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Single planet formation regime in the high-ionization environment:Possible origin of hot Jupiters and super-Earths

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Geoscience Frontiers 2018年第4期9卷 1023-1031页

作者： Yusuke Imaeda toshikazu ebisuzaki RIKEN

We studied the particle growth in a protoplanetary disk in a high-ionization environment and found that icy planet formation is inactive for a disk with an ionization rate 100 times higher than that of the present Solar System. In particular, in the case of M 〈 10^（-7.4）M_☉yr^（-1）, only rocky planet formation occurs. In such a case, all the solid materials in the disk drift inward, eventually reach the inner MRI front,and accumulate there. They form a dense, thin sub-disk of solid particles, which undergoes gravitational instability to form rocky planetesimals. The planetesimals rapidly grow into a planet through pebble accretion. Consequently, rocky planets tend to be much larger than planets formed through other regimes（tandem planet formation regime and dispersed planet formation regime）, in which icy planet formation actively takes place. These rocky planets may evolve into hot Jupiters if they grow fast enough to the critical core mass of the runaway gas accretion before the dispersal of the disk gas, or they may evolve into super-Earths if the gas dispersed sufficiently early.

关键词： Accretion disk Planet formation Magneto-rotational instability Galactic cosmic rays Hot Jupiter Super-Earth

Population synthesis of planet formation using a torque formula with dynamic effects

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Geoscience Frontiers 2017年第2期8卷 215-222页

作者： Takanori Sasaki toshikazu ebisuzaki Department of Astronomy Kyoto University RIKEN

Population synthesis studies into planet formation have suggested that distributions consistent with observations can only be reproduced if the actual Type Ⅰ migration timescale is at least an order of magnitude longer than that deduced from linear theories.Although past studies considered the effect of the Type I migration of protoplanetary embryos,in most cases they used a conventional formula based on static torques in isothermal disks,and employed a reduction factor to account for uncertainty in the mechanism details.However,in addition to static torques,a migrating planet experiences dynamic torques that are proportional to the migration rate.These dynamic torques can impact on planet migration and predicted planetary populations.In this study,we derived a new torque formula for Type Ⅰ migration by taking into account dynamic corrections.This formula was used to perform population synthesis simulations with and without the effect of dynamic torques.In many cases,inward migration was slowed significantly by the dynamic effects.For the static torque case,gas giant formation was effectively suppressed by Type I migration;however,when dynamic effects were considered,a substantial fraction of cores survived and grew into gas giants.

关键词： Planetary formation Population synthesis Type Ⅰ migration

Tandem planet formation for solar system-like planetary systems

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Geoscience Frontiers 2017年第2期8卷 223-231页

作者： Yusuke Imaeda toshikazu ebisuzaki RIKEN 2-1Hirosawa351-0198Japan

We present a new united theory of planet formation,which includes magneto-rotational instability（MRl） and porous aggregation of solid particles in a consistent way.We show that the ＂tandem planet formation＂ regime is likely to result in solar system-like planetary systems.In the tandem planet formation regime,planetesimals form at two distinct sites：the outer and inner edges of the MRl suppressed region.The former is likely to be the source of the outer gas giants,and the latter is the source for the inner volatile-free rocky planets.Our study spans disks with a various range of accretion rates,and we find that tandem planet formation can occur for M = 10^7.3- 10^-6.9Myr^-1.The rocky planets form between 0.4-2 AU,while the icy planets form between 6-30 All;no planets form in 2-6 AU region for any accretion rate.This is consistent with the gap in the solid component distribution in the solar system,which has only a relatively small Mars and a very small amount of material in the main asteroid belt from 2-6 AU.The tandem regime is consistent with the idea that the Earth was initially formed as a completely volatile-free planet.Water and other volatile elements came later through the accretion of icy material by occasional inward scattering from the outer regions.Reactions between reductive minerals,such as schreibersite（Fe-jP）,and water are essential to supply energy and nutrients for primitive life on Earth.

