another question, indicating that the asteroid transportation efficiency from the main belt could bemuch lower than what has been considered. Also, how much contribution the Oort Cloud cometary objects has over the total
population of NEO is not clear at all. In this presentation we will make a brief summary as to what is known and not known
about the dynamical evolution of NEOs, and tries to show several ways for future studies.
Keywords: near-Earth asteroids, collisional disruption

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MIS35-P04

Room:Convention Hall

Time:May 24 18:15-19:30

Dynamical evolution of captured Trojan asteroids
OHTSUKI, Keiji1∗ ; OKAYAMA, Hiroaki1
1

Kobe University

The Trojan asteroids orbit the Sun about the L4 and L5 Lagrangian points of Jupiter. These objects have a wide range of
eccentricities and inclinations, and are thought to be captured planetesimals. Since the origin of the Trojan asteroids is expected
to provide clues to the dynamical evolution of the planets and small bodies in the Solar System, various models have been
proposed, e.g., capture due to gas drag from the solar nebula, capture during Jupiter’s mass growth, or capture during smoothmigration of Jupiter. However, such models failed to reproduce some important characteristics of the present Trojan asteroids,
such as the total mass of the Trojans, the distribution of orbital elements, or the distribution of the libration amplitudes. On theother hand, recent models for the formation of the Solar System suggest that the giant planets likely experienced significant radialmigration and orbital instability after their formation. Recent studies of capture of Trojan asteroids based on such models of giant
planet migration show that icy planetesimals (or KOBs) originally in the outer Solar System can be captured into Jupiter’s Trojan
regions. Such models successfully explain the present total mass as well as the observed orbital characteristics of the Trojan
asteroids.
However, in such studies of capture of the Trojan asteroids, planetesimals were treated as test particles, thus gravitational
interactions between planetesimals are not taken into account. Also, effects of mutual gravity among asteroids are also neglectedin the studies of the stability of the Trojan asteroids after their capture into the Lagrangian points. Although effects of gravitational
interactions between sufficiently small asteroids may reasonably be neglected, there may have been significantly large objects inthe original swam of Trojan asteroids immediately after their capture. In the above-mentioned recent models of capture, Trojan
asteroids likely originated from the outer region of the Solar System, including the Kuiper belt. Among the current KBOs, there
are many objects that are much larger than the largest of the present Trojans, some of them being as large as 1,000km across indiameter. If such a large body is captured into the Trojan regions even temporarily, it may have a significant influence on thestability of other Trojan bodies, and some of them would be scattered out of the Trojan regions.
In the present study, we assume that a large body was captured into Jupiter’s Trojan region, and examine its dynamical influenceon other Trojan asteroids using orbital integration. From the results of our orbital integrations, we derive constraints on the massof bodies existed in the Trojan swarm in the past.
Keywords: asteroids, Trojan

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