关键词： Accretion disk Planet formation Magneto-rotational instability Origin of life

Origin of the Earth:A proposal of new model called ABEL

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Geoscience Frontiers 2017年第2期8卷 253-274页

作者： Shigenori Maruyama toshikazu ebisuzaki Earth-Life Science Institute Tokyo Institute of Technology Institute for Study of the Earth's Interior Okayama University RIKEN

The Earth was born as a dry planet without atmosphere and ocean components at 4.56 Ga,with subsequent secondary accretion of bio-elements,such as carbon（C）,hydrogen（H）,oxygen（O）,and nitrogen（N） which peaked at 4.37-4.20 Ga.This two-step formation model of the Earth we refer to as the advent of bio-elements model（ABEL Model） and the event of the advent of bio-elements（water component） as ABEL Bombardment.It is clear that the solid Earth originated from enstatite chondrite-like dry material based on the similarity in oxygen isotopic composition and among other isotopes.On the other hand,Earth＇s water derives primarily from carbonaceous chondrite material based on the hydrogen isotopic ratio.We present our ABEL model to explain this enigma between solid Earth and water,as well as secondary accretion of oxidizing bio-elements,which became a precursor to initiate metabolism to emerge life on a highly reductive planet.If ABEL Bombardment had not occurred,life never would have emerged on the Earth.Therefore,ABEL Bombardment is one of the most important events for this planet to evolve into a habitable planet.The chronology of ABEL Bombardment is informed through previous researches of the late heavy bombardment and the late veneer model.ABEL Bombardment is considered to have occurred during 4.37-4.20 Ga,which is the concept to redefine the standard late heavy bombardment and the late veneer models.Also,ABEL Bombardment is the trigger of the transition from stagnant lid tectonics to plate tectonics on this planet because of the injection of volatiles into the initial dry Earth.

关键词： Origin of Earth and life Advent of bin-elements （ABEL）Origin of water on the Earth ABEL Bombardment Habitable Trinity planet Initiation of plate tectonics

Nuclear geyser model of the origin of life:Driving force to promote the synthesis of building blocks of life

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Geoscience Frontiers 2017年第2期8卷 275-298页

作者： toshikazu ebisuzaki Shigenori Maruyama RIKEN Earth-Life Science Institute Tokyo Institute of Technology

We propose the nuclear geyser model to elucidate an optimal site to bear the first life.Our model overcomes the difficulties that previously proposed models have encountered.Nuclear geyser is a geyser driven by a natural nuclear reactor,which was likely common in the Hadean Earth,because of a much higher abundance of 235U as nuclear fuel.The nuclear geyser supplies the following：（1）high-density ionizing radiation to promote chemical chain reactions that even tar can be used for intermediate material to restart chemical reactions,（2）a system to maintain the circulation of material and energy,which includes cyclic environmental conditions（warm/cool,dry/wet,etc.）to enable to produce complex organic compounds,（3）a lower temperature than 100℃ as not to break down macromolecular organic compounds,（4）a locally reductive environment depending on rock types exposed along the geyser wall,and（5）a container to confine and accumulate volatile chemicals.These five factors are the necessary conditions that the birth place of life must satisfy.Only the nuclear geyser can meet all five,in contrast to the previously proposed birth sites,such as tidal flat,submarine hydrothermal vent,and outer space.The nuclear reactor and associated geyser,which maintain the circulations of material and energy with its surrounding environment,are regarded as the nuclear geyser system that enables numerous kinds of chemical reactions to synthesize complex organic compounds,and where the most primitive metabolism could be generated.

关键词： Origin of life Chemical evolution Natural nuclear reactor Aqueous electron Radiation chemistry

United theory of biological evolution:Disaster-forced evolution through Supernova,radioactive ash fall-outs,genome instability,and mass extinctions

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Geoscience Frontiers 2015年第1期6卷 103-119页

作者： toshikazu ebisuzaki Shigenori Maruyama RIKEN 2-1Hirosawa Earth-Life Science Institute Tokyo Institute of Technology

We present the disaster-forced biological evolution model as a general framework that includes Darwinian ＂phylogenic gradualism＂, Eldredge-Gould＇s ＂punctuated equilibrium＂, mass extinctions, and allopatric, parapatric, and sympatric speciation. It describes how reproductive isolation of organisms is established through global disasters due to supernova encounters and local disasters due to radioactive volcanic ash fall-outs by continental alkaline volcanism. Our new evolution model uniquely highlights three major factors of disaster-forced speciation： enhanced mutation rate by higher natural radiation level, smaller population size, and shrunken habitat size （i.e., isolation among the individual pop- ulations）. We developed a mathematical model describing speciation of a half-isolated group from a parental group, taking into account the population size （Ne）, immigration rate （m）, and mutation rate （μ）. The model gives a quantitative estimate of the speciation, which is consistent with the observations of speciation speed. For example, the speciation takes at least 105 generations, if mutation rate is less than 10 3 per generation per individual. This result is consistent with the previous studies, in which μ is assumed to be 10 3-10-5. On the other hand, the speciation is much faster （less than l0S generations） for the case that μ is as large as 0.1 in parapatric conditions （m 〈 μ）. Even a sympatric （m ~ 1 ） speciatiou can occur within 103 generations, if mutation rate is very high （μ- 1 mutation per individual per generation）, and if Ne 〈 20-30. Such a high mutation rate is possible during global disasters due to supernova encounters and local disasters due to radioactive ash fall-outs. They raise natural radiation level by a factor of 100-1000. Such rapid speciation events can also contribute to macro-evolution during mass extinction events, such as observed during the Cambrian explosion of biodiversity. A similar rapid speciation （though in a much smaller scale） also has been undergoing in cichlid fishes and g

关键词： Speciation Supernova encounter Continental alkaline volcanism Cichlid Great African ape Cambrian explosion

Nine requirements for the origin of Earth’s life:Not at the hydrothermal vent, but in a nuclear geyser system

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Geoscience Frontiers 2019年第4期10卷 1337-1357页

作者： Shigenori Maruyama Ken Kurokawa toshikazu ebisuzaki Yusuke Sawaki Konomi Suda M. Santosh Earth-Life Science Institute Tokyo Institute of Technology2-12-1 O-okayamaMeguro-kuTokyo 152-8551Japan Kochi University Kochi 780-8520Japan Novosibirsk State University Pirogova 1630090Russia Genome Evolution Laboratory National Institute of Genetics1111 YataMishimaShizuoka 411-8540Japan RIKEN 2-1 HirosawaWakoSaitama351-0198Japan Department of Earth and Planetary Sciences Graduate School of Science and EngineeringTokyo Institute of Technology2-12-1 OokayamaMeguro-kuTokyo 152-8550Japan Department of Earth Science and Astronomy Graduate School of Arts and Sciences The University of Tokyo3-8-1 KomabaMeguro-kuTokyo 153-8902Japan Geomicrobiology Research Group Institute for Geo-Resources and EnvironmentNational Institute of Advanced Industrial Science and Technology (AIST)Central 71-1-1 HigashiTsukubaIbaraki 305-8567Japan Centre for Tectonics Resources and ExplorationDepartment of Earth SciencesUniversity of AdelaideSA 5005Australia School of Earth Sciences and Resources China University of Geosciences (Beijing)29 Xueyuan RoadBeijing 100083China

The origin of life on Earth remains enigmatic with diverse models and debates.Here we discuss essential requirements for the first emergence of life on our planet and propose the following nine requirements:(1)an energy source(ionizing radiation and thermal energy);(2)a supply of nutrients(P.K.REE.etc.);(3)a supply of life-constituting major elements;(4)a high concentration of reduced gases such as CH4,HCN and NH3;(5)dry-wet cycles to create membranes and polymerize RNA;(6)a non-toxic aqueous environment;(7)Na-poor water;(8)highly diversified environments,and(9)cyclic conditions,such as dayto-night,hot-to-cold etc.Based on these nine requirements,we evaluate previously proposed locations for the origin of Earth’s life,including:(1)Darwin’s"warm little pond",leading to a"prebiotic soup"for life;(2)panspermia or Neo-panspermia(succession model of panspermia);(3)transportation from/through Mars;(4)a deepsea hydrothermal system;(5)an on-land subduct ion-zone hot spring,and(6)a geyser systems driven by a natural nuclear reactor.We conclude that location(6)is the most ideal candidate for the o rigin point for Earth’s life because of its efficiency in continuously supplying both the energy and the necessary materials for life,thereby maintaining the essential"cradle"for its initial development.We also emphasize that falsifiable working hypothesis provides an important tool to evaluate one of the biggest mysteries of the universe-the origin of life.

关键词： Origin of Earth's life Nuclear geyser system Emergence and evolution of life Falsifiability

Three-step modernization of the ocean:Modeling of carbon cycles and the revolution of ecological systems in the Ediacaran/Cambrian periods

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Geoscience Frontiers 2015年第1期6卷 121-136页

作者： Miyuki Tahata Yusuke Sawaki Yuichiro Ueno Manabu Nishizawa Naohiro Yoshida toshikazu ebisuzaki Tsuyoshi Komiya Shigenori Maruyama Department of Earth and Planetary Sciences Tokyo Institute of Technology Earth-Life Science Institute Tokyo Institute of Technology Precambrian Ecosystem Laboratory Japan Agency for Marine-Earth Science and Technology(JAMSTEC) Department of Environmental Science and Technology Tokyo Institute of Technology RIKEN 2-1 Hirosawa Department of Earth Science and Astronomy The University of Tokyo Research Center for the Evolving Earth and Planets Tokyo Institute of Technology

Important ecological changes of the Earth （oxidization of the atmosphere and the ocean） increase in nutrient supply due to the break-up of the super continent （Rodinia） and the appearance of multi-cellular organisms （macroscopic algae and metazoan） took place in the Ediacaran period, priming the Cambrian explosion. The strong perturbations in carbon cycles in the ocean are recorded as excursions in carbonate and organic carbon isotope ratio （δ13Ccarb and δ13Corg） from the Ediacaran through early Cambrian periods. The Ediacaran-early Cambrian sediment records of δ13Ccarb and δ13Corg, obtained from the drill-core samples in Three Gorges in South China, are compared with the results of numerical simulation of a sim- ple one-zone model of the carbon cycle of the ocean, which has two reservoirs （i.e., dissolved organic carbon （DOC） and dissolved inorganic carbon （DIC）. The fluxes from the reservoirs are assumed to be proportional to the mass of the carbon reservoirs. We constructed a model, referred to here as the Best Fit Model （BFM）, which reproduce δ13Ccarb and δ13Corg records in the Ediacaran-early Cambrian period noted above. BFM reveals that the Shuram excursion is related to three major changes in the carbon cycle or the global ecological system of the Earth：（1） an increase in the coefficient of remineralization by a factor of ca. 100, possibly corresponding to a change in the dominant metabolism from anaerobic respiration to aerobic respiration, （2） an increase of carbon fractionation index from 25‰, to 33‰, possibly corresponding to the change in the primary producer from rock-living cyanobacteria to free-living macro algae, and （3） an in- crease in the coefficient of the organic carbon burial by a factor of ca. 100, possibly corresponding to the onset of a biological pump driven by the flourishing metazoan and zooplankton. The former two changes took place at the start of the Shuram excursion, while the third occurred at the end of the Shuram excursion. The other two excursions are explained by the tentativ

关键词： Carbon cycle change Carbon isotope ratio Ediacaran to Cambrian Oxygen level Evolution of life

High-reliability zircon separation for hunting the oldest material on Earth： An automatic zircon separator with image-processing/microtweezers-manipulating system and double-step dating

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Geoscience Frontiers 2018年第4期9卷 1073-1083页

作者： Yukio Isozaki Shinji Yamamoto Shuhei Sakata Hideyuki Obayashi Takafumi Hirata Ken-ichi Obori Toshinori Maebayashi Satoshi Takeshima toshikazu ebisuzaki Shigenori Maruyama Department of Earth Science and Astronomy The University of Tokyo Komaba Meguro Tokyo 153-8902 Japan Department of Human Environmental Studies Yokohama National University Hodogaya Yokohama 240-8501 Japan Department of Chemistry Gakushuin University Mejiro Toshima Tokyo 171-8588 Japan Department of Geology and Mineralogy Kyoto University Kitashirakawa Oiwakecho Sakyo-ku Kyoto 606-8502 Japan Geochemical Research Center The University of Tokyo Hongo 7-3-1 Bunkyo Tokyo 113-0033 Japan Horiba Ltd. Kanda Chiyoda Tokyo 101-0063 Japan MicroSupport Co. Ltd. Hiratsuka Kanagawa 254-0035 Japan Mitani Corporation Takada Toshima Tokyo 171-0033 Japan Riken Wako Saitama 351-0198 Japan ELSI Tokyo Institute of Technology Meguro Tokyo 152-8551 Japan

Despite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine-an automatic zircon separator（AZS）. This is designed particularly for automatic pick-up of100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes：（1） image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray,（2） automatic capturing of the individual zircon grains with microtweezers, and（3） placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical（color） indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e.（1） direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing,（2） high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors（LA-MIC-ICPMS）.With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of 〉150 grains per hour can be achieved with sufficient precision（ca. 0.5%）.We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest（〉4.3 Ga） zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zircons that are over 4.0 Ga out of ca. 3800 checked

关键词： Hadean Zircon U-Pb dating LA-ICPMS Automatic mineral separator Primordial crust